EXPERIMENTAL PARTIAL MELTING OF THE ST. SEVERIN (LL) AND LOST CITY (H) CHONDRITES

Abstract

We have performed melting experiments between 1170-1325°C and at an oxygen fugacity of IW-1 on the ordinary chondrites Lost City (H) and St. Severin (LL) in order to investigate the origin of diogenites and their possible relationship to eucrites. Low-Ca pyroxene disappears from the Lost City residuum just above 1220°C at a Mg# near 66. Accordingly, Lost City does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites. In contrast, orthopyroxene in St. Severin persists up to about 1325°C, at which point it has a Mg# similar to that of typical diogenites (#75). This is only marginally compatible with diogenite origins, as the most magnesian pyroxenes from diogenites (Mg# 82) and howardites (Mg# 85) are more MgO-rich than any pyroxene formed from a St. Severin melt. Accordingly, if diogenites formed from a source having the bulk composition of an ordinary chondrite, than either the fO2 had to be lower than that inferred for eucrite formation or, more likely, the parent body had a lower FeSi ratio than that of an LL chondrite. Also, low temperature melts of St. Severin are depleted in elements compatible with pyroxene and so, while broadly eucritic, do not closely match the compositions of eucrites that are thought to be primary partial melts. Hence, it doesn't appear possible to produce both diogenites and eucrites from the same source region composition, if eucrites are primary partial melts.Reconnaissance experiments at higher oxygen fugacity (IW + 2) produce broadly angritic melts for Lost City, as was observed for Murchison and Allende. In contrast, our Na2O-poor St. Severin charge produced a broadly eucritic melt at this higher fO2. This difference in melting behavior is probably because LL chondrites have a much lower bulk FeSi ratio than the H, CM, or CV chondrites.