No abstract

Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plate boundaries. The subduction of oceanic lithosphere, occurring with increasing pressures and rising temperatures, causes liberation of H2O from the slab. Amphibole, which can be stable to the highest PT conditions among hydrous phases in the slab, decomposes at around 90 km depth. It follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab‐derived H2O is not a direct trigger for the production of arc magmas. Instead, the H2O released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals (serpentine, talc, amphibole, chlorite, and phlogopite). This hydrated peridotite is dragged downward on the slab toward higher PT regions and releases H2O to shallower potential magma source regions in the mantle wedge. Combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge, it is concluded that those minerals decompose beneath the forearc region. On the other hand, high PT experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front. Phlogopite in the peridotite decomposes to release H2O at a deeper level (about 200 km). H2O liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites. Along with the migration of H2O through the above processes, subduction components, especially large ion lithophile elements, can be overprinted on the magma source region, which governs the geochemical characteristics of arc magmas.

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Earth's solid inner core is mainly composed of iron (Fe). Because the relevant ultrahigh pressure and temperature conditions are difficult to produce experimentally, the preferred crystal structure of Fe at the inner core remains uncertain. Static compression experiments showed that the hexagonal close-packed (hcp) structure of Fe is stable up to 377 gigapascals and 5700 kelvin, corresponding to inner core conditions. The observed weak temperature dependence of the c/a axial ratio suggests that hcp Fe is elastically anisotropic at core temperatures. Preferred orientation of the hcp phase may explain previously observed inner core seismic anisotropy.

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The chemical compositions of primary magmas of olivine tholeiite (OTB), high‐alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water‐undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the mantle wedge.

No abstract

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian "Selli Level" black shale deposited during OAE1a in central Italy reveal two negative excursions in marine 187 Os/ 188 Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Level horizon, suggesting an order-of-magnitude increase in the global fl ux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratifi cation that triggered OAE1a.