Petrographic constraints on the formation of silica‐rich igneous rims around chondrules in CR chondrites

Abstract

In the CR (Renazzo‐like) chondrite group, many chondrules have successive igneous rim (IR) layers, with an outer layer that contains a silica mineral and/or silica‐rich glass (silica‐rich igneous rims, SIRs). Models for SIR formation include (1) accretion of Si‐rich dust onto solid chondrule surfaces, followed by heating and cooling and (2) condensation of SiO(gas) onto the surface of partially molten chondrules. We evaluate these models, based on a petrographic study of five Antarctic CR chondrites that have undergone minimal secondary alteration. We obtained electron microprobe analyses of minerals and glass with quantitative wavelength‐dispersive spectroscopy mapping, and identified silica polymorphs with Raman spectroscopy. Common SIRs contain silica, low‐Ca pyroxene, Ca‐rich pyroxene, Fe,Ni metal, ± glass ± plagioclase ± rare olivine. We also describe near‐monomineralic SIRs where a narrow zone of cristobalite occurs at the outer edge of the chondrule. All crystalline silica is cristobalite, except for one SIR that consists of tridymite. Some rims contain silica‐rich glass (>80 wt% SiO2) but no silica mineral. Features such as sharp interfaces and compositional boundaries between chondrules and SIRs indicate that SIRs were formed from solid precursors. Consideration of the stability fields of silica polymorphs and computed liquidus temperatures indicates that SIRs were heated to >1500°C for limited time periods, followed by rapid cooling, similar to conditions for chondrule formation. We infer that in the CR chondrule formation region, the same heating mechanism was repeated multiple times while the chemical composition of the nebular gas evolved to highly fractionated silica‐rich compositions.

Late Pebble Accretion of Comet 81P/Wild 2 Nucleus: Evidence from a Plagioclase-bearing Chondrule Fragment, Pyxie

“…The number in parentheses represents the number of fragments or chondrules included in the plot. Low-Ca pyroxene in the SIRs of CR chondrules (Krot et al 2004;Smith & Jones 2023) were plotted together with CR chondrules using a different color symbol.…”

“…Bulk composition of Pyxie. ARCs(Bischoff & Keil 1983;Sheng et al 1991;Krot & Rubin 1994;MacPherson & Huss 2005;Ebert & Bischoff 2016;Zhang et al 2020;Ebert et al 2022), IA/IAB/IB ferromagnesium chondrules(Jones & Scott 1989;Jones 1994;Tachibana et al 2003;Kita et al 2010), and SIRs of CR chondrules(Krot et al 2004;Smith & Jones 2023) are plotted for comparison. Iris, an iron-rich chondrulelike Wild 2 fragment, is also plotted(Gainsforth et al 2015).…”

“…Histograms of mean En (mol%) and Wo (mol%) contents in low-Ca pyroxene from Wild 2 fragments (SD), giant cluster (GC) anhydrous chondritic IDP, and individual primitive chondrite chondrules. Data sources: ordinary (O;Jones 1994Jones , 1996Huang et al 1996 ;Grossman et al 2002;Tachibana et al 2003;Pape et al 2019;Siron et al 2021Siron et al , 2022, enstatite (E;Grossman et al 1985;Weisberg et al 2011Weisberg et al , 2021, CV(Rubin & Wasson 1987;Jones & Schilk 2009;Rudraswami et al 2011;Hertwig et al 2018Hertwig et al , 2019a, CO(Rubin & Wasson 1988;Tenner et al 2013;Fukuda et al 2022), CM(Rubin & Wasson 1986;Brearley 1995;Chaumard et al 2018Chaumard et al , 2021, CA(Tenner et al 2017), CR(Krot et al 2004;Schrader et al 2014Schrader et al , 2015Schrader et al , 2017Tenner et al 2015;Smith & Jones 2023), and CH/CB chondrites(Krot et al 2007(Krot et al , 2010. The number in parentheses represents the number of fragments or chondrules included in the plot.…”