Making Crust in the Asteroid Belt: Evidence from Gra 06128/9 and Brachinites

GRA 06128/9: Since their discovery at Graves Nunataks icefield during the 2006/2007 Antarctic Search for Meteorites field season, GRA 06128 and GRA 06129 have received considerable scientific attention. Their elevated modal abundances of oligoclase (>75%), blocky or slab-like character, granoblastic textures and ‘andesitic’ bulk compositions set them apart from all other meteorite classes. It has been suggested, based on whole-rock and mineral compositions, Pb-Pb ages, ΔO, and highly siderophile element (HSE) data, that GRA 06128/9 represent fragments of ancient (4.52 ±0.06 Ga), evolved asteroidal crust [1]. Here, we critically assess how the GRA 06128/9 meteorites formed, how extensive such materials were in the early Solar System and whether there is a link between GRA 06128/9 and brachinite meteorites [2]. Methods: For mineral characterization, we used Cameca SX-50 and JEOL 8900 Superprobe electron microprobes, as well as an UP213nm laser-ablation system coupled to an Element 2 ICP-MS [1]. Wholerock Os isotopic and highly siderophile element abundance measurements were performed via N-TIMS (Triton) and solution-based ICP-MS (Element 2) methodologies, respectively [1]. Oxygen isotope analyses were performed via laser fluorination [3]. Spectral analysis of finely ground powders of the meteorites was made at the Brown University RELAB facility [4]. When And How Was Evolved Asteroidal Crust Formed? The GRA 06128/9 meteorites are characterized by granoblastic textures and two-pyroxene thermometry indicates equilibration temperatures between 650-850oC [1,5], implying significant metamorphism subsequent to igneous emplacement of the samples in their parent body. Since Pb diffusion occurs in phosphates at ≥500oC [6], the Pb-Pb age determined on Cl-apatite of 4.52 ±0.06 Ga indicates that crystallization, thermal metamorphism and cooling below 500oC all occurred within 100 Ma of Solar System formation [1]. The Sm-Nd age for the meteorites of 4.55 ±0.09 Ga [7] and the Al-Mg model age initial of 4.564 Ga [5] support this chronology. Petrogenesis of the meteorites is consistent with derivation from an oxidized, volatile-rich and undifferentiated chondritic source [1]. Non-modal batch melting models imply 10-30% partial melting to generate the GRA 06128/9 meteorites, in broad agreement with experimental work on the role of partial melting in L6 chondrites (Fig. 1).