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ON THE RARITY OF NEBULAR MATERIALS WITH SOLAR OXYGEN ISOTOPES.

Introduction: NASA’s Genesis mission collected and returned with samples of solar wind to Earth. Isotopic measurements revealed that solar wind is O-rich compared to sampled planets (i.e., Earth, Mars) and asteroids. The Sun’s DO (=dO–0.52 ́dO) value was inferred to be –28.4±3.6‰ (2s) [1]. While many nebular materials preserved in chondritic meteorites are more O-rich than Earth (e.g., [2-6]), compositions as O-rich as the Sun are exceedingly rare in the meteorite record. For example, most Ca-, Al-rich inclusions (CAIs), the oldest dated materials that formed in the Solar System [7], appear to have formed with a DO of ~ –24‰ to –23‰ [2–5], suggesting that a resolved difference exists between the Sun and many of these early formed objects. Here, we report preliminary O, Ca, Ti, and Al-Mg systematics for a rare CAI (1-2-4) from Murchison with solar O isotopic composition. We also explore the reasons for the scarcity of such objects in the meteorite record. Methods: O, Al-Mg, and Ca-Ti isotopes were analyzed with the UH Mānoa ims-1280 with conditions similar to those reported in [8], [9], and [5,6,10], respectively. Results: Petrologic characteristics: Mineralogically, CAI 1-2-4 resembles spinel-hibonite inclusions known as SHIBs in CM chondrites [11]. This angular, friable aggregate is ~100 μm across and consists of spinel and hibonite crystals (Fig. 1). Picked from an acid residue of Murchison, voids could be primary or the result of dissolution of silicate minerals. The hibonite grains in this CAI are zoned in MgO and TiO2, as seen in many SHIBs [5], but usually not in PLAC-type hibonites [6].