Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere

Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of the carbonatite melt, releasing CO₂-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions and reacting them with CO₂ at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with minor apatite (composition 71001*) and one without apatite (composition 70965*). At lower temperatures (900° C) the products were apatite + pargasite + magnesite harzburgite for runs using composition 71001*, and pargasite + dolomite lherzolite for runs using composition 70965*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine + orthopyroxene + spinel) and with lherzolite residue (olivine + orthopyroxene + clinopyroxene + spinel) were produced respectively. Phase compositions in reactants and products are consistent with the documented carbonatite-lherzolite reactions, and also permit estimation of the carbonatite melt compositions. In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral, sodic dolomitic melts in causing metasomatic changes in the lithosphere at P ≤ 2 GPa. The compositions of wehrlites imply fluxes of CO₂, released by metasomatic reactions, which are locally very large at around 5 wt% CO₂.