Chemical and isotopic constraints on the formation and crystallization of SA-1, a basaltic Allende plagioclase-olivine inclusion

Abstract

SA-1, an unusual basaltic plagioclase-olivine inclusion (POI) in Allende, has concentric textural and mineralogic zones, a fine-grained, 100μm outer border and a coarse-grained interior with subophitic texture. Fassaite, diopside and olivine from the exterior border and interior of SA-1 have uniform intrinsic mass fractionation with isotopically heavy Mg (F_(Mg) = 3.6 ± 1.8‰/amu). In contrast, spinels from the spinel-rich regions adjacent to the fine-grained border have normal Mg isotopic composition (F_(Mg) = 0.1 ± 1.5‰/amu). The cores of large calcic (An_(90,99)) plagioclase have no excess 26Mg, corresponding to ²⁶Mg*/²⁷Al < 3.7 × 10⁻⁶. The Mg isotopic heterogeneity in SA-1 requires initial cooling rates of spinel-rich regions adjacent to the fine-grained border to be greater than ∼75 °C/hr. In contrast, the subophitic texture of the interior suggests cooling rates of 5–20 °C/ hr. The minerals in SA-1 exhibit a wide range of REE abundances. Lanthanum concentrations vary from 1 × chondritic (ch) in early crystallizing diopside to 100 × ch in late crystallizing fassaite. Nepheline has 18–20 × ch LREE and 11–25 × ch HREE and iron-rich mesostasis is highly enriched in the REE with 270–400 × ch LREE and 230–280 × ch HREE. The complementary REE patterns of clinopyroxene and plagioclase and the enrichment of incompatible trace elements in the mesostasis and late crystallizing phases is consistent with closed system crystallization. The REE data for nepheline and the iron-rich mesostasis indicate these phases are in equilibrium and that nepheline crystallized from a melt. Influx of alkalies, minor Fe and halogens must have occurred during the last stages of crystallization or the inclusion must have been partially molten during Na influx as both anorthite (An₉₉) and nepheline are present in this inclusion. The preservation of isotopic heterogeneity in an inclusion that crystallized from a melt implies that melting was incomplete, allowing for survival of the relict spinels. The major and trace element abundances in SA-1 are inconsistent with formation as a mixture of nebular materials and suggest that SA-1 contains a chemically fractionated component produced by igneous differentiation.

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