Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..451t&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 451
Computer Science
Cosmic Rays, Exposure Ages, Inclusions, Shergottites
Scientific paper
The EETA79001 shergottite basalt contains xenoliths (to 1 cm diameter) that are comparable to the nonpoikilitic lithologies in the ALHA77005 and LEW88516 shergottites. The xenoliths are fragments of an olivine-low-Ca-pyroxene rock, with lesser augite, chromite, and maskelynite; their textural patterns suggest an original heteradcumulate igneous rock [1,2] similar to ALHA77005 and LEW88516 [3,4]. Mineral phases in the xenoliths have experienced some chemical exchange with the host basalt [5]; however, their core compositions, especially pyroxenes, are nearly identical to those in LEW88516, and somewhat less restricted than those in ALHA77005 (new data; [1]; [2]; G. McKay, personal communication). Olivines in the xenoliths contain multiphase inclusions, ellipsoidal aggregates (to 200 micrometers diameter) of augite, alkali-rich, and Si-rich glasses, chromite, and a phosphate, in radiating or variolitic textures. These are interpreted as magmatic inclusions, remnants of original magma trapped in the olivines, and are petrographically identical to magmatic inclusions in the olivines of ALHA77005 and LEW88516 [4,6,7]. Shock effects in the xenoliths include zones of brown pleochroism in olivine, mosaickism of both olivine and pyroxene, formation of maskelynite from original plagioclase, recrystallization, and melting; these same effects are present but more severe in ALHA77005 and LEW88516 [8,9]. These many similarities among ALHA77005, LEW88516, and the EETA79001 xenoliths suggest that all were derived from similar, closely related igneous rocks (radioisotope ratios apparently preclude a comagmatic origin for ALHA77005 and the EETA79001 xenoliths [7,10,11]). If these three lithologies are closely related, they may be contemporaneous, and thus older than the EETA79001 basalt (host to the xenoliths). This inference may be consistent with radiometric crystallization ages: for ALHA77005 the age is 187 +- 12 (2 sigma) m.y. [11,5] or 154 +- 6 (2 sigma) m.y. [7]; for the EETA79001 basalts, the age 173 +- 10 (2sigms) m.y. [10,5]. Re-examination of shergottite radio-chronologies might resolve this uncertainty. If ALHA77005, the EETA79001 xenoliths, and LEW88516 are closely related, it is likely that they came from the same region on their parent planet (Mars), and so were probably ejected into space by a single impact event. In this case, the 0.6-m.y. cosmic ray exposure age of EETA79001 must date an impact in space, as the cosmic ray exposure ages for ALHA77005 and LEW88516 are ~2.8 m.y. [12,13]. References: [1] Steele I. M. and Smith J. V. (1982) Proc. LPSC 13th, in JGR, 87, A375-A384. [2] McSween H. Y. Jr. and Jarosewich E. (1983) GCA, 47, 1501- 1513. [3] McSween H. Y. Jr. et al. (1979) Science, 204, 1201-1203. [4] Harvey R. P. et al. (1993) GCA, in press. [5] Jones J. H. (1986) GCA, 50, 969-977. [6] Shih C.-Y. et al. (1982) GCA, 46, 2323-2344. [7] Jagoutz E. (1989) GCA, 53, 2429-2441. [8] Ostertag R. et al. (1984) EPSL, 67, 162-166. [9] Keller L. P. and Treiman A. H. (1992) Meteoritics, 27, 242. [10] Wooden J. et al. (1982) LPSC XIII, 879-880. [11] Shih C.-Y. et al. (1982) GCA, 46, 2323-2344. [12] Bogard D. D. et al. (1984) GCA, 48, 1723. [13] Bogard D. D. and Garrison D. H. (1993) LPSC XXIV, 139-140.
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