Figure F33. Anhydrous major element compositions of hydrothermally altered peridotite samples from Sites 1268, 1270, and 1271, plotted in terms of mole fraction of SiO2 vs. mole fraction of MgO + FeO + CaO, compared to anhydrous compositions of end-member minerals (speciation as for Fig. F22). Tick marks along the vector connecting olivine and pyroxene indicate variations in normative orthopyroxene content, in terms of weight percent orthopyroxene/(orthopyroxene + olivine), assuming olivine Mg# = orthopyroxene Mg#. Most serpentinized dunites and harzburgites from Site 1271 have <30 wt% normative orthopyroxene and so do not require metasomatic increases in Si/(Mg + Fe) during alteration. Substantial divergence of the Site 1271 peridotite compositions from the MgO + FeO to SiO2 mixing line is due to the presence of interstitial gabbroic material, originally including plagioclase and/or igneous amphibole, especially in the impregnated dunite sample. Site 1270 dunites also include relatively abundant Cr-Al spinel.
