Systematic study of liquidas phase relations in olivine melilitite +H₂O +CO₂ at high pressures and petrogenesis of an olivine melilitite magma

Abstract

Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying ${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}}$, ${\text{x}}_{{\text{CO}}_{\text{2}} }$, and ${\text{f}}_{{\text{O}}_{\text{2}} }$. At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing ${\text{x}}_{{\text{CO}}_{\text{2}} }$ (${\text{x}}_{{\text{CO}}_{\text{2}} }$ = CO₂/CO₂ +H₂O+olivine melilitite). Olivine is the liquidus phase in the presence of H₂O alone and is joined by clinopyroxene at low ${\text{x}}_{{\text{CO}}_{\text{2}} }$. Increasing ${\text{x}}_{{\text{CO}}_{\text{2}} }$ eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO₂ brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO₂. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different ${\text{x}}_{{\text{CO}}_{\text{2}} }$. The field with olivine- +clinopyroxene becomes stable to higher ${\text{x}}_{{\text{CO}}_{\text{2}} }$ with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, ${\text{x}}_{{\text{CO}}_{\text{2}} }$∼ 0.08 and ${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}}$∼ 0.2 (i.e., 6–7% CO₂+ 7–8% H₂O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher ${\text{x}}_{{\text{CO}}_{\text{2}} }$ and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a ∼ 5% melt (inferred from K_{2 O} and P₂O₅ content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO₂ and 7–8% H₂O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO₂.