Melting of the Martian mantle from 1.0 to 4.5 GPa

摘要

To investigate the chemical composition of possible primitive melts of the Martian mantle, we performed melting experiments of a model Martian mantle derived by Dreibus and Wanke (DWM) at pressures from 1.0 to 4.5 GPa. The chemical compositions of partial melts are systematically related to pressure. The partial melts at pressure of 1.0 GPa in spinel stability field show high Al_2O_3 and low FeO contents. The partial melts at higher pressure in garnet stability field are, however, characterized by a relatively high FeO content, low Al_2O_3 content and high CaO/Al_2O_3 ratio. In garnet stability field, clinopyroxene (= Ca-rich phase) contribute significantly to melt formation near the solidus temperature, although garnet (= Al-rich phase) is stable at temperature above solidus. Therefore Al-poor and Ca-rich partial melt are formed at higher pressure. Comparing the shergottite chemistry with the chemical trends of the partial melts obtained by the experiments, we suggest that one of basaltic shergottite with a high Al_2O_3 and low CaO/Al_2O_3 ratio, QUE94201, resembles the composition of the DWM partial melts in major-element chemistry in a low degree (＜20%) partial melt of DWM at pressure of 1.0 GPa and temperature of 1360 ℃. We conclude that the olivine-poor (or olivine-free) basaltic magma with low CaO/Al_2O_3 ratio and high Al_2O_3 could be primitive melts derived from the upper mantle of Mars if the actual Martian mantle is similar in composition to DWM.