Experimental element-partitioning and phase-equilibrium studies relevant to subduction zones

Melting in subduction zones is thought to take place in the mantle wedge, triggered by influx of fluid from the subducting slab. Although the isotopic composition of island arc volcanics (IAV) indicates that there is some recycling of the subducted oceanic crust, there is no consensus as to its relative importance or mode of incorporation into the mantle-derived melts. One major obstacle to the development of models for magma genesis in subduction zones is a scarcity of experimental data relevant to the partitioning of trace elements between melts, fluids, and residual phases during melting or dehydration of the mantle or the slab. The aims of this project were: a) to determine experimentally the majorand trace-element compositions of partial melts and residual phases of hydrous peridotite; and b) to calculate the partition coefficients between hydrous fluid and basaltic melt, produced at high pressure and temperature. Using solid-media apparatus, synthetic trace-element-doped peridotitic starting materials were equilibrated between 1.8 GPa and 2.8 GPa, and 970°C and 1150°C under water-saturated conditions at an f0 2 equal to the NiJNiO buffer. Melt compositions were calculated from analyses of melt inclusions trapped in an olivine crystal included in the capsule. However, analysis by secondary ion mass spectrometry (SIMS) showed that many of these experiments had been contaminated by boron from the surrounding boron-nitride sleeve, apparently via holes which formed and subsequently sealed. Water had not always been lost from contaminated capsules, and hence their failure had not previously been suspected. Comparison with uncontaminated experiments shows that boron fluxes melting and thus invalidates the results from the contaminated runs. This may be a general experimental problem, affecting past studies. Some experiments performed at 1.8 GPa were not contaminated and showed that with increasing temperature, the solidus is encountered, followed by the amph-out, ilm-out, ru-out, cpx-out and opx-out curves, consistent with previous studies. Calculated melt compositions were tested by reversal, i.e. equilibration of the melt composition under similar conditions to the original experiment. A forward experiment conducted at 1.8 GPa, 1020°C produced ol, opx, cpx, amph, ru, ilm, sp, and basaltic melt (53.6% Si02, 22.7% A1 203). However the phases on the liquidus of the calculated melt composition were opx, cpx, gt, amph, and were more aluminous than the original phases. In reversal experiments performed without a separate olivine crystal, an extra 20 wt% forsterite component was necessary to stabilise olivine. The calculated melt is therefore deficient in olivine component and is more aluminous than the true melt. In order to maximise fluid-phase quench products for analysis, a basaltic composition was equilibrated with up to 50 wt% water at —2.0 GPa and 1010°C. A salt cell was used to avoid contamination. Mass balance, coupled with textural and compositional evidence, suggests that the small glass spheres are quenched representatives of the fluid at equilibrium. Glass spheres and melt were analysed by SIMS for Li, Be, B, Na, K, Ti, Rb, Sr, Y, Zr, Nb. Cs, Ba, REE, Hf, Ta, Pb, and U, and assuming a value of 12.7 wt% solute in the fluid, values of D (fluid/melt) were determined, and, with the exception of Cs, are less than unity. The lowest values are exhibited by the REE and Y. There is a positive relationship between D (fluid/melt) and ionic radius for the 1+ ions, and negative relationships for the 3+ and 4+ ions. There is no obvious relationship for the 2+ ions, although Be, with the smallest ion, has the largest value of D (fluid/melt). D (fluid/melt) is greater for Ta than for Nb, and both values are less than those of the 1+ ions. Values of D (olivine/fluid) were derived from analysis of olivine and coexisting glass spheres. D (mineral/fluid) was calculated by combining D (fluid/melt) with published D (mineral/melt) values. The compositions of melts and fluids in equilibrium with the slab and the mantle have been calculated for a variety of possible compositions and mineralogies. Although the trace-element composition of IAV can be reproduced by mixing slaband mantle-derived melts, it is thought that this is due to the similarities between IAV and subducted sediment. A more likely scenario, considering that the slab is thought to lose much of its mobile-element budget during metamorphism, is that the mantle melts after metasomatism by a LILE-rich slabderived fluid. The main conclusion is that the trace-element contents of IAV can be reproduced by this model if the mantle wedge is of a depleted MORB-source composition.