doi:10.1016/j.gca.2004.12.021 Hydrogen isotopic composition of water from fossil micrometeorites in howardites MATTHIEU GOUNELLE, 1,2,3, *CÉCILE ENGRAND, 1 OLIVIER ALARD, 4 PHILIP A. BLAND, 3,5 MICHAEL E. ZOLENSKY, 6 SARA S. RUSSELL, 3 and JEAN DUPRAT 1 1 CSNSM, Bâtiment 104, 91 405 Orsay Campus, France 2 Université Paris XI, Bâtiment 104, 91 405 Orsay Campus, France 3 Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK 4 Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 5 Department of Earth Science and Engineering, Exhibition Road, Imperial College, London SW7 2AZ, UK 6 KT, NASA Johnson Space Center, Houston, TX 77058, USA (Received June 23, 2004; accepted in revised form December 23, 2004) Abstract—We have measured the hydrogen isotopic composition (D/H ratios) of the water from 13 carbonaceous chondritic microclasts (CCMs, size 1 mm) trapped in two howardites (Kapoeta and Yamato- 793497) early in the evolution of Solar System. The division into tochilinite-rich; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich CCM types is corroborated by the hydrogen isotopic compositions. Both miner- alogy and hydrogen isotopic compositions demonstrate that tochilinite-rich CCMs represent CM2 chondritic matter. In contrast, there is no good match between the isotopic and mineralogical properties of the magnetite-rich CCMs and the known groups of carbonaceous chondrites, suggesting that magnetite-rich CCMs represent a new kind of chondritic matter, not yet sampled in meteorite collections. This demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete. The study of (micro)clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids. The average hydrogen isotopic composition of water belonging to CCMs, D/H = (152.0  4.8)  10 -6 (1 m ), is similar to that of Antarctic micrometeorites (AMMs), D/H = (161.2  3.8)  10 -6 (1 m ). The similarity, in terms of mineralogy and hydrogen isotopic composition, between CCMs and AMMs demon- strates that the composition of the micrometeorites has not been modified over the whole history of the Solar System. It indicates that the composition of the micrometeorite flux onto Earth has been, and is, dominated by a mixture of CM2-like; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich carbonaceous chondritic matter exemplified by CCMs found in howardites. Because CCMs have not suffered atmospheric entry, they provide an abundant source of pristine micrometeorites. The average D/H ratio of the whole population of CCMs is identical within errors to that of the Earth (149  3  10 -6 ). The match between the CCMs D/H ratio and that of the Earth is especially remarkable because 1) three different populations of CCMs are needed to make the D/H ratio of the Earth; 2) there is no single carbonaceous chondrite group for which a similar match exists. This observation suggests that CCMs population might be representative of the late veneer agent(s) that delivered water to the Earth. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Volatile-rich extraterrestrial materials provide a unique op- portunity for deciphering the origin of Solar System, the early evolution of asteroids, and the formation of the planets. Until the Stardust sample return mission brings back dust from the Wild 2 Jupiter family comet in 2006, the only volatile-rich extraterrestrial samples we have at hand are carbonaceous chondrites (e.g., McSween, 1979), cm-sized carbonaceous chondritic clasts within meteorites (e.g., Nakashima et al., 2003; Zolensky et al., 1992), micrometeorites collected in the polar ice caps (e.g., Maurette et al., 1991; Engrand and Mau- rette, 1998; Taylor et al., 1998; Nakamura, 1999; Duprat et al., 2003) and interplanetary dust particles (IDPs) in the strato- sphere (e.g., Zolensky et al., 1994; Rietmeijer, 1998). In addi- tion to these already well-characterized samples, a new source of volatile-rich extraterrestrial dust has been recently identified. Gounelle et al. (2003) have reported the finding of 71 sub- millimeter-sized carbonaceous chondritic microclasts (CCMs) in howardites. Howardites are achondritic meteorites members of the HED (howardites-eucrites-diogenites) clan, and believed to sample the regolith of a differentiated asteroid, possibly 4 Vesta (Binzel and Xu, 1993). Although mm- to cm-sized clasts in howardites have been known and studied for a long time (e.g., Wilkening, 1973; Zolensky et al., 1996b), submillimeter- sized clasts have been only recently identified and are very little studied. A mineralogical study has shown that CCMs are made of C2-like chondritic matter (Gounelle et al., 2003). These sam- ples are worth further study for a number of reasons. First, CCMs provide a new, abundant, source of carbonaceous, vol- atile-rich micrometeorites. Second, they might offer a view of the asteroid belt significantly different from that given by meteorites, and potentially less biased. Third, unlike polar micrometeorites, CCMs have not suffered alteration due to atmospheric entry (e.g., Toppani et al., 2001), so they are relatively pristine with regard to their bulk chemical composi- tion, mineralogy and isotopic composition. Fourth, CCMs have been shown to be fossil micrometeorites that were present in the inner Solar System very early in its history (Gounelle et al., 2003). As such, they can help us to probe the nature of Earth- * Author to whom correspondence should be addressed (gounelle@ csnsm.in2p3.fr). Geochimica et Cosmochimica Acta, Vol. 69, No. 13, pp. 3431–3443, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 3431