Meteoritics & Planetary Science, Volume 43, Number 9 (2008)

The Meteoritical Bulletin, No. 94, September 2008

2021-02-13T01:18:51Z

The Meteoritical Bulletin, No. 94, September 2008 Weisberg, M. K.; Smith, C.; Benedix, G.; Folco, L.; Righter, K.; Zipfel, J.; Yamaguchi, A.; Chennaoui Aoudjehane, H. The Meteoritical Bulletin 94 reports over 900 (119 non-Antarctic and 803 Antarctic) newly approved meteorite names and their recovery histories, macroscopic descriptions, petrography, mineral compositions, and geochemistry. Meteorites reported include lunar, Martian, howardites, eucrites, diogenites, mesosiderites, aubrites, irons, angrites, ureilites, acapulcoites, lodranites and H, L, LL, R, CO, and CV chondrites. Eight of the meteorites in this bulletin are falls, including 4 from India (Jodiya, Kaprada, Kavapura, Mahadevpur), 1 from Mali (Chergach), Turkey (Didim), Libya (Werdama), and the U.S. (Berthoud).

Correlated isotopic and microstructural studies of turbostratic presolar graphites from the Murchison meteorite

2021-02-13T01:18:31Z

Correlated isotopic and microstructural studies of turbostratic presolar graphites from the Murchison meteorite Croat, T. K.; Stadermann, F. J.; Bernatowicz, T. J. We present data from TEM and NanoSIMS investigations of Murchison (CM2) KFC1 presolar graphites. TEM examinations of graphite ultramicrotome sections reveal varying degrees of graphite disorder, leading to distinctions between well-graphitized onions, more turbostratic platy graphites, and the most disordered cauliflower graphites. Aside from their larger size, platy graphites are roughly similar in isotopic composition and in internal grain properties to the well-graphitized onions. Most carbide-containing platy graphites exhibit large s-process element enrichments (~200 solar Mo/Ti ratios), suggesting origins predominantly in AGB carbon stars. The degrees C isotopic distribution of platy graphites is similar to onions, with representatives in both 12C-depleted (5 < 12C/ 13C < 40) and 12C-enriched groups (100 < 12C/13C < 350) and a pronounced gap in the 40 < 12C/13C < 75 region that contains 75% of mainstream SiCs. The large 12C enrichments combined with the extreme s-process element enrichments suggest formation in an environment inhomogeneously enriched in the nucleosynthetic products of thermal pulses in AGB stars. In contrast, numerous scaly cauliflower graphites show 18O enrichments and lack s-process-enriched carbides, suggesting a SN origin, as was the case for many Murchison KE3 SN graphites. The more turbostratic graphites (platy and scaly) are on average larger than onions, likely resulting from formation in a gas with higher degrees C number density. Oxygen content increases progressively with increasing degree of graphite disorder, which can stabilize these grains against further graphitization and may be a reflection of higher O/C ratios in their formation environments.

The lunar-wide effects of basin ejecta distribution on the early megaregolith

2021-02-13T01:18:38Z

The lunar-wide effects of basin ejecta distribution on the early megaregolith Petro, N. E.; Pieters, C. M. The lunar surface is marked by at least 43 large and ancient impact basins, each of which ejected a large amount of material that modified the areas surrounding each basin. We present an analysis of the effects of basin formation on the entire lunar surface using a previously defined basin ejecta model. Our modeling includes several simplifying assumptions in order to quantify two aspects of basin formation across the entire lunar surface: 1) the cumulative amount of material distributed across the surface, and 2) the depth to which that basin material created a well-mixed megaregolith. We find that the asymmetric distribution of large basins across the Moon creates a considerable nearside-farside dichotomy in both the cumulative amount of basin ejecta and the depth of the megaregolith. Basins significantly modified a large portion of the nearside while the farside experienced relatively small degrees of basin modification following the formation of the large South Pole-Aitken basin. The regions of the Moon with differing degrees of modification by basins correspond to regions thought to contain geochemical signatures remnant of early lunar crustal processes, indicating that the degree of basin modification of the surface directly influenced the distribution of the geochemical terranes observed today. Additionally, the modification of the lunar surface by basins suggests that the provenance of lunar highland samples currently in research collections is not representative of the entire lunar crust. Identifying locations on the lunar surface with unique modification histories will aid in selecting locations for future sample collection.

Refractory inclusions in the CH/CB-like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography

2021-02-13T01:18:44Z

Refractory inclusions in the CH/CB-like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography Krot, A. N.; Ulyanov, A. A.; Ivanova, M. A. The CH/CB-like chondrite Isheyevo consists of metal-rich (7090 vol% Fe,Ni-metal) and metal-poor (720 vol% Fe,Ni-metal) lithologies which differ in size and relative abundance of Fe,Nimetal and chondrules, as well as proportions of porphyritic versus non-porphyritic chondrules. Here, we describe the mineralogy and petrography of Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in these lithologies. Based on mineralogy, refractory inclusions can be divided into hibonite-rich (39%), grossite-rich (16%), melilite-rich (19%), spinel-rich (14%), pyroxeneanorthite- rich (8%), fine-grained spinel-rich CAIs (1%), and AOAs (4%). There are no systematic differences in the inclusion types or their relative abundances between the lithologies. About 55% of the Isheyevo CAIs are very refractory (hibonite-rich and grossite-rich) objects, 20-240 micrometers in size, which appear to have crystallized from rapidly cooling melts. These inclusions are texturally and mineralogically similar to the majority of CAIs in CH and CB chondrites. They are distinctly different from CAIs in other carbonaceous chondrite groups dominated by the spinel-pyroxene +/- melilite CAIs and AOAs. The remaining 45% of inclusions are less refractory objects (melilite-, spinel- and pyroxene-rich CAIs and AOAs), 40-300 micrometers in size, which are texturally and mineralogically similar to those in other chondrite groups. Both types of CAIs are found as relict objects inside porphyritic chondrules indicating recycling during chondrule formation. We infer that there are at least two populations of CAIs in Isheyevo which appear to have experienced different thermal histories. All of the Isheyevo CAIs apparently formed at an early stage, prior to chondrule formation and prior to a hypothesized planetary impact that produced magnesian cryptocrystalline and skeletal chondrules and metal grains in CB, and possibly CH chondrites. However, some of the CAIs appear to have undergone melting during chondrule formation and possibly during a major impact event. We suggest that Isheyevo, as well as CH and CB chondrites, consist of variable proportions of materials produced by different processes in different settings: 1) by evaporation, condensation, and melting of dust in the protoplanetary disk (porphyritic chondrules and refractory inclusions), 2) by melting, evaporation and condensation in an impact generated plume (magnesian cryptocrystalline and skeletal chondrules and metal grains; some igneous CAIs could have been melted during this event), and 3) by aqueous alteration of pre-existing planetesimals (heavily hydrated lithic clasts). The Isheyevo lithologies formed by size sorting of similar components during accretion in the Isheyevo parent body; they do not represent fragments of CH and CB chondrites.