16:45 〜 17:00
[PPS10-30] A combined study of Be-B and Al-Mg systematics on CH and CH/CB CAIs
キーワード：宇宙線照射、原始太陽系円盤、難揮発性包有物、二次イオン質量分析計
Beryllium-10, which decays to 10B with a half-life of 1.4 Myr [1], is considered as a key indicator of irradiation processes in the Early Solar System (ESS). However, recent numerical studies [2, 3] have demonstrated that 10Be can be produced by stellar processes with neutrino reactions, which rendered reconsideration of the origin of 10Be in the ESS. In order to further understand the origin of 10Be, it is important to determine the accurate initial abundances of 10Be in several types of meteoritic components. Previous studies implied that CH and CB chondrites contain a high proportion of the outer solar system material based on their bulk Mg- and Cr-isotopic compositions and 15N-rich bulk compositions [e.g., 4, 5]. If this is correct, CH and CB CAIs may have information different from CAIs in other types of chondrites. In the present study, we have conducted Be-B and Al-Mg measurements on CH and CH/CB CAIs with newly determined Be/B relative sensitivity factors using synthetic glass standards.
We studied 8 CAIs from the Sayh al Uhaymir 290 (CH) and the Isheyevo (CB/CH) chondrites. Be-B and Al-Mg measurements were conducted with a NanoSIMS 50 at Atmosphere and Ocean Research Institute (AORI), The Univ. of Tokyo. Seven out of 8 CAIs show highly variable initial 10Be/9Be ratios ranging from 1.1 to 33 x 10-4. They cannot be explained by a molecular cloud origin [6, 7] or a stellar origin [3], suggesting that they have experienced solar cosmic ray irradiation near the proto-Sun. In contrast to Be-B systematics, all CAIs studied here do not show resolvable excesses in 26Mg. This could be attributed to: (1) heterogeneous distribution of 26Al in the protoplanetary disk, (2) formation prior to injection of 26Al, or (3) late formation after a significant decay of 26Al. (1) is unlikely because CH and CB/CH CAIs may have formed in the same region as that of CV CAIs (= near the proto-Sun) as inferred from the Be-B systematics [e.g., 8-12, this study]. (2) is possible because CH and CB/CH CAIs have highly refractory nature relative to CV canonical CAIs. (3) may be a simpler interpretation. If (3) is the case, the transportation mechanism from near the proto-Sun to the accretion region of CH and CB parent bodies must have existed at least until the timing of CH and CB/CH CAI formation.
[1] Korschinek G. et al. 2010. Nucl. Instrum. Methods Phys. Res. B. 268:187-191. [2] Yoshida T. et al. 2008. ApJ. 686: 448-466. [3] Banerjee P. et al. 2016. Nat. Commun. 10, 1038. [4] Van Kooten E. M. M. E. et al. 2016. PNAS. 113: 2011-2016. [5] Bonal L. et al. 2010. GCA. 74: 6590-6609. [6] Desch S. J. et al. 2004. ApJ: 602, 528-542. [7] Tatischeff V. et al. 2014. ApJ. 796,:124 (20pp). [8] McKeegan K. D. et al. 2000. Science. 289: 1334-1337. [9] Sugiura N. et al. 2001. MAPS. 36:1397-1408. [10] Chaussidon M. et al. 2006. GCA. 70:224-245. [11] Wielandt D. et al. 2012. ApJ. 748: L25 (7pp). [12] Gounelle M. et al. 2013 ApJ. 763: L33 (5pp).
We studied 8 CAIs from the Sayh al Uhaymir 290 (CH) and the Isheyevo (CB/CH) chondrites. Be-B and Al-Mg measurements were conducted with a NanoSIMS 50 at Atmosphere and Ocean Research Institute (AORI), The Univ. of Tokyo. Seven out of 8 CAIs show highly variable initial 10Be/9Be ratios ranging from 1.1 to 33 x 10-4. They cannot be explained by a molecular cloud origin [6, 7] or a stellar origin [3], suggesting that they have experienced solar cosmic ray irradiation near the proto-Sun. In contrast to Be-B systematics, all CAIs studied here do not show resolvable excesses in 26Mg. This could be attributed to: (1) heterogeneous distribution of 26Al in the protoplanetary disk, (2) formation prior to injection of 26Al, or (3) late formation after a significant decay of 26Al. (1) is unlikely because CH and CB/CH CAIs may have formed in the same region as that of CV CAIs (= near the proto-Sun) as inferred from the Be-B systematics [e.g., 8-12, this study]. (2) is possible because CH and CB/CH CAIs have highly refractory nature relative to CV canonical CAIs. (3) may be a simpler interpretation. If (3) is the case, the transportation mechanism from near the proto-Sun to the accretion region of CH and CB parent bodies must have existed at least until the timing of CH and CB/CH CAI formation.
[1] Korschinek G. et al. 2010. Nucl. Instrum. Methods Phys. Res. B. 268:187-191. [2] Yoshida T. et al. 2008. ApJ. 686: 448-466. [3] Banerjee P. et al. 2016. Nat. Commun. 10, 1038. [4] Van Kooten E. M. M. E. et al. 2016. PNAS. 113: 2011-2016. [5] Bonal L. et al. 2010. GCA. 74: 6590-6609. [6] Desch S. J. et al. 2004. ApJ: 602, 528-542. [7] Tatischeff V. et al. 2014. ApJ. 796,:124 (20pp). [8] McKeegan K. D. et al. 2000. Science. 289: 1334-1337. [9] Sugiura N. et al. 2001. MAPS. 36:1397-1408. [10] Chaussidon M. et al. 2006. GCA. 70:224-245. [11] Wielandt D. et al. 2012. ApJ. 748: L25 (7pp). [12] Gounelle M. et al. 2013 ApJ. 763: L33 (5pp).