Evidence from polymict ureilite meteorites for a disrupted and re-accreted single ureilite parent asteroid gardened by several distinct impactors

https://doi.org/10.1016/j.gca.2008.06.028Get rights and content

Abstract

Ureilites are ultramafic achondrites that exhibit heterogeneity in mg# and oxygen isotope ratios between different meteorites. Polymict ureilites represent near-surface material of the ureilite parent asteroid(s). Electron microprobe analyses of >500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites, suggesting derivation from a single parent asteroid. Many ureilitic clasts have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 unbrecciated ureilite (here termed the “Hughes cluster”). Some polymict samples also contain lithic clasts derived from oxidized impactors. The presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid.

In situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts, using a secondary ion mass spectrometer (SIMS) with precision typically better than 0.2–0.4‰ (2SD) for δ18O and δ17O. Oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the CCAM line, covering the range for unbrecciated ureilites, and show a good anti-correlation with mineral mg#. SIMS analysis identifies one ferroan lithic clast as an R-chondrite, while a second ferroan clast is unlike any known meteorite. An exotic enstatite grain is derived from an enstatite chondrite or aubrite, and another pyroxene grain with Δ17O of −0.4 ± 0.2‰ is unrelated to any known meteorite type.

Ureilitic olivine clasts with mg#s < 85 are much more common than those with mg# > 85 which include the melt-inclusion-bearing “Hughes cluster” ureilites. Thus melt was present in regions of the parent ureilite asteroid with a bulk mg# > 85 when the asteroid was disrupted by impact, giving rise to two types of ureilites: common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred. One or more daughter asteroids re-accreted from the remnants of the mantle of the proto-ureilite asteroid. Polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid, including impact-derived material from at least six different meteoritic sources.

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

References (69)

  • A.L. Jaques et al.

    The Nilpena ureilite, an unusual polymict breccia: implications for origin

    Geochim. Cosmochim. Acta

    (1982)
  • R.H. Jones et al.

    Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite

    Geochim. Cosmochim. Acta

    (2004)
  • N.T. Kita et al.

    Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

    Geochim. Cosmochim. Acta

    (2004)
  • A.N. Krot et al.

    Oxygen isotope compositions of chondrules in CR chondrites

    Geochim. Cosmochim. Acta

    (2006)
  • D.W. Mittlefehldt

    Fe–Mg–Mn relations of ureilite olivines and pyroxene and the genesis of ureilites

    Geochim. Cosmochim. Acta

    (1986)
  • V.K. Rai et al.

    Noble gases in ureilites: cosmogenic, radiogenic and trapped components

    Geochim. Cosmochim. Acta

    (2003)
  • S. Singletary et al.

    Experimental constraints on ureilite petrogenesis

    Geochim. Cosmochim. Acta

    (2006)
  • H. Takeda

    Mineralogy of Antarctic ureilites and a working hypothesis for their origin and evolution

    Earth Planet. Sci. Lett.

    (1987)
  • N. Torigoye-Kita et al.

    The 4.56 Ga U–Pb age of the MET 78008 ureilite

    Geochim. Cosmochim. Acta

    (1995)
  • W. Wahl

    Brecciated stony meteorites and meteorites containing foreign fragments

    Geochim. Cosmochim. Acta

    (1952)
  • P.H. Warren et al.

    Siderophile geochemistry of ureilites: a record of early stages of planetesimal core formation

    Geochim. Cosmochim. Acta

    (2006)
  • P. Warren et al.

    Explosive volcanism and the graphite–oxygen fugacity buffer on the parent asteroid(s) of the ureilite meteorites

    Icarus

    (1992)
  • M.K. Weisberg et al.

    The Carlisle Lakes-type chondrites: a new grouplet with high Δ17O and evidence for nebular oxidation

    Geochim. Cosmochim. Acta

    (1991)
  • J. Aléon et al.

    Calcium–aluminum-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

    Meteoritics Planet. Sci.

    (2002)
  • E. Asphaug et al.

    Hit-and-run planetary collisions

    Nature

    (2006)
  • G.K. Benedix et al.

    A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-winonaite parent body

    Meteoritics Planet. Sci.

    (2000)
  • J.L. Berkley

    Four Antarctic ureilites: petrology and observations on ureilite petrogenesis

    Meteoritics

    (1986)
  • A. Bischoff et al.

    Acfer 217—a new member of the Rumuruti chondrite group (R)

    Meteoritics

    (1994)
  • Brearley A. J. and Jones R. H. (1998) Chondritic meteorites. In Planetary Materials, (ed. J. J. Papike), Reviews in...
  • R.N. Clayton et al.

    Oxygen isotopic compositions of enstatite chondrites and aubrites

    Proc. Part 1, J. Geophys. Res.

    (1984)
  • H. Downes

    Shallow continental lithospheric mantle heterogeneity—petrological constraints

  • A.M. Fioretti et al.

    Primary melt inclusions in olivine, augite and orthopyroxene in ureilite FRO 90054

    Lunar Planet. Sci.

    (2000)
  • A.M. Fioretti et al.

    A contact between an olivine-pigeonite lithology and an olivine-augite-orthopyroxene lithology in ureilite FRO 93008: dashed hopes?

    Meteoritics Planet. Sci.

    (2001)
  • I.A. Franchi et al.

    Resolved sub-groups within the ureilite population

    Meteoritics Planet. Sci.

    (1997)
  • Cited by (0)

    View full text