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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] , [...]

Helium isotope ratios in oceanic glasses provide a high-integrity tracer of contributions from mantle plumes. Despite a diverse array of petrogenetic affinities, glasses from the central part of the Manus Basin - a backarc basin in the western Pacific - have typical plume (or hotspot) 3He/4He ratios that cluster around 12.2[R.sub.A] ([+ or -]1.0[R.sub.A], n = 18, where [R.sub.A] = 3He/4He of air), a value significantly higher than the range found in most mid-ocean-ridge basalts (MORB) ([8 [+ or -] 1][R.sub.A]). Lavas in other parts of the basin have MORB-like or lower 3He/4He values. A wide range of He concentrations characterizes the Manus Basin glasses: This is considered to reflect the high water content of some lavas, which promotes He loss through volatile degassing. For the most part, it is the degassed lavas that do not show the plume He isotope signature. Results of the present study, together with 3He/4He data for lavas and gases from islands to the south and east of the Bismark Sea, indicate that the focus of mantle plume upwelling is either the center of the Manus Basin or possibly the region to the northwest beneath the volcanic islands of the St. Andrew Strait. This region of plume or hotspot 3He/4He ratios coincides with a domain of anomalously low seismic velocities at the underlying core-mantle boundary, and indicates that the provenance of high-3He/4He magmas in the Manus Basin (and possibly elsewhere) is linked to this boundary layer - either by plume entrainment of lower mantle or, more speculatively, through addition of material from the core-mantle boundary.

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]