Oxygen Isotopic Variations in a Fluffy Type a Cai from the Vigarano Meteorite

Introduction: Coarse-grained Ca-Al-rich inclusions (CAIs) are classified into Type A, B and C [1]. Type B and compact Type A CAIs are believed that they were formed from molten drops [2, 3]. On the other hand, fluffy Type A CAIs are believed that they were formed by gas-solid condensation, because of their extremely irregular shapes and reversely-zoned melilite crystals [4]. Recent ion microprobe analyses show that 16 O-rich and 16 O-poor melilite coexist in a Type B or compact Type A CAI directly indicating oxygen isotopic exchange during the formation [e.g. 5]. However, such observation has not been made in fluffy Type A CAIs. Distributions of oxygen isotopes in Type A CAIs have been characterized as: spinel, hibonite and pyrox-ene are 16 O-rich (δ 17, 18 O ~-40‰) while melilite is usually 16 O-poor (δ 17, 18 O ~ 0‰) [6, 7]. Here we report coexistence of 16 O-rich and 16 O-poor melilite in a fluffy Type A CAI from the Vigarano meteorite. The discovery indicates that multiple heating processes in the solar nebula are required to form fluffy Type A CAIs. Analytical Techniques: A coarse-grained CAI (V2-01) for a thin section of the Vigarano CV3 meteorite was studied by petrographic microscope and a scanning electron microscope (JEOL JSM-5310LV) equipped with energy dispersive X-ray spectrometer (Oxford LINK ISIS). Oxygen isotopes were measured in situ by secondary ion mass spectrometry using multi-collection mode of the TiTech CAMECA IMS-1270. A Cs + primary beam (20 keV, 0.6-2 nA) of 15-20 µm diameter was used. Both 16 O-and 18 O-were measured using off-axis Fara-day cups at mass resolving power (MRP) ~ 2000, while 17 O-was measured on the axial electron multiplier at MRP ~ 4500, sufficient to separate interfering 16 OH-. Measurements of background noises of Faraday cups were made and signals of 16 O-and 18 O-were corrected. In order to correct instrumental mass fractionation and matrix effect among minerals, five terrestrial minerals were used as standards for each mineral species: a synthetic gehlenite and åkermanite, a Russian spinel, a Miyake-Jima anorthite and a Takashima augite.