Abstract:
We have experimentally determined the soli-dus position of model lherzolite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 (CMAS.CO 2) from 3 to 7 GPa by locating isobaric invariant points where liquid coex-ists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus re-actions at the solidus involving carbonate generates two invariant points, I 1A and I 2A , which mark the transition from CO 2 -bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO 2 , we ®nd I 1A at 2.6 GPa/1230 °C and I 2A at 4.8 GPa/1320 °C. The variation of all phase composi-tions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbon-atitic with SiO 2 contents of <6 wt%, CO 2 contents of ~ 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 magnesiocar-bonatites may well be an example of the eruption of such melts directly from their mantle source region as evi-denced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO 2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO 2 -rich and silica poor. The widespread evidence for the presence of CO 2 in both the oceanic and continental upper mantle implies that such low degree SiO 2 -poor carbonatitic melts are com-mon in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.
