82nd Annual Meeting of The Meteoritical Society 2019 (LPI Contrib. No. 2157)

6069.pdf

EXTREME 16O-RICH REFRACTORY INCLUSIONS IN THE ISHEYEVO CHONDRITE.
N. Sakamoto and N. Kawasaki, Hokkaido University, Sapporo 001-0021 Japan (naoya@ep.sci.hokudai.ac.jp)
Introduction: The oxygen three-isotope diagram shows fractionation and mixing of materials having different
origins. Whole-rock oxygen isotopic compositions of chondrite groups plotted on the diagram provide mixing lines
with different slopes (e.g. CO-CM 0.70, CR-CH 0.59) as a consequence of the interaction of primordial isotopic
reservoirs in the solar system [1]. CV chondrites lie close to the carbonaceous chondrite anhydrous mineral line(CCAM, slope 0.941) defined by separated minerals from Allende refractory inclusions (CAI), implying that theirisotopic compositions were determined by an exchange interaction between two reservoirs, one 16O-rich and theother 16O-poor [2]. An 16O-poor end member (Δ17O = +85‰) was found from cosmic symplectites composed ofmagnetite and iron sulfide, interpreted that the 16O-poor reservoir is H2O water [3]. Although several candidates
were reported for the 16O-rich reservoir from a chondrule named a006 (Δ17O = -37‰) [4], 4 refractory inclusions(Δ17O = -37 to -32‰) [5,6] from CH chondrites and the Sun (Δ17O = -28‰) [7] enriched in 16O relative to commonCAIs (Δ17O = -23‰) [e.g. 8], the nature of the primordial 16O-rich reservoir is poorly understood. In this study, wesurveyed extremely 16O-enriched refractory inclusions in the Isheyevo CH/CB carbonaceous chondrite.
Experimental: Thirteen thick sections of the Isheyevo chondrite were newly prepared. Al and/or Ca rich inclusions were picked up from combined X-ray elemental maps of the whole sections obtained by FE-SEM equippedwith a X-MaxN 150 EDS detector. Oxygen isotope analysis were performed by ims-1280 and ims-1280HR SIMS
instruments at Hokkaido University.
Results and Discussion: Oxygen isotopic compositions of 263 refractory inclusions were measured and 4 extreme 16O-rich inclusions were found. The extreme 16O-rich inclusions are composed of grossite core rimmed bythin layers of spinel, melilite and/or Ti-rich diopside. Although the grossite cores show heterogeneous oxygen isotopic composition ranging from Δ17O = -32 to -36‰, high precision isotopograph clearly revealed that the grossite
can be isotopically altered and the inclusions are enriched in 16O relative to common CAIs including the thin layers.
On the other hand, the spinel grains in the thin layer have uniform 16O-rich composition (Δ17O = -37‰) and fall on
somewhat lower than the CCAM line. In order to confirm the characteristic oxygen isotopic composition of the extremely 16O-rich material, the oxygen isotopic composition of olivine grains in the shell of the a006 chondrule were
measured with high precision condition using three Faraday cups. The oxygen isotopic composition of the olivine
grains in a006 chondrule (δ17O = -77.26‰, δ18O = -75.15‰, Δ17O = -38.18‰) was plotted close to the spinel data of
the Isheyevo inclusions and makes a regression line with olivine grains of a porphyritic olivine chondrule δ17O =(0.9882±0.0051) × δ18O – (3.00±0.25). Cosmic symplectites are located at the terminal of the line passing throughthe chondrules [9]. The extreme 16O-rich materials also located on the extension of the lower limit of the standard
deviation of the recommended group averages (area A, B, C and F) with a mass dependent fractionation line fromthe average of Genesis data [7]. If the extreme 16O-rich end member simply formed a line mixed with the primordial
16
O-poor water, CAIs in CV chondrites (Δ17O = -24‰) formed in a distinct oxygen reservoir slightly fractionatedfrom the Solar System line.
References: [1] Clayton and Mayeda (1999) Geochimica et Cosmochimica Acta 63:2089-2104. [2] Clayton et al.
(1977) Earth & Planetary Science Letters 34:209-224. [3] Sakamoto et al. (2007) Science 317:231-233. [4] Kobayashi et al. (2003) Geochemical Journal 37:663–669. [5] Gounelle et al. (2009) Astrophysical Journal 698:L18-L22.
[6] Krot et al. (2017) Geochimica et Cosmochimica Acta 201:185-223. [7] McKeegan et al. (2011) Science332:1528-1532. [8] Kawasaki et al. (2018) Geochimica et Cosmochimica Acta 221:318-341. [9] Ushikubo et al.
(2012) Geochimica et Cosmochimica Acta 90:242-264.