Petrology and bulk chemistry of Yamato‐82094, a new type of carbonaceous chondrite

Abstract

Abstract-Carbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y-82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y-82094 contains 3 vol% Ca-Al-rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y-82094 is within the range of CO and CV chondrites. Therefore, Y-82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO-rich rims on AOA olivine and the mode of occurrence of Ni-rich metal in the chondrules indicate that Y-82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe-Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high-temperature conditions, and accreted to the parent body of Y-82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y-82094 may represent a new, previously unsampled, asteroidal body.

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

“…Regarding Y‐82094 chondrules, their average diameter is 330 μm (Kimura et al. ), which is comparable to the value from R chondrites (400 μm), but differs from the values of CH (20 μm), CO (150 μm), CB (200 μm), CR (700 μm), and CV/CK (900 μm) chondrites, as well as chondrule average diameters from ordinary chondrites (450–550 μm) (Friedrich et al. ).…”

“…However, Y‐82094 has several characteristics that distinguish it from other chondrites, as summarized by Kimura et al. (). For example, the abundance of chondrules (78 vol%) versus matrix (11 vol%) in Y‐82094 is distinctly higher than that in CO, CV, and CR chondrites (45–60 vol% chondrules; 30–50 vol% matrix), as well as R chondrites (~50% chondrules; ~50% matrix), but is similar to CH chondrites (~70 vol% chondrules; 5 vol% matrix) and ordinary chondrites (60–80 vol% chondrules; 8–15% matrix) (Brearley and Jones ; Weisberg et al.…”

“…The low matrix abundance in Y‐82094 likely explains its bulk depletion in volatile elements (Kimura et al. ), as carbonaceous chondrite matrix is typically enhanced in volatiles relative to that in chondrules (Rubin and Wasson ; Bland et al. ; Huss et al.…”

“…From the grain size area and the number density of Ni‐rich metal within kamacite, Kimura et al. () established that the petrologic type of Y‐82094 is 3.2.…”

Petrology and geochemistry of chondrules and metal in NWA 5492 and GRO 95551: A new type of metal-rich chondrite

“…However, the R chondrites, which have a low abundance of chondrules as do the C chondrites, are inconsistent with this hypothesis. Additionally, Y 82094 is an unusual C chondrite-like meteorite with a low abundance of matrix (Kimura et al, 2014).…”