¹Jilly, C. E., ¹Huss, G. R., ¹Krot, A. N., ¹Nagashima, ²K., Yin, Q.-Z.,³Sugiura, N.

¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA
²Department of Earth and Planetary Sciences, University of California, Davis, Davis, California, USA
³Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, Bunkyo-ku, Tokyo, Japan

Radiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies. Dolomite is a well-documented secondary mineral in CM chondrites, and is thought to have formed by precipitation from an aqueous fluid on the CM parent body within several million years of accretion. The petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ, rather than in the nebular setting. Here, we present 53Mn-53Cr systematics for dolomite grains in Sutter’s Mill section SM51-1. The Mn-Cr isotope data show well-resolved excesses of 53Cr correlated with 55Mn/52Cr ratio, which we interpret as evidence for the in situ decay of radioactive 53Mn. After correcting for the relative sensitivities of Mn and Cr using a synthetic Mn- and Cr-bearing calcite standard, the data yield an isochron with slope corresponding to an initial 53Mn/55Mn ratio of 3.42 ± 0.86 × 10−6. The reported error includes systematic uncertainty from the relative sensitivity factor. When calculated relative to the U-corrected Pb-Pb absolute age of the D’Orbigny angrite, Sutter’s Mill dolomites give a formation age between 4564.8 and 4562.2 Ma (2.4–5.0 Myr after the birth of the solar system). This age is contemporaneous with previously reported ages for secondary carbonates in CM and CI chondrites. Consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation, probably occurring via heating from internal 26Al decay. The high-precision isochron for Sutter’s Mill dolomite indicates that late-stage processing did not reach temperatures that were high enough to further disturb the Mn-Cr isochron.

Reference
Jilly CE, Huss GR, Krot AN, Nagashima K, Yin Q-Z, Sugiura N (2014) 53Mn-53Cr dating of aqueously formed carbonates in the CM2 lithology of the Sutter’s Mill carbonaceous chondrite. Meteoritics & Planetary Science (in Press).
Link to Article [doi: 10.1111/maps.12305]

Published by arrangement with John Wiley & Sons

¹Dobrică, E., ¹Brearley, A. J.
¹Department of Earth and Planetary Sciences MSC03-2040, University of New Mexico, Albuquerque, New Mexico, USA

Mineralogic, textural, and compositional studies of black and white matrices in the unequilibrated ordinary chondrite Tieschitz (H/L, 3.6) show, for the first time in an ordinary chondrite, the presence of widespread, randomly distributed geode-like voids and veins. Scanning electron microscope (SEM) and transmission electron microscope (TEM) studies show that these voids and veins are partially or completely filled by sodic–calcic amphiboles (winchite and barroisite). The occurrence of amphiboles provides unequivocal evidence of the involvement of fluids in the metamorphic evolution of the parent body of Tieschitz. The presence of amphiboles as the main hydrous phases, rather than phyllosilicates, indicates that aqueous fluids were present at or close to the peak of thermal metamorphism, rather than during the waning stages of the cooling history of the parent body. In addition, ferrous olivine crystals, in association with the amphibole, also establish an important link between thermal metamorphism and hydrous phases formed at high temperatures. Mineralogic and textural evidence suggests that the white matrix and amphibole formed contemporaneously from the same hydrous fluid, prior to the formation of ferrous olivine crystals. Additionally, a dark inclusion identified in the host chondrite has mineralogic, petrologic, and bulk chemical characteristics that are similar to the black matrix of host Tieschitz, suggesting that this dark inclusion was emplaced before or during parent body metamorphism.

Reference
Dobrică E, Brearley AJ (2014) Widespread hydrothermal alteration minerals in the fine-grained matrices of the Tieschitz unequilibrated ordinary chondrite. Meteoritics & Planetary Science (in Press).
Link to Article [doi: 10.1111/maps.12335]

Published in Arrangement with John Wiley & Sons

^1,2Jenniskens, P. et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.
¹SETI Institute, Carl Sagan Center, Mountain View, California, USA
²NASA Ames Research Center, Moffett Field, California, USA

The Novato L6 chondrite fragmental breccia fell in California on 17 October 2012, and was recovered after the Cameras for Allsky Meteor Surveillance (CAMS) project determined the meteor’s trajectory between 95 and 46 km altitude. The final fragmentation from 42 to 22 km altitude was exceptionally well documented by digital photographs. The first sample was recovered before rain hit the area. First results from a consortium study of the meteorite’s characterization, cosmogenic and radiogenic nuclides, origin, and conditions of the fall are presented. Some meteorites did not retain fusion crust and show evidence of spallation. Before entry, the meteoroid was 35 ± 5 cm in diameter (mass 80 ± 35 kg) with a cosmic-ray exposure age of 9 ± 1 Ma, if it had a one-stage exposure history. A two-stage exposure history is more likely, with lower shielding in the last few Ma. Thermoluminescence data suggest a collision event within the last ∼0.1 Ma. Novato probably belonged to the class of shocked L chondrites that have a common shock age of 470 Ma, based on the U,Th-He age of 420 ± 220 Ma. The measured orbits of Novato, Jesenice, and Innisfree are consistent with a proposed origin of these shocked L chondrites in the Gefion asteroid family, perhaps directly via the 5:2 mean-motion resonance with Jupiter. Novato experienced a stronger compaction than did other L6 chondrites of shock-stage S4. Despite this, a freshly broken surface shows a wide range of organic compounds.

Reference
Jenniskens P et al. (2014) Fall, recovery, and characterization of the Novato L6 chondrite breccia. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12323]

Published by arrangement with John Wiley & Sons