Determination of carbon contents in the iron meteorites Toluca and Algarrobo

Abstract

The nuclear microprobe at the cyclotron of the Vrije Universiteit in Amsterdam, The Netherlands, was employed for a study of the iron meteorites Toluca and Algarrobo. Samples were irradiated with ~ 0.4 nA of 1.4 MeV deuterons. Carbon was detected by measuring the proton spectrum of the 12C(d, p) 13C nuclear reaction. The X-ray spectrum of Fe and Ni was measured simultaneously. Area scans of squares approximately 1 × 1 mm 2 were made with beam size approximately 20 × 20 μm 2. Spots chosen for the analysis contained areas of taenite, plessite and martensite surrounded by kamacite. Our area scans show clearly that regions of martensite are more C-rich than those of plessite and taenite with lowest C intensity in kamacite. At this point it is still impossible to determine the C-concentrations absolutely with good precision due to possible C-contamination during the target preparation and/or the C-deposition on the irradiated spot during the measurement. However, minimal concentrations of carbon in plessitic and martensitic areas were calculated assuming that all the C counts in kamacite areas come from contamination and using a Fe-C standard with 0.8% C. Further improvements of the technique are planned, including measuring immediately after polishing samples with Al 2O 3, as well as employing a cold trap inside of the irradiation chamber to reduce carbon deposition during measurement.