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This article is a companion to Shervais et al. (2020), https://doi.org/10.1029/2020GC009093 .

International audience; nternational Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Injection of volatile-rich mafic magma prior to an eruption may trigger episodes of volcanism and can act to transfer metals from depth. However, petrologic knowledge of the timescales from mafic injection to eruption have thus far been focussed on mineral-scale studies of chemical zoning patterns. The study of mafic enclaves dispersed within eruption products can provide insights into the interaction between deep and shallow reservoirs. We combine 238U-230Th-226Ra-210Pb isotope data with trace element concentrations across the interface of two contrasting mafic enclaves in contact with their host andesite from the 2010 eruption at Soufrière Hills Volcano (SHV), Montserrat to investigate the history of mass exchange between the mafic enclave and the andesite host. The application of these time-sensitive isotopes highlights complexities in the transfer of volatiles and metal elements between magmas and the enclaves' potential as eruption triggers. The enclaves exhibit (210Pb/226Ra)0 ratios >1 consistent with volatile input to the subsurface plumbing system a few decades prior to eruption. Samples of the andesitic host, however, which make up the bulk of the eruptive products, have (210Pb/226Ra)0 ≤ 1 suggesting no net volatile gain in the decades leading up to eruption, or that melt-volatile interaction is on a timescale unresolvable by 210Pbsingle bond226Ra systematics (i.e.

Arc basalts have a higher proportion of Fe in an oxidized state (Fe^(3+)) relative to Fe^(2+) compared to mid-oceanic ridge basalts (MORBs), likely because slab-derived fluids oxidize the mantle wedge where subduction zone magmas originate. Yet, the time scales over which oxygen fugacity of the mantle wedge changes during subduction initiation and margin evolution are unknown. Fe speciation ratios show that magmas produced during the early stages of subduction in the Mariana arc record oxygen fugacities ∼2× more oxidized than MORB. Mantle wedge oxygen fugacity rises by ∼1.3 orders of magnitude as slab fluids become more involved in melt generation processes, reaching conditions essentially equivalent to the modern arc in just 2–4 m.y. These results constrain existing models for the geochemical evolution of the mantle wedge and suggest that oxidation commences upon subduction initiation and matures rapidly in the portions of the mantle wedge that produce melts. This further implies that sulfide or other reduced phases are not present in the mantle wedge in high enough abundance to prevent oxidation of the magmas that form upon subduction initiation. The arc mantle source is oxidized for the majority of a subduction zone’s lifetime, influencing the mobility of multivalent elements during recycling, the degassing of oxidized volcanic volatiles, and the mechanisms for generating continental crust from the immediate onset of subduction.

We present new U-238-Th-230-Ra-226-Pb-210-Po-210, Sr-87/(86) Sr and Nd-143/(144) Nd isotopic data of whole-rock samples and plagioclase separates from volcanic deposits of the 2006 and 2010 eruptions at Merapi volcano, Java, Indonesia. These data are combined with available eruption monitoring, petrographic, mineralogical and Pb isotopic data to assess current theories on the cause of a recent transition from effusive dome-building (2006) to explosive (2010) activity at the volcano, as well as to further investigate the petrogenetic components involved in magma genesis and evolution. Despite the significant difference in eruption style, the 2006 and 2010 volcanic rocks show no significant difference in (U-238/(232) Th), (Th-230/Th-232) and ((226)-Ra/ Th-230) activity ratios, with all samples displaying U and Ra excesses. The Ra-226 and Pb-210 excesses observed in plagioclase separates from the 2006 and 2010 eruptions indicate that a proportion of the plagioclase grew within the decades preceding eruption. The 2006 and 2010 samples were depleted in Po-210 relative to Pb-210 ((Po-210/Pb-210)(i) < 1) at the time of eruption but were variably degassed (69%-100%), with the degree of Pb-210 degassing strongly related to sample texture and eruption phase. In good agreement with several activity monitoring parameters, Po-210 ingrowth calculations suggest that initial intrusion into the shallow magma plumbing system occurred several weeks to a few months prior to the initial 2010 eruption. The 2006 and 2010 samples show a wide range in (Pb-210/Ra-226) activity ratio within a single eruption at Merapi and are largely characterised by Pb-210 deficits ((Pb-210/Ra-226) < 1). Assuming a model of complete radon degassing, the 210 Pb deficits in the 2006 volcanic rocks indicate relatively longer degassing timescales of similar to 2-4 years than those given by the 2010 samples of similar to 0-3 years. The uranium-series and radiogenic isotopic data do not support greater crustal assimilation of carbonate material as the explanation for the more explosive behaviour of Merapi in 2010 (as has been previously suggested) and instead indicate that relatively rapid ascent of a more undegassed magma was the primary difference responsible for the transition in explosive behaviour. This interpretation is in good agreement with gas monitoring data, previous petrological studies (mineral, microlite and melt inclusion work) and maximum calculated timescale estimates using Fe-Mg compositional gradients in clinopyroxene, that also suggest more rapid movement of relatively undegassed magma in 2010 relative to 2006.

The forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81-06-26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well-studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48–43 Ma. This is the first report of boninite-like plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58–1.44, Nb/La: 0.18–0.79) and other features uniquely connected with boninites: TiO2 25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO2, forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low-velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites.

Mafic enclaves brought to the surface in volcanic eruptions offer insight into the deeper sections of a volcanic plumbing system, where the degree of interaction between deeper, mafic magma and the magma reservoir directly feeding a volcanic edifice is not well-constrained. At Soufriere Hills Volcano (SHV) on the island of Montserrat, Lesser Antilles, mafic enclaves have been a ubiquitous feature of the andesitic eruptions during the five phases of the 1995-2010 eruption. We use the short-lived isotopes 210Pb and 226Ra within the Uranium series decay chain, which are sensitive to volatile transfer and loss (due to the loss or gain of the intermediary daughter 222Rn), in both the enclaves and the host andesite to reveal significant time-related information. The sequentially erupted andesites are almost entirely in equilibrium or have deficits of 210Pb with the deficits becoming more pronounced over several eruptive phases. We model that the andesitic reservoirs involved were subject to continuous volatile loss both before and during the eruption. The majority of enclaves, however, have excesses of 210Pb, showing volatile enrichment that lasted over a decade. The highest (210Pb/226Ra)0 ratios are from enclaves in Phase II, indicating that the deeper mafic system was closed to fresh gas influx from Phase III onwards. Enclaves are modelled as being sporadically entrained in the andesite prior to eruption, suggesting that enclave formation is not a triggering mechanism for each eruptive phase.

Short-lived uranium-series isotope data from rocks erupted under explosive and effusive regimes are presented and used to provide constraints on the timescales of magmatic degassing and volatile fluxing during the 1999 and 2000 AD eruption of Mt. Cameroon. In contrast to the relatively homogenous major and trace element data of the analysed rocks, volcanic rocks from Mt. Cameroon reveal a spread of 230 Th– 226 Ra isotope data. Volcanic rocks erupted along the southwest rift in 1999 have ( 226 Ra/ 230 Th) ratios of ~ 1.25, whereas rocks erupted more axially in 2000 have relatively low ( 226 Ra/ 230 Th) ratios of 1.09–1.2 and concomitant low Ba/Th ratios. These differences imply separate magma chamber systems and probably reflect differences in the concentrations of water within the primary magmas, which led to different amounts of amphibole fractionation at depth. Variations in the ( 210 Pb/ 226 Ra) 0 ratios are used to track degassing or volatile accumulation within the magma system. The near equilibrium ( 210 Pb/ 226 Ra) values for effusively erupted rocks from Mt. Cameroon suggest that this magma resided for more than several decades and less than a few thousand years before it erupted. The small excesses of 210 Pb over 226 Ra in some samples indicate that some of this magma was fluxed by a Rn-bearing gaseous phase for weeks to years before the eruption. In contrast, most explosively erupted rocks from Mt. Cameroon have deficits of 210 Pb relative to 226 Ra that require years to decades of degassing before eruption. We suggest that the magmas erupted as scoria at Mt. Cameroon degassed as they rose from lithospheric depths. Deep degassing was CO 2 -dominated, whereas shallower magma degassing involved more water and crystallisation of an anhydrous mineral assemblage.

No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238 U– 230 Th– 226 Ra– 210 Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens ( Kent et al., 2000 ). We report 210 Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222 Rn from 226 Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8 ka, and likely as young as 1 ka, despite being located on crust that has a spreading age of ~75 ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies.

The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7 Ma. The overlying basalts have 40Ar/39Ar ages of 48–52 Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/39Ar age of 51.1 Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52 Ma. The change to flux melting and boninitic volcanism took 2–4 m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8 m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50 Ma suggests that the two events are intimately linked.

The silica-undersaturated Nyiragongo volcanics, located in the East African Rift, have globally unique chemical compositions and unusually low viscosities, only higher than carbonatite lavas, for terrestrial silicate magmas. We report 238U-230Th-226Ra-210Pb series disequilibria in 13 recent and prehistoric lava samples from Nyiragongo including those from the 2002 flank eruption and a 2003 lava lake sample. (230Th/238U) ranges from 0.90 to 0.97 in the recent lavas and from 0.94 to 1.09 in the prehistoric lavas. To explain the variable 230Th and 238U excesses in these lavas, we hypothesize that different processes with opposite effects in terms of fractionating Th/U in the mantle source are involved. These processes include: 1) low degree partial melting of a phlogopite-bearing mantle source (consistent with low K/Rb) with residual garnet (consistent with high chondrite-normalized Dy/Yb), to produce the observed 230Th excesses; and, 2) carbonate metasomatism for the 238U enrichment, consistent with high Zr/Hf in the Nyiragongo lavas. The Nyiragongo volcanics have higher (230Th/232Th) values than observed in most mantle-derived rocks, especially ocean-island basalts, suggesting that their mantle source was affected by carbonate metasomatism less than 300 ka ago. Several Nyiragongo samples display significant 226Ra excesses implying rapid magma transport (less than 8 ka) from the mantle source to the surface. Modeling the observed (226Ra/230Th) versus Zr/Hf correlation in the lavas indicates that the 2002, 2003 and a few prehistoric lavas incorporated 50–60% of a carbonate-metasomatized mantle source while the other prehistoric lavas show 10–22% contribution of this source. This result indicates that the Nyiragongo lavas were derived from a heterogeneous, non-uniformly carbonated mantle source. The 2002 lava shows (210Pb/226Ra) equilibrium, whereas the 2003 lava lake sample shows initial (210Pb/226Ra)

Measurements of 226Ra–210Pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide, 222Rn, follows the gas phase. We present measurements of 210Pb–226Ra disequilibria for whole-rock samples representing a time sequence of tephra through the June 15, 1991 cataclysmic eruption of Mount Pinatubo volcano, Philippines. Mount Pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption, and we can, therefore, isolate the 210Pb–226Ra disequilibria due to gas accumulation without complications due to 222Rn loss during degassing. Pyroclastic samples have (210Pb)/(226Ra)0 ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has (210Pb)/(226Ra)0 = 1.12. Previous uranium-series degassing studies have suggested that 210Pb excesses are created by rapid volatile transport (carrying the intermediate daughter 222Rn) and subsequent volatile accumulation and decay of 222Rn to 210Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222Rn large enough to cause measurable disequilibria in the 210Pb–226Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of 222Rn. Therefore, we present a model in which 210Pb–226Ra disequilibria is established during basaltic recharge of the Pinatubo reservoir. The relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210Pb excess in localized basaltic melt. Transport of volatiles and 210Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of 222Rn (3.8 days), but rather by the half-life of 210Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of 210Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in 210Pb values with time may suggest changing conditions of magma supply at volcanoes.

An 40 Ar/ 39 Ar age of 45·1 Ma determined for lavas from northern Saipan confirms that these high-silica rhyolites erupted during the ‘proto-arc’ stage of volcanism in the Izu-Bonin-Mariana system, which is characterized elsewhere by eruption of boninitic lavas. Incompatible trace element concentrations and Sr, Hf, Nd, and Pb isotope ratios for these rhyolites are transitional between those of c. 48 Ma boninitic lavas and post-38 Ma ‘first-arc’ andesites and dacites from Saipan and Rota that have typical subduction-related compositions. These transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust. A second stage of Rayleigh fractionation in the upper crust is required by SiO 2 concentrations that exceed 77 wt % and near-zero compatible element concentrations. First-arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first-arc magmas themselves. The mantle sources of the proto-arc and first-arc lavas from Saipan and Rota are similar to those of Philippine back-arc basin basalts based on Nd and Hf isotopic compositions. The Pb isotope compositions of these lavas are between those of Pacific sea-floor basalts and Jurassic and younger cherty and clay-rich sediments. This contrasts with the boninitic proto-arc volcanic rocks from Guam and Deep Sea Drilling Project Sites 458 and 459 that have Pb isotope compositions similar to Pacific basin basalts and volcaniclastic sediments. The preferred explanation for the difference in the nature of proto-arc volcanism between Saipan and other fore-arc locations is that the crust ceased extending 3-4 Myr earlier beneath Saipan. This was caused by a change from mantle upwelling, fore-arc extension, and shallow melting to an environment dominated by more normal mantle wedge convection, stable crust, and deeper melting.

A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40 Ar/ 39 Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74±0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0±0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84±0.14 to 43.88±0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply westdipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench.

Arenal volcano, located in northern Costa Rica, has been continuously erupting since 1968. Magmas during the first half of the eruption by volume (Stage 1: 1968–1971) were related by largely closed-system crystal fractionation that had produced a compositionally zoned magma chamber prior to 1968. It erupted downward from the most differentiated magma in 1968 to the most mafic by early 1971. In contrast, the second half of the eruption has been dominated by recharge and compositions have become more evolved with time (Stage 2: 1971–current). We base these conclusions on new major and trace element plus Sr–Nd–Hf–Pb isotope analyses of 56 whole rocks from throughout the eruption. Differentiates are enriched in incompatible elements in both stages, but compatible element concentrations drop much more during Stage 1 than 2. Changes during Stage 1 were successfully modeled using least squares and MELTS models despite the mineral complexity of the rocks. About 19% fractional crystallization of phenocryst phases (plagioclase > orthopyroxene > clinopyroxene > magnetite) is required, consistent with crystallization at 4 kb and from 1145 to 1088 °C of a melt initially containing 2.5 wt.% H2O at quite oxidizing conditions (QFM + 2). An implication is that most phenocrysts formed during decompression and degassing. Changes during the second stage were successfully modeled using EC-E′RAχFC with the ratio of recharge to crystallization decreasing from 17 to 5 over ∼ 30 years. Crystallization rates (dFS / dt) increase from 0.05 to 0.4%/a from closed- to open-system behavior and are even faster than inferred from U-series disequilibria. The recharging magma results from a smaller degree of flux melting of a mostly similar source than for the resident magma prior to the eruption, with the two events separated by ∼ 450 years. The most recent compositions have no precedent at Arenal.

We present comprehensive U-series data (238U–234U–230Th–226Ra–210Pb–210Po and (230Th)/(232Th)) for an andesite from an oceanic arc. The juvenile Anatahan andesite has U–Th systematics colinear with other historical Mariana volcanic rocks, and is most similar to those of the other volcano in the Mariana arc with a significant proportion of silicic andesite: Uracas. Like Uracas, the parental basalt for the Anatahan andesite was generated by relatively low degrees of flux melting from a source previously enriched in a sediment component from the subducting slab. However, the Anatahan andesite is much more strongly enriched in 226Ra over 230Th than Uracas lavas, and has one of the highest (226Ra)/(232Th) ratios of siliceous andesites globally. The long-lived disequilibria between 238U–230Th–226Ra in the Anatahan andesite are inherited from basalt genesis, not created during differentiation or eruption. Thus, the time between genesis of the parental basalt and eruption of andesite at Anatahan is shorter than for Uracas. Moreover, the near-equilibrium (210Pb)/(226Ra) value indicates that the magma body did not persistently lose or gain 222Rn for more than 2 years before eruption. This permits differentiation of the parental basalt to form andesite within this 2-year time period, although a differentiation time period between 100 and a few thousand years also is possible. The relative activities between 210Po and 210Pb suggest erupted scoria degassed Po less than most lavas despite eruption plume heights of ∼10 km, which further suggests an unusually rapid ascent before eruption. These data also show that juvenile material was ejected from the first day of the eruption. Phreatomagmatic ejecta overlying the main Anatahan scoria is strongly enriched in 210Po over 210Pb, indicating that a significant proportion of the Po degassed from rising magmas sublimes in its shallow fumarolic conduit system.

Young rhyolites and associated lavas andmagmatic enclaves from the Katmai---Novarupta volcanic system (Alaskan Peninsula), and the Crater Lake and Medicine Lake volcanic system (Cascades) were analyzed for U and Th isotope abundances, as well as major and trace element concentrations, to investigate the time-scales of the processes that lead to rhyolite generation in continental arcs. Basalts and basaltic andesites typically migrate from the mantle to the surface within several thousand years. Variations in (Th)/(Th) and (U)/(Th) ratios with SiO2 concentrations in intermediate lavas appear to result from crystal fractionation combined with assimilation of recently crystallized magmas. These data also suggest that 10---10 years of mafic magmatism are required at a volcanic center to generate silicic andesites and dacites. Rhyolite genesis involves varying proportions of crystal fractionation of intermediate magmas and assimilation of crust. The nearequilibrium (U)/(Th) ratios for all of the rhyolites suggest an average time since U was last fractionated from Th for the constituents making up these rhyolites of 410 years. Therefore, the residence times of continental magmas and their entrained crystals appear to increase by a minimum of 2---3 orders of magnitude with increasing SiO2 concentrations from basalt to rhyolite.

The bulk composition of magma erupted from Volcan Arenal has remained nearly constant (SiO2 = 53.6–54.9 wt%; MgO = 5.0–4.5 wt%) during almost 30 years of continuous activity (1969–1996). None the less, clinopyroxene (cpx) phenocrysts and their spinel inclusions record a much more complex open-system evolution in which steady-state production of the erupted basaltic andesitic magma is linked to episodic injections of basalt into Arenal's magma conduit/reservoir system. High-resolution major element zoning profiles (electron microprobe) on a large number of phenocrysts (>14,000 analyses), tied to back-scattered electron (BSE) images, have been used to assess the compositional characteristics of the magmatic end members as well as the timing and dynamics of magma replenishment events. No two cpx phenocrysts have exactly the same zoning profile. The vast majority of our analyses record the crystallization of cpx (Cr2O3

The Marsokhod rover returned data from six stations that were used to decipher the geomorphology and geology of a region not previously visited by members of the geomorphology field team. Satellite images and simulated descent images provided information about the regional setting. The landing zone was on an alluvial apron flanking a mountain block to the west and a playa surface to the east. Rover color images, infrared spectra analysis of the mountains, and the apron surface provided insight into the rock composition of the nearby mountains. From the return data the geomorphology team interpreted the region to consist of compressionally deformed, ancient marine sediments and igneous rocks exposed by more recent extensional tectonics. Unconsolidated alluvial materials blanket the lower flanks of the mountains. An ancient shoreline cut into alluvial material marks a high stand of water during a past, wetter climate period. Playa sediments floor a present-day, seasonally, dry lake. Observations made by the rover using panoramic and close-up (hand specimens—scale) image data and color scene data confirmed the presence of boulders, cobbles, and fines of various provinces. Rover traverses to sites identified as geologically distinct, such as fan, channel, shoreline, and playa, provided useful clues to the geologic interpretations. Analysis of local rocks was given context only through comparison with distant geologic features. These results demonstrated the importance of a multifaceted approach to site interpretation through comparison of interpretations derived by differing geologic techniques.

The carbon isotopic compositions of six stalagmites from five caves in the Ozark Highlands of central and southern Missouri and northern Arkansas provide a detailed record of early and late Holocene vegetation dynamics. A rapid decrease in speleothem ∂ 13 C values between ∼9500 and ∼8200 yr BP indicates a period of increased C3 vegetation, suggesting cool and/or moist conditions relative to the earliest Holocene and the prairie-dominated middle Holocene. A second negative ∂ 13 C excursion from ∼4500 to ∼3000 yr BP interrupts a predominantly C4-rich middle Holocene prairie environment that became established at ∼7500 yr BP. Speleothem mineralogical indicators of cave aridity do not support previous inferences of increased regional dryness between 3800 and 3100 yr BP.

Speleothems from a well-ventilated dolomitic cave in the Pokhara Valley, central Nepal, preserve a mineralogic record of Indian summer monsoon variability over the past 2300 yr. Annually deposited aragonite layers formed between 2300 and 1500 yr B.P., indicating reduced monsoon precipitation and increased cave aridity, whereas alternating calcite/aragonite laminae deposited after 1500 yr B.P. record elevated summer monsoon precipitation and increased cave humidity. Dense, optically clear calcite layers deposited from 450 ± 5 to 360 ± 20 yr B.P. (1550 to 1640 A.D.) indicate a less-evaporative cave environment and suggest moister and/or cooler conditions, possibly related to climatic change associated with the onset of the Little Ice Age.

Speleothem carbon and oxygen isotopic records from Onondaga Cave, south-central Missouri, and Beckham Creek Cave, north-central Arkansas, are compared with the Cupola Pond and Oldfield Swamp pollen series from southeastern Missouri and the Rodgers Shelter and Modoc Shelter vertebrate biostratigraphic sequences from central Missouri and southwestern Illinois. Similar, and roughly contemporaneous, shifts between deciduous forest and steppe indicators throughout the Holocene are revealed in each database. These independent proxies record steppe conditions between approximately 9000 and 1500 cal yr B.P. A shift toward lighter speleothem carbon may reflect a change from warm and dry to cool and dry conditions between 4500 and 3000 yr B.P. The sensitive response of speleothem δ13C to changes in vegetation emphasizes their importance as paleoclimate records in an area containing few other millenial-scale climate proxies.

Carbon and oxygen isotopic trends from seven Midwestern speleothems record significant offsets in the timing of middle-Holocene vegetation change. Interactions of dry Pacific and moist Gulf of Mexico air masses maintained a sharp moisture gradient across Iowa, Minnesota, and Wisconsin such that the arrival of prairie was offset by 2000 years between caves and pollen sites located only 50 km apart. Oxygen isotopes shift concomitantly with carbon in most cases, although these changes are believed to represent increased evaporative enrichment of 18O prior to infiltration during the prairie period.

This is the published version. Copyright 1998 American Geophysical Union. All Rights Reserved.

This paper presents the biotic, sedimentary, geomorphic, and climatic history of the upper part of the Roberts Creek Basin, northeastern Iowa for the late-glacial and Holocene, and compares these records with a C-O isotopic sequence from Coldwater Cave, 60 km northwest of Roberts Creek. the biotic record (pollen, vascular plant and bryophyte macrofossils, and insects) is preserved in floodplain alluvium that underlies three constructional surfaces separated by low scarps. Each surface is underlain by a lithologically and temporally distinct alluvial fill. The highest surface is underlain by the Gunder Member of the Deforest Formation, dating from 11,000 to 4000 yr BP; beneath the intermediate level is the Roberts Creek Member, dating from 4000 to 400 yr BP; and the lowest level is underlain by the Camp Creek Member, deposited during the last 380 yr. Pollen and plant macrofossils in the alluvial fill show that a typical late-glacial spruce forest was replaced by Quercus and Ulmus in the early Holocene. This early-to-middle Holocene forest became dominated by medic elements such as Acer saccharum, Tila americana, Ostyra virginiana, and Carpinus caroliniana as late as 5500 yr BP; in contrast, the closest sites to the west and north were at their warmest andmore » driest were covered by prairie vegetation between 6500 and 5500 yr BP. After 5500 yr BP, the forest in the roberts Creek area was replaced by prairie, as indicated by a rich assemblage of plant macrofossils, although only Ambrosia and Poaceae became abundant in the pollen record. The return of Quercus {approx} 3000 BP (while nonarboreal pollen percentages remained relatively high) indicates the oak savanna prevailed with little change until settlement time. 83 refs., 17 figs., 5 tabs.« less

The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4oAr/39Ar ages of variably tilted (0-90 o) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

[1] There has been much debate as to whether 210Pb-226Ra disequilibria in young volcanic rocks result from partial melting, cumulate interaction or magma degassing. Here we present new data from basalts erupted in 1978 from Ardoukoba volcano in the Asal Rift. The (210Pb/226Ra)t ratios are very low (0.2 to 0.6) and appear to correlate negatively with (226Ra/230Th). Invariant (230Th/238U) and (231Pa/235U) ratios require similar melting rates, porosities, and extents for all parental magmas. Thus, the range in (226Ra/230Th), which is negatively correlated with Th concentration, reflects fractional crystallization over millennia after the magmas were emplaced into the crust. This precludes the 210Pb deficits from resulting from partial melting. Instead, the 210Pb deficits must have formed subsequent to magma differentiation and are interpreted to reflect several decades of magma degassing. Many young basalts erupted in a variety of tectonic settings are similarly depleted in 210Pb with respect to 226Ra, suggesting that they continuously degas over a period of a few to several decades, perhaps reflecting the time required to rise to the surface from deeper reservoirs. In some of these basalts, gas accumulation leads to the shallowest, most evolved, and earliest erupting magmas having the highest (210Pb/226Ra) ratios and sometimes 210Pb excesses.

We report 238U-230Th-226Ra-210Pb-210Po, 232Th-228Ra and 235U-231Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas ( 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time-independent models. Interpretation of the 226Ra data requires time-dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these 210Pb deficits. The young coeval volcanism on Malumalu and Vailulu'u suggests the Samoa hot spot is currently migrating to the northeast due to dynamic interaction with the Tonga slab.

Lavas from Klyuchevskoy and Bezymianny volcanoes, Kamchatka, appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust. This fractionation may have occurred on timescales significantly less than 1000 years if observed 226 Ra excesses largely reflect variable residual porosity in the source melting region. Unlike most arc lavas, those with the highest MgO contents and Ba/Th ratios have the lowest 226 Ra excess. Forward models suggest that those portions of the source which had undergone the greatest addition of U by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity. At Kluchevskoy, a change from eruption of high-MgO to high-Al 2 O 3 basaltic andesites around 1945 is reflected in an increase in size of 226 Ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime. The eruption of andesites at Bezyminanny, simultaneous with the eruption of basaltic andesites at Klyuchevskoy, further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time. Thus, the melting processes beneath Klyuchevskoy and Bezyminanny are demonstrably complex. They have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the Central Kamchatka Depression. Finally, the 210 Pb data are, with one or two exceptions, in equilibrium with 226 Ra, suggesting that there was restricted relative magma-gas movement in this highly productive magmatic system.

Tham Khuyen Cave (Lang Son Province, northern Vietnam) is one of the more significant sites to yield fossil vertebrates in east Asia. During the mid-1960s, excavation in a suite of deposits produced important hominoid dental remains of middle Pleistocene age. We undertake more rigorous analyses of these sediments to understand the fluvial dynamics of Pleistocene cave infilling as they determine how skeletal elements accumulate within Tham Khuyen and other east Asian sites. Uranium/thorium series analysis of speleothems brackets the Pleistocene chronology for breaching, infilling, and exhuming the regional paleokarst. Clast analysis indicates sedimentary constituents, including hominoid teeth and cranial fragments accumulated from very short distances and under low fluvial energy. Electron spin resonance analysis of vertebrate tooth enamel and sediments shows that the main fossil-bearing suite (S1-S3) was deposited about 475 thousand years ago. Among the hominoid teeth excavated from S1-S3, some represent Homo erectus and Gigantopithecus blacki. Criteria are defined to differentiate these teeth from more numerous Pongo pygmaeus elements. The dated co-occurrence of Homo erectus and Gigantopithecus blacki at Tham Khuyen helps to establish the long co-existence of these two species throughout east Asia during the Early and Middle Pleistocene.

High-precision uranium-thorium mass spectrometric chronology and (18)O-(13)C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in dagger(18)O and dagger(13)C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on (18)O/(16)O ratios suggest that the climate warmed rapidly by about 3 degrees C at 5900 years before present and then cooled by 4 degrees C at 3600 years before present. Initiation of a gradual increase in dagger(13)C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 µg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess (210Pb)/(226Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ∼1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

What triggered the recent and ongoing eruption of Mount St. Helens? Is it new magma or just leftovers from the 1980s? How long will the eruption continue? Will the magma composition change, and will it become more explosive? The U.S. Geological Survey's (USGS) Cascades Volcano Observatory (CVO) hosted a three-day workshop at which 25 petrologists addressed these fundamental questions. The workshop began with a review of monitoring data by CVO staff and was followed by presentations and discussions of new petrologic data.

Volcanoes of the Pacific Ring of Fire and other convergent margins worldwide are familiar manifestations of nature's energy, account for about 25% of global volcanic outputs, dominate volcanic gas emissions to the atmosphere, and pose significant physical threats to a large human population. Yet the processes behind this prolific activity remain poorly understood. An international “State of the Arc” (SOTA) conference was held in August on the slopes of Mt. Hood, Oregon, to address current views on the energy and mass fluxes in volcanic arcs. This meeting brought together some 90 leading experts and students of subduction zones and their related magmatism.

In the Andean Central Southern volcanic zone, basalts from small eruptive centers near the large composite center Volcan Villarrica are poor in fluid mobile elements, such as B, Cs, Rb, K, Pb, Ba, and U, compared with concurrently erupted Villarrica basalts. New 1 0 Be and U-series isotopic data for these centers show that fluid mobile element-poor small eruptive center basalts have small 1 0 Be/ 9 Be ratios (1.6-1.9 × 10 - 1 1 ) and ( 2 3 8 U/ 2 3 0 Th) activity ratios near 1.0, whereas basalts from Villarrica show 2 3 8 U enrichment and larger 1 0 Be/ 9 Be ratios (4.0-6.4 × 10 - 1 1 ). These results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k.y. to 3 m.y. That these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting. In contrast, the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere. The results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales.

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical δ 13 C trends from ca. 7 to 2 ka. However, δ 18 O patterns differ by as much as 3‰ from 5.7 to 3.2 ka. These disparate δ 18 O values reflect evaporative 18 O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity.

Volcanic rocks exposed on Guam were erupted during the Late Middle Eocene (Facpi Fm.), Late Eocene-Oligocene (Alutom Fm.) and Miocene (Umatac Fm.). Four magma series are recognized: the boninite series (44 m.y.b.p.), the tholeiite and calc-alkaline series, which were erupted along with boninite series lavas at 32–36 m.y.b.p. and high-K lavas of the Umatac Fm. (14 m.y.b.p.). Isotope and and rare earth element (REE) characteristics of the four magma series are distinct. Boninite series lavas have U-shaped REE patterns, relatively low 143Nd/144Nd (0.51294–0.51298), and high 206Pb/204Pb (19.0–19.2). Tholeiite series lavas are LREE (light REE) depleted, and have high 143Nd/144Nd (0.51304–0.51306) and low 206Pb/204Pb (18.4–18.5). Calc-alkaline series lavas have Sr, Nd and Pb isotope ratios similar to tholeiite series lavas, but flat to U-shaped REE patterns. Umatac Fm. lavas are strongly LREE-enriched, and have higher 87Sr/ 86Sr (0.70375–0.70380) and 207Pb/204Pb relative to 206Pb/ 204Pb than Facpi and Alutom Fm. lavas.

Isotopic studies of rocks from oceanic island arcs such as the Marianas indicate that little, if any, recycling of continental material (e.g. oceanic sediments) occurs in these arcs. Because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs, an important question is whether the andesites of continental arcs are produced by a fundamentally different (more complex?) mechanism than the lavas of oceanic arcs. An excellent opportunity for study of this question is provided by the island of Sarigan, in the Mariana active arc, on which calc-alkaline andesites (including hornblende-bearing types) are exposed along with more mafic lavas. Available isotope data suggest the Sarigan lavas (including the andesites) were derived from mantle material with little or no involvement of continental components. Ratios of incompatible elements suggest that most of the Sarigan lavas were derived from similar source materials. Absolute abundances of incompatible elements vary irregularly within the eruptive sequence and indicate at least 5 distinct magma batches are represented on Sarigan. Major element data obtained on the lavas and mineral phases in them, combined with modal mineral abundances, suggest that the calc-alkaline nature of the volcanic rocks on Sarigan results from the fractional crystallization of titanomagnetite in combination with other anhydrous phases. Amphibole, although present in some samples, is mainly a late-crystallizing phase and did not produce the calc-alkline characteristics of these lavas. Gabbroic samples found in the volcanic sequence have mineralogc and geochemical characteristics that would be expected of residual solids produced during fractional crystallization of the Sarigan lavas. When combined, data on the lavas and the gabbros suggest the following crystallization sequence: olivine — plagioclase — clinopyroxene — titanomagnetite — orthopyroxene±hornblende, biotite and accessory phases. These results lead to the conclusion that calc-alkaline magmas can be generated directly from mantle sources.

The island of Guam is located at the southern end of the Mariana fore-arc and was the site of early arc volcanism between 43 and 32 m.y. ago. Two volcanic units were erupted during the early evolution of Guam: the late middle Eocene Facpi Formation and the late Eocene to early Oligocene Alutom Formation. The Facpi Formation is composed largely of interbedded boninite series pillow lavas, pillow breccias, and dikes, although arc tholeiite series rocks cap the formation in some areas. The Alutom Formation is composed of interbedded volcanic breccias, tuffaceous sandstones, lava flows, and sills, calc-alkaline, arc tholeiite, and boninite series compositions are all found in the Alutom formation. Primitive boninite series rocks are relatively high in SiO 2 , MgO, and Ni and low in A1 2 O 3 and TiO 2 contents compared to basaltic rocks from the arc tholeiite series. Additionally, they have low Ti/Zr ratios and high K/Zr, Rb/Zr, and Ba/Zr ratios. Although no calc-alkaline basalts are found in the early arc formations of Guam, basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc-alkaline series. Among the analyzed samples here are a few with anomalously high Y and rare-earth element (REE) contents. Chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase, clinopyroxene, and orthopyroxene. The arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase, clinopyroxene, orthopyroxene, and magnetite. The composition of the least silicic calc-alkaline series andesite can be modeled by fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite from a transitional basalt. Derivation of more silicic calc-alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende. All basalts from the early volcanic series of Guam were erupted in an arc setting. The boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels, whereas the arc tholeiite and calc-alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels. We suggest that because the calc-alkaline series parent basalts most likely were richer in incompatible high field-strength (HFS) elements and SiO 2 than were the tholeiitic parents, they were generated by lower degrees of melting.

Lavas from volcanoes in the Mariana arc show a range of K 2 O-SiO 2 trends even though the volcanoes are all nearly the same distance above the Benioff Zone. The preferred petrologic explanation for the K 2 O-SiO 2 variations invokes differences in the degree of partial melting required to produce lavas for each volcano from a relatively homogeneous source material. The Mariana arc volcanoes appear to evolve chemically in a manner similar to oceanic volcanoes (e.g., Hawaii) with early and late alkalic stages dominated by an intermediate tholeiitic stage. If volcanoes in other arcs also undergo this evolution, many of the anomalies in K 2 O-depth relations in modern arcs could be explained.