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Abstract: Subduction zones play a fundamental role in the geochemical cycle of carbon, and related arc volcanism is believed to exert primary control on atmospheric CO2 concentrations over geological time. Arc volcanism may have been particularly important in the most recent Greenhouse of the late Cretaceous, where it has been hypothesized that the subduction of the carbonate-rich Tethys contributed to overall higher volcanic CO2 outgassing rates and thus a warmer climate. To test this hypothesis, the decarbonation efficiencies of modern subduction zones were calculated through a geochemical database that compared subaerial arc CO2 fluxes with the subducting crust and sediment geochemistry. The modern data are used to postulate a CO2 recycling and degassing scenario for arc volcanism related to the closure of the Tethys. Our analysis indicates that the thermal structure of subduction zones controls the extent and depth of slab decarbonation, while the sediment geochemistry (e.g. the amount of carbonate sediment) may be of secondary importance. The calculated decarbonation efficiency of modern arcs ranges from 18 to 70%. Our calculations support recent models predicting carbon recycling through infiltration-driven decarbonation, and limited by water availability at sub-arc depths. This analysis allows us to make inferences about the potential volcanic CO2 flux from subduction of the Tethys during the Cretaceous, suggesting between an 8 and 222% increase over modern CO2 outgassing. We suggest that the primary reason for the increase in CO2 outgassing in the Cretaceous is contamination of arc magmas by platform carbonates in the overlying crust and increased decarbonation efficiency. [Copyright &y& Elsevier]

Abstract: This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O at 1 bar to 35 kbar and 1150 to 1450 °C on model rhyodacite (∼69 wt.% SiO2) and haplobasalt (∼54 wt.% SiO2) base melt compositions. At rutile saturation, the concentration of TiO2 in the model rhyodacite base melt increases at 1 bar from 3.27±0.03 wt.% at 1150 °C to 13.88±0.04 wt.% at 1450 °C. At 1350 °C it decreases from 8.89±0.08 wt.% at 1 bar to 2.06±0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO2 in rutile-saturated haplobasalt increases at 1 bar from 20.9±0.3 wt.% at 1300 °C to 39.0±0.3 wt.% at 1450 °C. At 1350 °C it decreases from 25.8±0.3 wt.% at 1 bar to 15.67±0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only ∼600 ppm TiO2. Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO2 that will be transferred to the mantle wedge will be small because so little TiO2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (∼22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB. [Copyright &y& Elsevier]

Teleseismic images suggest that the Tarim plate plunges ∼45°S, down to ∼300 km depth, beneath NW Tibet. The 410 km discontinuity shallows by ∼10 km under the plateau, implying ∼100°C cooler upper mantle. The deepest Moho on record (∼90 km) lies under W Qiangtang. It rises abruptly by ∼20 and ∼10 km beneath the Altyn Tagh Fault and Bangong Suture, respectively. Vp/Vs ratios are normal, except in the Yecheng flexural basin and deep under the south Karakax volcanics (∼1.92). W Kunlun’s Neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward-extruding thrust wedge of the Tarim crust. [Copyright &y& Elsevier]