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To better constrain Ca isotopic fractionation during volcanic magma differentiation, we report Ca isotope data for a series of volcanic glasses from the eastern Manus Basin that represent a continuum of compositions comprising basaltic andesite, andesite, dacite, and rhyolite. Our results show that these volcanic glasses exhibit ~0.25‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.81 ± 0.04 to 1.05 ± 0.04‰. The similar trace element patterns and Sr-Nd isotopic compositions exclude source heterogeneity as the cause of the varied Ca isotopic compositions. Due to their low K/Ca ratios and young age, the Ca isotopic variations cannot be attributed to 40K decay. Instead, the observed Ca isotopic variations are primarily controlled by their lithology. Specifically, δ44/40Ca of basaltic andesite (0.81 ± 0.04) and andesite (0.82 ± 0.01) are similar to the averages of reported mid-ocean ridge basalts (MORBs; 0.84 ± 0.10; 2SD, N = 26) and back-arc basin basalts (BABBs; 0.80 ± 0.08; 2SD, N = 21) in literature, but do not correlate with SiO2 or MgO contents, indicating the Ca isotopic fractionation is insignificant during intermediate-mafic magma evolution. As magma differentiation continues, δ44/40Ca of dacite and rhyolite evolves from 0.85 ± 0.01 to 1.05 ± 0.04‰. Moreover, δ44/40Ca of felsic glasses are negatively correlated with Eu/Eu*, which suggests that the increasing δ44/40Ca of felsic glasses is most likely related to plagioclase (Pl) fractional crystallization. Therefore, we conclude that Ca isotopes can behave differently at different stages of volcanic magma differentiation. Modeling of fractional crystallization using rhyolite-MELTS shows that the relative Ca fractions in crystallized minerals (e.g., Pl vs. Cpx) and the Ca isotopic fractionation factor between the residual melt and crystallized Pl (αmelt-Pl, which is likely related to the An number of Pl, magma temperature and kinetic effects during crystallization), provide a plausible explanation for the behavior of Ca isotopes during volcanic magma differentiation.

Hydrous minerals in the hybridized zone at the slab-mantle interface play a critical role in retaining and transporting fluid mobile elements in subduction zones. The observed high B and As concentrations and high δ11B values in arc magmas were previously thought to have been resulted from the mechanical transport of forearc serpentinites down to subarc depths by mantle corner flows or through subduction channels because B, As, and Sb are mostly released during early stages of subduction. This study provides new insights into the recycling of B, As, and Sb in subduction zones via an in-situ study on a set of carbonate-bearing serpentinite samples from the Mianlue melange complex in the Qinling Orogen. Petrological observations and bulk-rock Sr-Pb isotopes reveal that the Jianchaling serpentinites were formed by interactions between forearc peridotites and sediment-derived fluids at a shallow depth in the subduction channel. Some Jianchaling serpentinites experienced the lizardite to antigorite transition at 300–400 °C with infiltrations of CO2-rich fluids. In contrast, the Liangyazi antigorite serpentinites were produced at a greater depth. Antigorite grains in the Jianchaling serpentinites have lower Feapfu than those in the Liangyazi serpentinites and most antigorite with geochemical data in the literature. Meanwhile, B, As, and Sb concentrations of Fe-poor antigorite from the Jianchaling serpentines (385–653 ppm B, 198–334 ppm As, and 6.24–8.01 ppm Sb) are not only higher than those of the Fe-rich antigorite from the Liangyazi serpentinites but also 2–3.5 times higher than those of the surrounding lizardite. The lower modal percentage of spinel and ferritchromite in antigorite veinlets, together with the lower bulk-rock FeO content of antigorite-lizardite serpentinites than that of lizardite serpentinites from the Jianchaling region, indicates that some FeO might be lost during the lizardite to antigorite transition. The presence of carbonate minerals in the antigorite veinlets and the remarkably high B, As, and Sb concentrations in the Jianchaling antigorite indicate that the penetrating fluids during the lizardite to antigorite transition are most likely derived from sediment and metabasite decarbonations at

The westward subduction of the Pacific plate interacted with the northward subduction of the eastern Neo-Tethys plate in the Early Cretaceous. However, the subduction polarities between these two plates in the Early Cretaceous is not well constrained. The proto-Philippine arc is suggested to have originated at the Pacific – Neo-Tethys intersection in the SW Pacific in the latest Jurassic – Early Cretaceous, and can provide information about the ancient interaction between these two plates. In this study, the oldest known Early Cretaceous mantle-derived arc volcanic rocks and slab-melting generated adakites occurring in the Philippine arc have been studied for Pb, Sr, Nd and Hf isotopes to discriminate the provenance of their mantle sources and/or subduction components. The 126–118 Ma Kansi island arc volcanics have Pacific-type Pb isotopes, suggesting that the subduction components in the source are mainly from the subducted Pacific plate. Their Hf-Nd isotopes give support to a mantle source that is isotopically transitional Pacific-Indian type. The formation of the Kansi volcanics dates the subduction of the Pacific plate under the transitional Pacific-Indian mantle as Early Cretaceous. The subsequent generation of the 110 Ma Kansi adakitic diorites with the similar Pacific-type Pb isotopes indicates further partial melting of the subducted Pacific plate. In contrast, the post-110 Ma Lutopan adakitic diorites have Indian-type Pb isotopes, indicating the subsequent subduction of an Indian-type oceanic slab in the eastern Neo-Tethys tectonic regime. Therefore, the proto-Philippine arc is constrained to have originated at the intersection between Indian-type and Pacific-type oceanic crusts and mantle domains. The successive generation of these Early Cretaceous arc rocks in the Philippines indicates that a transformation of the tectonic regime from southwestward Pacific subduction to northeastward subduction of the eastern Neo-Tethys plate in the SW Pacific Ocean occurred at ca. 110 Ma, due to the abrupt change in drifting direction of the Pacific plate.

In order to investigate the behavior of potassium (K) isotopes during metamorphic dehydration and to further constrain its implications for K recycling in the subduction zone, we measured K isotopic compositions of whole-rocks and separated minerals (phengite, omphacite, amphibole) from Sumdo eclogites, Tibet, using a recently established high-precision analysis method. Our data reveal that the δ41K (‰) of the whole-rock eclogites (−1.64 to −0.24) displays dramatically lower values than observed in fresh mid-ocean ridge basalts (MORB) (−0.43 ± 0.17) and altered MORB (−0.76 to −0.11). In addition, the δ41K values of eclogites show a positive correlation with both K2O contents and K/Nb ratios, which suggests that the low δ41K values were most likely caused by dehydration during subduction. Thus, isotopically heavy K may be released into the mantle wedge, while the light component is subducted into the deep mantle. Therefore, the K isotope systems have the potential to trace subducted crustal materials and to create heterogeneity within the mantle. Mineral separates from Sumdo eclogites are highly heterogeneous in K isotopic compositions, ranging from −0.98 to +0.23 in phengite, from −1.25 to −1.10 in omphacite and from −1.16 to −0.09 in amphibole. The K isotope fractionations between amphibole and phengite and between amphibole and omphacite vary from −0.30 to +0.25 and from −0.04 to +0.63, respectively, indicating K isotopic disequilibrium between amphibole and omphacite/phengite, which might result from the multistage growth of minerals during subduction metamorphism. However, generally phengites show heavier K isotopic compositions than the coexisting omphacite (Δ41Kphengite-amphibole = +0.25) and amphibole (Δ41Kphengite-amphibole = +0.03 ∼ +0.30, except TB193 is −0.25), which may imply that the K isotopic fractionation is controlled by the difference in coordination numbers of K between phengite (6) and omphacite (7 to 8)/amphibole (8).

Calcium isotopes can potentially be used to trace many geological processes. Characterizing the Ca isotopic compositions of marine sediments is a precondition for using Ca isotopes to trace the recycling of marine sediments into the mantle and mantle-derived rocks. The best way to achieve this goal is to investigate different types of marine sediments that are adjacent to subduction zones and would undergo subduction in the future. Here, we present high-precision Ca isotopic data for marine sediments from the Deep Sea Drilling Project (DSDP) Sites 543 and 144 along the Lesser Antilles forearc region, to constrain the Ca isotope signatures of marine sediments with various components and assess their possible effects on δ44/40Ca of the local mantle and arc lavas via subduction. Our results show that the δ44/40Ca values of these marine sediments vary from 0.10 to 1.21‰ and depend on lithology. Carbonate-rich sediments from Site 144 exhibit δ44/40Ca values in the range of 0.10–0.58‰ with a weighted average value of 0.36‰, which is ∼0.6‰ lower than that of the mantle. In comparison, δ44/40Ca of clay-rich sediments from Site 543 range from 0.79 to 1.21‰ with a mantle-like mean value of 0.96‰. The data obtained here suggest that carbonate- and clay-rich marine sediments differ significantly both in Ca isotopic compositions and CaO contents. Recycling of these sediments will result in different effects on the isotopic compositions of the local mantle and probably affect the local arc lavas. Specifically, carbonate-rich sediments from Site 144 with high CaO contents and light Ca isotopic compositions can alter the local mantle to lower δ44/40Ca signatures, which is expected in the southern Lesser Antilles arc lavas.

Subduction related metasomatism has been proposed to be one of the most important processes that may cause Ca isotopic heterogeneities in the mantle. Back-arc basin basalts (BABBs) affected by different contributions of subducted materials have various geochemical compositions ranging from MORB-like to arc-like, so that they are ideal media to investigate the behavior of Ca isotopes during plate subduction. Here we report Ca isotopic compositions of a series of BABBs from the Lau Basin, Woodlark Basin and Coriolis Troughs in the southwestern Pacific. Our results show that δ44/40Ca of all the BABBs exhibit a narrow range of 0.73 ± 0.07 to 0.89 ± 0.06‰ with an average of 0.80 ± 0.08‰ (2SD, N = 21), which are lower than the estimated value of the Bulk Silicate Earth (BSE, 0.94 ± 0.05‰), but are similar to that of MORBs reported in the literature (0.83 ± 0.11‰, 2SD, N = 21). Even though the mantle sources of BABBs studied here have been influenced by various amounts of slab-derived fluids and/or sediments, there is no systematic variation of δ44/40Ca with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, Th/Yb, 87Sr/86Sr and 143Nd/144Nd). This suggests that either there is no Ca isotopic discrepancy between subducted components and mantle, and/or the amount of Ca that is transferred from the subducting Pacific slab to the studied BABB mantle sources is insignificant. Water depths at which subducting sediments were deposited control the components of marine sediments that may be subducted, which in turn determine how subduction affects Ca isotopic composition of the mantle. Because the southwestern Pacific has deeper water depths than the carbonate compensation depth, pelagic clays dominate the contribution from the subducted slab rather than carbonates, which leads to the negligible Ca isotopic fractionation during subduction along these trenches. The lack of correlations of δ44/40Ca with geochemical indicators of magma differentiation (e.g., MgO, CaO/Al2O3, Sr/Nd, Eu/Eu*) suggests that intermediate-mafic magma differentiation has insignificant effects on Ca isotopic compositions of BABBs. Therefore, we conclude that the observed offset of δ44/40Ca (∼0.14‰) between the mantle and BABBs studied here should be primarily controlled by Ca isotopic fractionation during partial melting without significant contributions from subducted materials.

We present high-precision iron isotope data for harzburgites and constituting minerals from the Yushigou supra-subduction zone ophiolite in the North Qilian orogen of northern Tibet to provide insights into iron isotope behavior in subduction zones. The Yushigou harzburgites represent typical melting residues of silica-enriched forearc mantle wedge with significant melt extraction and minimal melt percolation. The δ56Fe ranges from −0.096‰ to 0.110‰ in olivine, from −0.111‰ to 0.105‰ in orthopyroxene and from −0.141‰ to 0.054‰ in spinel. Coexisting minerals fall on a line with a slope of 1 in the δ-δ plot, indicating iron isotope equilibrium between mineral pairs. Iron isotopic fractionation between olivine and orthopyroxene is minor, which is consistent with theoretical predictions and previous observations. Good iron isotopic equilibrium between olivine and orthopyroxene is a further indicator that silica enrichment in arc peridotites is inherited from their mantle source. In contrast, significantly negative iron isotopic fractionation between spinel and olivine contradicts with most previous constraints, which may be ascribed to Cr substitution in spinel. Bulk rock harzburgites have δ56Fe ranging from −0.097‰ to 0.076‰, the average of which (0.002 ± 0.020‰) is similar to average δ56Fe determined for abyssal peridotites. Based on this inference, the lower δ56Fe of arc magmas than MORB should be due to smaller discrepancy in Fe3+/ΣFe between arc magmas and arc peridotites when compared with MORB and abyssal peridotites. Our modelling demonstrates that this can be achieved if arc magmas are produced by redox-buffered melting of silica-enriched mantle wedge while MORB are produced by redox-unbuffered melting of the depleted mantle.

Oxygen fugacity (fO2) is a key factor that controls the formation of porphyry Cu deposits. Porphyry Cu deposits are typically oxidized, but when and how porphyry magmas gain their high oxygen fugacity signatures, and how oxygen fugacity controls porphyry mineralization, remains obscure. To trace the origin of the high oxygen fugacity in porphyry Cu deposits, we determined trace element compositions and U-Pb ages of magmatic and inherited zircon from Dexing porphyry Cu deposit, calculated Ce4+/Ce3+ of zircons and estimated the oxygen fugacity of their parental magmas. The Ce4+/Ce3+ ratios of Middle Jurassic (∼170 Ma) magmatic zircons are high (550 on average), whereas the Ce4+/Ce3+ ratios of inherited zircons (200–880 Ma) are much lower (263 on average). The relationship suggests that the Dexing porphyry magma was highly oxidized when the Jurassic magmatic zircons crystallized (estimated fO2: ΔFMQ + 0.7 (±1.3) – ΔFMQ + 1.9 (±1.3)), which cannot be attributed to partial melting of the Neoproterozoic arc crust (estimated fO2: ΔFMQ − 2.4 (±1.1) – ΔFMQ + 0.7 (±1.2)) as proposed previously. Instead, the high fO2 (ΔFMQ + 1.5) of these magmas is a primary feature of the latest magmatism. Sulfate rather than sulfide is the dominant species at fO2 > ΔFMQ + 1.5. In general, the behavior of Cu and other chalcophile elements is controlled by sulfide, while the sulfur speciation is controlled by oxygen fugacity. Previous modeling results show that partial melting of mantle peridotite under high oxygen fugacity (even at >ΔFMQ + 1.5) cannot form Cu-rich magmas, which plausibly explains the lack of porphyry Cu deposits in normal arc rocks. This is because mantle peridotite has low Cu and S contents. Our modeling shows that partial melting of subducted oceanic crust, under oxygen fugacities higher than ΔFMQ + 1.5, is favorable for producing primary magmas with Cu contents sufficiently high for porphyry mineralization, which plausibly explains the close relationship between porphyry Cu deposits and oxidized magmas with adakitic affinities.

Previously published Cu partition coefficients (DCu) between silicate minerals and melts cover a wide range and have resulted in large uncertainties in model calculations of Cu behavior during mantle melting. In order to obtain true D Cu mineral / melt values, this study used Pt95Cu05 alloy capsules as the source of Cu to experimentally determine the DCu between olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (spl), garnet (grt) and hydrous silicate melts at upper mantle conditions. Three synthetic silicate compositions, a Komatiite, a MORB and a Di70An30, were used to produce these minerals and melts. The experiments were conducted in piston cylinder presses at 1.0–3.5 GPa, 1150–1300 °C and oxygen fugacities (fO2) of from ∼2 log units below to ∼5 log units above fayalite–magnetite–quartz (FMQ). The compositions of minerals and quenched melts in the run products were measured with EMP and LA-ICP-MS. Attainment of equilibrium is verified by reproducible DCu values obtained at similar experimental conditions but different durations. The results show that DCu for ol/, opx/, spl/ and possibly cpx/melt increase with increasing fO2 when fO2 > FMQ + 1.2, while DCu for cpx/ and spl/melt also increase with increasing Na2O in cpx and Fe2O3 in spinel, respectively. In the investigated P–T–fO2 conditions, the D Cu mineral / melt values are 0.04–0.14 for ol, 0.04–0.09 for opx, 0.02–0.23 for cpx, 0.19–0.77 for spl and 0.03–0.05 for grt. These results confirm that Cu is highly incompatible (DCu 70 ppm) arc basalts may form at oxidized, sulfide-absent conditions, which is consistent with the possibility of some high fO2 regions present in the arc mantle.

Porphyry copper deposits account for more than 80% of the world’s total Cu resources. However, the formation mechanism and controlling factors of porphyry copper deposits remain obscure. Previous studies have revealed that porphyry copper deposits are usually associated with oxidized, calc-alkalic, adakitic shallow intrusive rocks. Here we show that hematite–magnetite intergrowths are commonly found in porphyry copper deposits, suggesting high and fluctuating oxygen fugacity (fO2). Oxidation promotes the destruction of sulfides in the magma source, and thereby increases initial chalcophile element concentrations. Sulfide remains undersaturated during the evolution of oxidized sulfur-enriched magmas where sulfate is the dominant sulfur species, leading to high chalcophile element concentrations in evolved magmas. The final porphyry copper mineralization is controlled by sulfate reduction, which starts with magnetite crystallization, accompanied by decreasing pH and correspondingly increasing fO2. Hematite forms once sulfate reduction lowers the pH sufficiently and the fO2 reaches the hematite–magnetite oxygen fugacity buffer, which in turn increases the pH for a given fO2. The oxidation of ferrous iron during the crystallization of magnetite and hematite is the causal process of sulfate reduction and consequent mineralization. Therefore, the initial pH and fO2 ranges of porphyries favorable for porphyry copper mineralization are defined by the hematite–magnetite oxygen fugacity buffer and SO42−–HS−–S3− reaction lines. Adakitic rocks have higher initial contents of copper, sulfur and iron than normal arc rocks, and thus are the best candidates for porphyry copper deposits. These provide a plausible explanation for the formation of copper porphyry deposits. The hematite–magnetite intergrowth marks the upper limits of fO2 favorable for the mineralization, and thus may be a powerful tool for future prospecting of large porphyry copper deposits.

In order to fully assess the role of rutile in fractionation of Nb/Ta during partial melting of hydrous metabasalt, we have measured rutile – felsic melt partition coefficients (D values) for Nb and Ta with tonalitic to trondhjemitic compositions at 1.5–3.5 GPa, 900–1350 °C and ∼5.0–20 wt% H2O. DNb, DTa and DNb/DTa range from 17 ± 1 to 246 ± 13, 34 ± 2 to 232 ± 25 and 0.51 ± 0.04 to 1.06 ± 0.13, respectively. For the compositions investigated, melt composition appears to have no observable effect on the partitioning; the effect of pressure is also slight; whereas temperature and H2O have marked effects. DNb, DTa and DNb/DTa increase with decreasing temperature and H2O content, showing a reversal of DNb/DTa from 1.0. Using the data that approached equilibrium and obeyed Henry’s law, expressions describing the dependences of DNb, DTa and DNb/DTa on temperature, pressure and melt H2O content were obtained: (1) ln ( D Nb ) rutile/melt = - 2.846 ( ± 0.453 ) + 9621 ( ± 470 ) / T + 0.207 ( ± 0.101 ) P - 0.042 ( ± 0.009 ) H 2 O (2) ln ( D Ta ) rutile/melt = - 0.775 ( ± 0.398 ) + 6954 ( ± 413 ) / T + 0.140 ( ± 0.089 ) P - 0.009 ( ± 0.007 ) H 2 O (3) ln ( D Nb / D Ta ) rutile/melt = - 2.075 ( ± 0.289 ) + 2657 ( ± 301 ) / T + 0.075 ( ± 0.065 ) P - 0.033 ( ± 0.006 ) H 2 O where T is in K, P is in GPa, and H2O are in wt%. These expressions are applicable to calculate Nb and Ta partitioning coefficients between rutile and felsic (highly siliceous) melts and Nb/Ta fractionation during partial melting of metabasalt. The derived models suggest that fluid-present partial melting of metabasalt always leads to DNb/DTa lower than 1.0, whereas dehydration melting may lead to DNb/DTa higher than 1.0 at temperatures

Trace element and isotopic compositions of carbonate from ore bodies, country rock which hosts the ore bodies (H8 dolomite), a carbonatite dyke exposed in Dulahala near Bayan Obo, and rare earth element (REE)-rich dolomite in Bayan Obo have been determined to understand the genesis of the Bayan Obo Fe–Nb–REE ore deposit, the world’s largest resource of REE. The REE and trace element distribution patterns of samples from the REE-rich carbonatite dykes are identical to those of mineralized carbonate rocks, indicating a genetic linkage between the REE-rich carbonatite and mineralization in this region. By contrast, carbon and oxygen isotopes in the mineralized carbonate varied significantly, δ13C = −7.98‰ to −1.12‰, δ18O = 8.60–25.69‰, which are distinctively different from those in mantle-derived carbonatite. Abnormal isotopic fractionations between dolomite and calcite suggest that these two minerals are in disequilibrium in the carbonatite dyke, ore bodies, and H8 marble from Bayan Obo. This isotopic characteristic is also found in mineralized sedimentary marine micrite from Heinaobao, ∼25 km southeast of the Bayan Obo Fe–Nb–REE ore deposit. These facts imply that the carbonate minerals in the Bayan Obo deposit have resulted from sedimentary carbonate rocks being metasomatised by mantle-derived fluids, likely derived from a REE-enriched carbonatitic magma. The initial Nd isotope values of ore bodies and carbonatite dykes are identical, indicating that ore bodies, carbonatite dykes and veins may have a similar REE source.

We used positive thermal ionization mass spectrometry (PTIMS) to generate high precision δ11B records in Porites corals of the mid-late Holocene from the South China Sea (SCS). The δ11B values of the Holocene corals vary significantly, ranging from 22.2‰ to 25.5‰. The paleo-pH records of the SCS, reconstructed from the δ11B data, were not stable as previously thought but show a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values. The latter is likely caused by the large amount of anthropogenic CO2 emissions since the Industrial Revolution but variations of atmospheric pCO2 cannot explain the pH change of the SCS before the Industrial Revolution. We suggest that variations of monsoon intensity during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene. Results of this study indicate that the impact of anthropogenic atmospheric CO2 emissions may have reversed the natural pH trend in the SCS since the mid-Holocene. Such ocean pH records in the current interglacial period can help us better understand the physical and biological controls on ocean pH and possibly predict the long-term impact of climate change on future ocean acidification.

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite–quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H 2 O with solids up to 30–70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

It has long been proposed that MORB and OIB have constant supra-primitive mantle (PM) Nb/U values identical to each other. This fact together with complementary sub-PM values for the continental crust (CC), are taken as fundamental evidence, linking the mantle sources of MORB and OIB to the formation of the CC. Given that plate subduction at convergent margins is the major known process that dramatically fractionates Nb from U, and consequently that subducted oceanic slabs are the main primary carriers of supra-PM Nb/U, a constant supra-PM Nb/U in MORB mantle implies that the mixing of subducted oceanic crust is essentially finished or the newly recycled oceanic crust has Nb/U close to that of the mantle. The similarity between Nb and U as well as the constancy of Nb/U in MORB are revisited here based on MORB glass data obtained using laser ablation ICP-MS. The result shows that Nb/U is not correlated with Nb/Hf, supporting that Nb and U are similarly incompatible. Further investigation shows that Nb is not perfectly identical to, but is faintly more incompatible than U as indicated by the good correlation between log(U) and log(Nb) with a slope of 0.954, very close to 1. Nonetheless, the similarity between Nb and U is high enough, such that the average Nb/U value of MORB glasses should be very close to that of the MORB mantle. By contrast, the difference between Ce and Pb is more obvious. Ce is more incompatible than Pb with a slope of 1.13 in a log(Pb) versus log(Ce) diagram. Therefore, the Ce/Pb of MORB should be a little bit higher than that of the mantle source. The Nb/U value is not as uniform as expected for the similar incompatibility in studied MORB glasses, but varies by a factor of ∼2, suggesting that MORB mantle source is not yet homogenized in term of Nb/U. This indicates that the mixing back of subducted oceanic crust is still an ongoing process, i.e., subducted oceanic crust is recycling back after staying in the lower mantle for billions of years.

Major, trace and organic elements of a laterite profile developed on Neogene basalts in northern Hainan Island, South China were reported in this paper, the aim of which was to investigate element mobilization and re-distribution during extreme weathering. The results indicate that most of the elements have been mobilized and transferred downwards along the profile by aqueous solution. Organic matter (OM) can significantly improve the transport of insoluble elements. Among all the elements, Th is the least mobile. As for the general conservative elements during incipient chemical weathering, such as Fe, Ti, Zr, Hf, Nb and Ta, the removals are up to 20–40% in the upper profile. However, these elements behave as conservatively as Th in the lower profile. In the middle profile, oxic environment occurs, accompanied with significant OM decomposition. The Mn and Ce transferred downward are readily oxidized into insoluble Mn(IV) and Ce(IV) and precipitate in the oxic front. Important OM decomposition decreases the capacity of transfer of insoluble elements in aqueous solution. Consequently, Al significantly precipitates in the oxic front, and REEs, with the exception of Ce, precipitate largely in the OMdepleted layers. Co and U are also concentrated in the oxic front in association with Mn and Ce, respectively. However, Cr shows a negative correlation with Mn because its response to redox condition changes is reversed from that of Mn. Mn oxides/hydroxides, Fe oxides/hydroxides and secondary phosphate minerals other than clay minerals are potential hosts for REEs except for Ce in the profile; REEs with high concentrations in the profile seem closely associated with Mn oxides/ hydroxides. Remarkable, highly correlated, Ce and Gd anomalies are observed in the profile. Ce anomalies are caused by Ce precipitation in the oxic environment and successive decomposition of organic matter. Gd anomalies are likely to have resulted from lower stability constants of Gd–OM complexes compared to those of neighboring REEs. The overall elemental behaviors in this profile suggest that organic matter plays a very important role in the mobilization and re-distribution of the elements during extreme weathering.

We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6–10 lm (mostly 6 lm) and about 50–75 lm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 lm, with most around 1.5 lm, and lengths that vary dramatically from 6 to 50 lm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of � 30–150 lm and widths of � 10 to 50 lm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 lm and lengths of about 60–70 lm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th–U-total Pb isochron method (CHIME) yields an U–Pb isochron age of 202 ± 28 Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO2-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO2-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems. � 2007 Elsevier Ltd. All rights reserved.

Submarine volcanic glasses from the eastern Manus Basin of Papua New Guinea, ranging from basalt to rhyodacite, clarify the geochemical behavior of Cl in arc-type magmas. For the Manus samples, Cl is well correlated with non-volatile highly incompatible trace elements, suggesting it was not highly volatile and discounting significant seawater contamination. The Cl partition coefficient is close to but slightly lower than that of Nb and K2O, a behavior similar to that in mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). The similar incompatibilities of Cl and Nb imply that the Cl/Nb values of the eastern Manus Basin glasses reflect their magma source. For glasses from other west Pacific back-arc basins, Cl/Nb, Ba/Nb, and U/Nb increase towards the subduction trench, indicating increased contribution of a component enriched in Cl, Ba, and U, likely from subduction-released slab fluids. It is estimate that similar to 80% of the Cl in the Manus arc-type glasses was added directly from subducted slab-derived fluids. We have also modeled Cl behavior during magma evolution in general. Our results show that the behavior of Cl in magma is strongly influenced by pressure, initial H2O content, and the degree of magmatic fractionation. At early stages of magmatic evolution, for magmas with initial H2O content of < 4.0 wt%, Cl is highly incompatible under all pressures. By contrast, for more evolved magmas at moderately high pressure and high H2O contents, considerable amounts of Cl can be extracted from the magma once H2O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB because of their low H2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H2O content and depth of magma chambers. The behavior of Cl in the eastern Manus Basin magmas is consistent with low H2O contents (1.1-1.7 wt%) and evolution at low pressures (< 0.1 GPa). Modeling results also indicate that Cl will behave differently in intrusive rocks compared to volcanic rocks because of the different pressures involved. This may have a strong influence on the mechanisms of ore genesis in these two tectonic settings.

The formation of the continental crust (CC) is one of the most important processes in the evolution of the silicate Earth. Exactly how the CC formed is the subject of ongoing debate that focuses on its subchondritic Nb/Ta ratio. Nb and Ta are “geochemical identical twins,” so they usually do not fractionate from each other. Here, we show that rutile grains from hydrous rutile-bearing eclogitic layers recovered from drillcores in the Dabie-Sulu ultrahigh pressure terrain have highly variable Nb/Ta values (ranging from 5.4 to 29.1, with an average of 9.8 ± 0.6), indicating major fractionation of Nb and Ta most likely occurred during blueschist to amphibole–eclogite transformation in the absence of rutile. It is suggested that the released fluids with subchondritic Nb/Ta were transported to, and retained by, hydrous rutile-bearing eclogite in colder regions, resulting in suprachondritic Nb/Ta ratios for drier eclogite in hotter regions. Further dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta values to the CC due to the low solubility of Nb and Ta in fluids in the presence of rutile, while dehydration-melting results in a major component of the CC, the tonalite–trondhjemite–granodiorite (TTG) component, which is responsible for the low Nb/Ta of the CC. Consequently, residual eclogites have variable but overall suprachondritic Nb/Ta.