11:15 AM - 11:30 AM
[PPS07-15] Characteristics of Oxygen isotope ratios of chondrules and isolated olivine grains from Tagish Lake C2 chondrite
Keywords:Tagish Lake meteorite, Chondrule, Oxygen isotope, SIMS
Two polished epoxy mounts of the Tagish Lake meteorite, TL-KC-1 (8mm × 5mm) and TL-KC-2 (4mm × 3mm), were prepared for this study. Due to extensive aqueous alteration in the Tagish Lake parent body, phyllosilicates and other secondary phases such as magnetite and carbonates replace primary anhydrous phases. However, remaining olivine grains in chondrules and matrix tend to have sharp grain boundaries and no apparent evidence for the Mg-Fe exchange between olivine grains and matrix was observed. No alteration features were observed in these grains. Forty chondrules and isolated olivine grains (hereafter “ferromagnesian inclusions”) were selected for oxygen isotope measurements (34 type I [Mg#>= 90] and 6 type II [Mg#<90]). Oxygen isotope ratios of olivine grains were measured with an ion microprobe, CAMECA IMS 1280-HR at Kochi Institute. A 20 kV accelerated Cs+ primary beam (~30 pA, ~3 um in diameter) was used. The analytical conditions were similar to those in .
Oxygen three-isotope ratios of ferromagnesian inclusions from Tagish Lake are distributed along the PCM (primitive chondrule minerals) line . Most type I ferromagnesian inclusions are relatively 16O-rich (D17O = -7.4 to -3.9‰) and type II ferromagnesian inclusions are relatively 16O-poor (D17O >= -3‰). Of six type II ferromagnesian inclusions, two inclusions have D17O ~ 0‰, which are recognized in in Tagish Lake-like meteorites, CR chondrites, and cometary silicate particles, but are almost absent in other carbonaceous chondrites [e.g., 2-5, 7]. The present results are further evidence for the link between ferromagnesian inclusions accumulated into asteroids in the outer regions of the asteroid belt (possibly D-type asteroids) and silicate particles accumulated into cometary nuclei in the Kuiper belt. As discussed in [4, 5, 8], type II silicates are common in cometary silicate particles, which is distinct from ferromagnesian inclusions in Tagish Lake(-like) meteorites and CR chondrites. Most cometary silicate particles formed in H2O ice enriched oxidizing regions. Such regions were probably farther than where typical 16O-rich type I chondrules of carbonaceous chondrites formed.
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