TY - JOUR
T1 - Low-temperature aqueous alteration on the CR chondrite parent body
T2 - Implications from in situ oxygen-isotope analyses
AU - Jilly-Rehak, Christine E.
AU - Huss, Gary R.
AU - Nagashima, Kazu
AU - Schrader, Devin
N1 - Funding Information:
We thank the Johnson Space Center’s Meteorite Working Group for the Antarctic meteorite samples, and the Division of Meteorites, Department of Mineral Sciences, Smithsonian Institution for providing the Al Rais USNM 6997 samples. We also thank the Museum of Natural History Vienna for providing the Renazzo NHMW-N1126 and NHMW-N1127 samples. The authors also thank N. Kita for insight on magnetite fractionation, and M. Garcia for sharing his writing knowledge. This manuscript was improved by the helpful reviews from T. Tenner and two anonymous reviewers, and Associate Editor S. Russell. This work was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program – Grant NNX14AO29H , as well as NASA Cosmochemistry Grant NNX11AG78G (G. R. Huss, PI), and NAI cooperative agreement NNA04CC08A (K. Meech, PI). This manuscript is SOEST publication #10235 and HIGP publication #2262.
Publisher Copyright:
© 2017 Elsevier Ltd
PY - 2018/2/1
Y1 - 2018/2/1
N2 - The presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early Solar System; however, the process of aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) chondrites are particularly well-suited for the study of aqueous alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots of the alteration process through time. We studied oxygen isotopes in secondary-minerals from six CR chondrites of varying hydration states to determine how aqueous fluid conditions (including composition and temperature) evolved on the parent body. Secondary minerals analyzed included calcite, dolomite, and magnetite. The O-isotope composition of calcites ranged from δ18O ≈ 9 to 35‰, dolomites from δ18O ≈ 23 to 27‰, and magnetites from δ18O ≈ −18 to 5‰. Calcite in less-altered samples showed more evidence of fluid evolution compared to heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted on a single trend, with the exception of grains from the extensively hydrated chondrite MIL 090292. The MIL 090292 magnetite diverges from this trend, possibly indicating an anomalous origin for the meteorite. If magnetite and calcite formed in equilibrium, then the relative 18O fractionation between them can be used to extract the temperature of co-precipitation. Isotopic fractionation in Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (∼60 °C). Assuming that the CR parent body experienced closed-system alteration, a similar exercise for parallel calcite and magnetite O-isotope arrays yields “global” alteration temperatures of ∼55 to 88 °C. These secondary mineral arrays indicate that the O-isotopic composition of the altering fluid evolved upon progressive alteration, beginning near the Al Rais water composition of Δ17O ∼ 1‰ and δ18O ∼ 10‰, and becoming increasingly 16O-enriched toward a final fluid composition of Δ17O ∼ −1.2‰ and δ18O ∼ −15‰.
AB - The presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early Solar System; however, the process of aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) chondrites are particularly well-suited for the study of aqueous alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots of the alteration process through time. We studied oxygen isotopes in secondary-minerals from six CR chondrites of varying hydration states to determine how aqueous fluid conditions (including composition and temperature) evolved on the parent body. Secondary minerals analyzed included calcite, dolomite, and magnetite. The O-isotope composition of calcites ranged from δ18O ≈ 9 to 35‰, dolomites from δ18O ≈ 23 to 27‰, and magnetites from δ18O ≈ −18 to 5‰. Calcite in less-altered samples showed more evidence of fluid evolution compared to heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted on a single trend, with the exception of grains from the extensively hydrated chondrite MIL 090292. The MIL 090292 magnetite diverges from this trend, possibly indicating an anomalous origin for the meteorite. If magnetite and calcite formed in equilibrium, then the relative 18O fractionation between them can be used to extract the temperature of co-precipitation. Isotopic fractionation in Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (∼60 °C). Assuming that the CR parent body experienced closed-system alteration, a similar exercise for parallel calcite and magnetite O-isotope arrays yields “global” alteration temperatures of ∼55 to 88 °C. These secondary mineral arrays indicate that the O-isotopic composition of the altering fluid evolved upon progressive alteration, beginning near the Al Rais water composition of Δ17O ∼ 1‰ and δ18O ∼ 10‰, and becoming increasingly 16O-enriched toward a final fluid composition of Δ17O ∼ −1.2‰ and δ18O ∼ −15‰.
KW - Aqueous alteration
KW - CR chondrite
KW - Carbonaceous chondrites
KW - Oxygen isotopes
KW - Secondary processes
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U2 - 10.1016/j.gca.2017.10.007
DO - 10.1016/j.gca.2017.10.007
M3 - Article
AN - SCOPUS:85033489095
SN - 0016-7037
VL - 222
SP - 230
EP - 252
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -