^1,2M. Kimura,^2,3A. Yamaguchi,⁴M. Miyahara
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12738]
¹Faculty of Science, Ibaraki University, Mito, Japan
²National Institute of Polar Research, Tokyo, Japan
³Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, Japan
⁴Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Japan
Published by arrangement with John Wiley & Sons

Shock-induced features are abundantly observed in meteorites. Especially, shock veins, including high-pressure minerals, characterize many kinds of heavily shocked meteorite. On the other hand, no high-pressure phases have been yet reported from enstatite chondrites. We studied a heavily shocked EH3 chondrite, Asuka 10164, containing a vein, which comprises fragments of fine-grained silicate and opaque minerals, and chondrules. In this vein, we found a silica polymorph, coesite. This is the first discovery of a high-pressure phase in enstatite chondrites. Other high-pressure polymorphs were not observed in the vein. The assemblages and chemical compositions of minerals, and the occurrence of coesite indicate that the vein was subjected to the high-pressure and temperature condition at about 3–10 GPa and 1000 °C. The host also experienced heating for a short time under lower temperature conditions, from ~700 to ~1000 °C, based on the opaque minerals typical of EH chondrites and textural features. Although the pressure condition of the vein in this chondrite is much lower than those in the other meteorites, our results suggest that all major meteorite groups contain high-pressure polymorphs. Heavy shock events commonly took place in the solar system.

^1,2,3Ekaterina V. Korochantseva,^1,2,3Alexei I. Buikin,^1,3Jens Hopp,²Cyrill A. Lorenz,²Alexander V. Korochantsev,^4,5Ulrich Ott,^1,4Mario Trieloff
Meteoritics & Planetary Society (in Press) Link to Article [10.1111/maps.12732]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Vernadsky Institute of Geochemistry, Moscow, Russia
³Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany
⁴University of West Hungary, Szombathely, Hungary
⁵Max-Planck-Institut für Chemie, Mainz, Germany
Published by arrangement with John Wiley & Sons

Dhofar 280 recorded a complex history on the Moon revealed by high-resolution 40Ar-39Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic 38Ar, resulting in a (low-temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low-temperature) 40Ar-39Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP-rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low 40Ar/36Ar ratio. It is apparently a mixture of two components, one endmember with 40Ar/36Ar = 17.5 ± 0.2 and a second less well-constrained endmember with 40Ar/36Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (40Ar/36Ar)trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.

^1,2Christine E. Jilly-Rehak, ²Gary R. Huss, ²Kazuhide Nagashima
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.033]
¹Department of Geology and Geophysics, University of Hawai‘i at Mānoa, 1680 East-West Road, POST 701, Honolulu, HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East-West Road, POST 602, Honolulu, HI 96822, USA
Copyright Elsevier

We present 53Mn-53Cr ages of secondary carbonates in Renazzo-like (CR) chondrites, determined by secondary ion mass spectrometry. The timing of aqueous alteration in CR chondrites has been unconstrained in the literature. We measured 53Mn-53Cr isotope systematics in carbonates from three different CR-chondrite lithologies. Calcite in the interchondrule matrix of Renazzo, calcite in the matrix of GRO 95577, and dolomite in a dark inclusion of Renazzo all show excesses in 53Cr, interpreted as the daughter product from the decay of 53Mn. The Renazzo calcite yields an initial ratio of (53Mn/55Mn)0 = (3.6 ± 2.7) × 10-6, and the Renazzo dark inclusion dolomite ranges from (53Mn/55Mn)0 = (3.1 ± 1.4) × 10-6 (corrected to the RSF of a calcite standard) to (3.7 ± 1.7) × 10-6 (corrected to an inferred dolomite RSF). When anchored to the D’Orbigny angrite, the Renazzo carbonates yield ages between 4563.6 to 4562.6 Ma, or ∼ 4.3 to 5.3 Myr after the formation of CV CAIs. Calcite measured in the heavily altered specimen GRO 95577 yields a shallower slope of (53Mn/55Mn)0 = (7.9 ± 2.8) × 10-7, corresponding to a much younger age of 4555.4 Ma, or ∼ 12.6 Myr after CAI formation. The two Renazzo ages are contemporaneous with Mn-Cr ages of carbonates in Tagish Lake, CI, and CM chondrites, but the GRO 95577 age is uniquely young. These findings suggest that early aqueous alteration on chondritic parent bodies was a common occurrence, likely driven by internal heating from 26Al decay after accretion. The young carbonate ages of GRO 95577 suggest that either the CR parent body was sufficiently large to sustain heating from 26Al for ∼ 8 Myr, or that late-stage impact events supplied heat to the region where GRO 95577 originated.