Major-element trend for shergottite melts and their source materials

The major-element compositions of shergottite melts, plotted against their mg [Mg/(Mg+Fe)] atomic ratios, form a narrow trend, which is designated the shergottite melt trend". Although the mg ratios range from 0.7 to 0.2, the silica contents of the trend are nearly constant from 47 to 53wt%, indicating that the shergottite melts are basaltic, never andesitic nor komatiitic. The trend is enriched in FeO with the range from 15 to 22wt%, and poor in Al_2O_3 with the range from 4 to 14wt%. The melts for nakhlites are poorer in SiO_2 and Al_2O_3 and richer in FeO than the shergottite melt trend. Although the melts for chassignite are similar to the shergottite melt trend, the melts for chassignite and nakhlites are more enriched in K_2O contents than the shergottites melts, indicating that they have a different origin from the latter. The bulk major-element compositions of terrestrial basalts, lunar basalts, and eucrites are compared to the shergottite melt trend. The terrestrial basalts and komatiites are poorer in FeO and richer in Al_2O_3, CaO, and Na_2O than the shergottite melt trend. The lunar low-Ti mare basalts and eucrites have rather similar compositions to the shergottite trend. However, their alkali and P_2O_5 contents are low in comparison to the shergottite trend, reflecting their planetary compositions. A plausible source material for olivine-phyric shergottites was estimated, and it may be a plagioclase peridotite with an mg ratio of 0.81 depleted in alkalis, CaO and Al_2O_3 contents. The partial melting to produce the shergottite trend may have taken place at low pressures, whereas the nakhlites and chassignite may have been produced under moderate (1-30kb) pressure conditions. The chassignite may have fractionated under a low-pressure condition to produce a large amount of cumulus olivine, whereas the nakhlites have not.