The effects of solid solution on the stability of the 10 Å phase

Abstract:

Subduction of hydrous phases enables the transport of water into the typically anhydrous mantle. Dehydration of these phases has a dramatic expression at the Earth’s surface in the form of earthquakes and arc volcanism. Within a subduction zone, talc (Mg₃Si₄O₁₀(OH)₂) is found within the hydrothermally altered oceanic crust and pelagic sediments of the subducting slab, as well as the metasomatised mantle wedge. The 10 Å phase (Mg₃Si₄O₁₀(OH)₂.H₂O) forms from the breakdown of talc at low temperatures and pressures above 5 GPa, and is thought to be stable to depths up to 200 km¹. Previous work has focused on the stability of Mg end-members of talc and the 10 Å phase. However, natural samples of talc show deviation from end member composition, most significantly in the substitution of iron for magnesium. It is likely that the stability of iron-bearing talc and its high pressure equivalent, the 10 Å phase, will differ from that of Mg end-member compositions, as demonstrated by previous studies examining the effect of solid solution in phyllosilicates such as antigorite. High pressure experiments using the multi-anvil press at the University of Manchester have bracketed the reaction talc + H₂O = 10 Å phase for Mg end-member compositions. Samples of iron-bearing talc and the 10 Å phase have been synthesised at pressures of 2 GPa and 6.2 GPa, respectively. By performing phase equilibrium experiments on iron-bearing talc and 10 Å phase it is possible to quantify the effect of composition on the position of the reaction talc + H₂O = 10 Å phase, as well the thermal breakdown of the 10 Å phase to enstatite and coesite. Univarient breakdown reactions assume that the 10 Å dehydrates at one discrete depth, where the thermal stability has been exceeded. The results are expected to show that instead, fluid release from 10 Å phase dehydration is more likely to occur over a range of depths during subduction, dependant on the bulk composition and the thermal regime of the subduction zone.

1. Pawley, A.R. and Wood, B.J. 1995. The high-pressure stability of talc and 10 Å phase: potential storage sites for H2O in subduction zones. American Mineralogist, 80, 998-1003.