1999 Fall AGU Abstract (Navrotsky)

Bina, C. R., and A. Navrotsky, Till Hell freezes over - Ice in subducting slabs?, Eos, Transactions of the American Geophysical Union, 80, Fall Supplement, S12F-10, 1999.


During subduction of oceanic lithosphere, a series of progressive dehydration reactions occurs. The water liberated participates in geologically important processes. It has been invoked in melting point depression responsible for island-arc volcanism, in enhanced pore pressures responsible for intermediate-depth seismicity, and in transport of soluble chemical constituents from slab to mantle. While supercritical fluid water is liberated during dehydration of most slabs, the stable phase of H2O is solid ice VII along portions of the geotherm for the coldest slabs. The substitution of ice VII for fluid water as a product of dehydration reactions has significant implications for the physical processes by which H2O is generated, stored, transported, and released within cold subduction zones. The locations and slopes in P-T space of dehydration reactions change, potentially affecting depths of seismogenesis and magmagenesis. Significant amounts of ice VII can be accumulated during progressive dehydration of mineral solid solutions during subduction. As the sinking slab warms, melting of this pure ice will occur at a single temperature and pressure, releasing large amounts of water in a small spatial region over a short time. This univariant melting reaction has a significant positive volume change and may trigger seismicity. The initially chemically pure liquid water that is formed, being undersaturated with respect to all dissolved constituents, will react strongly with the surrounding rock, with possible implications for trace element distributions and metal transport. Accumulation of ice VII in a cooling planetary interior (e.g., Mars) may eventually lead to a decline in or cessation of tectonic activity.

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