Early formation of evolved asteroidal crust

Abstract

Mechanisms for the formation of crust on planetary bodies remain poorly understood. It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 +/- 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System.

Origin of felsic achondrites Graves Nunataks 06128 and 06129, and ultramafic brachinites and brachinite-like achondrites by partial melting of volatile-rich primitive parent bodies

“…Major-element analyses of GRA 06128/9 are reported in Day et al (2009a). Trace-element analyses of wholerock powders were performed by solution ICP-MS techniques at the University of Maryland.…”

“…We provide updated petrology, as well as new mineral and whole-rock trace-element abundances, including highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances, as well as Os and O isotope data for GRA 06128/9, six brachinites (Brachina; Elephant Morraine [EET] 99402/7; Northwest Africa [NWA] 1500; NWA 3151; NWA 4872; NWA 4882), and three brachinite-like achondrites (NWA 5400; NWA 6077; Zag (b)). The Re-Os isotopic data and HSE abundances were especially targeted in order to gain a better understanding of the silicate-sulphide-metal segregation processes that acted on partially-melted asteroidal bodies (e.g., Rankenburg et al, 2008;Walker et al, 2008;Day et al, 2009a).…”

Trachyandesitic volcanism in the early Solar System

“…This rock is texturally completely different from the felsic achondrite Graves Nunatak (GRA) 06128/9 (13) and granitic lithologies that occur as fragments in meteorite breccias (14). GRA 06128/9 shows a plutonic, granoblastic texture, 120°triple junctions between coexisting phases, and chemically equilibrated silicates (13). ALM-A clearly demonstrates that SiO 2 -rich lavas were formed on the UPB and sets new important constraints on ureilite petrogenesis.…”

Noble gases and halogens in Graves Nunataks 06129: The complex thermal history of a felsic asteroid crust

“…It was therefore thought prudent for further fragments to be smaller in mass. However, the size of the smaller fragments Y and Z ($0.5 mm in diameter) may be similar to the variable grain size of GRA 06129, ranging from 0.1 to 0.5 mm (Day et al, 2009). Therefore the larger fragment (fragment X) is likely to be more representative of the bulk rock composition than the smaller (Y and Z) fragments.…”

“…Analyses of newly identified ungrouped meteorites are of particular interest as they allow investigations of previously unsampled Solar System processes and environments; among these, the paired meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9) sample an ancient planetary crust. Unlike most achondritic meteorites, which are basaltic in composition, GRA 06128/9 is dominated by Na-rich plagioclase (Shearer et al, 2008(Shearer et al, , 2010(Shearer et al, , 2011Day et al, 2009Day et al, , 2012. Analyses of pyroxene exsolution lamellae have demonstrated that GRA 06129 crystallised within 15-20 m of the surface of its parent body (Mikouchi and Miyamoto, 2008), supporting an origin as felsic crustal material.…”