Figure F45. Anhydrous major element compositions of hydrothermally altered peridotite samples from Sites 1268, 1270, 1271, 1272, and 1274, plotted in terms of mole fraction of SiO2 vs. mole fraction of MgO + FeO, compared to anhydrous compositions of end-member minerals (speciation as for Figure F22). Tick marks along the vector connecting olivine and pyroxene are for variations in normative orthopyroxene content, in terms of weight percent pyroxene/(pyroxene + olivine), assuming olivine Mg# = pyroxene Mg#. Partially serpentinized "dunites" from Site 1274 have ~5–13 wt% normative pyroxene. Five of six partially serpentinized harzburgites from Site 1274 have 26–29 wt% normative pyroxene, more than expected for highly depleted residual peridotites, and so may have undergone metasomatic increases in Si/(Mg + Fe) during alteration. Divergence of a Site 1274 fault gouge mud from the MgO + FeO + CaO to SiO2 mixing line is probably due mechanical mixing of gabbroic and ultramafic material in the fault gouge.
