The composition of near-solidus melts of peridotite in the presence of CO2 and H2O between 40 and 60 kbar

Partial melting experiments at 40, 50 and 60 kbar pressure on three peridotite compositions with 0.5–0.63 wt.% H2O and 2.0–3.2 wt.% CO2 added indicate melting temperatures only marginally above continental geotherms. Most experiments were performed on a composition with 1.5 wt.% K2O added, which causes a further decrease of about 40 °C in melting temperature. Melts progress gradually from carbonate-rich to carbonated silicate in composition: near-solidus melts have Ca/(Ca + Mg) of 0.46–0.53, which fall to < 0.40 more than 50 °C above the solidus. With increasing temperature for the K-enriched peridotite HPK at 50 kbar, melts are characterised by strong increases in SiO2 (< 3 to > 30 wt.%) and Al2O3 (< 1 to > 9 wt.%) and concomitant decrease in CaO (> 20 to < 3 wt.%), with little change in MgO. K2O and TiO2 exhibit maxima at intermediate temperatures, reflecting the stability of phlogopite and ilmenite above the solidus, indicating the presence of up to 13 wt.% K2O and 2.6 wt.% TiO2 in carbonate-rich melts with only 13 wt.% SiO2. Of 30 trace elements, only Cr, Mn, Ni, and Zn are compatible in the residue, whereas U, Rb, Ba and the LREE are most enriched in the melts. Low melt fractions exhibit troughs in trace element patterns for Hf, Nb, Ta and Cs that are diminished or eliminated at higher degrees of melting. Apart from these features and larger LREE/HREE ratios, trace element compositions are similar to silicate melts of peridotite in CO2-free conditions.Partial melts of peridotite with CO2 and H2O are too low in MgO to resemble kimberlites, but will act as effective metasomatic agents enriching the lithosphere in K and carbonates, of relevance for ultramafic lamprophyres and kamafugites. Higher-degree melts (15–35%) have > 20 wt.% SiO2, are only mildly enriched in trace elements, and will not cause large time-integrated isotopic in-growth except over long geological time intervals. The temperature interval over which melts are carbonatitic with SiO2 contents < 10 wt.% narrows with increasing pressure from 30– 60 kbar, meaning that the metasomatic effects of carbonatitic melts may be greatest at 20–30 kbar.