Physics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..453w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 453-454
Physics
Ion Probe, Meteorites, Snc, Rare-Earth Elements, Shergottites, Trace Elements, Zagami
Scientific paper
An unusual olivine-rich lithology in Zagami (referred to as Zagami DN [1]) has recently been interpreted as a centimeter-sized pocket of late-stage melt by [2]; these authors outlined the major element chemical evolution of DN and speculated on its physical setting. To better understand its formation in the context of the crystallization history of the Zagami shergottite, we made ion microprobe measurements of REE and selected trace-element concentrations in the major mineral phases present in a thin section of Zagami DN and conducted an extensive search of Zagami hand samples to locate DN material in situ. Whitlockite in DN has a REE pattern that is identical to that of whitlockite in Normal Zagami (hereon referred to as NZ); however, absolute concentrations of REE in whitlockite are somewhat higher in DN (La ~520x CI) than in NZ (La ~460x CI) [3]. Apatite in DN has REE abundances (La ~27x CI) that are similar to those in an apatite measured in Shergotty (Sh10G in [4]). Also, maskelynite grains in NZ [3] and DN have similar REE concentrations (La ~0.26x CI; Eu/Eu* ~ 60). Olivine, found in a fayalite-rich intergrowth in DN, is considerably enriched in minor and trace elements such as Mn, Ti, Zr, and REE compared to the early-formed phenocrystic olivine in the harzburgitic shergottites (ALHA77005 and LEW88516) [5]. Pigeonite in DN (like pyroxene in all shergottites) is extremely zoned in its trace element composition. On average, concentrations of minor and trace elements such as Ti, Zr, Y, and REE are higher by factors of 2-4 in pigeonite in DN than in NZ. Moreover, on a plot of Zr concentrations vs. Ti concentrations (Fig. 1, which appears in the hard copy), pigeonite compositions in DN lie along an extension of the trend defined by pigeonite compositions in NZ. These data confirm that DN did indeed crystallize from an evolved, late-stage melt that had the same parent magma as the melt from which NZ formed. We examined 5.2 kg of Zagami hand samples, with a sawn surface area of ~910 cm^2. Zagami exhibits a strong preferred alignment of pyroxene laths, and impact melt veins parallel this direction of alignment. The alignment is apparent on orthogonal faces of a 1.85-kg slice, suggesting foliation rather than lineation. On a 2.7-kg end piece, a dramatic variation in grain size exists perpendicular to the direction of foliation. One extreme in this variation is a dark, mottled area containing large pyroxene phenocrysts in a very fine-grained matrix. The darker area may have formed after the settling of phenocrysts, leaving a late-stage melt from which the fine-grained, fayalite-rich DN material crystallized. Studies of thin sections are planned to further evaluate this hypothesis. Acknowledgments: Hand samples of Zagami were generously made available by R. Haag. References: [1] Vistisen L. et al. (1992) Physica Scripta, 46, 94-96. [2] McCoy T. J. et al. (1993) LPSC XXIV, 947-948. [3] Wadhwa M. and Crozaz G. (1993) LPSC XXIV, 1473-1474. [4] Lundberg L. L. et al. (1988) GCA, 52, 2147- 2163. [5] Harvey R. P. et al. (1993) GCA, submitted.
Crozaz Ghislaine
Keil Klaus
McCoy Timothy James
Wadhwa Meenakshi
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