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Meteoritics & Planetary Science 41, Nr 8, Supplement, A13–A199 (2006) http://meteoritics.org Abstracts 5367 5372 CHARACTERIZATION OF ASTEROIDAL BASALTS THROUGH ONSET OF AQUEOUS ALTERATION IN PRIMITIVE CR REFLECTANCE SPECTROSCOPY AND IMPLICATIONS FOR THE CHONDRITES DAWN MISSION N. M. Abreu and A. J. Brearley. Department of Earth and Planetary Sciences, P. A. Abell 1, D. W. Mittlefehldt1, and M. J. Gaffey2. 1Astromaterials Research University of New Mexico, Albuquerque, New Mexico 87131, USA. E-mail: and Exploration Science, NASA Johnson Space Center, Houston, Texas [email protected] 77058, USA. 2Department of Space Studies, University of North Dakota, Grand Forks, North Dakota 58202, USA Introduction: Although some CR chondrites show evidence of significant aqueous alteration [1], our studies [2] have identified CR Introduction: There are currently five known groups of basaltic chondrites that exhibit only minimal degrees of aqueous alteration. These achondrites that represent material from distinct differentiated parent bodies. meteorites have the potential to provide insights into the earliest stages of These are the howardite-eucrite-diogenite (HED) clan, mesosiderite silicates, aqueous alteration and the characteristics of organic material that has not angrites, Ibitira, and Northwest Africa (NWA) 011 [1]. Spectroscopically, all been affected by aqueous alteration, i.e., contains a relatively pristine record these basaltic achondrite groups have absorption bands located near 1 and 2 of carbonaceous material present in nebular dust. The CR chondrites are of microns due to the presence of pyroxene. Some of these meteorite types have special significance in this regard, because they contain the most primitive spectra that are quite similar, but nevertheless have characteristics (e.g., carbonaceous material currently known [3]. spectral slope, band depths, etc.) that may be used to differentiate them from Results: Matrix and fine-grained rims in CR chondrites MET 00426 each other. and QUE 99177 were examined by TEM, focusing particularly on EFTEM Spectral Characteristics: Laboratory analysis of spectral features (energy filtered TEM) and EFTEM spectral imaging (EFTEMSI) to locate from meteorites and terrestrial samples has been used in the past to help make and characterize carbonaceous material in situ. Our new observations are connections to various bodies located among the asteroid population [2–4]. consistent with earlier HRTEM and EFTEM studies, but we have made a Based on similar techniques, analyses of various spectra of basaltic number of significant new observations. As previously reported [2], abundant achondrites have demonstrated clear distinctions between some of these amorphous silica-bearing material and µm to nm-sized, well-faceted Fe groups. In particular, the spectral signatures of such basaltic achondrites as sulfides containing variable amounts of Ni were identified in the matrix and Ibitira, NWA 011, and HEDs are quite different when various spectral fine-grained rims of both meteorites. In addition, carbonaceous material is parameters such as band area ratio (BAR) and band centers are identified and widespread in all regions and is often associated with sulfides. However, compared. Although band centers for Ibitira and NWA 011 are the same as isolated, rounded, nm-sized C-rich hotspots were also common. The electron those of basaltic eucrites, the former two achondrites have distinctly lower energy loss spectra (EELS) of this material are most consistent with BAR. On a diagram of band center versus BAR, Ibitira and NWA 011 plot amorphous C or carbonaceous phases. EFTEM spectral maps served to outside the field of basaltic eucrites. Stannern-trend basaltic eucrites are unequivocally establish a spatial relationship between C and N. The one compositionally distinct from the majority of eucrites [5], but seem to be example of a fine-grained rim studied to date in QUE 99177 is more complex indistinguishable from them on a band center versus BAR plot. than the matrices documented above. This rim contains abundant fine- Implications: The revitalized Dawn mission is now scheduled for grained (few unit cells) phyllosilicates and wispy and rounded Fe sulfides. launch sometime between June and August 2007 [6]. Its first target is the Rare olivines and low-Ca pyroxenes (cpx-opx intergrowths) were also asteroid Vesta, which is often thought to be the parent body of the HEDs observed. The low-Ca pyroxenes are embedded within a groundmass of [7, 8]. Given the various distinct terrains previously discovered across the nanocrystalline phyllosilicates, but show only very limited evidence of surface of Vesta [9, 10], preliminary results from this study suggest that alteration. Dawn should be able to obtain spectra of sufficient quality to constrain the Discussion: In general, our TEM observations of MET 00426 and QUE possible types of basaltic achondrites that exist on the surface of Vesta. Since 99177 show that aqueous alteration of matrix was extremely limited and no single petrogenesis scheme is capable of explaining the formation of all localized. However, the one fine-grained rim studied so far in QUE 99177, basaltic eucrites, the identification of one or more of these meteorites on appears to have experienced more extensive aqueous alteration with Vesta will provide an important constraint on eucrite petrogenesis. comparatively extensive development of phyllosilicates. Both these Acknowledgments: Various portions of this work were supported by meteorites are breccias, so this phenomenon could be the result of regolith the NASA Cosmochemistry and the NASA Planetary Geology and mixing of materials with variable degrees of alteration. Alternatively, it is Geophysics programs. possible that under fluid-limited conditions, at the onset of aqueous References: [1] Mittlefehldt D. W. 2005. Meteoritics & Planetary alteration, hydration occurs heterogeneously within the fine-grained material. Science 40:655–677. [2] Gaffey M. J. 1976. Journal of Geophysical Research Factors such as the local porosity and permeability of the fine-grained 81:905–920. [3] Cloutis E. M. et al. 1986. Journal of Geophysical Research material will be important factors in controlling the extent of aqueous 91:11,641–11,653. [4] Gaffey M. J. et al. 1993. Icarus 106:573–602. [5] alteration. Observations of minimal replacement of anhydrous phases Mittlefehldt D. W. and Lindstrom M. M. 2003. Geochimica et Cosmochimica supports the assertion that fluid alteration proceeded by replacement of Acta 67:1911–1935. [6] Russell C. T. et al. 2004. Planetary and Space metastable, amorphous material first, followed by replacement of crystalline Science 52:465–489. [7] McCord T. B. et al. 1970. Science 168:1445–1447. grains as alteration became more advanced [4]. [8] Drake M. J. 2001. Meteoritics & Planetary Science 36:501–513. [9] References: [1] Weisberg M. K. et al. 1995. Proceedings of the NIPR Gaffey M. J. 1997. Icarus 127:130–157. [10] Binzel R. P. 1997. Icarus 128: Symposium on Antarctic Meteorites 8:11–32. [2] Abreu N. M. and Brearley 95–103. A. J. 2006. Abstract #2395. 37th Lunar and Planetary Science Conference. [3] Ash R. D. et al. 1993. Meteoritics 28:318. [4] Nuth J. et al. 2005. In Chondrites and the protoplanetary disk. 675 p. A13 © The Meteoritical Society, 2006. Printed in USA. A14 69th Meeting of the Meteoritical Society: Abstracts 5125 5098 Pb-Pb SYSTEMATICS OF MARTIAN METEORITES AND THE THE REGOLITH PORTION OF THE LUNAR METEORITE SAYH DIFFERENTIATION HISTORY OF MARS AL UHAYMIR 169 F. Albarède1, A. Bouvier1, 2, J. Blichert-Toft1, and J. D. Vervoort3. 1Ecole A. Al-Kathiri1, 2, E. Gnos1, and B. A. Hofmann3. 1Institut für Geologie, Normale Supérieure, 69007 Lyon, France. E-mail: [email protected]. Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland. 2Directorate 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, General of Commerce and Industry, Ministry of Commerce and Industry, USA. 3Department of Geology, Washington State University, Pullman, Salalah, Oman. 3Naturhistorisches Museum der Burgergemeinde Bern, Washington 99164, USA Bernastrasse 15, CH-3005 Bern, Switzerland Our new Pb isotope analyses of basaltic shergottites, in combination Introduction: Sayh al Uhaymir (SaU) 169 is a complete, light gray- with literature data, suggest that their ancient ~4.1 Ga Pb-Pb isochron age greenish stone (70 × 43 × 40 mm) with a mass of 206.45 g found in the reflects the magmatic emplacement of these rocks [1]. This age, however, is Sultanate of Oman in January 2002. The rock consists of two contrasting in apparent conflict with much younger ages of ~180 Ma of U-Pb, Rb-Sr, Sm- lithologies. Approximately 87 vol% consists of a holocrystalline, fine- Nd, and Lu-Hf mineral isochrons. We have argued that these younger ages grained poikilitic polymict KREEP-rich impact melt breccia, the other 13 record the last disturbance of disseminated phosphates by acidic brines vol% is shock-lithified regolith [1]. percolating beneath the Martian surface. The counter suggestion that the old Discussion: The regolith shows two formation stages and contains the Pb-Pb age reflects contamination by terrestrial Pb [2] has been dismissed in following clasts: Ti-poor to Ti-rich basalts, gabbros to granulites, and regolith [1] and conflicts with two additional observations: a) shergottite Pb does not breccias. The younger regolith additionally contains a highland gabbronorite 207 lie between modern terrestrial (SKE) and 180 Myr old radiogenic Pb
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