¹First E, ¹Hammer J
¹Department of Geology & Geophysics, University of Hawai‘i at Mānoa, 1680 East-West Road, POST 606A, Honolulu, Hawai‘i, USA

Dynamic crystallization experiments were performed on a liquid having the bulk composition of olivine-phyric shergottite Yamato 980459, to constrain the igneous thermal history of this meteorite. Key characteristics of the meteorite’s mineralogy and texture, including several morphologically distinct olivine and pyroxene crystal populations and a glassy mesostasis devoid of plagioclase, were replicated upon cooling from 1435 to 909 °C at 1 atmosphere under reducing conditions. Three sequential cooling ramps are required to produce synthetic samples with textures and compositions matching Yamato 980459. Olivine phenocrysts formed at

Reference
First E, Hammer J (2016) Igneous cooling history of olivine-phyric shergottite Yamato 980459 constrained by dynamic crystallization experiments. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12659]
Published by arrangement with John Wiley & Sons

¹Antoine S. G. Roth, ²Knut Metzler, ³Lukas P. Baumgartner, ¹Ingo Leya
¹Institute of Physics, University of Bern, Bern, Switzerland
²Institute for Planetology, University of Münster, Münster, Germany
³Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland

If chondrules were exposed to cosmic rays prior to meteorite compaction, they should retain an excess of cosmogenic noble gases. Beyersdorf-Kuis et al. (2015) showed that such excesses can be detected provided that the chemical composition of each individual chondrule is precisely known. However, their study was limited to a few samples as they had to be irradiated in a nuclear reactor for instrumental neutron activation analysis. We developed a novel analytical protocol that combines the measurements of He and Ne isotopic concentrations with a fast method to correct for differences in chemical composition using micro X-ray computed tomography. Our main idea is to combine noble gas, nuclear track, and petrography data for numerous chondrules to understand the precompaction exposure history of the chondrite parent bodies. Here, we report our results for a total of 77 chondrules and four matrix samples from NWA 8276 (L3.00), NWA 8007 (L3.2), and Bjurböle (L/LL4). All chondrules from the same meteorite have within uncertainty identical 21Ne exposure ages, and all chondrules from Bjurböle have within uncertainty identical 3He exposure ages. However, most chondrules from NWA 8276 and a few from NWA 8007 show small but resolvable differences in 3He exposure age that we attribute to matrix contamination and/or gas loss. The finding that none of the chondrules has noble gas excesses is consistent with the uniform track density found for each meteorite. We conclude that the studied chondrules did not experience a precompaction exposure longer than a few Ma assuming present-day flux of galactic cosmic rays. A majority of chondrules from L and LL chondrites thus rapidly accreted and/or was efficiently shielded from cosmic rays in the solar nebula.

Reference
Roth ASG, Metzler K,Baumgartner LP, Leya I (2016) Cosmic-ray exposure ages of chondrules. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12658]
Published by arrangement with John Wiley & Sons

¹Katrina D. van Drongelen, ²Douglas Rumble III,³Kimberly T. Tait
¹Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
²Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia, USA
³Royal Ontario Museum, Department of Natural History, Toronto, Ontario, Canada

Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high-En pyroxene), pyroxene-lath, olivine rich with symplectite intergrowths as a break-down product of a quickly cooled Fe-rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ18O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large-scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta.

Reference
van Drongelen KD, Rumble III D, Tait KT (2016) Petrology and oxygen isotopic compositions of clasts in HED polymict breccia NWA 5232. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12651]
Published by arrangement with John Wiley & Sons