Introduction: Almahata Sitta (AhS) is the first meteorite to originate from a spectrally classified asteroid (2008 TC3) [1-3], and provides an unprecedented opportunity to correlate properties of meteorites with those of their par-ent asteroid. AhS is also unique because its fragments comprise a wide variety of meteorite types. Of ~140 stones studied to-date, ~70% are ureilites (carbon-rich ultramafic achondrites) and 30% are various types of chondrites [4,5]. None of these stones show contacts between ureilitic and chondritic lithologies. AhS is classified as an anoma-lous polymict ureilite. It has been inferred that 2008 TC3 was loosely aggregated and porous, so that it disintegrated in the atmosphere and only its most coherent clasts fell as individual stones [2,5]. Understanding the structure and composition of this asteroid is critical for missions aimed at sampling asteroid surfaces. We are studying [6] the University of Khartoum (UoK) collection of AhS stones [1] to test hypotheses for the nature of asteroid 2008 TC3. We report the discovery of an AhS sample that consists of both ureilitic and chondritic materials. Sample and Methods: We received ~0.8 g of UoK sample AhS 91A as multiple fragments of various sizes. So far we have studied 7 of these by optical microscopy, FE-SEM, EMPA, Raman, and X-ray tomography, and an 8th is being analyzed for oxygen isotopes. Cr isotopes have been analyzed in a 9th fragment and are reported in [7]. Results: The dominant lithology in AhS 91A is a fine-grained C2 carbonaceous chondrite consisting of phyl-losilicates (tentatively, serpentine and saponite), magnetite, carbonate (magnesite), fayalitic olivine, ilmenite, Ca-phosphate, pyrrhotite and pentlandite (Fig. 1a,b). As shown in multiple fragments (Fig. 1b,c), this lithology contains coarse-grained clasts of olivine (up to ~1.8 mm), pyroxenes (up to ~0.5 mm), and albite (~0.5 mm) having composi-tions and textures consistent with being ureilitic. The olivines have characteristic ureilite reduction rims/zones [8] and exsolved chromite+pyroxene symplectites [9]. At least two different core olivine compositions were observed in different fragments: Fo ~79-80 and Fo ~84, both with high CaO (0.3-0.4 wt% ) and Cr2O3 (~0.6-0.7 wt. %) typical of ureilites. Two different pyroxene fragments have compositions: mg# ~79, Wo 9 and mg#~85, Wo 9, typical for ureilitic pigeonite. The albite grain is Ab 96-99, consistent with plagioclase in the ureilitic "albitic lithology" in po-lymict ureilites [10,11] and the ureilitic andesite found in AhS [12,13]. One fragment of the C2 lithology also con-tains a coarse-grained clast of olivine, pyroxenes and feldspathic material whose affinity is uncertain - its mineral compositions are consistent with ureilites but its texture is not familiar. In addition, a CT scan of one fragment (not yet studied by other methods) shows a clast that appears to contain multiple chondrules, suggesting the possibility that an OC lithology is also present. Discussion: The grain sizes and compositions of the olivine, pyroxene and albite clasts in 91A are not consistent with their belonging to the C2 lithology, and strongly suggest that they are ureilitic. AhS 91A is the first observed occurrence of ureilitic and chondritic materials in the same AhS stone [1,4,5], but does not show a simple contact between two lithologies. Rather, it appears to be an intimately mixed breccia of C2 material (and OC?) with clasts from several different ureilitic materials. It could represent well-gardened regolith from the immediate parent aster-oid of 2008 TC3 [5]. Cr isotopes [7] indicate that the C2 lithology is a type of CC-like material not previously known. This sample shows that reflectance spectra of ureilitic asteroids could exhibit hydration features. References: [1] Jenniskens P. et al. 2010. Nature 12, 458-488. [2] Jenniskens P. et al. 2010. MAPS 45, 1590-1617. [3] Shaddad M. et al. 2010, MAPS 45, 1557-1589. [4] Horstmann M. and Bischoff A. 2014. Chemie der Erde 74, 149-183. [5] Goodrich C.A. et al. 2015. MAPS 50, 782-809. [6] Fioretti A.M. et al. 2017. LPSC 48, #1846. [7] Sanborn M. et al. 2017, this meeting. [8] Mit-tlefehldt
A BRECCIA OF UREILITIC AND C2 CARBONACEOUS CHONDRITE MATERIALS FROM ALMAHATA SITTA: IMPLICATIONS FOR THE REGOLITH OF UREILITIC ASTEROIDS
A M Fioretti;
2017
Abstract
Introduction: Almahata Sitta (AhS) is the first meteorite to originate from a spectrally classified asteroid (2008 TC3) [1-3], and provides an unprecedented opportunity to correlate properties of meteorites with those of their par-ent asteroid. AhS is also unique because its fragments comprise a wide variety of meteorite types. Of ~140 stones studied to-date, ~70% are ureilites (carbon-rich ultramafic achondrites) and 30% are various types of chondrites [4,5]. None of these stones show contacts between ureilitic and chondritic lithologies. AhS is classified as an anoma-lous polymict ureilite. It has been inferred that 2008 TC3 was loosely aggregated and porous, so that it disintegrated in the atmosphere and only its most coherent clasts fell as individual stones [2,5]. Understanding the structure and composition of this asteroid is critical for missions aimed at sampling asteroid surfaces. We are studying [6] the University of Khartoum (UoK) collection of AhS stones [1] to test hypotheses for the nature of asteroid 2008 TC3. We report the discovery of an AhS sample that consists of both ureilitic and chondritic materials. Sample and Methods: We received ~0.8 g of UoK sample AhS 91A as multiple fragments of various sizes. So far we have studied 7 of these by optical microscopy, FE-SEM, EMPA, Raman, and X-ray tomography, and an 8th is being analyzed for oxygen isotopes. Cr isotopes have been analyzed in a 9th fragment and are reported in [7]. Results: The dominant lithology in AhS 91A is a fine-grained C2 carbonaceous chondrite consisting of phyl-losilicates (tentatively, serpentine and saponite), magnetite, carbonate (magnesite), fayalitic olivine, ilmenite, Ca-phosphate, pyrrhotite and pentlandite (Fig. 1a,b). As shown in multiple fragments (Fig. 1b,c), this lithology contains coarse-grained clasts of olivine (up to ~1.8 mm), pyroxenes (up to ~0.5 mm), and albite (~0.5 mm) having composi-tions and textures consistent with being ureilitic. The olivines have characteristic ureilite reduction rims/zones [8] and exsolved chromite+pyroxene symplectites [9]. At least two different core olivine compositions were observed in different fragments: Fo ~79-80 and Fo ~84, both with high CaO (0.3-0.4 wt% ) and Cr2O3 (~0.6-0.7 wt. %) typical of ureilites. Two different pyroxene fragments have compositions: mg# ~79, Wo 9 and mg#~85, Wo 9, typical for ureilitic pigeonite. The albite grain is Ab 96-99, consistent with plagioclase in the ureilitic "albitic lithology" in po-lymict ureilites [10,11] and the ureilitic andesite found in AhS [12,13]. One fragment of the C2 lithology also con-tains a coarse-grained clast of olivine, pyroxenes and feldspathic material whose affinity is uncertain - its mineral compositions are consistent with ureilites but its texture is not familiar. In addition, a CT scan of one fragment (not yet studied by other methods) shows a clast that appears to contain multiple chondrules, suggesting the possibility that an OC lithology is also present. Discussion: The grain sizes and compositions of the olivine, pyroxene and albite clasts in 91A are not consistent with their belonging to the C2 lithology, and strongly suggest that they are ureilitic. AhS 91A is the first observed occurrence of ureilitic and chondritic materials in the same AhS stone [1,4,5], but does not show a simple contact between two lithologies. Rather, it appears to be an intimately mixed breccia of C2 material (and OC?) with clasts from several different ureilitic materials. It could represent well-gardened regolith from the immediate parent aster-oid of 2008 TC3 [5]. Cr isotopes [7] indicate that the C2 lithology is a type of CC-like material not previously known. This sample shows that reflectance spectra of ureilitic asteroids could exhibit hydration features. References: [1] Jenniskens P. et al. 2010. Nature 12, 458-488. [2] Jenniskens P. et al. 2010. MAPS 45, 1590-1617. [3] Shaddad M. et al. 2010, MAPS 45, 1557-1589. [4] Horstmann M. and Bischoff A. 2014. Chemie der Erde 74, 149-183. [5] Goodrich C.A. et al. 2015. MAPS 50, 782-809. [6] Fioretti A.M. et al. 2017. LPSC 48, #1846. [7] Sanborn M. et al. 2017, this meeting. [8] Mit-tlefehldtI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
