Non-basaltic asteroidal magmatism during the earliest stages of solar system evolution: A view from Antarctic achondrites Graves Nunatak 06128 and 06129

The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 ± 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2-30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe-Ni-S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium- or alkali-rich chondritic parent body with additional incorporation of Fe-Ni-S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600 °C at an fO₂ of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years. The existence of assemblages similar to GRA on several other planetary bodies with different geochemical characteristics (ureilite, winonaites, IAB irons) implies that this type of early asteroidal melting was not rare. Whereas, eucrites and angrites represent extensive melting of a parent body with low concentrations of moderately-volatile elements, GRA represents low-degrees of melting of a parent body with chondritic abundances of moderately volatile elements. The interpretation of the low-temperature mineral assemblage is somewhat ambiguous. Textural features suggest multiple episodes of alteration. The earliest stage follows the interaction of magmatic assemblages with a Cl-rich fluid. The last episode of alteration appears to cross-cut the fusion crust and earlier stages of alteration. Stable isotopic measurements of the alteration can be interpreted as indicating that an extraterrestrial volatile component was preserved in GRA.