The Petrography of Meteoritic Nano-diamonds

Introduction: At least some meteoritic nano-diamonds are likely of presolar origin because of their association with anomalous Xe-HL and Te isotopic components indicative of a supernova (SN) origin [1,2]. But the abundance of Xe is such that only ~1 in 10 6 nano-diamonds contains a Xe atom, and the bulk 13 C/ 12 C composition of nano-diamond acid residues is chondritic (solar) [3]. Therefore, it is possible that a significant fraction of meteoritic nano-diamonds formed within the solar nebula. Nano-diamonds have recently been detected for the first time within the accretion discs of young stars by the Infrared Space Observatory (ISO) [4]. No comparable evidence of nano-diamonds in the interstellar medium has yet been found. We have identified nano-diamonds in acid etched thin-sections of meteorites, polar micrometeor-ites, and interplanetary dust particles (IDPs) with the goal of determining their distribution as a function of heliocentric distance [5]. (It is assumed the meteorites and the polar micrometeorites are from asteroids at 2-4 AU and at least some of the IDPs are from comets at >50AU). We found that nano-diamonds are heterogeneously distributed throughout carbon-rich meteoritic materials (we identified them in some IDPs and not in others), and that their abundance may actually decrease with heliocentric distance, consistent with the hypothesis that some of them formed within the inner solar system and not in a presolar (SN) environment. In order to gain further insight about the origins of meteoritic nano-diamonds we are currently investigating their distribution in unetched thin-sections. We have examined a chondritic cluster IDP (U220GCA), fragments of the Tagish Lake (CM1) meteorite, and a SN graphite spherule (KE3d8) isolated from the Mur-chison (CM) meteorite. We selected U220GCA because its nano-diamond abundance (in acid etched thin-sections) appears to be as much as ~10X higher than in Murchison matrix, Tagish Lake because it has a higher reported nano-diamond abundance than other carbonaceous chondrites (~3650-4330 ppm) [6], and KE3d8 because it is a carbon spherule with an isotopic composition suggesting that it is a bone fide presolar SN grain [7]. Experimental procedures: Fragments of IDP U220GCA, Murchison matrix, and Tagish Lake were embedded in sulfur and thin sectioned using ultrami-crotomy. SN graphite spherule Ked38 was embedded and sectioned in epoxy. The sections were mounted on continuous carbon films on gold and copper mesh TEM grids and examined using a 400 keV