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

Abstract

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.