Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias

Abstract

Abstract-Recent results of isotopic dating studies ( 182 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 calciumaluminum-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.

An igneous-textured clast in the Peace River meteorite: insights into accretion and metamorphism of asteroids in the early solar system

“…Xenolithic clasts, such as the one reported here, have been reported by Dodd and Jarosewich (1976), Chou et al (1981), Casanova et al (1987), Hutchison et al (1988), Sack et al (1994), and Sokol et al (2007). Of these, the clasts found in St. Mesmin (Dodd and Jarosewich 1976) and those found in Cabezo de Mayo (L-LL6) and Sevilla (LL4), first reported in abstract form by Casanova et al (1987), seem to bear the closest resemblance to the Peace River clast under consideration here.…”

“…Studies of the clasts provide insights into processes on the asteroid parent body, or, in the case of xenoliths, insights into processes from a different parent body and the nature and timing of the exchange of material between asteroids (e.g., Bischoff et al 2006Bischoff et al , 2010Sokol et al 2007;Zolensky and Ivanov 2003). For this reason the study of clasts that differ in texture and composition in chondrite meteorites such as those of Peace River are of interest, even if the meteorite is not classified as a breccia.…”

Evidence for K‐rich terranes on Vesta from impact spherules

“…For example, rare iron meteorites display some inclusions that contain K-feldspar or K-rich glasses (e.g., Mittlefehldt et al [1998]; Ruzicka et al [2006]). Four small "granitic" clasts have been found in some LL chondritic breccias (Bischoff et al [1993]; King Sokol et al [2007]). The other known extraterrestrial granites were collected on the Moon during the Apollo missions (Papike et al 1998).…”

Classification of stony meteorites and chondrules – the case of meteorite Jesenice

“…Starost hondritov je bila dolo~ena z absolutnimi datacijami z radioaktivnimi izotopi Sr, Pb in Nd na povpre~no 4.56 milijarde let, kar ustreza starosti Zemlje in astrofizikalnim ocenam starosti Sonca (SearS, 2004). Hondriti naj bi nastali s procesi akrecije (Sokol et al, 2007), aglomeracije, bre~izacije in kon~nega oblikovanja (angl. final assembly) (SearS, 2004).…”