EXPERIMENTAL RECONSTRUCTION OF SODIC DOLOMITIC CARBONATITE MELTS FROMMETASOMATISED LITHOSPHERE

Investigations of peridotite xenolith suites have identified a composi tional trend from lherzolite to magnesian wehrlite in which clinopyrox ene increases at the expense of orthopyroxene and aluminous spinel, an d in which apatite may be a minor phase. Previous studies have shown t hat this trend in mineralogy and chemical composition may result from reaction between sodic dolomitic carbonatite melt and lherzolite at pr essures around 1.7 to 2 GPa. This reaction results in decarbonation of the carbonatite melt, releasing CO2-rich fluid. In this study, we hav e experimentally reversed the decarbonation reaction by taking two nat ural wehrlite compositions and reacting them with CO2 at a pressure of 2.2 GPa and temperatures from 900 to 1150 degrees C. Starting materia ls were pargasite-bearing wehrlites, one with minor apatite (compositi on 71001) and one without apatite (composition 70965*). At lower temp eratures (900 degrees C) the products were apatite + pargasite + magne site harzburgite for runs using composition 71001, and pargasite + do lomite Iherzolite for runs using composition 70965. At and above 1000 degrees C, carbonatite melt with harzburgite residue (olivine + ortho pyroxene + spinel) and with lherzolite residue (olivine + orthopyroxen e + clinopyroxene + spinel) were produced respectively. Phase composit ions in reactants and products are consistent with the documented carb onatite/lherzolite reactions, and also permit estimation of the carbon atite melt compositions. In both cases the melts are sodic dolomitic c arbonatites. The study supports the hypothesis of a significant role f or ephemeral, sodic dolomitic melts in causing metasomatic changes in the lithosphere at P less than or equal to 2 GPa. The compositions of wehrlites imply fluxes of CO2, released by metasomatic reactions, whic h are locally very large at around 5 wt% CO2.