Evidence from polymict ureilite meteorites for a disrupted and re-accreted single ureilite parent asteroid gardened by several distinct impactors
Introduction
Ureilites are the second largest group of achondrites. They are ultramafic meteorites composed of mostly of olivine and pyroxene with lesser amounts of elemental carbon, sulfide and metal (Goodrich, 1992, Mittlefehldt et al., 1998). Their high carbon content, roughly 3 wt% on average, distinguishes ureilites from other achondrites. The carbon occurs mostly as graphite, but shock-produced diamond and lonsdaleite are present in many ureilites. We will refer to the assemblage of carbon minerals as “carbon phases”. The silicate mineral compositions of ureilites clearly indicate the loss of a basaltic component through igneous processing, yet the suite is very heterogeneous in mg# (molar 100 * (MgO/(MgO+FeO))). Another distinguishing characteristic of ureilites is their extremely heterogeneous oxygen isotopic compositions that are very similar to those of the carbonaceous chondrite anhydrous mineral (CCAM) line (Clayton and Mayeda, 1988, Clayton and Mayeda, 1996) which may reflect heterogeneity in their chondritic precursors. The mg# heterogeneity may have been inherited from the nebula (Clayton and Mayeda, 1988, Clayton and Mayeda, 1996) or may have resulted from combined igneous and redox processes acting on the parent asteroid (Walker and Grove, 1993, Singletary and Grove, 2003, Goodrich et al., 2007).
Despite numerous studies, the exact origin of ureilites remains unclear. It is generally accepted that typical ureilites are the basalt-depleted remains of partial melting of a chondritic precursor (Mittlefehldt et al., 1998). Some ureilites, particularly augite-bearing ureilites, may be partially of cumulate origin (Goodrich et al., 2001). The ∼4.56 Ga U–Pb age (Torigoye-Kita et al., 1995) and recent studies of short-lived chronometers (Goodrich et al., 2002, Kita et al., 2003, Lee et al., 2005) indicate that the parent asteroid of the ureilites differentiated very early in the history of the Solar System. Therefore, they contain important information about processes that formed small rocky planetesimals in the early Solar System.
Because of the compositional heterogeneity, it has not yet been firmly established whether ureilites were derived from a single parent asteroid or from multiple parents (see Warren et al., 2006). Indeed, the wide variation in mineral mg#s and oxygen isotope ratios could be readily explained by an origin in multiple compositionally similar parent asteroids that had experienced a similar evolution. If all ureilites are derived from a single parent asteroid, then it cannot have achieved isotopic and chemical homogenization, i.e., it did not experience a magma-ocean stage. On the other hand, if they are derived from numerous different asteroids with different Fe/Mg ratios and oxygen isotope compositions, then the processes that formed them must have been extremely common in at least one region of the early Solar System. In either case they form a crucial test of our understanding of the formation of achondritic planetesimals from chondritic precursors. This study attempts to investigate the origin of ureilites and determine whether there are multiple parent asteroids for ureilites or just one, by examining the compositions of minerals in polymict ureilites (i.e., regolith breccias from asteroidal surfaces) and comparing them to unbrecciated (also known as “monomict”) ureilites.1
There are currently 203 recognized ureilites (census through Meteoritical Bulletin, No. 91), which reduce to approximately 140 individual samples when pairing is taken into account. Most ureilites are unbrecciated. The cores of silicate minerals in each unbrecciated ureilite are homogeneous in cation composition. Thus, only a maximum of ∼140 data points are available from these rocks to constrain the composition of the ureilite parent asteroid(s). However, there are 17 known brecciated ureilites, most being polymict, although some of these are paired. Each thin-section of a polymict ureilite may contain hundreds of clasts of ureilitic material, and multiple thin-sections greatly increase the number of clasts available for analysis. Polymict ureilites also contain lithic clasts of material indigenous to the ureilite parent asteroid but not known as discrete meteorites. Therefore the potential for studying the parent asteroid(s) of ureilites is much greater if polymict samples are analyzed.
We have undertaken a detailed study of mineral compositions in polymict ureilite meteorites that provide information about the regolith of their parent asteroid(s). We have analyzed over 500 mineral or lithic clasts from six polymict ureilites. Our goals were to evaluate whether ureilites were derived from a single parent asteroid, to gain further understanding of the evolution of the parent asteroid, and to understand ureilite petrogenesis.
Section snippets
Electron microbeam techniques
The polymict ureilites were investigated petrographically by SEM, using back-scatter electron images. The electron microprobe analyses were done using the Cameca SX100 wavelength dispersive electron microprobe at NASA Johnson Space Center. An average was taken of three spots per grain. Analytical conditions were 20 kV, 40 nA, 1 μm beam for olivine and pyroxene. For olivine, counting times were 120 s for Ca, Cr and Mn, and 40 s for Mg, Si and Fe. For pyroxenes, counting times were 120 s for Mn, 100 s
Petrography of selected polymict ureilites
Samples investigated in this study include two from Elephant Moraine Antarctica (EET 83309, EET 87720), two from Australia (North Haig, Nilpena) and two from Libya (DaG 999, DaG 1000). The Elephant Moraine samples were obtained as interior chips. Dar al Gani ureilites were purchased as small slices. Two sections were investigated from each of EET 87720 (sections 13, 41), EET 83309 (sections 50, 51), North Haig (A, B) and Nilpena (A, B), and one section from each of the two DaG samples. The four
Olivine compositions
Olivine core compositions of 34 unbrecciated ureilites were previously analyzed at JSC (Hudon and Mittlefehldt, data in EA-2) using the analytical protocols used in this study. Almost all olivines from these samples fall on the previously established ureilite Fe/Mn vs. Fe/Mg trend of Goodrich et al. (2004) (see Fig. 1 of EA-2). Olivine core mg#s range from 76 (Graves Nunataks (GRA) 98032) to 95 (Allan Hills (ALH) 82130), thus covering the entire range of known ureilite compositions. Olivines
Discussion
Our petrographic observations, and electron microprobe and high precision SIMS oxygen isotope data on mineral and lithic clasts from polymict ureilites, coupled with literature data, allow us to examine several issues regarding the origin of ureilites. Here we will primarily address (i) whether there are distinct groupings of ureilites, (ii) whether the ureilite suite was derived from more than one parent asteroid, (iii) the types of impactors that gardened ureilites to produce the polymict
Conclusions
A petrological study of ureilitic silicate mineral clasts in six polymict ureilites has revealed that each polymict ureilite contains a wide range of olivine and pyroxene compositions, exactly covering the Fe–Mg–Mn compositional range seen among unbrecciated ureilites. The olivine mg# distribution in the studied polymict ureilites is statistically indistinguishable from that of the unbrecciated ureilite suite, indicating that they sample the same olivine population. High precision in situ SIMS
Acknowledgments
We thank the Meteorite Working Group for providing the EET samples, Alex Bevan for the sections of North Haig, and Cyrena Goodrich for loaning the sections of Nilpena and providing data for Fig. 3. Barbara Cohen kindly provided the data for minerals in melt clasts. DaG samples were purchased from Erich Haiderer. We thank Craig Schwandt and GeorgAnn Robinson for help with the SEM and electron microprobe, respectively. Reviews by Steve Singletary, Ed Scott and Yukio Ikeda were stimulating and
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