Petrography and Origin of the Unique Achondrite Gra

GRA 06128 & 06129 are paired achondrites [1], with unique mineral proportions (75% oligoclase), mineral compositions, and oxygen isotope ratios. They appear to represent alkalic igneous rock from a hitherto unsampled differentiated parent body, modified significantly by thermal and shock metamorphism. Samples and Methods. Bulk samples were examined at JSC during splitting for consortium analyses. Microscope and BSE images here are on thick section GRA06128,40. Chemical analyses of minerals were acquired at Johnson Space Center with the Cameca SX100, operated at 15 kV. Feldspar was analyzed with a defocused 5 m beam @ 5 nA; other minerals were analyzed with a focused beam @ 20 nA. Mössbauer spectra were obtained at ARES, JSC [2]. Intrinsic radioactivity was measured in the low-level counting facility at ARES JSC [3]. An estimated abundance of 26 Al of ~70 dpm/kg is within the range determined for eucrites. Bulk Sample: The meteorites are slab-shaped gray rocks with partial fusion crusts of shiny black glass. They consist of massive rock (grain sizes ~500m) gradational and interlayered with strongly foliated easily split rock (grain sizes ~ 50 m). The foliation is defined by parallel fractures; no lineations were seen. The meteorites are extensively rusted, and emit a sulfurous odor on crushing. Thin Section: GRA06128 consists mostly of oli-goclase feldspar (75% area), with lesser proportions of mafic silicates (olivine and two pyroxenes, ~20% total), and percent levels of Ca-phosphates, sulfides and chromite (Fig. 1). Small grains of Ni-rich metal are reported [1]. Alteration is pervasive, and appears as rusty deposits on grain boundaries and cracks. Our section contains masses (to cm) of nearly pure oligoclase (Fig. 1) that are separated by lenses and angular masses of ferromagnesian minerals and sulfides. The largest oligoclase-rich area (Fig. 1) is nearly square, and is composed of several tabular crystals with smaller irregular grains between them. The oligoclase-rich areas contain masses (to 500 m) of mafic minerals, sulfides and oxides, similar to the ma-fic clots that bound the oligoclase areas. Oligoclase grains contain abundant small ellipsoidal inclusions of sulfide minerals, commonly marking planes or curving surfaces in the section. Oligoclase has a constant composition across the section: Ab 82.3 An 16.0 Or 01.7. Clots of mafic material among the oligoclase-rich areas are lens-shaped or blocky, and up to a cm long; individual grains are in the 100 m size range. Some areas show a poikilitic texture of rounded sili-cate and oxide grains …