Search

Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific plate subducting beneath Kamchatka. This paper presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are magnesian (Mg# = 60–75 mol %) medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = 52–59 mol %) basaltic andesites are present among the monogenic cones of Kharchinsky Lake. The rare rock varieties include the high-K basalts–basaltic andesites of dikes in the center of Kharchinsky volcano and the magnesian andesites (Mg# = 58–61 mol %) of the extrusions of Zarechnу volcano. The distribution of trace-element contents in these samples demonstrates enrichment in large-ion lithophile elements and light REEs at depletion in high field strength elements and heavy REEs, as is typical of arc rocks. The high-K basalts and basaltic andesites show anomalous enrichment in Ba (>1000 ppm), Th (>3.8 ppm), U (>1.8 ppm), Sr (> 800 ppm, Sr/Y > 50), and light REE (La > 20 ppm), and their compositions are close to those of low-Si adakites. The basalts and basaltic andesites contain phenocrysts of high-Mg olivine (up to Fo92.6) and clinopyroxene (Mg # up to 91 mol %). The rocks show petrographic and geochemical evidence of fractional crystallization, along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and an elevated Fe/Mn ratio (up to 80), which were interpreted as evidence of the participation of a pyroxenite source in the magma generation processes. The use of the Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivine associated with different sources: peridotite and pyroxenite, which were formed by a reaction between mantle-wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other lines of evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka Depression at the Kurile–Kamchatka and Aleutian subduction zone junction and testify to a significant heterogeneity of the mantle in this area. [ABSTRACT FROM AUTHOR]

New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka. [ABSTRACT FROM AUTHOR]

Anhydrite has become increasingly recognized as a primary igneous phase since its discovery in pumices from the 1982 eruption of El Chichón, Mexico. Recent work has provided evidence that immiscible sulfate melts may also be present in high-temperature, sulfur-rich, arc magmas. In this study we present partition coefficients for 37 trace elements between anhydrite, sulfate melt and silicate melt based on experiments at 0.2–1 GPa, 800–1200 °C, and fO2 > NNO+2.5. Sulfate melt–silicate melt partition coefficients are shown to vary consistently with ionic potential (the ratio of nominal charge to ionic radius, Z/r) and show peaks in compatibility close to the ionic potential of Ca and S. Partition coefficients for many elements, particularly REE, are more than an order of magnitude lower than previously published data, likely related to differences in silicate melt composition between the studies. Several highly charged cations, including V, W, and Mo are somewhat compatible in sulfate melt but are strongly incompatible in anhydrite. Their concentrations in quench material from natural samples may help to fingerprint the original presence of sulfate melt. Partition coefficients for 2+ and 3+ cations between anhydrite and silicate melt vary primarily as a function of the calcium partition coefficients (DCaAnh-Sil) and can be described in terms of exchange reactions involving the Ca2+ site in anhydrite. Trivalent cations are dominantly charge-balanced by Na1+. Most data are well fit using a simple lattice-strain model, although some features of the partitioning data, including DLaAnh-Sil > DCaAnh-Sil, suggest the occurrence of two distinct anhydrite Ca-sites with slightly different optimum radii at the experimental conditions. The ratio DSrAnh-Sil / DCaAnh-Sil is shown to be relatively insensitive to silicate melt composition and should vary from 0.63–0.53 between 1200–800 °C, based on a simple, "one-site" lattice strain model. Comparison to DSrAnh-Sil and DCaAnh-Sil calculated for natural anhydrite suggests that in most cases, including the S-rich eruptions of Pinatubo and El Chichón, the composition of anhydrite is consistent with early crystallization of anhydrite close to the liquidus of silicate melt with a composition approximately that of the bulk erupted material. This illustrates how anhydrite (and perhaps sulfate melt) provides a mechanism to transport large quantities of sulfur from significant depth to the eruptive environment. [ABSTRACT FROM AUTHOR]

Arc basalts typically have higher Fe3+/ΣFe ratios but lighter Fe isotopic compositions than mid-ocean ridge basalts (MORB); however, the reasons for these differences are unclear. Here, we report new Fe isotope data for fresh lavas from the Mariana subduction zone, along with available Fe-B-Sr-Nd isotope and B/Nb ratio data for global arc basalts, to constrain the role of serpentinite dehydration in sub-arc mantle oxidation. The basaltic lavas from the Mariana Arc, like other arc basalts from cold subduction zones in the west Pacific, have lighter Fe isotope compositions (δ56Fe = 0.02 ± 0.02‰; 1SD; n = 9) than the MORB-like δ56Fe values of the middle Mariana Trough basalts (δ56Fe = 0.09 ± 0.02‰; 1SD; n = 8). Notably, the significant negative correlations between the δ56Fe values and the proxy for the addition of serpentinite-derived fluids (i.e., B isotopes and B/Nb ratios) indicate that the serpentinite-derived fluid contribution is a first-order control on the across-arc Fe isotope variations. Dehydration of subducted slab serpentinites or dragged-down forearc serpentinites at sub-arc depths can release isotopically light Fe fluids in the form of Fe2+–SO x complexes. Sulfate-rich fluids containing adequate Fe to metasomatize the sub-arc mantle can cause light Fe isotope compositions and high Fe3+/ΣFe ratios in arc basalts. Alternatively, the low δ56Fe values and high Fe3+/ΣFe ratios of arc magmas may be generated by direct melting of the mélange, which includes dragged-down sulfate-rich forearc serpentinites. Comparatively, serpentinite-derived components are absent at back-arc depths because of the earlier breakdown of serpentine minerals. Accordingly, the back-arc basin basalts exhibit MORB-like δ56Fe-δ11B values and B/Nb–Fe3+/ΣFe ratios. Therefore, the across-arc Fe isotope variations and the decoupling of Fe3+/ΣFe and δ56Fe in global arc magmas can be explained by the different contributions of subducted serpentinites. [ABSTRACT FROM AUTHOR]

Magma-derived fluids containing chlorine and sulfur are critical for the transport of ore metals to porphyry ore-forming environments. However, experimental data on the speciation and the solubility of ore metals in these fluids at conditions relevant to the arc magmatism are scarce. In particular, the effect of redox conditions on ore metal speciation and solubilities in sulfur-bearing fluids has not yet been experimentally constrained. We performed experiments to determine the effect of oxygen fugacity (f O 2) on the solubility of Cu, Ag and Au in high-temperature, low- density, low-salinity fluids and hypersaline brine. The experiments were conducted at T = 900 °C, P = 2000 bar varying f O 2 in 7 steps between 0.5 log units below the Ni-NiO buffer (NNO − 0.5) to NNO + 2.5. A prototype rapid-quench Molybdenum-Hafnium Carbide (MHC) externally heated pressure vessel assembly was used, which was fitted with a Shaw membrane for precise control of f O 2. The fluid phase was sampled as synthetic fluid inclusions (SFI) by in situ fracturing of quartz chips during the experiments. As capsule material, Au 97 Ag 2 Cu 1 alloy was used, which imposed activities of 0.962, 0.0082 and 0.0097 for Au, Ag and Cu, respectively. The apparent solubility of Cu and Ag at the imposed metal activities increases by a factor of 7 in the H 2 O-NaCl-KCl-S low-salinity fluid with f O 2 increasing from NNO − 0.5 to NNO + 2.5. The addition of 0.198 m HCl increases the overall solubility of Cu by a factor of 1.2–2.4. The apparent solubility of Au decreases by a factor of 9 as f O 2 changes from NNO − 0.5 to NNO + 2.5. The relationship between the logarithms of the apparent Cu and Ag solubilities and f O 2 is linear and the slope of the fitted line corresponds to 1+ oxidation state of these metals indicating that they are dominantly complexed by ligands that are S-free (e.g., chloride). Thermodynamic model calculations indicate that the dominant species for Cu and Ag are NaCuCl 2 and NaAgCl 2 , respectively. For Au, the dominant species is predicted to be NaAu(HS) 2 at low and intermediate f O 2 conditions, and AuCl and NaAuCl 2 at oxidizing conditions. The measured gold solubilities at intermediate f O 2 do not indicate significant Au complexation with S species containing S in intermediate oxidation states. Considering previous studies on silicate melts and our experimental data for volatiles, we conclude that Cu and Ag likely have constant fluid/melt partition coefficients in the typical f O 2 range of arc magmatism because they dissolve in the same oxidation state in the fluid and the melt (1+) and are dominantly chloride complexed in the fluid. [ABSTRACT FROM AUTHOR]

The South Aegean Volcanic Arc overlies a slowly subducting, cool slab of oceanic-to-transitional crust, and hosts the hazardous Christiana–Santorini–Kolumbo volcanic field. In order to investigate the primitive melts feeding the volcanic field, we present major and trace element analyses of 130 olivine-hosted melt inclusions from Santorini, integrated with previously published H2O and CO2 data. Following post-entrapment corrections, we identify four endmember primitive melt types preserved in Fo ≥ 80 olivines, ranging from low-K island-arc basalts with La/Yb ~ 1.5 and 1.5–3.0 wt% H2O to andesites with La/Yb ~ 6–10 and 3.0–3.5 wt% H2O. They are consistent with melting at 1.3 to 2.3 GPa and 1350–1440 °C of variably depleted peridotitic mantle fluxed by slab-derived melts and fluids. The chemical signatures of sediment melts dominate, while those of fluids derived from the ocean crust are low compared to global datasets. This is consistent with thick sediment accumulations observed in the Hellenic trench, and with low calculated fluid fluxes from the downgoing slab. The low H2O contents estimated for the primary melts (0.8–1.8 wt%) may imply a component of decompression melting beneath the arc. Coupled with a well-constrained chronostratigraphic context, the melt inclusion archive provides a time series of mantle-derived input into the silicic crustal magmatic system over the last 530 ka. Primitive melts with La/Yb ≤ 5 have been erupted encased in olivines over the last 530 ky, without any evident time variation. Melt inclusions with La/Yb > 5 have, on the other hand, been restricted to two periods: (1) prior to the onset of major explosive volcanism at ~ 360 ka, and (2) the products of the 3.6 ka Late-Bronze-Age eruption and the 22-to-3.6 ka inter-Plinian period immediately preceding it. The observations may be explained by time-varying differential extraction of melts from deep storage zones in the mantle or lower crust, related to lithospheric rifting and caldera collapse events. Temporal variations in the supplies of slab-derived melts and fluids may also play a role. [ABSTRACT FROM AUTHOR]

• Progressive δ56Fe in the convergent margins magmas reflects continental crustal oxidation. • Fractionation of amphibole from magmas can controls the removal of the light Fe isotopes. • Control of amphibole fractionation on oxidation widely impacts worldwide arc magmas. • Amphibole can act synergistically to the formation of porphyry deposits. During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe2+-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential. [ABSTRACT FROM AUTHOR]

Subducted slabs, which include altered-oceanic crust and overlying sediments, impart geochemical and redox fingerprints to arc magmas and affect long-term geochemical evolution of the deep mantle. Yet, the oxygen fugacity of the subducting slab remains poorly constrained. Light Rare Earth Element (LREE) to chalcophile element (ChE) ratios of arc magmas may serve as redox proxies for downgoing slabs because of the difference in the compatibility of these groups of elements between sulfide (e.g., pyrrhotite: Po) and sulfate (e.g., anhydrite: Anh) whose presence in the subducting crustal lithologies depend on the oxygen fugacity of the subducting slab. However, evaluating LREE/ChE of the arc magma require a complete understanding of element partitioning between residual phases in the subducting slab and the slab derived partial melts or fluids. Although previous studies have explored trace element partitioning between sulfide and silicate melts, similar data are not available for anhydrite-melt systems at the conditions of fluid-present slab melting. Here we performed laboratory experiments at 2 GPa and 900–1000 °C to investigate the partitioning of 26 lithophile and chalcophile elements between anhydrite and a hydrous silicic slab melt. Phases were analyzed using EPMA and LA-ICPMS. Sr, Y, Ba, and the REEs were found to be compatible in anhydrite while the other lithophile and chalcophile elements behave oppositely. Since Ce is compatible in anhydrite and Mo and Cu are not, we can use the Ce/Cu and Ce/Mo ratios of the near-primary arc magmas to try to fingerprint processes involved in modifying the sub-arc mantle by reduced or oxidized slab melt components. The bulk D ( D ¯ ) for Ce, Cu, and Mo were calculated using the mineralogical modes from partial melting experiments carried out on subducting sediments and Altered-Oceanic Crust (AOC). The differences in D ¯ Ce / D ¯ Mo and D ¯ Ce / D ¯ Cu values for anhydrite-saturated and sulfide-saturated subducting lithologies indicate preferential partitioning of Ce in sulfide-saturated sediment melts, whereas Mo and Cu are partitioned into anhydrite-saturated melts. Arc localities featuring slab melt signature like Marianas, L. Antilles, Kurile, Cascades, and Mexico show that the Ce/Mo and Ce/Cu ratios can be attributed to the mixing of depleted mantle-derived melts and sulfide-saturated slab melts for some arcs (e.g., Mexico and Cascades), whereas for lavas from Marianas and Kurile, most of the Ce/Mo and Ce/Cu values needs the mixing of depleted mantle-derived melts and anhydrite-saturated slab melts. Our experimental data and the resulting geochemical modeling, therefore, suggest that oxygen fugacity of subducting crusts is likely variable from one subduction zone to another. [ABSTRACT FROM AUTHOR]

Chlorine isotopes have emerged as a new geochemical tool over the past 15 years. Most of the data consist of bulk rock data, with a minority carried out in situ on melt inclusions using secondary ion mass spectrometry. More data are necessary to understand the relationship between δ37Cl measured in melt inclusions and that in bulk rocks from the same volcanic center. Here we have analyzed a suite of melt inclusions entrapped in olivine Fo63-85, as well as some from clinopyroxene crystals, from a single hand-sample from the Vancori unit of Stromboli, Aeolian Islands. The 27 selected melt inclusions have major element compositions ranging from high potassium alkali basalt to evolved shoshonite. Their δ37Cl vary from −2.6 ± 0.1‰ to +1.2 ± 0.2‰, a far larger range than for Stromboli bulk rocks. In this dataset, the δ37Cl variation in melt inclusions is not related to Cl degassing, or to fractional crystallization. Instead, correlations between δ37Cl and S/Cl, K2O and trace element ratios suggest mixing of two Cl endmembers with distinct δ37Cl signatures. A first endmember is characterized by high potassium alkali basalt compositions, high Ba/La (∼28), high S/Cl, and high δ37Cl (>1‰), confirming the influence in the mantle source of an aqueous fluid and providing a new constraint on its composition: that it derives from the breakdown of amphibole. The second endmember has a more evolved composition, high La/Yb, low S/Cl, and low δ37Cl (

Exposures of arc crustal sections represent rare opportunities to directly evaluate lower crustal magmatic processes and their link to arc products in the middle and upper crust. Within the southernmost Sierra Nevada batholith, the Bear Valley Intrusive Suite (BVIS) exposes a contemporaneously constructed ~ 30 km thick intrusive suite, and thus is ideal for this type of examination. Here we present detailed petrography and mineral major and trace element data for the BVIS. The deepest exposed portion of the BVIS (8–9 kbars) is composed of heterogeneous mafic igneous intrusions of olivine metagabbro, olivine-hornblende orthopyroxenite, olivine-bearing hornblende norite, hornblende norite, hornblende gabbronorite, hornblendite and hornblende gabbro. Shallower crustal intrusions (3–7 kbars) are comparatively homogeneous and dominated by hypersthene-bearing and hypersthene-free tonalites. Using amphibole-plagioclase geothermometry, we show that the mafic lower crustal intrusions crystallized over a wide temperature range from 850 to 1070 °C, highlighting mafic igneous fractionation during isobaric cooling in the lower crust of the Sierran arc, while tonalitic liquids were emplaced at temperatures < 800 °C in the middle and upper crust. Calculated trace element melt compositions in equilibrium with amphibole in lower crustal gabbros are similar to measured tonalite bulk compositions and support the generation of tonalites through fractionation of the observed gabbros. Further, petrography and mineral chemistry suggest multiple distinct crystallization sequences recorded in the different types of gabbro, requiring the presence of coexisting parental melts with contrasting compositions and H2O contents. Using available experimental data, we develop a model by which mixing of variably fractionated dry and wet magmas with similar viscosities followed by crystallization-differentiation in the deep crust to explain the formation of uniform tonalitic melts at shallower crustal levels in the BVIS. This process also explains the unusual predominance of orthopyroxene in the BVIS, and the limited aluminum enrichment compared to experimental differentiation sequences of hydrous basalts. Considering the similar geochemical characteristics of intermediate and felsic igneous rocks from the Sierra Nevada batholith and the Cascades, mixing magmas of variable H2O contents in the lower crust represents a viable petrological process to produce SiO2-rich liquids that may be more common than previously recognized. [ABSTRACT FROM AUTHOR]

Calcium (Ca) is a fluid-mobile element and plays a pivotal role in regulating carbon cycling through the precipitation of calcium carbonate. Calcium isotopes have been proposed as a promising tracer for deep carbon recycling because of the higher Ca content and distinctive Ca isotope composition in carbonate relative to the mantle. To test the effect of carbonate recycling on Ca isotope composition in arc magma and determine the fate of carbonate in subduction zones, we analyzed a suite of magmatic rocks from Central America, Southern Lesser Antilles, and Central-eastern Aleutian arcs. These three island arcs have variable thermal regimes from intermediate to cool and sediment types from carbonate-rich to carbonate-poor. Thirty-six arc magmas display δ44/40Ca values from 0.70‰ to 1.00‰ which are similar to the range of fresh mid-ocean ridge basalts (MORBs). Their average δ44/40Ca (0.82 ± 0.12‰, 2SD, n = 36) is lower than the estimates of Bulk Silicate Earth (~0.94‰, BSE) and can be ascribed to the effect of mantle partial melting. Despite strong evidence of C and Sr isotopes that various degrees of carbonate metasomatism occurred in the source of these rocks, they do not show Ca isotope variation. Based on the flux model, the buffering effect of the mantle wedge can homogenize the different carbonate Ca isotope signals at subduction zones where carbonate recycling efficiency is lower than 30% or the carbonates are mainly brought by carbonate-bearing altered oceanic crust. For subduction zones with carbonate recycling efficiency higher than 30%, the MORB-like δ44/40Ca in arc magmas may also reflect the storage of Ca in the mantle wedge and the retention of Ca in the descending slab beyond sub-arc depth. Combined with the fact that carbonate Ca isotope signals have been observed in OIBs and intra-plate igneous rocks, our new data indicates that most subducted Ca may finally enter the deep mantle via corner flow or within the descending slab. [ABSTRACT FROM AUTHOR]

Magmatic sulfur plays an important role in affecting mantle oxidation state, volcanic eruption, formation of ore deposits, and global climate change. To better understand the sulfur cycle in subduction zones and to constrain the sulfur concentration at anhydrite saturation (SCAS) in subducting slab-derived silicate melts, forty-three experiments were conducted at 0.5–5 GPa and 900–1200 °C using a piston cylinder and a multi-anvil apparatus. The experimentally produced silicate melts are rhyodacitic to rhyolitic in composition, and the measured SCAS values range from 170 to 3500 ppm. The SCAS values increase with increasing temperature and the water and CaO content of the silicate melts, but the effect of pressure varying from 0.5 to 5 GPa is negligible. Using our new and all available literature SCAS data (n = 252), we tested the accuracy of all previous SCAS models that were calibrated for predicting SCAS in silicate melts at various conditions. We find that the Z–T model (Zajacz and Tsay, 2019) works as the greatest SCAS model in capturing all SCAS data with a mean and median absolute error of 5% and 4%, respectively. The success of the Z–T model in capturing all SCAS data demonstrates its robustness in predicting SCAS in silicate melts relevant for magmatism in subduction zones. Applying the Z–T model to slab melting reveals that slab-derived silicate melts of global subduction zones can dissolve 130–1200 ppm S6+, but they cannot contribute enough sulfur to explain the estimated sulfur abundance (200–500 ppm) in the metasomatized sub-arc mantle, which thus requires the addition of sulfur by slab-derived aqueous fluids. The addition of slab S6+ can cause oxidation of the sub-arc mantle in an fO 2 range of FMQ+0.5 to FMQ+2, consistent with the f O 2 values observed for the metasomatized sub-arc mantle peridotites. However, during partial melting of the metasomatized sub-arc mantle, S2− would play as a reducer and the f O 2 of primitive arc basalts cannot be higher than FMQ+0.5 to FMQ+1, which is consistent with the sub-arc mantle f O 2 inferred from the V–Sc, Fe–Zn, V–Ga, and Cu–Re systematics of primitive arc basalts. The f O 2 above FMQ+1 of arc basalts may thus be obtained during magmatic differentiation in the lithosphere. We finally modeled the fate of S6+ during the differentiation of parental arc basalts with fO 2 varying from FMQ+0.5 to FMQ+1.5 in a thickened continental arc setting. We find that significant fractions of S6+ in the parental arc basalts are converted into S2− and lost in sulfides during arc magmatic differentiation, and the estimated 400 ppm sulfur in Earth's continental crust implies that Earth's continental crust cannot have formed from arc basalts with f O 2 significantly higher than FMQ+0.5 to FMQ+1. [ABSTRACT FROM AUTHOR]

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting. [ABSTRACT FROM AUTHOR]

To constrain the behavior of chalcophile (sulfide-loving) elements during arc magmatic differentiation and to understand the formation conditions of Earth's continental crust, the partition coefficients (D) of Mn, Co, Cu, Zn, As, Se, Mo, Ag, Cd, Sn, Sb, Te, Re, Au, Pb, and Bi between monosulfide-solid-solution (MSS), Cu-rich sulfide liquid (SL; containing 11–45 wt.% Cu), and hydrous silicate melt (SM) of basaltic to dacitic compositions were determined at 1000–1200 °C, 0.5–1.0 GPa, and f O 2 1–1.5 log units above the fayalite–magnetite–quartz (FMQ) buffer. The D SL / S M values are 16–160 for Co, 1100–8400 for Cu, 50–220 for Se, 1200–5900 for Ag, 50–1800 for Cd, 700–3300 for Te, 15–510 for Re, 5700–90,000 for Au, 20–440 for Pb, and 140–3300 for Bi. The D SL / S M values for Mn, Zn, As, Mo, Sn, and Sb are below 1–40. The D MSS / S M values are 55–260 for Co, 530–1700 for Cu, 74–110 for Se, 30–110 for Ag, 4–40 for Cd, 15–70 for Te, 200–5900 for Re, and 140–270 for Au. The D MSS / S M values for Mn, Zn, As, Mo, Sn, Sb, Pb, and Bi are below 1–3. The D SL / S M of Au increase with increasing Cu content of the sulfide liquid, but the D SL / S M of the other elements little affected by the Cu concentration in the sulfide liquid. Because of their distinct dissolution mechanisms in the silicate melt, the D SL / S M and D MSS / S M of Mn, Co, Zn, Cd, Sn, and Pb are mainly controlled by the silicate melt FeO tot content ([FeO tot); the D SL / S M and D MSS / S M for Re, Mo, As, Sb, and Bi are mainly controlled by [FeO tot and f O 2 ; the D SL / S M and D MSS / S M for Cu, Ag, and Au are mainly controlled by [FeO tot and the content of reduced sulfur in the silicate melt; and the D SL / S M and D MSS / S M for Se and Te are mainly controlled by f O 2. Using all available D SL / S M and D MSS / S M data, a partitioning model was developed for predicting D SL / S M and D MSS / S M of chalcophile elements as a multi-function of temperature, pressure, f O 2 , and silicate melt and sulfide compositions. Sulfide phase relations suggest that the sulfides precipitating from arc magmas containing >100 µg/g Cu in the silicate melt occur as Cu-rich sulfide liquid, whereas the sulfides precipitating from arc magmas containing 30–70 µg/g Cu in the silicate melt occur as mixed MSS and Cu-rich sulfide liquid. Modeling the Cu evolution trends of global arc magmas illustrates that the precipitating sulfides are dominantly MSS in continental arcs with a crustal thickness of >30 km, with the proportion of sulfide liquid being less than 20%; whereas, in island arcs with a crustal thickness of <20 km, the proportion of sulfide liquid may reach up to 90%. Applying the model to predict the evolution trends of Ag, As, Sn, Sb, Se, Mo, Re, Mo, Au, Pb, and Bi in global arc magmas under various f O 2 conditions, we find that when no more than 10% of the precipitating sulfides are sulfide liquid, the chalcophile element patterns of oxidized magmas (0–1 log unit above FMQ) in continental arcs match that of Earth's bulk continental crust, which implies that Earth's continental crust formed mainly in oxidized continental arcs. [ABSTRACT FROM AUTHOR]

Under the right circumstances, fluid-fluxed crustal melting occurs, as demonstrated by some amphibolite-facies migmatites, but the relatively low temperatures involved make this mode of formation an unsatisfactory model for most silicic magma genesis in arcs. The concept of silicic arc magma formation through either partial melting of metabasalts or fractionation of hydrous basaltic parent magmas should also be treated with scepticism, as both these processes produce sodic and moderately to strongly peraluminous liquids that are chemically unlike most arc silicic rocks and glasses. Furthermore, if the silicic magmas are formed in the deep crust, the fractionation model predicts evolved liquids with extreme H2O contents. Thus, the answer to the question posed in the title is that neither of these two models, in isolation, seems likely for the formation of the majority of the more silicic magmas in arc environments. Given the totality of the evidence, we favour models in which high-T processes dominate. These could include fluid-absent partial melting of older non-basaltic arc crust, entrainment of source-derived crystal cargos, hybridisation with mantle-derived magmas and, in some cases, crystal fractionation of andesitic magmas at shallow crustal levels. [ABSTRACT FROM AUTHOR]

Recent geodynamic models and geothermometers suggest that slabs in intermediate to hot subduction zone cross the water-saturated basalt solidus, indicating that hydrous silicate melts are important agents of mass transfer from slab to mantle wedge beneath arcs. Yet the effects of basaltic crust-derived hydrous melt fluxing on mantle wedge melting are poorly known. Here we present the melting phase relations of a depleted peridotite + a MORB-derived hydrous silicate melt at a melt:rock mass ratio of 0.1 and 0.05 (3.5 and 1.7 wt.% H 2 O, respectively) to simulate fluid-present partial melting of a depleted peridotite, which has been metasomatized by a hydrous silicate melt derived from subducting basaltic crust. Experiments were performed at 2–3 GPa and 900–1250 °C in a piston cylinder, using Au and Au 75 Pd 25 capsules. Amphibole (7–10 wt.%) is stable up to 1000 °C at 2 and 3 GPa coexisting with an assemblage dominated by olivine and opx and with minor fractions of cpx and garnet at 3 GPa. The apparent fluid-saturated solidus of our bulk composition is located at 1000–1050 °C, coinciding with the exhaustion of amphibole at 2 and 3 GPa. Amphibole is exhausted between 0 and 5 wt.% melting at 2 and 3 GPa and dominates the melting reactions in this melting interval along with opx, generating SiO 2 and Al 2 O 3 -rich, and FeO*- and MgO-poor primitive andesites under fluid-saturated conditions. The melting reactions during low-degree, fluid-saturated melting are incongruent, consuming opx and producing olivine + SiO 2 -rich melts and is observed over a wide range of starting compositions and pressures from this study and others. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 – 4.2 wt.% H 2 O can reproduce the major oxide spread and trends of primitive arc magmas globally. From this comparison, it is clear that differences solely in the pressure of hydrous mantle melting, where the partial melts are fluid-under saturated, can account for the first order trends observed in experimental and natural data, with differences in temperature and composition contributing to the compositional spread within these trends. The ubiquity of andesite genesis over a wide range of pressures and bulk compositions during aqueous fluid-saturated melting suggests that the relative rarity of primitive andesitic melt flux through the crust could be related to the fact that such melts are only produced at the base of the mantle wedge where temperatures are relatively low. As fluid-saturated andesitic melts ascend into the hotter core of the mantle wedge, they are likely consumed by higher-degree, fluid-undersaturated melting generating more common hydrous basaltic melts. [ABSTRACT FROM AUTHOR]

• Fluid signatures in Izu arc lavas associated with isotopically heavy Mo. • Slab-derived fluids have heavy Mo isotopic composition of δ98/95Mo = 0.1–0.25‰. • Co-variations with radiogenic isotopes link fluid input and mantle heterogeneity. • Stronger control of slab fluids over more depleted mantle sources. • Heavy Mo lost through slab fluids leaves light δ98/95Mo signature in residual slab. Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their δ98/95Mo (the relative difference in measured 98Mo/95Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143Nd/144Nd and 176Hf/177Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites. [ABSTRACT FROM AUTHOR]

The fractionation of hornblende is a common phenomenon in arc magmas, and gives rise to a number of notable geochemical characteristics. However, it is often cryptic, with limited direct petrographic evidence for the fractionation, or even presence of amphibole in volcanic suites. Newly identified hornblendites in the SE Tibet represent direct evidence for hornblende fractionation in the Gangdese arc. The hornblendites have age ranges from 95.3 ± 0.2 to 92.6 ± 0.2 Ma, recording magmatic activities over a ca. 2 Myr interval. Textural relationships indicate that hornblendites form by peritectic reactions between an evolving melt and earlier-formed clinopyroxenites. Relict clinopyroxenes and primitive pargasite record melt evolution from high Sr/Y basaltic andesite to dacite, during cooling from ∼1050 °C to ∼850 °C in the mid to lower crust. The clinopyroxene precursors interacted with water-rich high SiO 2 fluids/melts at ca. 800 °C. Zircons have high δ18O values (up to 7.62‰) and positive ε Hf (t) values (up to +13.0), but bulk rock data show high 87Sr/86Sr ratios (up to 0.7093) and negative ε Nd (t) values (ca. −3.2). This decoupled signature implies the primary magmatic source was metasomatized by 5–10% recycled subducted sediments. Magmas have a high oxygen fugacity (ΔNNO can up to +2.40) through a combination of metasomatism during subduction, and subsequent melt-mush interaction that is recorded by zoned hornblendes. Metasomatism, widespread and long-term fractionation of amphibole ± garnet ± clinopyroxene, and high oxygen fugacity, are all positive indicators for the fertility of arc magmas with respect to porphyry copper formation. This study demonstrates that late-stage melt-mush interaction can contribute to peritectic hornblendite formation and facilitate magma fertility. [ABSTRACT FROM AUTHOR]

The behavior of Re in magmas is of great significance to understanding the distribution of Re in Earth's different reservoirs. In order to better constrain the behavior of Re in magmas of different tectonic settings, the partition coefficients (D) of Re and Mo between sulfide liquid (SL), monosulfide solid solution (MSS), and basaltic to dacitic melts (SM) were determined using a piston-cylinder apparatus. The experiments were conducted at 1050-1200 °C, 0.7-1.5 GPa, and oxygen fugacity (f O 2) of ∼FMQ−1.8 to FMQ+1.5. The D values range from 0.11 to 76 for D Mo SL / SM , 0.32 to 219 for D Mo MSS / SM , 45 to 76000 for D R e SL / SM , and 280 to 160000 for D R e MSS / SM . All the D values increase with decreasing f O 2 and the FeO content in silicate melt. We also find that D R e SL / SM and D R e MSS / SM are strongly correlated with D Mo SL / SM and D Mo MSS / SM regardless of the temperature, pressure, silicate melt composition, and f O 2. This correlation, which can be explained by the similar dissolution behaviors of Re and Mo in silicate melt, provides an approach to use D Mo SL / SM or D Mo MSS / SM to predict D R e SL / SM or D R e MSS / SM in magmas. The newly obtained D R e SL / SM , D R e MSS / SM , D Mo SL / SM , and D Mo MSS / SM , in conjunction with D C u SL / SM , were applied to constrain the behavior of Re during magmatic genesis and differentiation. The results show that the observed Re contents in primitive MORBs can be explained only if the Re abundance in the depleted mantle is between 0.12 and 0.28 ppb. To satisfactorily explain both the Cu and Re contents in primitive arc basalts, (1) the f O 2 of the subarc mantle during melting must be between FMQ and FMQ+1, and (2) the subducting slab must contribute both Re and S to the subarc mantle, although the extent of slab contribution differs between arcs. During arc magmatic differentiation, a significant fraction of Re is sequestered by sulfide, which results in a heterogeneous distribution of Re in the continental crust and the formation of Re-rich cumulates in the deep crust. The heterogeneous distribution of Re in the continental crust may lead to overestimations or underestimations of the Re abundance in the continental crust. The delamination of such Re-rich cumulates can form Re-rich and radiogenic Os-rich reservoirs in the depleted mantle, which partially explains the "missing Re" problem. Our study illustrates that sulfides play a key role in controlling the behavior of Re in magmas of mid-ocean ridge and arc settings and consequently, the distribution of Re in Earth's crust and mantle. • Sulfide/silicate melt D s for Re and Mo vary with f O 2 and the melt FeO content. • A strong correlation exists between the sulfide/silicate melt D s for Re and Mo. • Re abundance in the depleted mantle is >0.12 ppb, explaining the MORB Re contents. • Subarc mantle melting occurs at FMQ-FMQ+1, correlated with slab S, Re contribution. • Sulfide is an important host for Re during arc magmatic differentiation. [ABSTRACT FROM AUTHOR]

This paper presents UPb zircon dating and element compositions for Miocene intrusive rocks in NE Tafresh situated in the central Urumieh‐Dokhtar Magmatic Arc. These intrusive rocks, consisting of granodiorite and diorite, were emplaced during the Early Miocene (19.07–20.37 Ma), following extensive submarine volcanic activity in the Eocene. In normalized multi‐element diagrams, all the analysed rocks are characterized by enrichments in large ion lithophile elements (e.g., Ba, Rb, and Sr) and depletions in high field strength elements (e.g., Nb, Ta, and Hf) and display geochemical features typical of subduction‐related calc‐alkaline arc magmas. The enrichment of light rare earth elements and flat heavy rare earth elements patterns reflect amphibole fractionation from relatively hydrous, calc‐alkalic magmas. The geochemical features and ages of the Tafresh intrusive rocks suggest that the Neo‐Tethys Ocean did not close completely in the region until the Miocene and is consistent with a diachronous collision starting in the NW and closing later in the SE. [ABSTRACT FROM AUTHOR]

Geochemical characteristics of arc magmas reflect incorporation of subducted materials to their mantle wedge sources in subduction zones. Subduction component addition has been proposed to modify the Zn isotopic budget of arc magmas. However, the lack of a systematic study on Zn isotopic compositions of arc magmas hampers a better understanding of Zn isotope behavior in subduction zones. To address this issue, we have determined Zn isotopic compositions of 37 well-characterized arc rocks from the Kamchatka and Central-Eastern Aleutian arcs. These rocks record contributions of fluids and melts derived from altered oceanic crust (AOC) without overprints of sediment melts and thus allow focus on the potential effects of AOC-derived fluids and melts on the Zn isotopic budget of arc magmas. For comparison, nine basalts from the Gakkel, Mid-Atlantic and Southeast Indian Ridges, and the Lau Basin and nine adakites from Central America were also analyzed. Rocks from the Kamchatka-Aleutian arcs have δ 66 Zn from 0.16 to 0.31‰ that are mostly similar to those of mid-ocean ridge basalts (MORBs), back-arc basin basalts (BABBs), and adakites (δ 66 Zn = 0.23–0.33‰), but a significant number of arc samples also display δ 66 Zn higher than that of the depleted MORB-type mantle (DMM), indicating Zn isotope fractionation during magmatic processes and/or modifications of the mantle wedge Zn isotopic budget by incorporation of AOC-derived fluids and melts. The lack of correlations of δ 66 Zn with geochemical indicators of magma differentiation (e.g., MgO, SiO 2 , and Zn/Fe T ) indicate that fractionation of olivine, pyroxene, and magnetite has a limited effect on the Zn isotopic compositions of arc magmas. Even though the mantle sources of arc rocks investigated here are strongly affected by AOC-derived fluids and melts that have higher δ 66 Zn compared to the DMM, we observe no systematic variations of δ 66 Zn with indicators of subduction components (e.g., Ba/La, Ba/Th, Sr/Y, Hf/Lu and 87 Sr/ 86 Sr). This suggests that insignificant transport of Zn from the subducting Pacific slab to the Kamchatka and Central-Eastern Aleutian mantle wedge. Our model calculations suggest that the observed offset of δ 66 Zn between the mantle and arc magmas can be attributed to isotope fractionation during partial melting with no need for contributions from subduction components. [ABSTRACT FROM AUTHOR]

Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their environmental impact. To date, bromine partitioning between silicate melts and the gas phase is very poorly constrained, with the only relevant experimental studies limited to investigation of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments were performed using natural silicate glasses with mafic, intermediate and silicic compositions. For each composition, experiments were run with various Br contents in the initial fluid (H 2 O–NaBr), at T – P conditions representative of shallow magmatic reservoirs in volcanic arc contexts (100–200 MPa, 900–1200 °C). The resulting fluid/melt partition coefficients (D Br f/m ) are: 5.0 ± 0.3 at 1200 °C–100 MPa for the basalt, 9.1 ± 0.6 at 1060 °C–200 MPa for the andesite and 20.2 ± 1.2 at 900 °C–200 MPa for the rhyodacite. Our experiments show that D Br f/m increases with increasing SiO 2 content of the melt (as for chlorine) and suggest that it is also sensitive to melt temperature (increase of D Br f/m with decreasing temperature). We develop a simple model to predict the S–Cl–Br degassing behaviour in mafic systems, which accounts for the variability of S–Cl–Br compositions of volcanic gases from Etna and other mafic systems, and shows that coexisting magmatic gas and melt evolve from S-rich to Cl–Br enriched (relative to S) upon increasing degree of degassing. We also report first Br contents for melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and calculate the mass of bromine available in the magma reservoir prior to the eruptions under consideration. The discrepancy that we highlight between the mass of Br in the co-existing melt and fluid prior to the Merapi 2010 eruption (433 and 73 tons, respectively) and the lack of observed BrO (from space) hints at the need to investigate further Br speciation in ‘ash-rich’ volcanic plumes. Overall, our results suggest that the Br yield into the atmosphere of cold and silicic magmas will be much larger than that from hotter and more mafic magmas. [ABSTRACT FROM AUTHOR]

Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts' parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts' parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6-2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H loss) and/or dehydration (HO loss), as recorded by their variable HO contents and D/ H and Fe/Fe ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi's often erratic eruptive behaviour. [ABSTRACT FROM AUTHOR]

The Aeolian arc (Italy) is characterized by some of the strongest along-the-arc geochemical variations in the planet, making it an ideal location to study the effect of subducting components in modifying the mantle source of island arc melts. Here, we use high-precision element concentrations in primitive phenocrystic olivine from basalts along the arc to elucidate the effects of mantle source modification by the subduction process. Olivines from this arc have Ni concentrations and Fe/Mn ratios that show similarity to peridotite sources that melted to produce mid-ocean ridge basalts. Nevertheless, they also have systematically lower Ca concentrations and Fe/Mn ratios that broadly overlap with olivines from the available global arc array. These phenocrysts also do not show significant variations in Ca as a function of olivine forsterite content. The global data suggest that all olivines crystallizing from island-arc melts have suppressed Ca concentrations and Fe/Mn ratios, relative to olivines derived from melts at intraplate and mid-ocean ridge settings suggesting elevated H 2 O concentrations and higher oxidation state of the equilibrium melts. Based on olivine chemistry, we interpret a predominantly peridotite source (fluxed by subduction fluids) beneath the Aeolian Arc and also for other examples of arc-related lavas. [ABSTRACT FROM AUTHOR]

Intra-crustal differentiation of primary arc magmas has been suggested to play an important role in continental crust growth. Here we use the zinc (Zn) isotopic systematics of deep continental crustal rocks and island arc basalts to further assess this differentiation model. We present high-precision Zn isotope data for late Archaean deep crustal xenoliths from the southeastern North China craton and early Cretaceous island arc basalts from the Central Philippines. These xenoliths have chemical compositions broadly similar to the globally average middle and lower continental crust and display trace elemental patterns typical of arc magmas (e.g., enrichment in LILEs and depletion in HFSEs). We obtain average δ66Zn JMC-Lyon values of 0.31 ± 0.03‰ (2se, n = 28) and 0.27 ± 0.04‰ (2se, n = 7) for the lower and middle continental crust, respectively, both of which are slightly but statistically higher than the global island arc magma average (δ66Zn = 0.22 ± 0.01‰, 2se, n = 64). This difference cannot be attributed to diffusion, metamorphism, metasomatism and fractionation of clinopyroxene or Fe Ti oxide of the lower crustal rocks or their igneous protoliths. Instead, the gradual increase of δ66Zn with decreasing (Dy/Yb) N (subscript N represents chondrite-normalization) in the studied lower crustal xenoliths points to fractionation of isotopically light amphibole during intra-crustal differentiation of their igneous protoliths. In addition, we find that island arc magmas generated in thicker arcs have systematically higher δ66Zn than those in thinner arcs, and (La/Yb) N and (Dy/Yb) N ratios display negative and positive correlations with MgO contents in global island arc lavas, respectively. These observations collectively suggest that amphibole fractionation plays an important role in shifting the chemical compositions of arc lavas and making the andesitic bulk continental crust. • A new estimate of average δ66Zn of the lower continental crust is 0.31 ± 0.03‰ (2se). • The average δ66Zn of global island arc lavas (0.22 ± 0.01‰, 2se) is lower than that of the average lower continental crust. • δ66Zn values of island arc lavas positively correlate with arc thickness. • Amphibole fractionation exerts an important control on arc magma differentiation and continental crustal formation. [ABSTRACT FROM AUTHOR]

While sulfur isotopes have proven powerful tracers of sulfur (re)cycling within subduction zones, the origin of the 34S-enrichment seen in arc magmas remains a subject of debate, with competing hypotheses implicating both mantle and crustal processes. Herein, we investigate these competing models through the study of the Early–Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains, south-central Alaska, reporting the sulfur isotope systematics of rocks spanning a depth transect that extends from the upper mantle (∼0.9–1.2 GPa, ∼30–35 km) up into the mid-crust (∼0.2–0.5 GPa, ∼5–15 km). Marked by an increase in δ34S values from −1.28 to +5.61‰, these data reveal a significant isotope effect associated with melt differentiation, with 32S being preferentially sequestered from primitive basaltic melts into ultramafic and mafic cumulates, yielding progressively more evolved melts enriched in 34S. Leveraging a quantitative petrological model that constrains the liquid line of descent and sulfide saturation as a function of sulfur content, oxygen fugacity, pressure and temperature, we then demonstrate that, in the presence of dissolved oxidized sulfur species, the saturation of immiscible magmatic sulfides is capable of generating the observed sulfur isotope fractionation. While fluid saturation is unlikely to occur at lower crustal depths, ascending melts will almost certainly experience fluid saturation and degassing. As a result, sulfide immiscibility and magma degassing are not mutually exclusive and may, instead, represent complementary processes that combine to explain the observed range of positive δ34S compositions. In Talkeetna, crustal assimilation is unlikely to have played a role to increase δ34S values as there was little to no assimilation of pre-existing oceanic crust containing seawater sulfate during the formation of the Talkeetna Arc section. Furthermore, our model suggests that primitive melt in isotopic equilibrium with the most primitive ultramafic cumulates have mantle-like δ34S values between ∼0 and +1.1‰, which suggest limited input of 34S-enriched slab-derived sulfur into the sub-arc mantle. Collectively, these findings emphasize that mantle-like δ34S values for primitive arc melt may be common, while revealing the underappreciated importance of deep crustal crystallization of immiscible magmatic sulfides to generate positive δ34S compositions in erupted arc magmas. [ABSTRACT FROM AUTHOR]

Highly siderophile elements such as Pt and Re are important tracers for Earth's crust–mantle differentiation. The behavior of Pt and Re in oxidized arc magmas are thought to be controlled by their strong partitioning into sulfides and/or magnetite. Disequilibrium fractionation of sulfide liquid was proposed to explain the Pt systematics in magmas when using sulfide liquid–silicate melt partition coefficients of Pt (D Pt Sul / Sil ) of 105. However, the segregating sulfides are coexisting monosulfide-solid-solution (MSS) and Cu-rich sulfide liquid in thin island arcs, but mainly as MSS in thick continental arcs. Here we experimentally determine the MSS–silicate melt partition coefficients of Pt (D Pt MSS / Sil ) and magnetite–silicate melt partition coefficients of Re (D Re Mag / Sil ) at 0.4–0.5 GPa, 1020–1100 °C, and oxygen fugacity (f O 2) from below FMQ-1.7 to ∼FMQ+2.5, using a piston cylinder apparatus. The results show that D Pt MSS / Sil range between 20–8700 and increase with increasing Pt concentration (20–530 μg/g) in MSS at a given f O 2 , which can be explained by the presence of Pt-rich nuggets in MSS and the non-Henrian law behavior of D Pt MSS / Sil . This finding suggests that Pt in oxidized arc magmas is much less chalcophile than previously thought. The obtained D Re Mag / Sil increase from 0.5 to 2.9 with f O 2 decreasing from ∼FMQ+2.5 to FMQ+1, suggesting that Re is incompatible to slightly compatible in magnetite in oxidized arc magmas. Re dissolves as Re4+ in magnetite through the substitution of Re4+ + Fe2+ for 2Fe3+. The application of our new D s together with previous D s to a magmatic differentiation model, which involves an equilibrium fractionation of MSS and/or sulfide liquid, magnetite, and silicate minerals, can fully reproduce the evolution trends of Pt, Re, and Cu in oxidized, sulfide-saturated arc magmas. Our model results demonstrate that equilibrium fractionation of sulfides predominantly controls the behavior of Pt and Re in oxidized arc magmas with sulfide saturation. • Determine monosulfide-solid-solution–silicate melt partition coefficients of Pt (D P t M S S / S i l). • D P t M S S / S i l of 20–8700 explained by non-Henrian law and presence of Pt nuggets in MSS. • Magnetite–silicate melt D R e are between 0.5–2.9 in oxidized arc magmas. • Equilibrium sulfide fractionation controls the Pt and Re behavior in oxidized arc magmas. [ABSTRACT FROM AUTHOR]

Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (e.g., serpentinite, altered oceanic crust – AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ 98 / 95 Mo) and B concentrations on Aleutian lavas (n = 59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR) (n = 6), AOC (n = 2) and Pacific sediments (DSDP 183, ODP 886) (n = 5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (e.g., B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ 98 / 95 Mo and low B/Ce values in the western section of the arc (B/Ce = 0.15–1.07; δ 98 / 95 Mo =−0.38 to +0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce = 1.20–2.60; δ 98 / 95 Mo = +0.03 to +0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce = 0.76 and δ 98 / 95 Mo = −0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ 98 / 95 Mo (−0.47 to −0.32 and +0.17 to −1.9‰, respectively), while serpentinites have extremely high δ 98 / 95 Mo (up to +1.09‰) and high B/Ce (∼22000). Based on the low δ 98 / 95 Mo in sediments and AOC, and lack of correlation between along-arc δ 98 / 95 Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ 98 / 95 Mo but high Mo enrichments (e.g., Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ 98 / 95 Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ 98 / 95 Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones. • Mo isotope compositions trace source characteristics in magmatic systems. • Serpentinites are a heavy-Mo isotope and high B/Ce source in subduction magmas. • Slab melting under eclogitic facies leads to nearly absent Mo isotope fractionation. [ABSTRACT FROM AUTHOR]

To understand fully the role of amphibole in the fractionation of Nb/Ta during arc magma evolution, we conducted experiments with mid-K and high-K basalts to determine amphibole/melt Nb, Ta and Ti partition coefficients (DNb, DTa and DTi) at variable conditions of bulk TiO2, P, T, H2O and fO2. The experimental results show that, at crustal pressures, amphibole is the most important crystalline phase in hydrous basaltic systems. The amphibole/melt Nb, Ta, and Ti partitioning results are 0.16-0.90 for DNb, 0.13-0.68 for DTa, 1.81-10.63 for DTi and 0.76-2.81 for DNb/DTa. Bulk TiO2 and fO2 show no observable effects. T and H2O, in addition to the compositions of amphibole and melt, are the main affecting factors. DNb, DTa, DTi and DNb/DTa increase with decreasing temperature, amphibole Mg# and melt H2O content and increasing melt polymerization. During cooling and crystallization of arc magmas at crustal pressures, amphibole Mg# decreases and melt polymerization increases, leading to significant increase in amphibole/melt DNb, DTi and DNb/DTa. Nb/Ta fractionation in evolved melts will thus be enhanced with crystallization progress. Meanwhile, melt H2O content will increase with the degree of crystallization, which slows down the increase in these D values. Therefore, the trend and extent of Nb/Ta fractionation in the melt by amphibole critically depends on temperature and melt H2O content. Only low temperatures or low H2O contents at high temperatures lead to high D values. For arc magmas with an average H2O of ~3.9wt %, DNb and DNb/DTa are in general >0.40 and >1.20, respectively, which explains why amphibole fractionation results in lower Nb/Ta ratios in evolved arc magmas. The bulk Nb/Ta fractionation trend during arc magma evolution appears to be generally controlled by fractional crystallization of amphibole. Experimental and modeling results suggest that amphibole is a main fractionating phase during arc magma evolution and continental crust formation. [ABSTRACT FROM AUTHOR]

The formation of the continental crust at subduction zones involves the differentiation of hydrous mantle-derived magmas through a combination of crystallization and crustal melting. However, understanding the mechanisms by which differentiation occurs at depth is hampered by the inaccessibility of the deep crust in active continental arcs. Here we report new high-pressure electrical conductivity and petrological experiments on hydrated andesitic melt from Uturuncu volcano on the Bolivian Altiplano. By applying our results to regional magnetotelluric data, we show that giant conductive anomalies at mid-crustal levels in several arcs are characterized by relatively low amounts of intergranular andesitic partial melts with unusually high dissolved water contents (≥8 wt.% H 2 O). Below Uturuncu, the Altiplano-Puna Magma Body (APMB) displays an electrical conductivity that requires high water content (up to 10 wt.%) dissolved in the melt based on crystal-liquid equilibria and melt H 2 O solubility experiments. Such a super-hydrous andesitic melt must constitute about 10% of the APMB, the remaining 90% being a combination of magmatic cumulates and older crustal rocks. The crustal ponding level of these andesites at around 6 kbar pressure implies that on ascent through the crust hydrous magmas reach their water saturation pressure in the mid-crust, resulting in decompression-induced crystallization that increases magma viscosity and in turn leads to preferential stalling and differentiation. Similar high conductivity features are observed beneath the Cascades volcanic arc and Taupo Volcanic Zone. This suggests that large amounts of water in super-hydrous andesitic magmas could be a common feature of active continental arcs and may illustrate a key step in the structure and growth of the continental crust. One Sentence Summary: Geophysical, laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the Central Andes, Cascades and Taupo Volcanic Zone image the ponding of super-hydrous andesitic melts which contributes to the growth of continental crust. [ABSTRACT FROM AUTHOR]

Melt inclusion data from primitive arc basalts from Mexico and Kamchatka show clear positive correlations of 'fluid mobile element'/H2O ratios with the Cl/H2O ratio, suggesting that the trace element content of subduction zone fluids is strongly enhanced by complexing with chloride. This effect is observed for large-ion lithophile (LILE) elements, (e.g., Rb and Sr), but also for the light rare earth elements (REE, e.g., La and Ce) as well as for U. The correlations of these elements with Cl/H2O cannot be explained by the addition of sediment melts or slab melts to the mantle source, since Cl has no effect on the solubility or partitioning of these elements in silicate melt systems. On the other hand, the observed relationship of trace element abundance with Cl is consistent with a large body of experimental data showing greatly enhanced partitioning into aqueous fluid upon addition of chloride. Accordingly, it appears that a dilute, Cl-bearing aqueous fluid is the main carrier of LILE, light REE, and U from the slab to the source of melting in arcs. Moreover, elevated Ce/H2O ratios clearly correlate with fluid salinity and therefore are not suitable as a 'slab geothermometer.' From a synopsis of experimental and melt inclusion data, it is suggested that the importance of sediment or slab melting in the generation of arc magmas is likely overestimated, while the effects of trace element scavenging from the mantle wedge may be underestimated. Moreover, establishing reliable data sets for the fluid/mineral partition coefficients of trace elements as a function of pressure, temperature, and salinity requires additional efforts, since most of the commonly used experimental strategies have severe drawbacks and potential pitfalls. [ABSTRACT FROM AUTHOR]

Magmas play a key role in the genesis of epithermal and porphyry ore deposits, notably by providing the bulk of ore metals to the hydrothermal fluid phase. It has been long shown that the formation of major deposits requires a multi-stage process, including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments. Both aspects have been intensively studied for most of noble metals in subsurface conditions, whereas the effect of pressure on the concentration (i.e., solubility) of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected. Here, we present new experimental data aiming to constrain the processes of gold (Au) dissolution in subduction-linked magmas along a range of depth. We have conducted hydrous melting experiments on two dacitic/adakitic magmas at 0.9 and 1.4 GPa and ∼1000 °C in an end-loaded piston cylinder apparatus, under f O 2 conditions close to NNO as measured by solid Co–Pd–O sensors. Experimental charges were carried out in pure Au containers, the latter serving as the source of gold, in presence of variable amounts of H 2 O and, for half of the charges, with elemental sulfur (S) so as to reach sulfide saturation. Au concentrations in melt quenched to glass were determined by LA-ICPMS. When compared to previous data obtained at lower pressures and variable redox conditions, our results show that in both S-free and sulfide-saturated systems pressure has no direct, detectable effect on melt Au solubility. Nevertheless, pressure has a strong, negative effect on sulfur solubility. Since gold dissolution is closely related to the behavior of sulfur in reducing and moderately oxidizing conditions, pressure has therefore a significant but indirect effect on Au solubility. The present study confirms that Au dissolution is mainly controlled by f O 2 in S-free melts and by a complex interplay of f O 2 and melt S 2 − concentration in sulfide-saturated melts, at given temperature. In addition, we propose that the transition from sulfide (S 2 − ) to sulfate (S 6+ ) species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase. If this is true, this may have important consequences during mantle melting and magma ascent. In particular, if mantle melting occurred in moderately oxidizing conditions, a small degree of partial melting would allow the primary melts to become Au-enriched but the relatively high pressure would move the sulfide-sulfate transition to more oxidizing conditions, making the primary melts saturated with sulfide phases that would sequester gold from the melt. During magma ascent, the decreasing pressure would favor the destabilization of sulfides and the release of gold to the silicate melt. However, at shallow levels, decreasing pressure, magma evolution, and varying redox conditions would be continuously competing to concentrate or fractionate gold. [ABSTRACT FROM AUTHOR]

Sulfur is important to a range of subduction zone processes, but the factors controlling the sulfur content of primitive arc magmas are poorly understood. In particular, uncertainties about the oxidation state of primary melts in the sub-arc mantle hinder efforts to understand the behavior of sulfur in arc magmas. Here, we use olivine-hosted melt inclusion data from 32 arc segments globally to characterize sulfur contents of magmas from a variety of subduction zones and identify the key processes that control the sulfur content of arc magmas. Average primary magma sulfur contents estimated from these data range from 466 ± 220 ppm (Mariana Forearc) to 4 , 264 ± 819 ppm (Ecuador). Primitive and more evolved magmas in both hot- and cold-slab subduction zones commonly have higher sulfur contents than mid-ocean ridge basalt (MORB) magmas. In most cases, these elevated sulfur contents require a subduction-modified mantle source with more sulfur than MORB-source mantle. Correlations between magma S and Cl concentrations and S/Dy and Th/Yb ratios in the global data set confirm that sulfur is partially sourced from the subducting slab. By comparing melt inclusion sulfur contents to sulfur solubility limits imposed by sulfide saturation, we find that 88% of arc magmas in our compilation require conditions more oxidizing than the quartz-fayalite-magnetite buffer (QFM). The relationship between S/Dy and calculated magmatic oxygen fugacity (f O2) suggests that slab-derived sulfate is responsible for the higher oxidation state of arc magmas compared to MORB. Model calculations of redox equilibrium between S and Fe species in basaltic melts when sulfate is added to the mantle wedge demonstrate the feasibility of this causal link, which can explain the common range of arc magma f O2 values observed for subduction zones globally. Correlations between magma sulfur contents, f O2 , and the proportion of added slab-derived material inferred from trace elements can drive associations between high Sr/Y magmas and fertile porphyry ore deposits, as well as the MORB-like Cu contents of arc magmas. • Slab-derived sulfur plays an important role in determining the f O2 and sulfur contents of arc magmas. • 88% of primary arc magmas have sulfur contents that require f O2 greater than QFM, and f O2 values extend up to QFM + 1.5. • The upper limit of relative f O2 values in arc magmas is likely controlled by S-Fe redox interactions during mantle melting. • Slab-derived sulfur and its f O2 effects can explain the MORB-like Cu contents of arc magmas. [ABSTRACT FROM AUTHOR]

Porphyry deposits supply most of the world's Cu and Mo resources. Over 90% of the porphyry deposits are found at convergent margins, especially above active subduction zones, with much fewer occurrences at post-collisional or other tectonic settings. Porphyry Cu–(Mo)–(Au) deposits are essentially magmatic–hydrothermal systems, which are generally initiated by injection of oxidized magmas saturated with metal-rich aqueous fluids, i.e., the parental magmas need to be water rich and oxidized with most of the sulfur appearing as sulfate in the magma. Sulfur is the most important geosolvent that controls the behavior of Cu and other chalcophile elements, due to high partition coefficients of chalcophile elements between sulfide and silicate melts. Small amount of residual sulfides can hold a large amount of Cu. Therefore, it is essential to eliminate residual sulfides to get high Cu contents in magmas for the formation of porphyry deposits. Sulfate (SO 4 2 − ) is over 10 times more soluble than sulfide (S 2 − ), and thus the solubility of sulfur depends strongly on sulfur speciation, which in turn depends on oxygen fugacities. The magic number of oxygen fugacity is log f O 2 > FMQ + 2 (i.e., ΔFMQ + 2), where FMQ is the fayalite–magnetite–quartz oxygen buffer. Most of the sulfur in magmas is present as sulfate at oxygen fugacities higher than ΔFMQ + 2. Correspondingly, the solubility of sulfur increases from ~ 1000 ppm up to > 1 wt.%. Oxidation promotes the destruction of sulfides in the magma source, and thereby increases initial chalcophile element concentrations, forming sulfur-undersaturated magmas that can further assimilate sulfides during ascent. Copper, Mo and Au act as incompatible elements in sulfide undersaturated magmas, leading to high chalcophile element concentrations in evolved magmas. The final porphyry mineralization is controlled by sulfate reduction, which is usually initiated by magnetite crystallization, accompanied by decreasing pH and correspondingly increasing oxidation potential of sulfate. Hematite forms once sulfate reduction lowers the pH sufficiently, driving the oxidation potential of sulfate up to the hematite–magnetite oxygen fugacity (HM) buffer, which is ~ ΔFMQ + 4. Given that ferrous iron is the most important reductant that is responsible for sulfate reduction during porphyry mineralization, the highest oxygen fugacity favorable for porphyry mineralization is the HM buffer. In addition to the oxidation of ferrous iron during the crystallization of magnetite and hematite, reducing wallrock may also contribute to sulfate reduction and mineralization. Nevertheless, porphyry deposits are usually mineralized in the whole upper portion of the pluton, whereas interactions with country rocks are generally restricted at the interface, therefore assimilation of reducing sediments is not likely to be a decisive controlling process. Degassing of oxidized gases has also been proposed as a major process that is responsible for sulfate reduction. Degassing, however, is not likely to be a main process in porphyry mineralization that occurs at 2–4 km depths in the upper crust. Sulfide formed during sulfate reduction is efficiently scavenged by aqueous fluids, which transports metals to shallower depths, i.e., the top of the porphyry and superjacent wallrock. According to traditional views, sulfide saturation and segregation during magma evolution is not favorable for the formation of porphyry Cu ± Au ± Mo deposits. This is the main difference between porphyry deposits and Ni–Cu sulfide deposits. Nevertheless, in places with thick sections of reducing sediments, e.g., the western North America, sulfide saturation and segregation may occur during evolution of the magma, forming Cu-rich cumulates at the base of plutons. These Cu-rich sulfides may evolve into porphyry mineralization or even control the ore-forming process [ABSTRACT FROM AUTHOR]

The geochemical behavior of Re in oxidized arc magmas (Eastern Manus basin) and MORB has been reexamined. In the early differentiation stage of oxidized arc magmas, Yb, Cu, Au, Ag, and Re have a bulk partition coefficient (D) between the crystallized mineral assemblage (olivine, clinopyroxene, and plagioclase) and the residual melt on the order of 0.33 = D Yb ~ D Au > D Re > D Ag > D Cu , while in the late stage they have a bulk D between the crystallized mineral assemblage (clinopyroxene, plagioclase, magnetite, and monosulfide solid solution — MSS) and the residual melt on the order D Re > D Cu > D Au ≥ 1 > D Yb = 0.29 > D Ag . In oxidized arc magmas, Cu, Au, and Ag are primarily controlled by crystalline MSS, whereas Re is primarily controlled by magnetite and it has a MSS/silicate melt partition coefficient significantly lower than 20. Unlike oxidized arc magmas, most MORB are saturated with sulfide liquid. Based on the positive correlation of log (D Cu / D Re ) and ∆FMQ regardless of f S 2 , log (D Cu / D Re ) = 0.997ΔFMQ + 0.181 (R 2 = 0.99) (D = partition coefficient between sulfide liquid and silicate melt; FMQ = fayalite–magnetite–quartz oxygen buffer), the partition coefficient of Re between sulfide liquid and silicate melt for MORB is estimated to be in the range of 600–10,000 at f O 2 of FMQ to FMQ − 1, increasing with decreasing f O 2 . These partitioning data constrain that during MORB differentiation Re may be as compatible as Cu or Ag at relatively oxidized conditions (FMQ − 0.5 < f O 2 < FMQ) or as Au at relatively reduced conditions (FMQ − 1 < f O 2 < FMQ − 0.5), and its concentration in MORB maybe expected to decrease with decreasing MgO in a similar manner as Cu, Ag, or Au. Yb is an incompatible element during MORB differentiation. Consequently, during MORB differentiation Yb/Re ratio may be expected to increase with decreasing MgO. However, the previously observed nearly constant Yb/Re ratio in a suite of MORB indicates similar incompatibility of Yb and Re and thus a minor role of sulfide liquid in controlling the geochemical behavior of Re during MORB differentiation. More work is required to resolve this paradox. [ABSTRACT FROM AUTHOR]

Ion microprobe measurements of H and B isotopic ratios and H2O, B and trace element contents are reported here for a series of melt inclusions typical of alkaline basalts of Aoba Island in the central part of Vanuatu arc (Southwestern Pacific). The melt inclusions, hosted in olivine Fo86–90, display large ranges in trace element concentrations and hydrogen (δD from − 48.2 to + 61.7‰) and boron (δ11B from − 11.9 to + 6.4‰) isotopic compositions. The high deuterium enrichment (δD ≥ 0‰) observed in a small subset of melt inclusions requires a proton diffusion loss through the olivine network, in addition to late-stage magma interactions with aqueous saline fluids. These melt inclusions are therefore not considered as representative of the magma from which the olivine grew. In most melt inclusions, positive correlations between H2O, K2O, Ba and Sr lead us to determine the K2O/H2O (1.5 ± 0.2), H2O/Ba (46 ± 3 × 10−4) and H2O/Sr (29 ± 2 × 10−4) ratios of Aoba basalts. Overall correlations between δ 11 B, B/Nb, and B/Nd testify to the mixing between slab-derived fluids, preferentially enriched in δ11B and fluid mobile elements and a relatively depleted MORB-type mantle wedge beneath Aoba Island. Heavy δ11B (on average 5.4 ± 0.7‰) indicate slab-derived fluids, possibly involving serpentine, which would have a mean δD value of − 28.4 ± 7‰. The chemical and isotopic variability recorded by Aoba magmas (melt inclusions) is consistent with the geodynamic context of ridge-arc collision in the central segment of Vanuatu arc. [ABSTRACT FROM AUTHOR]

Understanding magmatic processes such as crystallization and melting recorded in natural samples requires calibration of mineral–melt trace element partition coefficients (D) and their dependence on temperature, pressure, oxygen fugacity (fO2) and chemical composition. However, few experimental studies have focused on measuring trace element partition coefficients for a large number of trace elements, in the various minerals present in basaltic rocks, and under diverse conditions, particularly of variable fO2. Twenty-seven 0.1 MPa experiments provide partition coefficients for major elements and Sc, Ti, V, Mn, Co, Ni, Ga, Sr, Y, Nb, Ba, Ce, Nd, Eu, Gd, and Yb for the mineral phases olivine, plagioclase, orthopyroxene and clinopyroxene. The experimental conditions range from 1150 to 1190 °C with oxygen fugacities from QFM to NNO+2. Run products were analyzed by laser-ablation ICP-MS. The new partition coefficients, combined with previously published data, can be used to model crystallization processes at low pressure. Partitioning of multivalent cations V, Fe and Eu varies as a function of the redox conditions, consistent with previous work, and can be used to constrain oxidation states of magmatic source regions. The V/Yb ratio is shown to be a useful proxy for oxidation state. The V/Yb ratio varies during mantle melting as a function of oxidation state of the mantle source, and it is not modified during fractional crystallization of olivine ± plag ± cpx. V/Yb increases from MORB, BABB to arc lavas, suggesting a progressive increase of fO2 from QFM to NNO+2. Apparent fO2 of arc lavas, however, is quite variable. These results demonstrate that sub-arc mantle displays a larger range of redox conditions toward a more oxidized mantle than the MORB mantle. [ABSTRACT FROM AUTHOR]

• Melting experiments were performed using peridotite + H 2 O ± CO 2 starting compositions at mantle wedge P-T conditions. • Experiments with XCO 2 > 0.10 produce melts significantly more CaO rich and SiO 2 poor than all primitive arc magmas. • Slab-derived fluids fluxing mantle wedges are CO 2 -poor. • A new subduction zone mass balance model suggests that 34–86% of subducted carbon will enter the deep mantle. The extent of CO 2 transfer from subducting lithologies to the overlying mantle wedge in general and to the arc magma source regions in particular remains debated. The limit of CO 2 transfer to the sub-arc mantle could be estimated if the effects of CO 2 on the primary hydrous melt compositions of mantle wedge can be assessed in relation to the observed compositions of primitive arc magmas. Here we present new piston cylinder and multi-anvil experiments using Au 75 Pd 25 and Au capsules on four depleted peridotite + H 2 O ± CO 2 starting compositions with 3.5 wt.% H 2 O and XCO 2 [= molar CO 2 / (CO 2 + H 2 O)] of 0–0.17. Experiments were performed at 2–4 GPa and 1200 °C to constrain how the presence of variable CO 2 in slab-derived aqueous fluids affects the composition of peridotite partial melts. All experiments consisted of low degree melts (<10 wt.%) in equilibrium with olivine + orthopyroxene ± clinopyroxene. Melts at 2–4 GPa are basaltic for XCO 2 of 0–0.10 and become SiO 2 -poor and CaO-rich at XCO 2 > 0.10. Comparison between our experimental partial melt compositions with a global dataset of the most primitive arc magmas suggests that the upper limit of XCO 2 in fluids inducing melting in mantle wedges is ∼0.10 at 2–4 GPa. We apply these new constrains to an H 2 O and CO 2 mass balance model for subduction zones and estimate that at least 34–86% of CO 2 entering subduction zones bypasses the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab. [ABSTRACT FROM AUTHOR]

A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U–Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ± 3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (ϵNd(t) = +2.2 to +4.0, (87Sr/86Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc–alkaline character and have E-MORB-like and OIB signatures, with a marked negative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.70377–0.70469, ϵNd(t) = 1.0–5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These features suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar. [ABSTRACT FROM AUTHOR]

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high XMg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (˜1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas. [ABSTRACT FROM AUTHOR]

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly HO) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal HO contents (≤4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene-hosted melt inclusions can preserve HO contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m s). The absence of noticeable HO loss in some of the inclusions requires post-entrapment diffusion coefficients (≤1 × 10 m s) at least several order of magnitude smaller than experimentally determined H diffusion coefficient in pyroxenes. The HO content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive HO reequilibration, are observed around 1 and 0.2 wt%. HO diffusive loss is also consistent with the manifest lack of correlations between HO and CO or S contents. The absence of textural evidence supporting post-entrapment HO loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that HO diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour-saturated magmas. [ABSTRACT FROM AUTHOR]