HR: 0800h
AN: V41A-1425    [Abstracts]
TI: Reliable Estimates of Mantle Water-Activity from Amphibole-Bearing Rocks
AU: * Lamb, W M
EM: lamb@geo.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics Texas A&M University, College Station, TX 77843-3115 United States
AU: Popp, R K
EM: popp@geo.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics Texas A&M University, College Station, TX 77843-3115 United States
AB: Determining values of H₂O activity (aH₂O) for mantle rocks will yield a better understanding of those mantle processes that are controlled, in part, by the availability of H₂O (e.g., melting and deformation). One approach for estimating aH₂O from amphibole-bearing mantle rocks is to use a variety of H₂O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole and other phases. A self-consistent thermodynamic database (THERMOCALC, Holland and Powell, 1990) can be used to determine values of aH₂O from H₂O-buffering equilibria as a function of P and T. We used a mantle peridotite assemblage, which includes amphibole, from Dish Hill (sample DH101-E, McGuire et al., 1991) to calculate aH₂O using this approach. The stability of spinel in this sample limits the pressure (P) of mineral equilibration to approximately 4 to 28 kbar at temperatures (T) reasonable for the equilibration of this mineral assemblage (830 to 900°C, respectively based on 2-pyx. thermometry). Given this range in P, THERMOCALC yields two invariant points, each at a fixed value of T and aH₂O at any given P (isobaric invariant points). These isobaric invariant points involve the following end-member components: Point A = diopside (di), Ca-tschermaks, enstatite (en), spinel (sp), pargasite, tremolite (tr), glaucophane, tshermakite (ts), and aH₂O; Point B = forsterite, en, di, sp, tr, ts, and aH₂O. Mineral assemblage A yields temperatures that are consistent with 2-pyroxene geothermometry (within ñ50°C over the entire P-range of spinel stability). In the range 4 to 28 kbar and 830 to 900°C the value of aH₂O inferred from assemblage A is 0.004 to 0.025. Assemblage B is consistent with the results of 2 pyroxene thermometry only at relatively high pressure (approx. 21 to 28 kbar) at values of aH₂O that range from approx. 0.15 to 0.38. We suggest that mineral assemblage A is more resistant to retrograde re-equilibration than is assemblage B and that, in this example, the most robust determination of the temperature of mineral equilibration is from 2-pyroxene thermometry. If this is the case then amphibole equilibria from assemblage A yields the most reliable estimate of aH₂O (0.004 to 0.025). Another method for determining aH₂O from amphibole equilibria relies on the equilibrium between iron oxy-component and hydroxy-component, as described in the dehydrogenation/ oxidation reaction Fe²⁺ + OH^- = Fe³⁺ + O^2- + 1/2 H₂. The proportions of iron oxy- and hydroxy-component in amphibole defines the equilibrium fH₂ at fixed T and P (Popp et al., 1995, 2004). Using the amphibole composition from the Dish Hill sample, combined with an fO₂ estimated from olivine-spinel oxygen barometry, the activity of H₂O was calculated over the range of relevant P-T conditions. Activities of H₂O in the range 0.01 to 0.02 occur in the same P-T region as those defined by the dehydration equilibria. Thus, the concordance of these three different approaches, 2-pyroxene thermometry, amphibole dehydration equilibria, and amphibole iron oxy-hydroxy equilibrium, yields a reliable aH₂O estimate of approx. 0.015 for this assemblage.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3621 Mantle processes (1038)
DE: 3660 Metamorphic petrology
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005