Hematite solubility in NaCl- and CaSiO3-bearing aqueous fluids at 10 kbar and 800 C

A distinguishing characteristic of arc-related magmas is their high oxidation state, manifest as elevated Fe3+/Fe(total) relative to OIB and MORB. However, there is no consensus on the process and agents responsible for arc-magma oxidation. The subducting slab represents an obvious source of oxidized material, particularly lithologies rich in ferric iron-bearing minerals and sulfates produced through seawater interaction. Fe3+ and SO4}2- represent the most readily available agents for mantle-wedge oxidation. Hydrous fluids and brines equilibrated with these oxidized portions of the slab may be effective oxidizing agents. To assess this possibility, we measured the solubility of hematite in NaCl-H2O fluids at 10 kbar and 800°C. Experiments were conducted in a piston-cylinder apparatus with graphite-NaCl furnace assemblies. Solubility was determined by weight loss of either sintered reagent-grade hematite pellets or rounded specular hematite fragments (1 wt% TiO2). The ultra-fine grained reagent hematite tended to recrystallize during experiments; however, in some cases the final pellets were quite friable, leading to material loss during weighing. These experiments were discounted because their weight changes were spurious. Two methods were employed to control fO2. Either a sealed Mn2O3-filled Pt capsule was run inside the fluid filled outer capsule, buffering at the Mn2O3-Mn3O4 equilibrium, or the outer capsule was packed in MnO2 powder inside the graphite heater assembly, acting as a H2 sink. External MnO2 resulted in some recrystallization of the Pt capsules. However, hematite solubility was identical for both methods, suggesting both techniques control fO2 to a similar degree. The solubility of hematite in H2O-NaCl fluids was measured from 0-72 wt% NaCl. Hematite solubility in pure H2O was 5.5×10-4 molal total Fe, with a detection limit of 8.3×10-5. With increasing NaCl conentration, hematite solubility increased from ~0.001 molal total Fe at 13 wt% NaCl to ~0.0105 molal total Fe at 72 wt% NaCl. The solubility enhancement of approximately an order of magnitude is comparable to the enhancement seen for wollastonite and corundum following the addition of NaCl. Experiments to determine the effect of hematite solubility in pure H2O as a function of fO2 are ongoing. Additional experiments were performed wollastonite undersaturated H2O-NaCl-CaSiO3 fluids. At a fixed NaCl mole fraction of 0.1, increasing CaSiO3 content of the fluid results in an increase in hematite solubility. At 0.089 molal CaSiO3, fractionally below andradite saturation, hematite solubility is 0.01 molal total Fe, which is approximately double that in the CaSiO3-free system (0.005 molal). This enhancement in solubility could either be due to an increase in solution pH or the formation of silicate complexes in solution, presumably an andradite-like molecule. Given the ubiquity of garnets in eclogite facies rocks, fluids equilibrated with andradite-component-bearing garnets represent a more realistic analogue of ferric iron-bearing fluids than those equilibrated with hematite alone. The important roles of pH and silicate complexing indicates that experimental characterization of the ferrous/ferric ratio of slab fluids should concentrate on complex mineral assemblages (eg. cpx-garnet) rather than single mineral systems.