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Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias
Author(s) -
Sokol Anna K.,
Bischoff Addi,
Marhas Kuljeet K.,
Mezger Klaus,
Zinner Emst
Publication year - 2007
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2007.tb00575.x
Subject(s) - chondrite , geology , geochemistry , igneous rock , parent body , chondrule , breccia , achondrite , radiogenic nuclide , enstatite , meteorite , astrobiology , mantle (geology) , physics
— Recent results of isotopic dating studies ( 182 Hf‐ 182 W, 26 Al‐ 26 Mg) and the increasing number of observed igneous and metamorphosed fragments in (primitive) chondrites provide strong evidence that accretion of differentiated planetesimals predates that of primitive chondrite parent bodies. The primitive chondrites Adrar 003 and Acfer 094 contain some unusual fragments that seem to have undergone recrystallization. Magnesium isotope analyses reveal no detectable radiogenic 26 Mg in any of the studied fragments. The possibility that evidence for 26 Al was destroyed by parent body metamorphism after formation is not likely because several other constituents of these chondrites do not show any metamorphic features. Since final accretion of a planetesimal must have occurred after formation of its youngest components, formation of these parent bodies must thus have been relatively late (i.e., after most 26 Al had decayed). Al‐Mg isotope data for some igneous‐textured clasts (granitoids and andesitic fragments) within the two chondrite regolith breccias Adzhi‐Bogdo and Study Butte reveal also no evidence for radiogenic 26 Mg. As calculated from the upper limits, the formation of these igneous clasts, the incorporation into the parent body regolith, and the lithification must have occurred at least 3.8 Myr (andesite in Study Butte) and 4.7 Myr (granitoids in Adzhi‐Bogdo) after calcium‐aluminum‐rich inclusions (CAI) formation. The absence of 26 Mg excess in the igneous inclusions does not exclude 26 Al from being a heat source for planetary melting. In large, early formed planetesimals, cooling below the closure temperature of the Al‐Mg system may be too late for any evidence for live 26 Al (in the form of 26 Mg excess) to be preserved. Thus, growing evidence exists that chondritic meteorites represent the products of a complex, multi‐stage history of accretion, parent body modification, disruption and re‐accretion.