Meteoritics & Planetary Science 45, Nr 10–11, 1638–1656 (2010) doi: 10.1111/j.1945-5100.2010.01108.x
Asteroid 2008 TC3—Almahata Sitta: A spectacular breccia containing many different ureilitic and chondritic lithologies
Addi BISCHOFF1*, Marian HORSTMANN1, Andreas PACK2, Matthias LAUBENSTEIN3, and Siegfried HABERER4
1Institut fu¨ r Planetologie, Wilhelm-Klemm-Str. 10, 48149 Mu¨ nster, Germany 2Geowissenschaftliches Zentrum, Universita¨ tGo¨ ttingen, Goldschmidtstrasse 1, 37077 Go¨ ttingen, Germany 3Laboratori Nazionali del Gran Sasso—I.N.F.N., S.S.17 ⁄ bis, km 18+910, I-67010 Assergi (AQ), Italy 4Haberer-Meteorites, Ruhbankweg 15, 79111 Freiburg, Germany *Corresponding author. E-mail: [email protected] (Received 24 February 2010; revision accepted 11 August 2010)
Abstract–Asteroid 2008 TC3 impacted Earth in northern Sudan on October 7, 2008. The meteorite named Almahata Sitta was classified as a polymict ureilite. In this study, 40 small pieces from different fragments collected in the Almahata Sitta strewn field were investigated and a large number of different lithologies were found. Some of these fragments are ureilitic in origin, whereas others are clearly chondritic. As all are relatively fresh (W0–W0 ⁄ 1) and as short-lived cosmogenic radioisotopes were detected within two of the chondritic fragments, there is strong evidence that most, if not all belong to the Almahata Sitta meteorite fall. The fragments can roughly be subdivided into achondritic (ureilitic; 23 samples) and chondritic lithologies (17 samples). Among the ureilitic rocks are at least 10 different lithologies. A similar number of different chondritic lithologies also exist. Most chondritic fragments belong to at least seven different E-chondrite rock types (EH3, EL3 ⁄ 4, EL6, EL breccias, several different types of EL and EH impact melt rocks and impact melt breccias; some of the latter are shock-darkened). In addition, two H-group ordinary chondrite lithologies were identified, and one sample of a chondrite type that is so far unique. The latter has some affinities to R chondrites. Oxygen isotope compositions of 14 fragments provide further fundamental information on the lithological heterogeneity of the Almahata Sitta meteorite. Based on the findings presented in this study, the reflectance spectrum of asteroid 2008 TC3 has to be evaluated in a new light.
INTRODUCTION collected in the Almahata Sitta strewn field and found a large number of different lithologies. Some of these Asteroid 2008 TC3 was the first asteroid detected in fragments are ureilitic in origin, whereas others are space before impacting Earth and fell in the Nubian clearly chondritic. As all are relatively fresh (W0–W0 ⁄ 1; Desert of northern Sudan on October 7, 2008. Hundreds see the Samples and Analytical Techniques section), there of mostly small fragments were recovered. The meteorite are strong indications that most, if not all belong to the called Almahata Sitta was classified as a polymict ureilite Almahata Sitta meteorite fall. (Jenniskens et al. 2009) based on the study of only one In this study, we will present mineralogical and meteorite fragment from the strewn field. The analysis by oxygen isotope data for several Almahata Sitta meteorite these authors shows that this meteorite is ‘‘an achondrite, fragments and the results of a study of short-lived a polymict ureilite, anomalous in its class: ultra-fine- cosmogenic radioisotopes of two chondritic fragments. grained and porous, with large carbonaceous grains.’’ We Some preliminary data have been published by Bischoff have studied 40 small pieces from different fragments et al. (2010) and Horstmann and Bischoff (2010a).