Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 from 3 to 7 GPa

We have experimentally determined the solidus position of model Iherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I-1A and I-2A, which mark the transition from CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing Iherzolite respectively. In CMAS.CO2, we find I-1A at 2.6 GPa/1230 degrees C and I-2A at 4.8 GPa/1320 degrees C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbon-atitic with SiO2 contents of <6 wt%, CO2 contents of similar to 45 wt%, and Ca/(Ca + Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO2-rich and silica poor. The widespread evidence for the presence of CO2 in both the oceanic and continental upper mantle implies that such low degree SiO2-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.