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Clarification of the Influence of Water on Mantle Wedge Melting

David H Green, William O Hibberson. Anna Rosenthal and Istvan Kovacs

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 

Water is a significant component in primitive island arc magmas and its ubiquitous presence is attributed to release of water from dehydration reactions in subducted oceanic crust and lithosphere.  Water released from the subducted slab is inferred to be transferred as aqueous vapour or water-rich melt into the overlying peridotite of the mantle wedge.  Because of the inverted temperature gradient inferred for the mantle wedge immediately above the subducted slab, access of aqueous vapour or water-rich melt will initiate melting close to the water-saturated peridotite solidus. 

The location of a region of water-saturated mantle melting, if it exists, can be predicted if we know the P,T dependence of the water-saturated peridotite solidus and can model the temperature distribution in a particular subduction setting.  We have confirmed the results of a number of experimental studies in the 1970's which defined the P,T conditions for the water-saturated solidus of lherzolite up to 3GPa.. We conducted 60 experiments from 1.5 GPa to 6 GPa using different water contents and several bulk compositions. Electron microprobe analyses of 4-7 phases in each experiment document systematic compositional changes in co-existing phases. In addition Fourier Transform Infra-Red (FTIR) spectroscopy was used to measure the water contents of nominally anhydrous minerals (commonly abbreviated NAMS) in 25 of the experiments. The solidus decreases rapidly from ~1100°C at atmospheric pressure to 0.5 GPa, 1000°C, and continues to decrease slightly to a minimum of 970°C at 1.5 to 2 GPa.  We demonstrate that for hydrous silicate melt, the fluid-saturated solidus of lherzolite model mantle composition with small (0.2-2%) water contents and very small carbon content, is ~1010°C at 2.5 GPa, ~1210°C at 4GPa. and at least 1375°C at 6GPa..

The melt composition at the water-saturated solidus at 2.5GPa is a very silica-undersaturated olivine nephelinite and is extremely silica-undersaturated at higher pressure. We also used olivine single crystal discs and either olivine aggregates or carbon sphere aggregates as melt and fluid traps forming interstitial  films or inclusions within olivine. For several experiments with high water contents, the capsule was pierced under high vacuum at room temperature and the vapour released was analysed by gas chromatography. We have conducted layered experiments  for the purpose of measuring the water content of nominally anhydrous minerals under conditions where we were simultaneously observing melting, water-rich vapour, pargasite or phlogopite in fertile lherzolite. We obtained data using the layered capsules with 'sensor' layers of olivine, low-Al and high-Al orthopyroxene and clinopyroxene, at pressures of 1.5, 2.5, 4, and 6 GPa..

Allowing for the uncertainty in calibrations in the quantification of IR spectra, our results show that if water contents in fertile mantle lherzolite (i.e. HZ1 Lherzolite, MORB Pyrolite, MM3 lherzolite ) are as low as 100-250 ppm H2O, then pargasite is stable at 2.5 GPa and melting begins at the 'fluid-absent lherzolite+ H2O dehydration solidus' which is close to 1100°C for these compositions. With increasing water content the proportion of pargasite at the solidus increases to ~10-15% (i.e. with 1500-2000 ppm H2O in the lherzolite) but the water content of NAMS remains unchanged. At higher water contents a water-rich vapour is present and melting begins at the vapour-saturated solidus with pargasite stable at and slightly above the solidus. Our data on the water content in olivine in the sequence from the first appearance of a water-rich vapour (e.g. between 0.073% and 0.145% H2O at 2.5Gpa, 1000°C) to 'leached' experiments with 14.5% H2O show little change with increasing bulk water content, suggesting that water activity is effectively buffered by the pargasite-bearing assemblage.

At >3GPa, pargasite is unstable and with water contents of 100-250 ppm or more, melting begins at the vapour-saturated solidus which for a water-rich vapour and fertile lherzolite composition is at ~1225 °C at 4Gpa and ~1375°C at 6GPa. The data also show that if a melt is formed at the vapour-saturated solidus at >3GPa ('incipient melting regime') and migrates out of the vicinity, then the water content retained in the residual but still fertile lherzolite (in nominally anhydrous minerals) is 100-250ppm H2O. Decompression melting of such residual lherzolite at temperatures in the 'major melting regime' i.e. slightly above the anhydrous solidus,  will produce magmas at ~10% or ~20% melting containing 0.1-0.25% or 0.05-0.13% H2O respectively, i.e. controlled by the residual water contents retained in NAMS. Such magmas would have incompatible element contents reflecting the history of their source including the loss of very small melt fraction(s) in the garnet lherzolite stability field. These characteristics match those of N-MORB or D-MORB, whereas E-MORB characteristics reflect source lherzolite to which a migrating incipient melt has been added.