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GENESIS OF THE CUMULATE EUCRITES SERRA DE MAGÉ AND MOORE COUNTY: A GEOCHEMICAL STUDY
Author(s) -
Ma M.S.,
Schmitt R.A.
Publication year - 1979
Publication title -
meteoritics
Language(s) - English
Resource type - Journals
eISSN - 1945-5100
pISSN - 0026-1114
DOI - 10.1111/j.1945-5100.1979.tb00481.x
Subject(s) - plagioclase , pigeonite , fractional crystallization (geology) , geochemistry , geology , partial melting , mineralogy , basalt , quartz , augite , paleontology
Major, minor and trace element abundances have been determined by instrumental neutron activation analysis (INAA) in whole rock and plagioclase separates of Serra de Magé (SdM). The whole rock contains 52% normative plagioclase and its chondritic normalized REE abundance pattern shows a large Eu anomaly, dominated by the plagioclase REE distribution, and nearly unfractionated La‐Sm and Sm‐Lu abundances. The plagioclase separates contained ∼ 6% pyroxenes and exhibited a typical plagioclase REE distribution. The REE abundances in the derivative equilibrium magmas from which SdM and Moore County (MC) plagioclases crystallized have been estimated from the plagioclase data and the plagioclase mineral/liquid partition coefficients. The REE distributions in possibly earlier parental magmas were calculated by assuming that various degrees of plagioclase and pigeonite (plagioclase/pigeonite = 1) fractional crystallization had been operative prior to the crystallization of SdM and MC. The calculated La/Sm and Sm/Yb ratios for the earlier magmas are essentially the same as the equilibrium magmas over a wide range (10–95%) of the assumed fractional crystallization. Considering the REE distributions and the Fe/Fe+Mg ratios, calculation shows that there is no simple genetic relationship between MC and SdM via fractional crystallization processes. A hypothesis for the derivation of these cumulate eucrites in the plutonic environment from residual diogenitic liquid, which was produced by the extensive partial melting of an eucritic source material followed by the crystallization of diogenite, also fails to account for the fractionated REE patterns calculated for the equilibrium and the possible parental magmas for either SdM or MC. Equilibrium non‐modal partial melting calculations indicate that SdM and MC could be genetically related by a factor ∼ 6 difference in the degrees of partial melting from a similar source material. However, this common source material which should contain > 30% high‐Ca clinopyroxene and has a chondritic normalized La/Yb ∼ 3, is different than that proposed for the non‐cumulate eucrites.