MICHAEL E. LIPSCHUTZ, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Meteorites yield a wealth of information concerning Solar System origin, geochemical evolution of primitive parent objects, and irradiation histories of material in space. Since the numerous (approximately 16,000) antarctic meteorites include many of rare or unique types, current extraterrestrial materials research attention focuses on this population. Antarctic meteorites also have the potential for providing unique information on ice-sheet dynamics.
Techniques used for studying extraterrestrial materials are applicable to studies of terrestrial geologic samples, and our analytical capabilities at Purdue have dual purposes. Our research activities involve the acquisition and genetic interpretation of chemical compositional data particularly at trace or ultratrace levels in rocks and other natural materials. Trace elements are especially valuable since a small absolute compositional change produced by some force is magnified into a large relative change.
Our analytical capabilities, previously limited to accelerator mass spectrometry (AMS) for quantifying cosmogenic radionuclides, radiochemical neutron activation analysis (RNAA) for quantifying 15 volatile trace and ultratrace elements, and electron microprobe analysis (in concert with computer modeling) to study compositional changes during igneous processing, have increased.
During the past year, we continued our AMS upgrade (Elmore et al. 1997), acquired a VG Elemental PQ3 inductively coupled plasma mass spectrometer (ICP-MS) and MicroProbe UV laser ablation unit, and a complementary x-ray fluorescence (XRF) system. We are now completing the last of the laboratory renovations housing our RNAA, AMS, ICP-MS, and XRF facilities and their associated chemical preparation areas.
Much genetic information concerning meteorites that we uncovered reflects our statistical capabilities to treat RNAA data using the multivariate statistical techniques of linear discriminant analysis and logistic regression. At the request of the series editors, Wolf and Lipschutz (1995a) reviewed both standard multivariate statistical analysis and our adaptations thereof.
Since my last review (Lipschutz 1995), we have studied the 12 Martian meteorites, six of which are antarctic. Our RNAA data for the Martian lherzolite Yamato (Y)-793605 (cf. Mittlefehldt et al. 1997) demonstrate its resemblance to the other two known lherzolites, Allan Hills (ALH) A77005 and Lewis Cliffs (LEW) 88516. Cluster analysis of whole-rock RNAA data for all 12 Martian meteorites, classifies them into the same six groups [shergottites, nakhlites, lherzolites, orthopyroxenite, chassignite, and shergottitelike Queen Alexandra (QUE) 94201] established petrographically and by refractory element data. Apparently, each of these six groups formed in separate igneous chambers in Mars, closed to volatile transfer (Wang, Mokos, and Lipschutz in press).
Having mineral chemistry data for shergottites, Ghosal et al. (in press) used the MELTS program developed here to study their origin. We found that the oxygen fugacity of the shergottite source region was more reducing, 1-4 log10 units lower than others suggested, and that Mars has a more Earthlike mantle than previously proposed. Subsequent in situ Martian surface rock analyses by Pathfinder apparently support the latter suggestion.
In the last year, we published seven studies dealing with ordinary chondrites, five dealing with antarctic samples. In studying igneous inclusions in three antarctic ordinary chondrites by RNAA, we found that troctolitic inclusions from the L6 chondrites Y-75097 and Y-793241 apparently formed from protoliths of nonchondritic composition, whose origin might be nebular or of secondary (parent body) origin. The igneous inclusion from the H5 chondrite Y-794046 was different: it derives from an H chondrite melt that lost approximately 90 percent of its siderophiles and chalcophiles, presumably in immiscible iron-iron sulfide (Fe-FeS) eutectic melt (Mittlefehldt et al. 1995).
Using AMS, we identified an H chondrite from Victoria Land, ALH 88019, having a terrestrial age of 2.03±.15 million years, essentially twice the age of the previous record-holder, and also reported additional information on its size and irradiation history (Scherer et al. 1997). This long age provides one boundary condition for ice-sheet history in the Allan Hills region of Antarctica.
To test effects of weathering on RNAA data for antarctic ordinary chondrites, we obtained Fe3+ data (using Mössbauer) for 33 antarctic H4-6 chondrites for which we already had RNAA data. Using multivariate statistical techniques, we found no significant dependence of composition on weathering (Wolf and Lipschutz 1995b). Thus, highly significant compositional difference between falls and antarctic H4-6 chondrites with terrestrial ages of 50,000 years or more suggests that the Earth's sampling of H chondrite source objects has varied on the 50,000-year scale (cf. Wolf and Lipschutz 1995c). Based on multivariate statistical analysis of RNAA data, Lipschutz, Wolf, and Dodd (1997) demonstrate that 13 H4-6 chondrites that fell in May 1855-1895 and 17 H4-6 chondrites that fell in September and October 1812-1992 can be distinguished compositionally from 33 random falls of H chondrites between 1773 and 1970 (Wolf et al. 1997). Identification of these two meteoroid streams demonstrates source variations on the hundred-year scale (cf. Lipschutz et al. 1997).
We obtained RNAA and mineral-chemical data for igneous regions in seven, nonantarctic, ordinary chondrites: an H5, H6, two L5, and three L6 (Yolcubal et al. 1997). Use of the MELTS program indicates that igneous regions in six chondrites derive from melting of chondrite types like their hosts and provide detailed information on their metamorphic and magmatic histories. The igneous region in the L6 chondrite Chantonnay, however, apparently derives from an H chondrite source. Only the L6 chondrite Chico evidenced volatile loss; heating involving the other six was short-lived, closed-system, or both.
We have revisited the idea that surfaces of C, G, B, and F asteroids represent thermally metamorphosed material excavated from the interiors of these asteroids by comparing the ultraviolet, visible, near infrared, and 3-micrometer (µm) reflectance spectra of seven selected C, G, B, and F asteroids with spectra from 29 carbonaceous chondrites and artificially heated samples (Hiroi et al. 1996). Twelve of these (including three thermally metamorphosed ones) are antarctic. Relationships between the various spectra produce results suggesting that the larger asteroids represent heated inner portions of even larger bodies. Thus, common CI and CM chondrites may derive from the lost outer portions of these bodies, which escaped extensive, late-stage heating episodes (Hiroi et al. 1996).
A paper describing the petrology of the known CM chondrites, most of which are antarctic, is currently being revised in the light of reviewers' comments (Zolensky et al. 1997). As previously found from RNAA data, CM chondrites not only exhibit petrographic characteristics traditionally defined as type 2 according to the traditional chondritic classification, but extend even to type 1, while filling in the putative hiatus between them (Zolensky et al. 1997).
Finally, Sinha, Sack, and Lipschutz (1997) reported microprobe data for eight antarctic ureilites and used previous empirical data to determine conditions during which ureilite smelting occurred. We found that these eight igneous rocks derive from three or more disconnected parent regions:
This research was supported in part by National Science Foundation grants EAR 92-19083 and EAR 93-05859, National Aeronautics and Space Administration grant NAGW-3396, Department of Energy grants DE-FG07-80ER1 072SJ and DE-FG02-95NE38135, and the W.M. Keck Foundation.
Elmore, D., X. Ma, T. Miller, K. Mueller, M. Perry, F. Rickey, P. Sharma, P. Simms, M. Lipschutz, and S. Vogt. 1997. Status and plans for the PRIME Lab AMS Facility. Nuclear Instruments and Methods in Physics Research , B123(1-4), 199-204.
Ghosal, S., R.O. Sack, M.S. Ghiorso, and M.E. Lipschutz. In press. Evidence for a reduced, Fe-depleted Martian mantle source region from Shergottites. Contributions to Mineralogy and Petrology .
Hiroi, T., C.M. Pieters, M.E. Zolensky, and M.E. Lipschutz. 1996. Thermal metamorphism of the C, G, B, and F asteroids seen from the 0.7- µ m, 3- µ m, and UV absorption strengths in comparison with carbonaceous chondrites. Meteoritics and Planetary Science , 31(3), 321-327.
Lipschutz, M.E. 1995. Meteorite studies: Terrestrial and extraterrestrial applications, 1995. Antarctic Journal of the U.S. , 30(5), 69-70.
Lipschutz, M.E., S.F. Wolf, and R.T. Dodd. 1997. Meteoroid streams as sources for meteorite falls: A status report. Planetary and Space Sciences , 68(5), 605-637.
Mittlefehldt, D.W., M.M. Lindstrom, M.-S. Wang, and M.E. Lipschutz. 1995. Geochemistry and origin of achondritic inclusions in Yamato 75097, 793241 and 794046. Proceedings of the NIPR Symposium on Antarctic Meteorites , 8, 251-271.
Mittlefehldt, D.W., S. Wentworth, M.-S. Wang, M.M. Lindstrom, and M.E. Lipschutz. 1997. Geochemistry of and alteration phases in Martian lherzolite Y-793605. Antarctic Meteorite Research , 10, 108-123.
Scherer, P., L. Schultz, U. Neupert, M. Knauer, S. Neumann, I. Leya, R. Michel, J. Mokos, M.E. Lipschutz, K. Metzler, M. Suter, and M. Kubik. 1997. Allan Hills 88019: An antarctic H-chondrite with a very long terrestrial age. Meteoritics and Planetary Science , 32(6), 769-773.
Sinha, S.K., R.O. Sack, and M.E. Lipschutz. 1997. Ureilite meteorites: Equilibration temperatures and smelting reactions. Geochimica et Cosmochimica Acta , 61(19), 4235-4242.
Wang, M.-S., J.A. Mokos, and M.E. Lipschutz. In press. Volatile trace elements in and cluster analysis of Martian meteorites. Meteoritics and Planetary Science .
Wolf, S.F., and M.E. Lipschutz. 1995a. Multivariate statistical techniques for trace element analysis. In H. Hyman and M. Rowe (Eds.), Advances in analytical geochemistry (Vol. 2). Greenwich, Connecticut: J.A.I. Press.
Wolf, S.F., and M.E. Lipschutz. 1995b. Chemical studies of H chondrites—7. Contents of Fe3+ and labile trace elements in antarctic samples. Meteoritics , 30(6), 621-624.
Wolf, S.F., and M.E. Lipschutz. 1995c. Meteoroid streams: Evidence for meteorites recovered on Earth. Earth, Moon and Planets , 68(1-3), 605-637.
Wolf, S.F., M.-S. Wang, R.T. Dodd, and M.E. Lipschutz. 1997. Chemical studies of H chondrites—8. On contemporary meteoroid streams. Journal of Geophysical Research , 102(E4), 9273-9288.
Yolcubal, I., R.O. Sack, M.-S. Wang, and M.E. Lipschutz. 1997. Formation conditions of igneous regions in ordinary chondrites: Chico, Rose City and other heavily shocked H and L chondrites. Journal of Geophysical Research , 102(E9), 21589-21611.
Zolensky, M.E., D.W. Mittlefehldt, M.E. Lipschutz, M-S. Wang, R.N. Clayton, T.K. Mayeda, M.M. Grady, C. Pillinger, and D. Barber. 1997. CM chondrites exhibit the complete petrologic range from type 2 to 1. Geochimica et Cosmochimica Acta , 61(23), 5099-5115.