Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.264m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 264
Computer Science
1
Scientific paper
Ureilites are an enigmatic group of achondritic meteorites, mainly made up of olivine, pigeonite, and elemental carbon. While multistage igneous processes have been advocated, based on trace element data (Goodrich et al., 1987) oxygen isotopes clearly speak of heterogenous accretion for ureilites (Clayton and Mayeda, 1988). Main group ureilites revealed nitrogen components with delta^15N ranging from +19.7 to -83.1o/oo (Grady et al., 1985; Grady and Pillinger, 1986) while polymict ureilites have a ^15N-enriched component with delta^15N >= 540o/oo (Grady and Pillinger, 1988). Here we present the nitrogen results for Lahrauli ureilite which fell in 1955 (Bhandari et al., 1981). N and noble gases have been extracted by a stepwise pyrolysis at temperatures (degrees C) of 800, 1000, 1200, 1300, 1400, 1500 and 1700 and analysed by standard procedures (Murty and Goswami, 1991). An initial combustion step was carried out at 400 degrees C in 100 m Torr O2 to get rid of surficial contaminants. We discuss only the N results here. The total N in Lahrauli amounts to 11.3 ppm with delta^15N = - 72o/oo. The delta^15N of the 800 degrees C fraction is 5.5o/oo and progressively decreases to -107o/oo in the 1400 degrees C fraction and subsequently increases to -89o/oo in the 1700 degrees C fraction. This delta^15N trend is most likely due to mixing between a heavy N component (delta^15N>=5.5o/oo) that has a low temperature release and a light N component (delta^15N<=-107o/oo) that is more retentively sited. Increase in the delta^15N, beyond 1400 degrees C is most likely due to release of spallation nitrogen. The spallogenic ^15N needed to cause such an increase is consistant with the spallation ^21Ne present in Lahrauli. This is the lightest N-component yet observed in a bulk ureilite as well as in a bulk stony meteorite. The unusual stony meteorite Acapulco also exhibits a similar N isotopic pattern (Sturgeon and Marti, 1991). The light N-component cannot be accounted for, if carbonaceous chondrites (which have delta^15N>=40o/oo), are the main precursor material for ureilites. Preservation of isotopically distinct N-components, will be consistant with the heterogenous accretion of the ureilite parent body, as suggested by oxygen isotopes (Clayton and Mayeda, 1988). N isotopic studies in separated phases, aimed at identifying the carriers of different N-components are in progress. References: Bhandari, N., Shah V.G., and Graham A. (1981) Meteoritics 16, 185; Clayton, R.N. and Mayeda, T.K. (1988) Geochim. Cosmochim. Acta 52, 1313; Goodrich, C.A., Jones, J.J., and Berkley, J.L. (1987) Geochim. Cosmochim. Acta 51, 2255; Grady, M.M., Wright, I.P., Swart, P.K., and Pillinger, C.T. (1985) Geochim. Cosmochim. Acta 49, 903; Grady, M.M. and Pillinger, C.T. (1986) Meteoritics 21, 375; Grady, M.M. and Pillinger, C.T. (1988) Nature 331, 321; Murty, S.V.S. and Goswami, J.N. (1991) 22nd Lunar Planet. Sci. Conf. (Abstract) 947; Sturgeon, G. and Marti, K. (1991) Proc. Lunar Sci. Conf. 21, 523.
Bhandari Narendra
Murty Sripada V. S.
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