Search

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] , [...]

A survey of academic authors who work with instructional designers at Central Queensland University (Australia) found that 75% thought assigning instructional designers to faculties rather than having them remain in a centralized distance-education facility would be an improvement. Discusses the significance of the findings and compares selected findings to a similar study at the University of Tasmania. (Author/PEN)

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 [...]

Reports observations of primary teachers (n=14) efforts to provide activity-based lessons for their pupils. Teachers' beliefs that their lessons were student-centered were not confirmed by the observations of the lessons. Discusses implications of the findings for inservice teacher education and parental involvement. (MDH)

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.

Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus Si[O.sub.2] relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this 'cryptic amphibole fractionation.' If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective filter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fluids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle. Keywords: amphibole, recycling, crustal origin, rare earth elements, magma, basalt, fractionation, arcs.

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] , [...]

The site of Pella, located in the foothills of the east Jordan valley, was a prosperous city–state throughout the Middle Bronze Age (MBA, ca. 2000–1500 BCE). As part of a widespread trading network, Pella enjoyed extensive socio‐economic relationships with Egypt, Cyprus, and the Aegean, Anatolia, and Babylonia during this period. We report isotopic analysis (87Sr/86Sr, δ18O, and δ13C) from enamel of 22 human permanent second molars of which 13 second lower molars were used for an additional biodistance analysis based on ASUDAS. The multidisciplinary approach investigates the ancestral background of MBA Pella and the degree of temporary or more permanent relocation from other settlements. Ancillary to carbonate isotope analysis for migration investigation, dietary information in the form of δ13Ccarbonate was also collected. δ13Ccarbonate values (mean −12.3‰ ± 0.4 SD) suggest a uniform diet reliant on C3 cereals and legumes as crops and animal fodder, adhering to expected Bronze Age Levantine dietary norms. Two methods are used to identify non‐locals. Using a biospheric baseline, three individuals with non‐local 87Sr/86Sr ratios are identified. Bagplot analysis of both 87Sr/86Sr and δ18O data suggests that three individuals (14%) grew up elsewhere; two individuals who were already identified as 87Sr/86Sr outliers using biospheric data and one more with outlying δ18O values. All individuals identified as non‐locals, using either method, are from one tomb, Tomb 62. The dental nonmetric traits indicated diverse morphology and subsequent ancestry for Tomb 62 (11/13), whereas primary burials (2/13) clustered together. The commingled condition of Tomb 62 material prevented a more exhaustive biodistance analysis, but the tentative results coincide with interpretations of the tomb. Significant movements of populations throughout the Middle Bronze Age are evidenced through funerary rituals and architecture, and this study demonstrates that Pella, thought to be peripheral, nonetheless had some permanent movement evidenced through isotopes and ancestry analysis. [ABSTRACT FROM AUTHOR]