Cai-like Particle among Antarctic Micrometeorites

Introduction: Antarctic micrometeorites (AMMs) are interplanetary dust particles collected in the Antarctic ice cap, and belonging to the size range 25–400 μm [1]. They might represent the bulk of extraterrestrial matter accreting on Earth today [2]. They bear strong resemblances with low petrographic type, hydrous carbonaceous chondrites [1]. Compared to IDPs they lack chondritic porous particles [3]. Antarctic micrometeorites might be of cometary or asteroidal origin, or a mixture thereof. Given the discovery of calcium-aluminium-rich (CAI) particles among the Stardust samples [4], we have characterized a CAI-like particle among the AMMs collection. Results: Micrometeorite 98-03-04 was collected at the Astrolabe glacier during the field season 1997–1998 [5]. After extraction under a binocular microscope, it was embedded in epoxy and polished. SEM examination has revealed an elongated, 100 μm across, compact particle with some holes, probably due to loss of grains. It is made of diopside (En53– 60Wo39–50), spinel and other Ca-Al-rich minerals too small to be analyzed with the electron microprobe, all set up in a phosphorus-rich chondritic matrix and surrounded by a partial magnetite rim. Pyrrhotite is present at the boundary of the Ca-Al-rich minerals and the chondritic matrix. In a previous polishing plan, olivine (Fo68–97) was also found. The bulk oxygen isotopic composition, measured at the CRPG Nancy with the CAMECA 1270 ion probe, and expressed relative to the Standard Mean Ocean Water, is δ17O = −36.6 ± 0.1 ‰, δ18O = −32.3 ± 0.4 ‰, i.e., ∆17O = −19.9 ± 0.5 ‰. Discussion: The compact nature and mineralogy of micrometeorite 9803-04 sets its apart from others CAI-like micrometeorites which are usually made of spinel (±perovskite) and iron-rich phyllosilicates [6, 7], or of isolated spinel and hibonite [8]. Though it lacks anorthite, it is reminiscent of the CAI particle found among Stardust samples. The oxygen isotopic composition of 98-03-04 is similar to other CAIs among Antarctic micrometeorites [6, 7, 9] or in carbonaceous chondrites [10], suggesting the existence of one common reservoir for all CAIs, possibly close to the Sun [11]. In this context, it will be of uttermost importance to measure the oxygen isotopic composition of CAIlike particles among Stardust samples. References: [1] Engrand C. and Maurette M. 1998. Meteoritics & Planetary Science 33:565–580. [2] Duprat J. et al. 2006. Abstract #5279. Meteoritics & Planetary Science. This issue. [3] Gounelle M. et al. 2005. Meteoritics & Planetary Science 40:917–932. [4] Zolensky M. E. et al. Abstract #1203. 37th Lunar and Planetary Science Conference. [5] Gounelle M. et al. 1999. Abstract. Meteoritics & Planetary Science 34:A46. [6] Hoppe P. et al. 1995. 26th Lunar and Planetary Science Conference. pp. 623–624. [7] Kurat G. et al. 1994. Abstract. Meteoritics & Planetary Science 29:487–488. [8] Gounelle M. 2000. Ph.D. thesis, Université Paris, Paris, France. [9] Engrand C. et al. 1999. Geochimica et Cosmochimica Acta 63:2623–2636. [10] Clayton R. N. 1993. Annual Review of Earth and Planetary Science 21: 115–149. [11] Shu F. H. et al. 2001. The Astrophysical Journal 548:1029– 1050. 5187 DEPTH-DEPENDENT FRACTIONATION OF LIGHT SOLAR WIND NOBLE GASES IN A GENESIS TARGET A. Grimberg1, F. Bühler2, P. Bochsler2, D. S. Burnett3, H. Baur1, and R. Wieler1. 1Isotope Geology, ETH Zürich, Switzerland. E-mail: [email protected]. 2Physikalisches Institut, University of Bern, Bern, Switzerland. 3GPS, CalTech, Pasadena, California, USA