The effect of carbon dioxide on phase relationships for synthetic lherzolite and harzburgite

Abstract

The system CaO-MgO-SiO2-CO2 includes mineral assemblages corresponding to model lherzolite: forsterite(Fo) + orthopyroxene(Opx) + clinopyroxene(Cpx), and model harzburgite: Fo + Opx, as well as model websterite and wehrlite. When fully carbonated, the peridotites are converted to limestones: dolomite(Do) + magnesite(Mc) + quartz(Qz), or Mc + Qz. When partly carbonated, the peridotites are converted to carbonate-lherzolite and magnesite-harzbuigite, which cannot coexist with CO2. Available experimental and calculated reaction data are presented for carbonate lherzolite: (6) Opx + Do = Cpx + Fo + CO and (6A) Opx + Cc = Cpx + Fo + CO2,where Do is dolomite and its solid solution, and Cc is magnesium calcite; for magnesite-harzburgite: (3) MC + En = Fo + CO2; for websterite + carbonate: (O) Mc + Cpx = Do + Opx and (01) Do + Cpx = Cc + Opx; and for carbonate-wehrlite: (9) Do + Cpx = Fo + Cc + CO2. Conditions for the occurrence of dolomite(stoichiometric)-lherzolite are evaluated. Comparison of fossil geotherms deduced from kimberlite nodules with the phase diagrams for model harzburgite and lherzolite, and solidus curves with H2O present, indicates that partially melted lherzolite may coexist with solid magnesite-harzbuigite between about 175 and 195 km depth. Dissociation of magnesite could disrupt the harzburgite nodules during eruption, distributing low-calcium garnet through kimberlite.