TY - JOUR
T1 - Extended chondrule formation intervals in distinct physicochemical environments
T2 - Evidence from Al-Mg isotope systematics of CR chondrite chondrules with unaltered plagioclase
AU - Tenner, Travis J.
AU - Nakashima, Daisuke
AU - Ushikubo, Takayuki
AU - Tomioka, Naotaka
AU - Kimura, Makoto
AU - Weisberg, Michael K.
AU - Kita, Noriko T.
N1 - Funding Information:
We thank the ANSMET Program and Meteorite Working Group for providing the meteorite thin sections for this study. We thank John Fournelle for assistance with EPMA measurements, and Jim Kern for technical assistance during SIMS operations. Dr. Akira Yamaguchi helped to analyze phases by Raman spectroscopy at NIPR . We thank Kazuhide Nagashima and two anonymous reviewers, as well as associate editor Dimitri Papanastassiou, for constructive comments that improved the quality of this submission. This work is supported by NASA ( NNX11AG62G , NNX14AG29G , NK; 80NSSC18K0589 , MKW) and a Grant-in-aids of Monkashou, Japan (No. 26400510 to MK). WiscSIMS is partly supported by NSF ( EAR03-19230 , EAR10-53466 , EAR13-55590 ). Through Los Alamos National Laboratory , this document is approved for unlimited release under LA-UR-18-30036 .
Funding Information:
We thank the ANSMET Program and Meteorite Working Group for providing the meteorite thin sections for this study. We thank John Fournelle for assistance with EPMA measurements, and Jim Kern for technical assistance during SIMS operations. Dr. Akira Yamaguchi helped to analyze phases by Raman spectroscopy at NIPR. We thank Kazuhide Nagashima and two anonymous reviewers, as well as associate editor Dimitri Papanastassiou, for constructive comments that improved the quality of this submission. This work is supported by NASA (NNX11AG62G, NNX14AG29G, NK; 80NSSC18K0589, MKW) and a Grant-in-aids of Monkashou, Japan (No. 26400510 to MK). WiscSIMS is partly supported by NSF (EAR03-19230, EAR10-53466, EAR13-55590). Through Los Alamos National Laboratory, this document is approved for unlimited release under LA-UR-18-30036.
Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2019/9/1
Y1 - 2019/9/1
N2 - Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#’s of 99.0–99.2 and Δ17O of −4.2‰ to −5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range from (3.5 ± 1.3) × 10−6 to (6.0 ± 3.9) × 10−6. This corresponds to formation times of 2.2 (−0.5/+1.1) Myr to 2.8 (−0.3/+0.5) Myr after CAIs, using a canonical (26Al/27Al)0 of 5.23 × 10−5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#’s (94.2–98.7) and generally higher Δ17O (−0.9‰ to −4.9‰) than chondrules with resolvable excess 26Mg. Their inferred (26Al/27Al)0 upper limits range from 1.3 × 10−6 to 3.2 × 10−6, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00–3.05 chondrites. We conclude that the resolvable (26Al/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (26Al/27Al)0, Mg#, and Δ17O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ17O ∼ −5‰ chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ∼100×. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94–99 chondrules with higher Δ17O (−5‰ to −1‰), with low (26Al/27Al)0, and hence no resolvable excess 26Mg. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (26Al/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al/27Al)0. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.
AB - Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#’s of 99.0–99.2 and Δ17O of −4.2‰ to −5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range from (3.5 ± 1.3) × 10−6 to (6.0 ± 3.9) × 10−6. This corresponds to formation times of 2.2 (−0.5/+1.1) Myr to 2.8 (−0.3/+0.5) Myr after CAIs, using a canonical (26Al/27Al)0 of 5.23 × 10−5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#’s (94.2–98.7) and generally higher Δ17O (−0.9‰ to −4.9‰) than chondrules with resolvable excess 26Mg. Their inferred (26Al/27Al)0 upper limits range from 1.3 × 10−6 to 3.2 × 10−6, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00–3.05 chondrites. We conclude that the resolvable (26Al/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (26Al/27Al)0, Mg#, and Δ17O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ17O ∼ −5‰ chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ∼100×. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94–99 chondrules with higher Δ17O (−5‰ to −1‰), with low (26Al/27Al)0, and hence no resolvable excess 26Mg. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (26Al/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al/27Al)0. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.
KW - Cosmochemistry
KW - Early Solar System chronology
KW - Protoplanetary disk evolution
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U2 - 10.1016/j.gca.2019.06.023
DO - 10.1016/j.gca.2019.06.023
M3 - Article
AN - SCOPUS:85068453197
SN - 0016-7037
VL - 260
SP - 133
EP - 160
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
ER -