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 Iherzo lite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa b y locating isobaric invariant points where liquid coexists with olivin e, orthopyroxene, clinopyroxene, garnet and carbonate. The intersectio n of two subsolidus reactions at the solidus involving carbonate gener ates two invariant points, I-1A and I-2A, which mark the transition fr om CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-b earing Iherzolite respectively. In CMAS.CO2, we find I-1A at 2.6 GPa/1 230 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 SiO 2 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); compositional ly they resemble natural magnesiocarbonatites. Volcanic magnesiocarbon atites may well be an example of the eruption of such melts directly f rom their mantle source region as evidenced by their diatremic style o f activity and lack of associated silicate magmas. Our data in the CMA S.CO2 system show that in a carbonate-bearing mantle, solidus and near -solidus melts will be CO2-rich and silica poor. The widespread eviden ce for the presence of CO2 in both the oceanic and continental upper m antle implies that such low degree SiO2-poor carbonatitic melts are co mmon in the mantle, despite the rarity of carbonatites themselves at t he Earth's surface.