Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..420p&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 420
Other
Augite, Cumulates, Eucrites, Exsolution, Ion Probe, Orthopyroxene, Partition Coefficients, Pyroxenes, Rare-Earth Elements, Trace Elements
Scientific paper
In order to improve our understanding of trace-element partitioning during pyroxene exsolution processes, we have examined solid-solid trace-element partitioning between lamellae-host pairs in Binda. Binda is a brecciated monomict cumulate eucrite with textures that show evidence of extensive subsolidus reequilibration of pyroxenes. Orthopyroxenes in Binda typically have coarse "remanent (001)" exsolution lamellae of augite formed during inversion of primary monoclinic pigeonite to orthopyroxene. The coarse lamellae exsolved on the (001) planes of monoclinic pigeonite. Later and thinner (100) augite exsolution lamellae in orthopyroxene were not analyzed for trace elements by SIMS. Two pyroxene grains with wider than 50-micrometer exsolution lamella were selected for trace-element analyses using SIMS. These analyses were performed on a Cameca 4f ion probe at the UNM/SNL Ion Microprobe Facility, a joint operation of the Institute of Meteoritics, UNM, and Sandia National Laboratories. The rare earth elements (REE), as well as Sr, Zr, and Y, were measured. Complimentary major-element analyses were measured using a JEOL 733 electron probe with an Oxford-Link imaging/analysis system. The results of the CI chondrite normalized (average CI [1]) trace-element analyses are shown in Fig. 1. Points for Gd have been estimated. Pattern shapes are in agreement with those expected for clino- and orthopyroxene. Both sets of patterns show HREE enrichment over LREE with a negative Eu anomaly. Abundances of REE in augite are greater than those found in host orthopyroxenes, consistent with pyroxene crystal chemical rationale whereby the Ca in the M2 site of augite "prop" open their structures to incorporate more REE [2,3]. The depleted LREE reflects the fact that the larger REE tend to be excluded from the M2 sites in pyroxenes. Strontium follows Ca in augite, showing a greater abundance than that found in the orthopyroxenes. This reflects the geochemical similarity of Sr^2+ and Ca^2+. Figure 1 clearly demonstrates that augite is an important repository for REE in solid-solid partitioning at subsolidus conditions. Subsolidus reequilibration temperatures were calculated for the lamellae-host pairs using two-pyroxene geothermometry [4]. Temperatures ranging from 900 degrees C to 850 degrees C were calculated for grain 1, whereas temperatures from 850 degrees C to 700 degrees C were determined for grain 2. Calculated temperatures are consistent with equilibrium pigeonite/augite subsolidus phase relations [5]. Differences in calculated temperatures and abundances of the REE between the two measured grains may be a function of their crystallization history, reflecting different temperatures of equilibration and/or slight differences in melt compositions. Our study demonstrates the importance of considering the partitioning of trace elements during pyroxene exsolution. These results indicate that it would be extremely difficult to use the trace-element concentrations in inverted pigeonites to back-calculate parental melt compositions. Acknowledgments: We thank M. N. Spilde for technical assistance and the Smithsonian Institution for the thin section of Binda. This work was supported by NASA grant NAGW-3347 (J.J. Papike) and NGT-70223 (NASA GSRP training grant for A. Pun). References: [1] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. [2] Papike J. J. et al. (1988) LPS XIX, 901-902. [3] Shearer C. K. et al. (1989) GCA, 53, 1041-1054. [4] Lindsley D. H. (1983) Am. Mineral., 68, 477-493. [5] Papike J. J. and Cameron M. (1976) Rev. Geophy. Space Phys., 14, 37- 80. Fig. 1 appears here in the hard copy.
Layne Graham D.
Papike James J.
Pun Aurora
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