Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.589t&link_type=abstract
Meteoritics, vol. 30, no. 5, page 589
Mathematics
Logic
Element Partitioning, Elements, Trace, Eucrites, Magma Compositions: Retrieval, Metamorphism, Meteorites, Differentiated
Scientific paper
Compositions of extraterrestrial magmas are commonly derived from mineral compositions using, using experimentally determined mineral/basalt partition coefficients, Dmineral/basalt [1]. However, Dmineral/basalts cannot be applied to minerals which have experienced post-magmatic (subsolidus or metamorphic) chemical equilibration [2]. A failure to recognize post-magmatic equilibration can lead to wildly erroneous estimates of magma compositions and unrealistic scenarios of magmatic and planetary evolution [3]. To judge the effects of subsolidus chemical equilibration, consider REE distributions in a eucrite basalt, formed from a magma with CREE = 10 x CI. Let this magma crystallize and chemically equilibrate just below its solidus to a rock consisting of 49.5% plagioclase, 49.5% pigeonite, 0.1% whitlockite (a Ca phosphate), and 0.9% minor phases no REE content (silica, Fe metal, troilite); exact proportions are not critical. The total REE content ofthe rock is unchanged at 10 x CI, and distributions of REE among its minerals can be calculated from solidus-temperature Ds, e.g., Dpigeonite/plagioclase = Dpigeonite/basalt / Dplagioclase/basalt (where Dmineral/basalts are chosen to reflect the same magma compositions and temperature). REE abundances in minerals of this equilibrated rock (Figure 1 [5]) are significantly higher than they would be in the presence of magma. For instance, if this eucrite basalt system consisted of 50% magma, 25% pigeonite, and 25% plagioclase, one calculates C(La)Pigeonite = 0 04 x CI and C(La)Plagioclase = 0.8 x CI; with no magma present (Figure 1), C(La)Pigeonite = 0.4 x CI and CLaPIagioclase = 9 x CI! In the absence of magma, the incompatible REE must go somewhere!! If a mineral grain from this rock were used with Dmineral/basalts to derive a magma composition, that "Hparent basalt" would be rich in REE (130-200 x CI), enrichmed in light REE (La/Lu = 1.6 x CI), and strongly depleted in Eu. Compare this to the original eucrite, with REE at 10 x CI, no LREE enrichment, and no Eu depletion (Figure 1). The petrogenesis and planetologic implications of this "parent magma" must be fundamentally different from those of the original simple basalt. If the rock equilibrated at a temperature far below its solidus, each mineral species would appear to have had a different parent magma (because of the various temperature dependencies of the Dmineral/mineral(s)). In the above example, using deep-subsolidus Dmineral/mineral [6,7,8] and Dmineral/basalt, one would infer that each mineral had a distinct parent magma, with REE abundances in the order [whitlockite magma] ~? [augite magma] > [opx magma] > [plag magma]. This order looks like, but is not, the order of igneous crystallization. Figure 1: Calculated REE abundances in minerals of a eucrite basalt (CREE = 10 X CI), chemically equilibrated without magma present just below its solidus temperature. Also shown are calculated REE abundances in the "parent magma" that would be inferred by assuming that the mineral compositions reflected equilibration with a magma. References: [1] Jones J. H. (1995) ch. 7, Handbook of Physical Constants, Vol. 3, Chapter 7, AGU. [2] Barnes S. J. (1986) Contrib. Mineral. Petrol., 93, 524-531. [3] Consolmagno G. J. and Drake M. J. (1977) GCA, 41, 1271-1282. [4] Treiman A. H., GCA, submitted. [6] Floss C. (1991) Ph.D. thesis, Washington University, St. Louis, Missouri. [7] Pun A. and Papike J. J. (1994) LPS XXV, 1111-1112. [8] Stosch H.-G. (1982) GCA, 46, 793-811.
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