(2020) Younger Al-Mg Ages of Chondrules in CO Chondrites Than L/LL Chondrites
Kita N, Fukuda K, Siron G & Kimura M
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01g: Plenary Hall, Thursday 25th June 22:12 - 22:15
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How do your Al-Mg ages of OC chondrules compare with other isotopic systems? (e.g., Hf-W)
Al-Mg system and Hf-W have been compared for CR chondrules and they seem to show agreements, though there is a caveat using Al-Mg data of chondrules without resolvable 26Mg excess. Fo OC chondrules, we may compare Hf-W study for types 4-6 chondrites, suggesting ~2 Ma to 2.7 Ma for chondrule formation (Hellman et al. 2019). Though it is not direct comparison and need thermal modeling, estimated time generally agree with Al-Mg ages of UOCs. Thermal history of OC suggest accretion time of 2.14±0.1 Ma (Sugiura and Fujiya, 2014), which agree very well with new Al-Mg chondrule data by Siron et al. (2020). Because of relatively narrow range of Al-Mg ages in UOCs, pooled chondrule analyses using other chronometers will provide useful comparisons.
If you chose chondrules from the upper and lower ends of the Type I and II ranges defined by Kurahashi et al. 2008, would you also get a larger range of formation times? Do you have any plans to measure more chondrules?
It is possible to find relatively older chondrule, similar to those found in In Acfer 094. Youngest chondrules in Y-81020 are small chondrule fragments with Na-rich plagioclase that are exposed to matrix. Y-81020 is very pristine, yet subtype 3.05. I have some concerns to use this meteorite for Na-rich plagioclase analyses (even though I obtained fairly good isochron). We have a plan to study DOM 08006 (CO3.01) with minimal thermal metamorphism to answer this question.
If chondrule formation generally progressed with greater heliocentric distance (or less given the previous presentation), what physical or chemical disk properties would this correspond to/correlate with? Does this tell us anything about the chondrule forming process?
Considering that chondrule formation many need large planetesimal or planetary embryos, either by bow shock, by gravitational instability, or by impact, timing of chondrule formation would be related to growth of planetary bodies in each regions. Growth was earlier in inner disk than outer disk where CC parent bodies formed. Mg# of chondrule minerals are systematically different between OC and CC. OC show a broad distribution of Mg# (100-70), while CC show a sharp peak at very high Mg# 99-98. OC chondrules formed in more dust-rich regions than most chondrules in CC.
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