Ion microprobe analyses of carbon in Fe–Ni metal in iron meteorites and mesosiderites

Abstract

Carbon concentrations in kamacite, taenite, and plessite (kamacite-taenite intergrowths) were measured in 18 iron meteorites and 2 mesosiderites using the Cameca ims 1280 ion microprobe at the University of Hawai‘i with a 5–7μm beam and a detection limit of <1ppm. Our goal was to investigate the effects of carbon on the microstructure of iron meteorites during cooling and to evaluate how carbon partitions between metal phases and other carbon-bearing minerals (graphite, haxonite, cohenite) in various meteorite groups. Carbon concentrations range from ∼100 to ∼1000ppm in taenite and plessite in groups IAB, IIICD, and IIIAB, which contain graphite and/or carbides, but only 2–6ppm in groups IVA, IVB and the ungrouped iron, Tishomingo, which lack graphite and carbides. Carbon contents in kamacite range from ∼2 to ∼10ppm in most studied meteorites, including IIAB, but higher abundances were found in kamacite from IAB Pitts subgroup meteorites Pitts and Woodbine (12–15ppm). Our carbon abundances for kamacite are lower than most published ion probe data, indicating that earlier carbon measurements had contamination problems.Grains of taenite and fine-grained plessite in carbon-rich meteorites, which all have normal M-shaped nickel profiles due to slow cooling, have diverse carbon contents and zoning profiles. This is because taenite decomposed by diverse mechanisms over a range of temperatures, when nickel could only diffuse over sub-μm distances. Carbon diffusion through taenite to growing carbides was rapid at the upper end of this temperature range, but was very limited at the lower end of the temperature range. In mesosiderites, carbon increases from 12ppm in tetrataenite to 40–115ppm in cloudy taenite as nickel decreases from 50% to 35%. Low carbon levels in tetrataenite may reflect ordering of iron and nickel; higher carbon in cloudy taenite is attributed to metastable bcc phase, possibly martensite, with ∼300ppm carbon intergrown with tetrataenite. Pearlitic plessite, which only forms in carbon-rich irons, contains much less carbon than martensitic plessite: 10–20ppm and 300–500, respectively, in IAB irons. Pearlitic plessite consists of μm-scale intergrowths of low-nickel kamacite and tetrataenite, which formed during cooling from ∼450 to 300°C when haxonite was forming. Martensitic plessite decomposed to tetrataenite and metastable high-nickel kamacite at temperatures below 300°C, which depended on nickel content. Carbon accumulated in untransformed taenite when haxonite growth ceased, producing M-shaped carbon profiles.Bulk carbon concentrations inferred from our ion probe data are 3–4ppm in IVA, IVB, and Tishomingo, which has IVB-like depletions of moderately volatile siderophiles. Published bulk carbon contents of IVA and IVB irons are >10 times higher suggesting contamination problems. Our ion probe analyses and observations of carbide and graphite show that bulk carbon decreases with decreasing germanium and other moderately volatile elements from group IAB, through IIAB and IIIAB, to group IVA and IVB. These trends may have been inherited from fractionated chondritic precursors, or may have been produced by impacts that caused volatile loss, separation of mantle from core material, and relatively rapid cooling of irons poor in volatiles and carbon.