Experimental partial melting of the Allende (CV) and Murchison (CM) chondrites and the origin of asteroidal basalts

The compositions of experimental partial melts from the Allende (CV) and anhydrous Murchison (CM) chondrites depend strongly on ambient oxygen fugacity (fo₂). At low fo₂ (IW-1) partial melts are tholeiitic and resemble the compositions of eucrites, whereas at high fo₂(IW+2) melts are silica undersaturated and resemble the compositions of angrites. In particular, the major element composition of our (1170°C, IW-1) melt of Murchison is strikingly similar to natural eucrites. Allende partial melts, while broadly eucritic, do not match most eucrites in detail. Conversely, IW+2 Allende partial melts are more like angrite compositions than are partial melts of Murchison. Chromium contents of eucrites and of our spinel-saturated Murchison and Allende experiments are extremely similar, suggesting that eucrites are also spinel saturated. This observation favors a partial melting origin for eucrites such as Sioux County, as opposed to an origin by fractional crystallization. However, most of the main group eucrites have slightly lower Mg#'s and slightly higher CaO/A1₂O₃ ratios than Sioux County, suggesting that, relative to Sioux County, the majority of the main group eucrites either have experienced small amounts of fractional crystallization or were produced by slightly smaller amounts of partial melting. If the eucrites were produced by simple partial melting, the MnO content of the eucrite parent body must be rather high compared to either Allende or Murchison. In contrast to comparisons between our IW-1 experiments and eucrites, none of our IW+2 experiments have exactly reproduced a natural angrite composition. There are also minor but significant discrepancies between our partial melting experiments and experiments on angrite compositions. While these differences may, in part, be due to our poor knowledge of the angrite parent body, they may be explicable if at least some of the angrites are not primitive partial melts but, rather, impact melts.