Petrology and geochemistry of D'Orbigny, geochemistry of Sahara 99555, and the origin of angrites

Abstract

Abstract-We have done a detailed petrologic study of the angrite, D'Orbigny, and geochemical study of it and Sahara 99555. D'Orbigny is an igneous-textured rock co m posed of Ca-rich olivine, Al-Tidiopside-hedenbergite, subcalcic kirschsteinite, two generations of hercynitic spinel and anorthite, with the mesostasis phases ulvospinel, Ca-phosphate, a silico-phosphate phase and Fe-sulfide. We report an unknown Fe-Ca-Al-Ti-silicate phase in the mesostasis not previously found in angrites. One hercynitic spinel is a large, rounded homogeneous grain of a different composition than the euhedral and zoned grains. We believe the former is a xenocryst, the first such described from angrites.The mafic phases are highly zoned; mg# of cores for olivine are -64, and for clinopyroxene -58, and both are zoned to Mg-free rims . The Ca content of olivine increases with decreasing mg#, until olivine with -2 0 mol% C a is overgrown by subcalcic kirschsteinite with about 30-35 mol% Ca. Detailed zoning sequences in olivine-subcalcic kirschsteinite and clinopyroxene show slight compositional reversals . There is no mineralogic control that can explain these reversals, and we believe they were likely caused by local additions of more primitive melt during cry sta llization of D'Orbigny. D'Orbigny is the most ferroan angrite with a bulk rock mg# of32. Compositionally, it is virtually identical to Sahara 99555; they are the first set of compositionally identical angrites. Comparison with the other angrites shows that there is no simple petrogenetic sequence, partial melting with or without fractional crystallization, that can explain the angrite suite. Angra dos Reis remains an anomalous angrite.Angrites show no evidence for the brecciation, shock, impact metamorphism, or thermal metamorphism that affected the howardite, eu crite, diogenite (HED) suite and ordinary chondrites. This suggests that the angrite parent body may have followed a fundamentally different evolutionary path than did these other parent bodies.

Formation of vesicles in asteroidal basaltic meteorites

“…On 4 Vesta, the dikes that formed eucrites may have been trapped at significant depths, providing an ideal location for metamorphism to explain the finegrained, granoblastic texture of Ibitira. In contrast, the angrites, which exhibit evidence of relatively rapid cooling [34], may have been emplaced in dikes closer to the surface. A long period of metamorphism in Ibitira almost certainly erased any signature of the minute amounts of gas that formed the vesicles.…”

“…This smaller size is also consistent with other basaltic asteroids that could be eucrite parent asteroids. The angrites have a liquidus temperature of ∼1155°C but, owing to their SiO 2 -poor compositions [34], are eight times more fluid (2.06 Pa s) than eucrites. We present the calculation only for CO 2 , owing to the higher CO 2 /CO ratio in the oxidized (IW+1) angrites compared to the reduced eucrites.…”

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite

“…Among the quenched angrites dated, we used D'Orbigny as an age anchor for calculating model ages for the following reasons: (1) it is relatively unmetamorphosed, and therefore preserves the chronological records largely undisturbed 57 ; (2) it has a precise U-isotope-corrected Pb-Pb absolute age (4, Supplementary Fig. 1) could result in a 1.4 Myr age difference, as discussed in the Supplementary Methods.…”

Iron isotope fractionation in planetary crusts

“…In addition, three modern island arc basalts (IABs) from New Britain (NMNH 116852-1, 116852-3 and 116852-11) were also analyzed here to test the accuracy of the measurements (see Dauphas et al (2009a) for a detailed discussion about IABs). (Jarosewich, 1990); NWA1950 (Gillet et al, 2005); Los Angeles (Warren et al, 2000); EETA79001, ALHA77005, Nakhla, ALH84001 (Lodders, 1998); Mil03346 (Day et al, 2006); GRO95555, MET00436, MET00424 ; NWA1461 (Warren et al, 2009);Shalka (McCarthy et al, 1972); ALHA77256 (Sack et al, 1991); NWA1240, NWA049 (Barrat et al, 2003); Cachari (Barrat et al, 2000); Agoult, Dag945 (Yamaguchi et al, 2009); Nuevo Laredo, NWA4523 (Barrat et al, 2007b); D'Orbigny, Sahara99555 (Mittlefehldt et al, 2002);LEW86010 (McKay et al, 1988); NWA1670 (Jambon et al, 2008); NWA1296 (Jambon et al, 2005); NWA2999 (Gellissen et al, 2007). i Weathering conditions are from MetBase (version 7.1).…”