Carbon in Metal of Primitive Ordinary Chondrites

Details Carbon in Metal of Primitive Ordinary Chondrites Carbon in Metal of Primitive Ordinary Chondrites

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.553m&link_type=abstract

Meteoritics, vol. 30, no. 5, page 553

Computer Science

2

Carbon, Chondrites, Ordinary, Graphite, Iron-Nickel Metals, Meteorites, Bishunpur

Scientific paper

Abundant graphite inclusions have recently been found in Fe-Ni metal grains of the Bishunpur (L3.1) chondrite [1]. Their presence in metal is correlated with the concentrations of minor elements (Co, Cr), which has been interpreted as evidence that carbon played a role in redox processes during chondrule formation [2]. The origin of these inclusions is not clear. In a companion paper (Mostefaoui and Perron, this conference) their characterization has been attempted by Raman spectrometry. Here we try i) to ascertain whether they are related to the rest of the carbon in the chondrite, ii) to determine the carbon content of metal grains. Graphite has been occasionally observed in the metal of ordinary chondrites (OC) [3] but no systematic search has apparently been conducted. We surveyed the metal of a number of chondrites and found graphite in 10 OCs, all of type 3 (Bishunpur, Inman, Kohar, Manych, Massenya, Mezo-Madaras, Moorabie, Ngawi, Semarkona, Tieschitz) and in Kainsaz (CO3). We have quantified the abundance of graphite-bearing metal by the number of metal grains with visible graphite inclusions (regardless of the size of the grains and of the inclusions) per cm2 of polished surface, normalized to the metal abundance of the chondrite, taken from [4]. Fig. 1 shows the values obtained, normalized to Bishunpur, plotted as a function of total carbon content, taken from [4] and references therein, for 10 chondrites (the total carbon content of the 11th, Massenya, is unknown). The line is a regression through 6 points. Four points have not been taken into account as i) Kainsaz, a carbonaceous chondrite, may not follow the OC trend; ii) Semarkona metal contains abundant carbides [5], which may result in a lower graphite abundance; iii) Kohar and Moorabie, whose graphite is essentially in veins extending into silicates, show abundant signs of shock effects (the link between graphite and shock is not clear, but there is some evidence of concentration of carbonaceous matter in shock veins [6]). If the correlation shown in Fig. 1 is real, it is a strong argument in favor of a common origin for graphite inclusions in metal and the bulk of carbon. Preliminary values of C concentration have been obtained with the ion probe at Nancy for 7 metal grains in Bishunpur. They range from *0.1wt%C for kamacite in reduced chondrules (FeO-poor silicates, Cr-rich metal)

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