Lunar and Planetary Science Conference 48, 1378. 201

Ishida H.

Lee J.I.

Nakamura T.

Nakashima D.

Park C.

Sakamoto N.

Yoshizaki T.

学術論文 (Article)Lunar and Planetary Science Conference 48, 1378. 2017journal articl

Yoshizaki, T.

Nakashima, D.

Nakamura, T.

Park, C.

Sakamoto, N.

Ishida, H.

Lee, J.I.

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Oxygen isotopic heterogeneity in anultrarefractory phase bearing CAI from areduced type CV3 chondrite RBT 04143著者 Yoshizaki T., Nakashima D., Nakamura T., ParkC., Sakamoto N., Ishida H., Lee J.I.year 2017URL http://hdl.handle.net/10097/00122396OXYGEN ISOTOPIC HETEROGENEITY IN AN ULTRAREFRACTORY PHASE BEARING CAI FROM A REDUCED TYPE CV3 CHONDRITE RBT 04143. T. Yoshizaki1, D. Nakashima1, T. Nakamura1, C. Park2, N. Sakamoto3, H. Ishida1, and J. I. Lee2, 1Division of Earth and Planetary Materials Science, Tohoku University, Sendai, Miyagi 980-8578, Japan (tacasy22@dc.tohoku.ac.jp); 2Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea; 3Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo 001-0021, Japan.   Introduction: Calcium-aluminum-rich inclusions (CAIs) are thought to be the oldest solids [e.g., 1] and the key to understanding of physicochemical conditions in the early Solar System. Ultrarefractory (UR) miner-als enriched in Sc, Zr, and/or Y rarely occur both in the melted and unmelted CAIs, thought to have formed at higher temperature compared to normal CAI minerals (e.g., melilite, spinel and Al-Ti-diopside).  Therefore, UR phases can potentially provide constraints on high temperature processes occurred in the nebula gas. However, it has not been well constrained whether UR minerals formed from a gas or a melt [e.g., 2, 3]. In addition, the origin of heterogeneous oxygen isotopic composition observed in some UR-phase-rich inclu-sions [e.g., 3, 4] is poorly understood. Here we report petrology, mineral chemistry and oxygen isotopic compositions of the ultrarefractory phase bearing CAI R3C-01 from Roberts Massif (RBT) 04143 (CVred; [5]).  Sample and Analytical Methods: Petrology and mineralogy of the CAI R3C-01 in a thin section of RBT 04143 was studied using FE-SEM at Tohoku University (TU). Quantitative X-ray microanalyses of CAI minerals and X-ray elemental mapping were per-formed at TU and Korea Polar Research Institute with FE-EPMAs using WDS detectors. Grain boundaries of minerals were determined by crystal orientation map-ping with EBSD system equipped with the TU FE-SEM. Oxygen isotopic compositions of individual minerals and quantitative oxygen isotope images (isotopographs) in R3C-01 were obtained with the iso-tope microscope system at Hokkaido University, con-sisting of the Cameca ims-1270 and SCAPS ion imager. Analytical conditions are similar to those in [6]. Results: Ultrarefractory phase bearing CAI R3C-01 is an irregular-shaped, compound inclusion that consists of five lithological units. All units consist of melilite, spinel, perovskite and Al-Ti-rich pyroxene with various modal abundances. Outer margin of the inclusion is surrounded by Wark-Lovering (WL) rim and forsteritic (mostly Fa<1) olivine rim. Some olivine grains from the outermost part of the rim have low-iron, manganese-enriched (LIME) compositions. R3C-01-u1 is the largest unit of R3C-01, consisting of fine (< 50 µm) grains of melilite, spinel, minor perovskite, Al-Ti-rich pyroxene (Figs. 1 and 2), and refractory metal nuggets (not shown). Anorthite replacing melilite as thin layers (< 10 µm) at some boundaries between WL rim and the outer margin of R3C-01-u1 is the only sec-ondary phase in this inclusion. Other secondary phases such as nepheline, sodalite and hydrous silicates are not observed. Some spinel grains form framboids en-closing melilite (Fig. 1). Chemical compositions of spinel and perovskite in R3C-01-u1 are similar to those in normal refractory inclusions [e.g., 7]. Reversely-zoned, Åk-rich melilite (typically Åk15-20 in the core and Åk5 at the rim) and unzoned Åk-poor melilite (typ-ically Åk5) are heterogeneously distributed in the inclu-sion (Fig. 2). Most Al-Ti-rich pyroxenes in R3C-01-u1 are adjacent to perovskite and/or spinel. They are Sc-poor, Al-Ti-rich diopside (grossmanite (CaTi3+AlSiO6) [8] and kushiroite (CaAlAlSiO6) [9]) with 0.1-0.5 wt% Sc2O3 and 0.2-1.4 wt% V2O3. On the other hand, some Al-Ti-rich pyroxene grains isolated in reversely-zoned melilite are Ti-V-rich davisite (CaScAlSiO6) [10], highly enriched in Sc and V (6.2-8.9 wt% Sc2O3 and 2.2-3.7 wt% V2O3). Ti-V-rich davisite were not ob-served in other units of R3C-01. Oxygen isotopographs of R3C-01-u1 minerals show that the CAI is mostly composed of two isotopi-cally distinct regions: 16O-poor (−20 ≤ δ18O ≤ 0‰) region that consists of reversely-zoned melilite (Åk15-20 at core to Åk5 at rim) and Ti-V-rich davisite; and 16O-rich (−50 ≤ δ18O ≤ −40‰) region that consists of unzoned, gehlenitic melilite (Åk5), Al-Ti-rich diopside and spinel (Fig. 2). The δ18O values change sharply at the grain boundaries between 16O-rich and 16O-poor melilites. These two chemically and isotopically dis-tinct regions distribute randomly in this inclusion.  Discussion: Heterogeneous distribution of oxygen isotopic compositions have been often observed in coarse-grained CAIs from CV3 chondrites and FGIs from higher petrologic type chondrites [11; references therein]. Such oxygen isotopic heterogeneities are con-sidered to be resulted from isotope exchange with 16O-poor gas during nebular reheating processes and/or with 16O-poor fluid during fluid-assisted thermal met-amorphism on the chondrite parent bodies [e.g., 11]. Sharp changes of δ18O values at grain boundaries (Fig. 2) suggest that this inclusion have never experi-enced partial melting that could result in oxygen iso-tope variation within a single crystal [12]. Presence of 1378.pdfLunar and Planetary Science XLVIII (2017)reversely zoned melilite, that cannot be explained by crystallization from a melt [13], is also inconsistent with partial melting. This inclusion may have escaped from parent body alteration, given (1) the lack of alter-ation products including alkali- and/or iron-bearing phases [e.g., 14] and (2) presence of LIME olivine, which is easily lost during parent body metamorphism [15]. In addition, oxygen isotope compositions of meli-lite do not correlate with the distance from the outer margin of the inclusion as reported in some CV CGIs [16, 17]. The random distribution of 16O-rich regions and 16O-poor regions and abrupt changes of δ18O val-ues at the grain boundaries in the CAI are inconsistent with gas-solid exchange and subsequent diffusion of oxygen isotopes during nebular reheating events [16, 1 Fig. 1. Combined elemental map in Mg (red), Ca (green), Al (blue) and Ti (pink) Kα X-rays image of R3C-01-u1. The analysis areas for isotopography (Fig. 2) are also shown by white circles.  7]. Therefore, it is suggested that oxygen isotopic com-positions of minerals in the CAI have never been modi-fied by these processes. We argue that isotopically and mineralogically dis-tinct two regions in R3C-01-u1 have condensed sepa-rately from different gaseous reservoirs, and subse-quently aggregated to form the inclusion. Occurrence of 16O-poor Ti-V-rich davisite in 16O-poor reversely-zoned melilite suggests that Ti-V-rich davisite con-densed prior to condensation of reversely-zoned meli-lite from 16O-poor nebular gas. Alternatively, Ti-V-rich davisite could be relict that originated from an UR or normal CAI [18]. However, lack of other possible 16O-depleted relict grains in R3C-01-u1 and absence of davisite with similar chemical composition in previous-ly-studied inclusions might be inconsistent with this scenario. In any cases, it is suggested that spatially and/or temporally distinct 16O-rich and 16O-poor gase-ous reservoir may have existed in the period of CAIs and UR phase formation. References: [1] Connelly J.N. et al. (2012) Science, 338, 651-655. [2] El Goresy A. et al. (2002) GCA, 66, 1459-1491. [3] Ivanova M.A. et al. (2012) MAPS, 47, 2107-2127. [4] Aléon J. et al. (2010) MAPS, 73, Ab-stract#5185. [5] Ishida et al. (2012) Polar Sci., 6, 252-262. [6] Park C. et al. (2012) MAPS, 47, 2070-2083. [7] Brearley A.J. and Jones R.H. (1998), Rev. Mineral. Geochem., 36, 3.1-3.398. [8] Ma C. and Rossman G.R. (2009) AM, 94, 1491-1494. [9] Kimura M. et al. (2009) AM, 94, 1479-1482. [10] Ma C. and Rossman G.R. (2009) AM, 94, 845-848. [11] Yurimoto H. et al. (2008) Rev. Mineral. Geochem., 68, 141-186. [12] Yurimoto H. et al. (1998) Science, 282, 1874-1877. [13] MacPherson G.J. and Grossman L. (1984) GCA, 48, 29-46. [14] Krot A.N. et al. (1998) MAPS, 33, 1065-1085. [15] Komatsu M. et al. (2015) MAPS, 50, 1271-1294. [16] Simon J.I. et al. (2011) Science, 331, 1175-1178. [17] Simon J.I. et al. (2016) GCA, 186, 242-276. [18] Lin Y. et al. (2003) MAPS, 38, 407-417.   Fig. 2. BSE image, EBSD crystal orientation map of melilite, Åk con-tent map of melilite and δ18O isotopograph. Dav = Ti-V-rich da-visite; di = Al-Ti-rich diopside; mel = melilite; sp = spinel. 1378.pdfLunar and Planetary Science XLVIII (2017)

Oxygen isotopic heterogeneity in an ultrarefractory phase bearing CAI from a reduced type CV3 chondrite RBT 04143

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=124983&item_no=1&attribute_id=18&file_no=1

Oxygen isotopic heterogeneity in an ultrarefractory phase bearing CAI from a reduced type CV3 chondrite RBT 04143

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ