Mesosiderite Clasts with the Most Extreme Positive Europium Anomalies Among Solar System Rocks

Abstract

Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the Earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.

Achondrites

“…However, differences in detail between most mesosiderite lithic clasts, and eucrites and diogenites, suggest that the later history of mesosiderites was different. Mittlefehldt (1979, Mittlefehldt et al (1992), and Rubin and Mittlefehldt (1992) argued that many of the mesosiderite mafic clasts were remelted after metal-silicate mixing. This process formed magnesian basalts and gabbros with low FeO/MnO, high-modal abundances of tridymite and/or whitlockite, and LREE-depleted patterns with high Eu/Sm.…”

Differentiation history of the mesosiderite parent body: constraints from trace elements and manganese-chromium isotope systematics in Vaca Muerta silicate clasts

“…These investigations have shown that although there are broad similarities in the petrographic and geochemical characteristics of mesosiderite silicates with those of the achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association, there are also systematic differences that make it improbable that these materials originated on the same parent body. There has been limited work done so far on characterizing the distributions of trace elements that are diagnostic indicators of igneous petrogenesis and postmagmatic processes (such as the rare earth elements) within minerals in silicate clasts (Crozaz et al, 1985;Kennedy et al, 1992;Mittlefehldt et al, 1992). These studies have shown that the mesosiderite silicate clasts are characterized by distinctive trace element characteristics unlike those seen in the HEDs, some of which are attributed to their primary petrogenesis and others to later metamorphism (Crozaz et al, 1985;Kennedy et al, 1992).…”

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

“…During this period, heating and melting events, which they attribute to impacts, caused metamorphism and extensive melting in some mesosiderites. In addition, basinforming impacts on the mesosiderite asteroid are thought to have melted the crust creating large melt sheets (Mittlefehldt et al, 1992).…”

“…In Vaca Muerta, a few clasts, which mostly have gabbroic textures, show extreme depletions of REEs with very high EdSm ratios, including the highest known EdSm ratios in solar system rocks. Mittlefehldt et al (1 992) and Rubin and Mittlefehldt ( 1992) argue that these REE patterns were not established by reactions between clasts and matrix or during weathering. They infer instead that the REE patterns result from complex igneous processing prior to the formation of mesosiderites.…”