Formation conditions of plagioclase-bearing type I chondrules in CO chondrites: A study of natural samples and experimental analogs

Abstract

Chondrites are samples from undifferentiated asteroids that contain components that formed in the early solar system. One of the components found in chondrites is chondrules, which are small igneous spherules that formed from the melting of precursor dust ball assemblages during very short, high-temperature events in the solar nebula. Chondrules typically contain olivine and pyroxene, a glassy mesostasis, Fe-Ni metal, and sulfides. Type I chondrules contain FeO-poor and volatile-poor mineral assemblages. Approximately 10% of type I chondrules, including type IA, IAB, and IB textures, also contain igneous plagioclase. While dynamic cooling experiments have put constraints on the formation conditions of chondrules based on olivine and pyroxene textures and morphologies, plagioclase has not been produced in previous experimental chondrule analogs. In this study, we investigated common chondrule textures in CO chondrites and determined mineral and bulk compositions for plagioclase-bearing type I chondrules in CO chondrites. We also performed one-atmosphere dynamic cooling experiments in order to establish formation conditions for type I chondrules. We attempted to optimize vi viii conditions for plagioclase nucleation and growth by conducting experiments at slow cooling rates, low quench temperatures, in the presence of a Na-rich atmosphere, and with anorthite seeds present in the starting material. Experimental run products closely resemble textures of type I chondrules in ordinary and carbonaceous chondrites. Olivine is commonly poikilitically enclosed in euhedral low-Ca pyroxene. Ca-pyroxene appears as overgrowths on larger low-Ca pyroxene grains. Compositions of olivine, pyroxene, and mesostasis from experimental charges are also very similar to those observed in natural chondrules. Therefore, peak temperatures (1500-1600°C) and slow cooling rates (5-25°C/hr) used are plausible conditions for type I chondrule formation. These conditions are also predicted by the shock wave model for chondrule formation. While our experiments were conducted at conditions that we considered optimized for plagioclase crystallization, plagioclase was not observed in any experiment. Defining the conditions necessary for plagioclase nucleation may place important constraints on chondrule thermal histories. vii ix TABLE OF CONTENTS LIST OF FIGURES .

Terminal particle from Stardust track 130: Probable Al-rich chondrule fragment from comet Wild 2

“…9). These elevated MgO levels are comparable to the MgO contents reported from anorthites in AOAs (Komatsu et al, 2001;Weisberg et al, 2004;Krot et al, 2004b), Al-rich chondrules (MacPherson and Huss, 2005) and plagioclase-bearing type I FMG chondrules (Kurahashi et al, 2008;Ushikubo et al, 2012;Wick and Jones, 2012) from the lowest metamorphic grade chondrites (CR and type 3.0-3.1 OC and CC) (Fig. 9).…”

“…When compared to AOAs and chondrules, Bidi olivine MnO abundances are most similar to Al-rich and plagioclase-bearing type I FMG chondrules (Sheng, 1992;Kita et al, 2000;Krot et al, 2004b;MacPherson and Huss, 2005;Kurahashi et al, 2008;Ushikubo et al, 2012;Wick and Jones, 2012) whose average olivine MnO contents are <0.1 wt%. In contrast, Cr 2 O 3 abundances in Bidi olivine are relatively high with measured concentrations ranging up to 0.8 wt% (Fig.…”

“…High-Ca pyroxene compositions from AOAs (solid ellipse, Fig. 6) and type I plagioclase-bearing FMG chondrules (dotted ellipse) have more restricted ranges in Ca content but have more similar Fe/ Mg ratios when compared to Bidi pyroxenes (Komatsu et al, 2001;Aléon et al, 2002;Wick and Jones, 2012).…”

Relationships between type I and type II chondrules: Implications on chondrule formation processes

“…Comparison of chondrule petrologic textures and Fe-Mg profiles measured in type II chondrules with dynamic crystallization experiment analogues have shown that porphyritic textures and Fe-Mg igneous zonation could be duplicated by relatively slow cooling from their peak temperatures at rates ≈ 1 -10 2 K/h in a matter of several tens of hours Jones and Lofgren, 1993;Lofgren, 1996;Desch and Connolly, 2002;Hewins et al, 2005;Lauretta et al, 2006;Wick and Jones, 2012). Despite the fact that shock models explain satisfactorily such slow cooling of chondrules over tens of hours , it is worth noticing here the important caveat that such chondrule thermal histories were deduced from closed system experiments in which only kinetic effects (cooling rate) governed the crystal growth rate and the corresponding texture of the chondrule analogues.…”

Metasomatism in the Early Solar System: The Record from Chondritic Meteorites

“…Metasomatic alteration in CO chondrules involving Na-and Fe-bearing fluids is apparent in rare plagioclase-rich chondrules and plagioclase-bearing type I chondrules (Kurat and Kracher 1980;Palme and Wlotzka 1981;Ikeda 1982;Jones and Brearley 1994;Jones 1997;Tomeoka and Itoh 2004;Wick 2010). Kurat and Kracher (1980) first described plagioclase-rich chondrules in Lancé (CO3.4) as basaltic clasts, based on their mineralogical and textural characteristics.…”