Minor Elements and Oxygen Isotopes in Forsterite Grains in Julesburg and Allende

Details Minor Elements and Oxygen Isotopes in Forsterite Grains in Julesburg and Allende Minor Elements and Oxygen Isotopes in Forsterite Grains in Julesburg and Allende

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.528s&link_type=abstract

Meteoritics, vol. 29, no. 4, page 528

Computer Science

Allende, Chondrites, Unequilibrated, Forsterite, Isotopes, Oxygen, Julesburg, Olivine

Scientific paper

Steele [1] recognised a type of Forsterite, characterised by blue CL, and very low iron, that appears to be a component common to CO, CV, and UOC meteorites. This "blue olivine" has high levels of refractory minor elements, may be a precursor material for chondrules, and is often surrounded by more Fe-rich olivine. The grains and their rims might therefore preserve a record of processes that took place prior to the formation of the different meteorite groups. Chondrules from ordinary and carbonaceous chondrites differ in their oxygen isotopic composition [2] and so we have begun a study of blue olivines to establish the relationship between this relict precursor material in different meteorite classes and their relationship to chondrules. The blue olivines were found using the high energy electron gun used for charge compensation of insulators on the Manchester Isolab 54 ion probe. We scanned the electron beam (1 microamp current, spot size 400 micron diameter) over the sample and noted grains that luminesced blue. These were then examined by SEM/EDX to identify those that were forsterite. We have so far found one blue olivine in Julesburg (L3.6) and two in Allende. Most of our attention so far has been directed to the Julesburg grain. This grain is 300 x 400 microns in size, and has a complete, 40 micron thick, iron rich (Fa20-25) rim. We have measured the minor element (Fe, Ca, Al, Mn) distribution using wavelength dispersive spectroscopy on a Cameca Camebax electron microprobe. The core, which contains up to 1.5 wt% CaO and 0.9 wt% Al2O3, is generally iron poor and only part of it luminesces blue. Areas with more than 1.5 wt% FeO do not luminesce. Oxygen isotope analyses are being carried out on the Manchester ion probe. This instrument employs a number of novel features designed to facilitate accurate measurement of oxygen isotope ratios. These include simultaneous collection of all three isotopes, and detection of 18O and 17O with a new type of electron multiplier that has been found to show excellent long term stability. We have obtained reproducible 18O/16O ratios (+1-1.5 permil over several days) from insulators (quartz and olivine) [3] and we are undertaking measurements of 18O/16O and 17O/16O in the Julesburg and Allende blue olivine grains. References: [1] Steele I. (1986) GCA, 50, 1379-1395. [2] Grossman J. N. et al. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), 619-659, Univ. of Arizona. [3] Lyon I. C. et al. (1994) Rapid Communications in Mass Spectrometry, submitted

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