Figure F22. Schematic pressure-temperature diagram showing conditions for representative background alteration mineral assemblages in the Site U1415 gabbroic rocks. 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 the database version 1.02 (Berman, 1988, 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 Trommsdorff and Evans (1974). Because of the lack of appropriate constraints, pressure conditions were inferred from the assumption that gabbros originated at a normal lower crustal level and were exposed at the present position by a tectonic event under a low-temperature condition. This uncertainty is indicated by the question marks on the pressure 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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