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Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging.sup.1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform.sup.2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc.sup.3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine--that is, hydrated mantle rather than crust or sediments--is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards. Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust., Author(s): George F. Cooper [sup.1] [sup.6] , Colin G. Macpherson [sup.2] , Jon D. Blundy [sup.1] , Benjamin Maunder [sup.3] , Robert W. Allen [sup.3] , Saskia Goes [sup.3] , [...]

Three prehistoric sites in the Upper Mun River Valley of north-eastern Thailand have provided a detailed chronological succession comprising 12 occupation phases. These represent occupation spanning 2300 years, from initial settlement in the Neolithic (seventeenth century BC) through to the Iron Age, ending in the seventh century AD with the foundation of early states. The precise chronology in place in the Upper Mun River Valley makes it possible to examine changes in social organisation, technology, agriculture and demography against a background of climatic change. In this area the evidence for subsistence has been traditionally drawn from the biological remains recovered from occupation and mortuary contexts. This paper presents the results of carbon isotope analysis to identify and explain changes in subsistence over time and between sites, before comparing the results with two sites of the Sakon Nakhon Basin, located 230km to the north-east, to explore the possibility of regional differences. Keywords: Southeast Asia, Thailand, Upper Mun River Valley, Sakon Nakhon Basin, Neolithic, isotope analysis, dietary change, Introduction Variations both over time and between sites in carbon, oxygen and strontium isotope signatures from human teeth recovered from five prehistoric settlements on the Khorat Plateau of north-eastern Thailand [...]

Plate tectonic processes result in recycling of crust and lithosphere into Earth's mantle. Evidence for long-term preservation of recycled reservoirs in the mantle comes from the enriched isotopic character of oceanic island basalt (OIB) lavas. Although recycled constituents can explain much of the geochemical variation in the OIB-source mantle, it has been shown that direct melting of these components would lead to magmas with evolved compositions, unlike OIB. Instead, it has been argued that either metasomatic pyroxene-rich peridotite that has inherited the trace element and isotopic character of subducted materials, or high-temperature intramantle metasomatism of lithosphere can explain OIB compositions. To test these models, we present new oxygen and osmium isotope data for lavas from the Canary Islands of El Hierro and La Palma. These islands have distinct [sup.18]O/[sup.16]O and [sup.187]Os/[sup.188]Os compositions that can be explained through melting of pyroxenite-enriched peridotite mantle containing

The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. Lithosphere is thicker beneath the younger, southern part of the arc, causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section, basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. Local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help to explain the andesitic composition of bulk continental crust. Keywords: subduction initiation, arc lithosphere erosion, crust growth, adakitic magmatism, high-Mg# andesite.

Whether the downgoing slab in modern subduction zones is able to melt is an important unresolved aspect of plate tectonics. Rare arc lavas with high Sr/Y ratios and low Y and heavy rare earth element concentrations, called adakites, are often attributed to melting of young subducted crust in subduction zones. Osmium isotopic data obtained for 13 adakite samples from Mindanao, Philippines, conflict with the slab-melting model. Ten of the samples have unradiogenic [sup.187Os]/[sup.188Os] ratios inconsistent with partial melting of the ca. 50 Ma Philippine Sea lithosphere subducting beneath Mindanao. The Os isotope signatures are similar to those of young mid-oceanic-ridge basalts and normal arc rocks, consistent with a mantle source. In addition to implications for mass transfer in subduction zones, the apparent association of adakites with Cu-Au deposits makes understanding their origin, and that of the associated metals, economically interesting. This study suggests that a reevaluation is required of the slab-melting hypothesis in general, and that the metals associated with the Mindanao adakites come from the mantle. Keywords: adakites, osmium, Philippines, subduction.

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit [sup.3]He/[sup.4]He ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High [sup.3]He/[sup.4]He ratios must indicate derivation from a mantle source with high time-integrated [sup.3]He/(U+Th) relative to depleted MORB-source mantle. The location of the high [sup.3]He/[sup.4]He mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earth's mantle. It has been proposed that the high [sup.3]He/[sup.4]He reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have [sup.3]He/[sup.4]He ratios more radiogenic than MORB, strongly arguing against a widespread high [sup.3]He/[sup.4]He source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high [sup.3]He/[sup.4]He in any samples known to originate from this environment. Therefore, volcanic rocks with [sup.3]He/[sup.4]He greater than MORB [sup.3]He/[sup.4]He are likely to sample a mantle source with high time-integrated [sup.3]He/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the [sup.3]He/[sup.4]He distribution in MORB. Keywords: helium, isotopes, mantle, continental, intraplate, volcanics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper., Author(s): George F. Cooper [sup.1] [sup.6] , Colin G. Macpherson [sup.2] , Jon D. Blundy [sup.1] , Benjamin Maunder [sup.3] , Robert W. Allen [sup.3] , Saskia Goes [sup.3] , [...]

Commercial helium systems have been found to date as a serendipitous by-product of petroleum exploration. There are nevertheless significant differences in the source and migration properties of helium compared with petroleum. An understanding of these differences enables prospects for helium gas accumulations to be identified in regions where petroleum exploration would not be tenable. Here we show how the basic petroleum exploration playbook (source, primary migration from the source rock, secondary longer distance migration, trapping) can be modified to identify helium plays. Plays are the areas occupied by a prospective reservoir and overlying seal associated with a mature helium source. This is the first step in identifying the detail of helium prospects (discrete pools of trapped helium). We show how these principles, adapted for helium, can be applied using the Rukwa Basin in the Tanzanian section of the East African Rift as a case study. A thermal hiatus caused by rifting of the continental basement has resulted in a surface expression of deep crustal gas release in the form of high-nitrogen gas seeps containing up to 10% 4He. We calculate the total likely regional source-rock helium generative capacity, identify the role of the Rungwe volcanic province in releasing the accumulated crustal helium and show the spatial control of helium concentration dilution by the associated volcanic CO2. Nitrogen, both dissolved and as a free-gas phase, plays a key role in the primary and secondary migration of crustal helium and its accumulation into what might become a commercially viable gas pool. This too is examined. We identify and discuss evidence that structures and seals suitable for trapping hydrocarbon and CO2 gases will likely also be efficient for helium accumulation on the timescale of the Rukwa Basin activity. The Rukwa Basin prospective recoverable P50 resources of helium have been independently estimated to be about 138 BSCF (billion standard cubic ft: 2.78 × 109 m3 at STP). If this volume is confirmed it would represent about 25% of the current global helium reserve. Two exploration wells, Tai 1 and Tai 2, completed by August 2021 have proved the presence of seal and reservoir horizons with the reservoirs containing significant helium shows. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series [ABSTRACT FROM AUTHOR]

We present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s−1) above a lower gradient Layer 3 (0.2 s−1). Here, PmP coincides with the 7.5 km s−1 contour, and velocity increases to >7.8 km s−1 within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s−1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide "domal" morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults. Key Points: We identify that magmatic and tectonic modes of slow Atlantic crustal accretion are distinct and equal, based on seismic characteristicsThe structure of crustal discontinuities is controlled by the accretion mode of neighboring segments, independent of ridge offset and orderTwo buried oceanic core complexes in mature Atlantic Ocean crust are identified, based on their dimensions and seismic structure [ABSTRACT FROM AUTHOR]

We present a spatial analysis of volcano distribution and morphology in the young, intraoceanic Mariana Arc. Both the quality of fit to idealized models and the divergence from those ideals indicate that Mariana Arc volcanoes are arranged into five great circle segments, rather than a single small circle or multiple small circles. The alignment of magmatic centers suggests that magma transport is controlled by the stress regime in the deep crust and/or lithospheric mantle of the Philippine Sea Plate, into which the arc is emplaced, and that arc-normal tension is the dominant process operating in the deep lithosphere along the whole arc. Volcano morphologies indicate that the stress regime in the shallow crust varies between arc-normal tension and compression, which also implies that the stress field can vary with depth in the arc lithosphere. We show that this horizontal and vertical stress partitioning can be related to the changing dip of the subducting plate and the breadth of the zone where it is coupled with the overriding plate. The variation in stress regime is consistent with both the distribution of seismicity in the Philippine Sea Plate and with the structural fabrics of the nonvolcanic part of the plate margin to the south. Our analysis suggests that the upper plate exerts the principal control on the distribution of volcanoes in the Mariana Arc. Where tension in the deeper parts of arc lithosphere is sufficiently concentrated, then a distinct volcanic front is produced. [ABSTRACT FROM AUTHOR]

New 40Ar/39Ar ages are presented for volcanic rocks from the Karacadağ Volcanic Complex (KVC) in SE Anatolia. These extend the oldest age recognised for the KVC for lavas from the Siverek Phase plateau basalt back to at least 17 Ma. Our data agree with prior studies that the second, Karacadağ Phase of igneous activity began at about 4.5 Ma with a peak between 2 and 1 Ma, but in contrast to previous work we find that this included volcanism on the Mt. Karacadağ ridge from the earliest part of this phase. Construction of the Mt. Karacadağ ridge coincided with development of the prominent east-west striking Abd El Aziz and Sinjar compressional antiforms, which lie approximately 100 km to the south. These provide complementary evidence for the orientation of compressional deformation across a broad zone of the leading edge of the Arabian Plate as it collided with Anatolia between 4 and 1 Ma. The new data also indicate that the compression-aligned deformation of the foreland magmatism may have permitted tapping of melts from greater depth in the probably asthenospheric mantle than occurred prior to 4 Ma. However, magmas from throughout the history of the KVC contain some components derived from the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Climate and tectonics have complex feedback systems which are difficult to resolve and remain controversial. Here we propose a new climate-independent approach to constrain regional Andean surface uplift. 87Sr/86Sr and 143Nd/144Nd ratios of Quaternary frontal-arc lavas from the Andean Plateau are distinctly crustal (>0.705 and <0.5125, respectively) compared to non-plateau arc lavas, which we identify as a plateau discriminant. Strong linear correlations exist between smoothed elevation and 87Sr/86Sr (R2 = 0.858, n = 17) and 143Nd/144Nd (R2 = 0.919, n = 16) ratios of non-plateau arc lavas. These relationships are used to constrain 200 Myr of surface uplift history for the Western Cordillera (present elevation 4200 ± 516 m). Between 16 and 26°S, Miocene to recent arc lavas have comparable isotopic signatures, which we infer indicates that current elevations were attained in the Western Cordillera from 23 Ma. From 23-10 Ma, surface uplift gradually propagated southwards by ~400 km. [ABSTRACT FROM AUTHOR]

We have used multiple-regression methods to calibrate new, pressure-independent empirical chemometric equations to calculate the major element composition of basanitic to rhyolitic melts in equilibrium with calcic amphibole. The equations are based on amphibole stoichiometric formula components ± temperature from published experimental P- T- X data and avoid some problems of previous studies associated with uncertainties in pressure determination. Compared with the pressure-dependent equations of , tests run using an independent data set indicate that the new equations yield improved precision and accuracy, in particular for SiO2, TiO2, CaO, and K2O. The results are only marginally more precise when temperature is used as a dependent variable, demonstrating that temperature has a relatively minor role in controlling amphibole crystal chemistry compared with melt composition. This allows us to accept a small decrease in precision in excluding temperature from the analysis, which is very convenient for application of the equations to natural amphiboles where temperature is typically unknown. Using the new chemometric equations, reconstructed melt compositions in equilibrium with the rims of amphiboles in pumice clasts of the Ongatiti ignimbrite are in good agreement with coexisting matrix glass compositions, lending support for our analysis. The compositionally variable cores of the amphiboles give predicted melt compositions with large compositional variations from andesitic (63 wt% SiO2) to high-Si rhyolite. These compositional variations in the predicted melt compositions suggest that there may be a range of heterogeneous melts undergoing progressive differentiation within a major crustal magma storage region underneath the volcano. The results support the existence of genuine intermediate composition melts within the storage region. Interaction between these stored melts, disaggregating mush fragments and replenishing magmas gives rise to the chemical complexity observed in erupted magmas. We also used our multiple regression model to predict the compositions of melts that were in equilibrium with amphiboles in plutonic nodules from Grenada lavas. The predicted melts cover a wide range of compositions, perhaps as a result of in situ fractionation, but are consistent with melt inclusions hosted in those cumulates, as reported by . Overall, our new pressure- and temperature-independent equations resolve issues associated with previous pressure-dependent studies and represent a useful tool for further investigation of crustal processes at subduction zones. [ABSTRACT FROM AUTHOR]

ABSTRACT Objectives The nature of the agricultural transition in Southeast Asia has been a topic of some debate for archaeologists over the past decades. A prominent model, known as the two-layer hypothesis, states that indigenous hunter-gatherers were subsumed by the expansion of exotic Neolithic farmers into the area around 2000 BC. These farmers had ultimate origins in East Asia and brought rice and millet agriculture. Ban Non Wat is one of the few archaeological sites in Southeast Asia where this model can potentially be tested. The site is located in the Mun River valley of Northeast Thailand, and divided into 12 phases that span over 2,000 years, from about 1750 BC to the end of the Iron Age (ca. 500 AD). These phases exhibit successive cultural changes, and current interpretation of the site is of an early hunter-gatherer population, with agriculturalists immigrating into the later phases. Methods We analyzed strontium, oxygen, and carbon isotopes in tooth enamel from over 150 individuals, dating from the Neolithic to Iron Age, to assess extrinsic origins and differences in diet between early and later phases. Results We find evidence of dietary and cultural differences between groups at Ban Non Wat during its early occupation, but little evidence for immigration from distinct environments beyond the Khorat Plateau of Northeast Thailand. Conclusions The lack of consistent isotopic differences between early and later Neolithic occupants at Ban Non Wat means that the site does not conclusively support the two-layer hypothesis. Am J Phys Anthropol 158:141-150, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The position of the intertropical convergence zone is an important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere temperature contrasts. The release of aerosols by human activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s (refs , , , , ) by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval, but this proposed shift remains equivocal. Here we reconstruct monthly rainfall over Belize for the past 456 years from variations in the carbon isotope composition of a well-dated, monthly resolved speleothem. We identify an unprecedented drying trend since ad 1850 that indicates a southward displacement of the intertropical convergence zone. This drying coincides with increasing aerosol emissions in the Northern Hemisphere and also marks a breakdown in the relationship between Northern Hemisphere temperatures and the position of the intertropical convergence zone observed earlier in the record. We also identify nine short-lived drying events since ad 1550 each following a large volcanic eruption in the Northern Hemisphere. We conclude that anthropogenic aerosol emissions have led to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics. [ABSTRACT FROM AUTHOR]

Abstract: New analyses of major and trace element concentrations and Sr, Nd and Pb isotopic ratios are presented for Plio-Pleistocene basalts and basaltic andesites from the Semporna peninsula in Sabah, Borneo, at the southern end of the Sulu Arc. Depletion of high field strength elements (HFSE), which is characteristic of many subduction-related magmatic suites, is present in more evolved Semporna rocks but is associated with radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios and results from contamination of mafic melt by, possibly ancient, crustal basement. The most mafic lavas from Semporna, and elsewhere in the Sulu Arc, display no HFSE depletion relative to other elements with similar compatibility. High-Nb basalt from Semporna formed when mantle resembling the source of Ocean Island Basalt (OIB) upwelled into lithospheric thin spots created during earlier subduction. This mantle did not experience enrichment by fluids or melt derived from subducted crust. The presence of similar lavas throughout the Sulu Arc and around the South China Sea suggests that the OIB-like component resides in the convecting upper mantle. Depletion of light rare earth elements, with respect to other incompatible elements, throughout the Sulu Arc could result from melt–mantle interaction during magma transport through the lithosphere. Such depletion is absent in suites from the South China Sea, where magma probably migrated along large, lithosphere-penetrating structures. Semporna high-Nb basalts are not associated with adakitic magmatism which is a frequent, but not ubiquitous, association in some active subduction zones. Both geochemical signatures are developed early in the history of a melt pulse, either in the source (high-Nb basalt) or during deep differentiation (adakite). Preservation of these distinctive geochemical signatures is favoured in settings that minimise (1) interaction with other, more copious melt types, and/or (2) subsequent differentiation in the shallow crust. Where found, the high-Nb basalt–adakite association is a result of transport through favourable lithospheric conditions and not due to any link between their mantle sources. [Copyright &y& Elsevier]

A letter to the editor is presented in response to the article "The population history of Mainland Southeast Asia: two layer model in the context of Northern Vietnam," by H. Matsumura and colleagues in the 2011 issue.

We report an extensive helium isotope survey of basaltic to andesitic lavas from the Lesser Antilles island arc—an arc system with well-documented evidence of crustal contamination. Given the sensitivity of helium isotopes as a tracer of the effects of crustal additions, our aim is to evaluate the relationship of 3He/4He ratios to other indices of contamination processes such as oxygen and strontium isotopes. To this end, we have carried out 53 3He/4He analyses on separated minerals (olivines and pyroxenes) from throughout the arc, which we compare with whole-rock strontium and phenocryst oxygen isotope measurements. We show that the three isotopic tracers show coherent patterns throughout the Lesser Antilles, indicating a regional control on crustal contamination. The southern section of the arc (Grenada to Martinique) shows clear evidence for major crustal contamination in all three isotopic systems with results for our samples in the range 3He/4He(olivine) 3·6–7·6RA, δ18O(olivine) 4·74–5·76‰, and 87Sr/86Sr(whole-rock) 0·703970–0·705463. We suggest that terrigenous sediments incorporated into the arc crust are the principal contaminant. In contrast, there is minimal contamination in the northern part of the arc [Martinique–(Dominica)–Guadeloupe to Saba] with results for the samples in the range 3He/4He(olivine) 6·8–8·4RA, δ18O(olivine) 5·01–5·29‰, and 87Sr/86Sr(whole-rock) 0·703221–0·703843. In this part of the arc, only pyroxene 3He/4He values (1·4–6·8RA) consistently record evidence of relatively minor, late-stage additions from the arc crust. Martinique appears to represent the transition between the two sections of the arc. The tracer with the greatest sensitivity appears to be helium isotopes, as contamination is seen first and most prominently in this system. [ABSTRACT FROM PUBLISHER]

Presents information on a study which was conducted in the Macus Basin, a backarc basin in the western Pacific, to analyze helium isotope rations in oceanic glasses. What these isotope ratios provide; Geological settings of the Macus Basin; Findings of the study.