Figure F26. Conditions for representative background alteration mineral assemblages in Site U1415 gabbroic rock. Temperature conditions are estimated from thermodynamic calculations of reaction curves using simplified bulk compositions. Reaction curves were drawn using the TWQ software (version 2.3) with database version 1.02 (Berman, 1991). Dotted curves = displacement caused by the substitution of Mg by Fe in mafic minerals coexisting with Fo80 olivine, calculated using the distribution coefficients of Evans and Trommsdorff (1974). Because of the lack of appropriate constraints, pressure conditions were inferred from the assumption that gabbros originated at mid- to lower crustal depths and were exposed at the present position by a tectonic event under low-temperature conditions. This uncertainty is indicated by the question marks on the vertical scale. A. CaO-Al2O3-SiO2-H2O system with additional reactions forming prehnite + chlorite assemblage. B. CaO-MgO-Al2O3-SiO2-H2O system (modified after Nozaka and Fryer, 2011).
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