Introduction: There is an increasing number of Cr-O-Ti isotope studies [1-6] that show solar system materials are divided into two main populations, one carbonaceous chondrite (CC)-like and the other is non-carbonecous (NC)-like, with minimal mixing attributed to a gap opened in the propoplanetary disk due to Jupiter's formation [5,7,8]. The Grand Tack model [9] suggests there should be large-scale mixing between S-, and C-type asteroids, an idea supported by our recent work on chondrule ?17O-?54Cr isotope systematics [10]. The Almahata Sitta (AhS) meteorite provides a unique opportunity to test the Grand Tack model. The meteorite fell to Earth in October 2008 and has been linked to the asteroid TC3 2008 which was discovered just prior to the fall of the AhS stones [11-13]. The AhS meteorite is composed of up to 700 individual pieces with ~140 of those pieces having some geochemical and/or petrologic studies [14]. Almahata Sitta is an anomalous polymict ureilite with other meteorite components, including enstatite, ordinary, and carbonaceous chondrites with an approximate abun-dance of 70% ureilites and 30% chondrites [14,15]. This observation has lead to the suggestion that TC3 2008 was a loosely aggregated rubble pile-like asteroid with the non-ureilite sample clasts within the rubble-pile (e.g., [14,15] and references therein). Due to the loosely-aggregated nature of AhS, the object disintegrated during atmospheric entry resulting in the weakly held clasts falling predominantly as individual stones in the AhS collection area. How-ever, recent work [16] has identified one sample of AhS, sample 91A, which may represent two different lithologies co-existing within a single stone. The predominate lithology type in 91A appears to be that of a C2 chondrite based on mineralogy but also contains olivine, pyroxene, and albite that have ureilite-like compositions. Previous Cr isotope investigations into AhS stones are sparse and what data is available show nearly uniform isotopic composition similar to that of typical ureilites with negative ?54Cr values [17,18]. Methods: The aliquot for Cr isotopic analysis (24.77 mg) was prepared from a small chip of material from Al-mahata Sitta 91A. Chromium was separated from the sample matrix using a three-column chromatography proce-dure [19]. The Cr isotopic composition was measured using a Thermo Triton Plus thermal ionization mass spec-trometer at the University of California, Davis. Results and Discussion: The Cr isotopic composition of AhS 91A shows clearly resolved excesses in both 53Cr and 54Cr with an ?53Cr = +0.09 ± 0.05 and ?54Cr = +1.83 ± 0.08. This is the highest ?54Cr value observed thus far in any CC-like reservoir. In fact it is the highest ?54Cr in any bulk planetary materials in hand. This ?54Cr is clearly re-solved from ureilites that have negative ?54Cr ~ -0.90 [18] as the extreme end member of the NC reservoir in terms of ?54Cr value. The difference indicates that the AhS 91A fragment originated from a Cr isotopic reservoir distinct from ureilties. The recent work studying the bulk mineralogy inferred that AhS 91A was likely comprised of mate-rial similar to C2 chondrites [16]. However, the ?54Cr of AhS 91A is higher than CI, CM, and CR chondrites and has a ?54Cr composition higher than any observed in any carbonaceous materials ([4,6] and references therein). This would indicate that the material that comprises AhS 91A is not typical CI-like material and instead is a carbona-ceous chondrite material that we have yet to sample in our collections as a separate meteorite. Oxygen isotopes are still pending, but even if oxygen isotopes were similar to CI, CM, or CR chondrites or another carbonaceous chon-drite group, the difference in ?54Cr would indicate a unique source. The observation of a different ?54Cr in AhS 91A compared to ureilites or the other AhS samples [17] has implications for large scale nebular wide mixing between NC and CC-like reservoirs, as suggested by the Grand Tack model [9]. This is supported by a rubble-pile like na-ture of the AhS asteroid. References: [1] Yin Q.-Z. et al. (2009) LPS XL, A2006. [2] Warren P. H. (2011) EPSL 311:93-100. [3] Sanborn M. E. et al. (2014) Goldschmidt Conf., A2171. [4] Sanborn M. E. et al. (2015) LPS XLVI, A2259. [5] Williams C. D. et al. (2016) Goldschmidt Conf., A3415. [6] Yin Q.-Z. et al. (2017) LPS XLVIII, A1771. [7] Morbidelli A. et al. (2015) Icarus 258:418-429. [8] Andrews S. M. et al. (2016) ApJ 820:L40. [9] Walsh K. J. et al. (2011) Nature 475:206-209. [10] Defouilloy C. (2016) Goldschmidt Conf., A629. [11] Jenniskins P. et al. (2009) Nature 458:485-488. [12] Jenniskins P. et al. (2010) Meteor. Planet. Sci. 45:1590-1617. [13] Shaddad M. H. et al. (2010) Meteor. Planet. Sci. 45:1557-1589. [14] Horstmann M. and Bischoff A. (2014) Chemie der Erde 74:149-183. [15] Goodrich C. A. et al. (2015) MAPS 50:782-809. [16] Goodrich C. A. (2017) 80th MetSoc, this meeting. [17] Qin L. et al. (2010) Meteor. Planet Sci. 45:1771-1777. [18] Yamakawa A. et al. (2010) Astrophysics J. 720:150-154. [19] Ya-makawa A. et al. (2009) Analytical Chem. 81:9787-9794.
A CASE FOR NEBULA SCALE MIXING BETWEEN NON-CARBONACEOUS AND CARBONACEOUS CHONDRITE RESERVOIRS: TESTING THE GRAND TACK MODEL WITH CHROMIUM ISOTOPIC COMPOSITION OF ALMAHATA SITTA STONE 91A
2017
Abstract
Introduction: There is an increasing number of Cr-O-Ti isotope studies [1-6] that show solar system materials are divided into two main populations, one carbonaceous chondrite (CC)-like and the other is non-carbonecous (NC)-like, with minimal mixing attributed to a gap opened in the propoplanetary disk due to Jupiter's formation [5,7,8]. The Grand Tack model [9] suggests there should be large-scale mixing between S-, and C-type asteroids, an idea supported by our recent work on chondrule ?17O-?54Cr isotope systematics [10]. The Almahata Sitta (AhS) meteorite provides a unique opportunity to test the Grand Tack model. The meteorite fell to Earth in October 2008 and has been linked to the asteroid TC3 2008 which was discovered just prior to the fall of the AhS stones [11-13]. The AhS meteorite is composed of up to 700 individual pieces with ~140 of those pieces having some geochemical and/or petrologic studies [14]. Almahata Sitta is an anomalous polymict ureilite with other meteorite components, including enstatite, ordinary, and carbonaceous chondrites with an approximate abun-dance of 70% ureilites and 30% chondrites [14,15]. This observation has lead to the suggestion that TC3 2008 was a loosely aggregated rubble pile-like asteroid with the non-ureilite sample clasts within the rubble-pile (e.g., [14,15] and references therein). Due to the loosely-aggregated nature of AhS, the object disintegrated during atmospheric entry resulting in the weakly held clasts falling predominantly as individual stones in the AhS collection area. How-ever, recent work [16] has identified one sample of AhS, sample 91A, which may represent two different lithologies co-existing within a single stone. The predominate lithology type in 91A appears to be that of a C2 chondrite based on mineralogy but also contains olivine, pyroxene, and albite that have ureilite-like compositions. Previous Cr isotope investigations into AhS stones are sparse and what data is available show nearly uniform isotopic composition similar to that of typical ureilites with negative ?54Cr values [17,18]. Methods: The aliquot for Cr isotopic analysis (24.77 mg) was prepared from a small chip of material from Al-mahata Sitta 91A. Chromium was separated from the sample matrix using a three-column chromatography proce-dure [19]. The Cr isotopic composition was measured using a Thermo Triton Plus thermal ionization mass spec-trometer at the University of California, Davis. Results and Discussion: The Cr isotopic composition of AhS 91A shows clearly resolved excesses in both 53Cr and 54Cr with an ?53Cr = +0.09 ± 0.05 and ?54Cr = +1.83 ± 0.08. This is the highest ?54Cr value observed thus far in any CC-like reservoir. In fact it is the highest ?54Cr in any bulk planetary materials in hand. This ?54Cr is clearly re-solved from ureilites that have negative ?54Cr ~ -0.90 [18] as the extreme end member of the NC reservoir in terms of ?54Cr value. The difference indicates that the AhS 91A fragment originated from a Cr isotopic reservoir distinct from ureilties. The recent work studying the bulk mineralogy inferred that AhS 91A was likely comprised of mate-rial similar to C2 chondrites [16]. However, the ?54Cr of AhS 91A is higher than CI, CM, and CR chondrites and has a ?54Cr composition higher than any observed in any carbonaceous materials ([4,6] and references therein). This would indicate that the material that comprises AhS 91A is not typical CI-like material and instead is a carbona-ceous chondrite material that we have yet to sample in our collections as a separate meteorite. Oxygen isotopes are still pending, but even if oxygen isotopes were similar to CI, CM, or CR chondrites or another carbonaceous chon-drite group, the difference in ?54Cr would indicate a unique source. The observation of a different ?54Cr in AhS 91A compared to ureilites or the other AhS samples [17] has implications for large scale nebular wide mixing between NC and CC-like reservoirs, as suggested by the Grand Tack model [9]. This is supported by a rubble-pile like na-ture of the AhS asteroid. References: [1] Yin Q.-Z. et al. (2009) LPS XL, A2006. [2] Warren P. H. (2011) EPSL 311:93-100. [3] Sanborn M. E. et al. (2014) Goldschmidt Conf., A2171. [4] Sanborn M. E. et al. (2015) LPS XLVI, A2259. [5] Williams C. D. et al. (2016) Goldschmidt Conf., A3415. [6] Yin Q.-Z. et al. (2017) LPS XLVIII, A1771. [7] Morbidelli A. et al. (2015) Icarus 258:418-429. [8] Andrews S. M. et al. (2016) ApJ 820:L40. [9] Walsh K. J. et al. (2011) Nature 475:206-209. [10] Defouilloy C. (2016) Goldschmidt Conf., A629. [11] Jenniskins P. et al. (2009) Nature 458:485-488. [12] Jenniskins P. et al. (2010) Meteor. Planet. Sci. 45:1590-1617. [13] Shaddad M. H. et al. (2010) Meteor. Planet. Sci. 45:1557-1589. [14] Horstmann M. and Bischoff A. (2014) Chemie der Erde 74:149-183. [15] Goodrich C. A. et al. (2015) MAPS 50:782-809. [16] Goodrich C. A. (2017) 80th MetSoc, this meeting. [17] Qin L. et al. (2010) Meteor. Planet Sci. 45:1771-1777. [18] Yamakawa A. et al. (2010) Astrophysics J. 720:150-154. [19] Ya-makawa A. et al. (2009) Analytical Chem. 81:9787-9794.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
