Orthopyroxenes as Recorders of Diogenite Petrogenesis: Nature and Causes of Intragrain Chemical Variability

Details Orthopyroxenes as Recorders of Diogenite Petrogenesis: Nature and Causes of Intragrain Chemical Variability Orthopyroxenes as Recorders of Diogenite Petrogenesis: Nature and Causes of Intragrain Chemical Variability

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.417p&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 417

Computer Science

Achondrites, Diffusion, Diogenites, Orthopyroxene

Scientific paper

Diogenites can as a first approximation be considered as orthopyroxene cumulates in differentiated asteroids [1]. Orthopyroxene is a very powerful mineral recorder in mafic cumulates as demonstrated by Lambert and Simmons [2], who "inverted" REE data from cores of orthopyroxene crystals to estimate the melt compositions parental to the various cumulate horizons in the Stillwater Complex, Montana. These authors demonstrated that the rims of orthopyroxene grains are enriched in REE relative to cores, most likely because of subsolidus orthopyroxene/trapped melt exchange. These authors also noted the importance of working with elements with slow diffusion rates (e.g., REE) relative to those with rapid diffusion rates (e.g., Fe^2+, Mg). In most diogenites, the primary pyroxene is orthopyroxene, while in Fe- enriched diogenites (e.g., some assemblages in Garland and Yamato 75032 [3]) the primary pyroxene phase is pigeonite. In slowly cooled cumulate assemblages, monoclinic pigeonite inverts to orthopyroxene with complex augite exsolution lamellae, and the trace-element records of the parental magmas are essentially lost. Our recent SIMS studies indicate a strong fractionation of REE into the augite lamellae. Most of the orthopyroxene grains that we examined in our original suite of 13 diogenites have fine (100) augite lamellae. These lamellae can easily be reintegrated with the host orthopyroxene by using a relatively broad EMP or SIMS beam of 15-20 micrometers. In this study, we consider the nature and causes of OPX intragrain chemical variations. Our previous studies [4] showed that Cr and Al showed significant variations among the different diogenites and that specific diogenites trend toward one of several different trajectories on a Cr-Al plot. We also found significant chemical variability in individual OPX grains. To date, we have identified three types of intragrain chemical variations: (1) smooth trends from core to rim, (2) periodic linear variations [e.g., (100) augite lamellae in an OPX host], and (3) relatively random variability (e.g., exsolution of spinel). Figure 1 (an OPX from Garland) provides evidence for smooth core-to-rim diffusion profiles. Iron and Mn increase while Mg and Cr decrease from core to rim. Also, Al remains relatively constant, Si decreases, and Ti shows an increase, near the grain edge, on the core-to-rim traverse. These chemical systematics were likely caused by a reaction between the cumulate OPX grain and trapped melt. The trapped melt was apparently enriched in Fe and Mn and depleted in Cr relative to OPX. In conclusion, postcrystallization resetting of cumulate grain chemical systematics must be carefully assessed before the data can be "inverted" to estimates of parental melt compositions. Acknowledgment: This research was supported by NASA grant NAGW-3347 and the Institute of Meteoritics. References: [1] Shearer C. K. et al. (1993) LPS XXIV, 1289-1290. [2] Lambert D. D. and Simmons E. C. (1987) Am. J. Sci., 287, 1-32. [3] Takeda H. et. al. (1979) Proc. 3rd Symp. Antarc. Met., 82-108. [4] Papike J. J. et. al. (1993) LPS XXIV, 1109-1110. Fig. 1 appears here in the hard copy.

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