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

Abstract

Abstract-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 C isotopic distribution of platy graphites is similar to onions, with representatives in both 12 C-depleted (5 < 12 C/ 13 C < 40) and 12 C-enriched groups (100 < 12 C/ 13 C < 350) and a pronounced gap in the 40 < 12 C/ 13 C < 75 region that contains 75% of mainstream SiCs. The large 12 C 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 18 O 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 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.

Quantifying the distribution of nanodiamonds in pre-Younger Dryas to recent age deposits along Bull Creek, Oklahoma Panhandle, USA

“…Lattice fringes often cross entire grains, providing evidence that these crystals are not mixtures of disordered graphite, graphene, and graphane, as observed in other natural and anthropogenic nanoscale carbons (e.g., refs. [37][38][39][40][41][42]. A number of other internal textures were observed.…”

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

“…The 16 O/ 18 O ratios range from 300 to 550 ( 16 O/ 18 O = 499), but are normal within error for most of the samples. Hoppe et al (1995) and Croat et al (2008) concluded that the O-isotopic composition reflects dilution with normal O at some time after grain formation. The dilution process probably occurred in the solar nebula, in the parent asteroid, and in the laboratory.…”

Presolar Grains

“…While all these grains, with the exception of iron oxide, as well as presolar Si 3 N 4 (Nittler et al, 1995), were located by single grain analysis of acid residues, presolar silicates were discovered by isotopic imaging of chemically untreated interplanetary dust particles (IDPs (See Chapter 1.8)) , meteoritic grain-size separates and polished sections (Nagashima et al, 2004;Nguyen and Zinner, 2004), and Antarctic meteorite samples (Yada et al, 2008). Finally, titanium-, zirconium-, and molybdenum-rich carbides, cohenite ((Fe,Ni) 3 C), kamacite (Fe-Ni), rutile (TiO 2 ), oldhamite (CaS), elemental-iron and ruthenium-iron metal, and iron and nickel silicide (Fe 2 Si, Ni 2 Si) were found as tiny subgrains inside of SiC grains and graphite spheres (Bernatowicz et al, 1991Croat et al, 2003Croat et al, , 2008Croat et al, , 2011aCroat et al, ,b, 2012Hynes et al, 2010Hynes et al, , 2011. While TiC inside of a SiC grain (Bernatowicz et al, 1992) could have formed by exsolution, there can be little doubt that interior grains in graphite must have formed prior to the condensation of the spherules.…”