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Constraints on the cooling rate from 16O-rich perovskite in a compact type A CAI from Allende

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Constraints on the cooling rate from 16O-rich perovskite in a compact type A CAI from Allende
Other Titles
아옌데 운석의 컴팩트 타입 A CAI에서 산출되는 16O가 부화된 페로브스카이트를 이용한 냉각속도 계산
Park, Changkun
Naoya, Sakamoto
Shigeyuki, Wakaki
Sachio, Kobayashi
Noriyuki, Kawasaki
Hisayoshi, Yurimoto
CTACooling rateperovskite
Issue Date
Park, Changkun, et al. 2019. Constraints on the cooling rate from 16O-rich perovskite in a compact type A CAI from Allende. The 82nd Annual Meeting of the Meteoritical Society. Hokkaido University, Sapporo, Japan. 2019.07.08~2019.07.12.
Introduction: Coarse-grained type A Ca-Al-rich inclusions (CAIs) in CV3 chondrites consist of mostly melilite and minor amount of spinel, perovskite, Al-Ti-diopside, and hibonite. Oxygen isotopes of the CAI minerals except perovskite have been extensively studied; those of spinel and hibonite are uniformly 16O-enriched (~24‰ in Δ17O), whereas those of melilite and Al-Ti-diopside are relatively 16O-depleted [1]. Only few studies reported 16O-depleted compositions for perovskite grains in compact type A CAIs from CV chondrites [2-4]. Several mechanisms have been proposed to explain O-isotopic variation among and within the individual minerals: 1) gas-melt interaction in the 16O-depleted nebula gas [e.g., 5, 6], 2) diffusive exchange between CAI minerals and 16O-poor nebular gas by multiple reheating events [7], postcrystallization O-isotope exchange during fluid-rock interaction on the parent body [8, 9]. Since perovskite is a common primary phase in most CAI varieties, further investigation on O-isotopic compositions for perovskite may shed a light on the O-isotope evolution of CAI minerals. Experimental: A compact type A CAI (ON01) from Allende was studied for mineral chemistry and oxygen iso-topes. Spot analyses and quantitative imaging for oxygen isotopes of perovskite and enclosing melilite carried out with the Hokudai isotope microscope system consisted of the Cameca ims-1270 and a stacked CMOS-type active pixel sensor (SCAPS) ion imager. The analytical procedure is described in the previous study [10]. Results and Discussion: Perovskite are small (generally <20 μm in diameter) and enclosed by large (mostly 300 500 μm) melilite. Quantitative O-isotope imaging and spot analyses on the individual perovskite inclusions clearly shows that O-isotopes become heavier from the crystal core towards rim. The O-isotopic compositions of perovskite vary from 22‰ to 1‰ in Δ17O, whereas those of enclosing melilite are homogeneously 16O-depleted (Δ17O≥5‰). The O-isotope dichotomy between perovskite and melilite can constrain their origin. If fluid-assisted thermal meta-morphism on the CV parent body resulted in almost complete O-isotope exchange of melilite with 16O-depleted fluid [9], O-isotopes of perovskite is also expected to have changed to 16O-depleted, based on its much smaller grain size and much faster O-isotope diffusivity relative to melilite [11-14]; thus it may not be the case in spite of the lack of experimental data on O-isotope diffusion under wet condition. If multiple reheating events in the 16O-depleted gase-ous reservoir [7] changed O-isotopes of entire melilite crystals, O-isotopic compositions of perovskite enclosed by the melilite should have also become 16O-depleted; thus it is also not the case. The most plausible scenario to ex-plain the 16O-enriched perovskite enclosed by 16O-depleted melilite is that the perovskite is relic as already men-tioned by previous study [15] and the melilite has crystallized from the melt that exchanged its O-isotopes with sur-rounding 16O-depleted gas. O-isotope diffusive exchange between melilite and relic perovskite likely resulted in O-isotope variation within the perovskite, and thus can constrain the cooling rate of the CAI. The estimated cooling rate of the CAI by numerical modeling is 1000 5000 K/hr at peak temperature of ~1600K, the melting tempera-ture of perovskite in compact type A CAI composition [15]. Such fast cooling rate of the CAI is comparable with <3000 K/hr estimated by the cooling experiment on perovskite reproducing (101) twins that is commonly observed in type A CAIs from CV chondrites [16, 17]. It is worthy to note that relatively slow cooling rate for type B CAI melts (0.5 50 K/hr [18]) mostly relied on the crystallization of euhedral anorthite.
Conference Name
The 82nd Annual Meeting of the Meteoritical Society
Conference Place
Hokkaido University, Sapporo, Japan
Conference Date
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