Early generation of a refractory inclusions-enriched H-chondritic parent body: A safe harbor for Ca, Al-rich inclusions [Early generation of a refractory inclusions-enriched H- chondritic parent body: A safe harbor for Ca,Al-rich inclusions ]

Ebert, Samuel; Nagashima, Kazuhide; Krot, Alexander N.; Wakita, Shigeru; Barrat, Jean-Alix; Bischoff, Addi

Abstract

Calcium-aluminum-rich inclusions (CAIs) commonly observed in chondritic meteorites are the oldest dated solids formed in the Solar System. Short-lived isotope chronologies (26Al-26Mg, 182Hf-182W) suggest a ~2 Ma gap between the formation of CAIs and the accretion of the final chondrite parent bodies. One thin section, 3.27 cm2 in size, of an ordinary chondrite NWA 3358 (H3.1) studied contains 52 refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)) comprising 0.14% of its area, which is the highest abundance of refractory inclusions among non-carbonaceous chondrites containing on average ~ 0.009 area % of CAIs and AOAs. In combination with a low chondrule/matrix ratio of ~1.5, this makes NWA 3358 a unique ordinary chondrite. The aqueously-formed fayalites (Fa99) in NWA 3358 have the inferred initial 53Mn/55Mn ratio of (5.56±0.44)´10-6 which is the highest measured value for secondary minerals in chondrites and corresponds to the formation time of ~1.0-1.5 Ma after CAIs. Based on the 53Mn-53Cr chronology of fayalite formation and the thermal modeling, we infer that the first-generation of an H chondrite parent body, ~6-12 km in diameter, accreted within 1.0 Ma after formation of CAIs, filling the gap of ~2 Ma between CAIs and the earliest chondrite parent bodies. This early accretion provides a possible mechanism of CAIs/AOAs storage in the inner solar nebula and could explain the high amount of refractory inclusions in NWA 3358. A later destruction of these first-generation bodies may also explain the presence of CAIs and chondrules of different ages within later formed chondrite parent bodies.