^1,2A. I. Sheen,^1,2C. K. F. Tirona,^1,2K. T. Tait,^1,2,3L. F. White,^1,2B. C. Hyde,^1,2S. Korchinos
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70028]
¹Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada
²Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
³School of Physical Sciences, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Olivine is a major constituent in ureilites and commonly defines macroscopic fabric via shape-preferred orientation of elongate grains. In this study, we examined olivine fabric (crystallographic preferred orientation, or CPO) and microstructures in the unbrecciated olivine-pigeonite ureilites Northwest Africa (NWA) 7059 and Nova 018 using electron backscatter diffraction (EBSD) analysis. Point-per-grain orientation data of NWA 7059 indicate a <010> lineation subparallel to grain elongation. Misorientation data of two grains in NWA 7059 indicate dominant activity of (010) [100], (001) [100], (100) [001], and {hk0} [001] slip systems. Nova 018 displays an axial-[010] fabric, with misorientations indicating (010) [001], {hk0} [001] slips, and formation of (010) twist boundaries. Axial-[010] fabric in Nova 018 is consistent with compaction of residual olivine during melt extraction. The <010> lineation in NWA 7059 is unlike typical ureilite fabric and requires a [010] Burgers vector, uncommon in terrestrial samples. Rotational axis analysis of 2°–10° misorientations in olivine shows that the relative proportion of [001] slips and [100] slips in both ureilites are similar to warm-shocked ordinary chondrites, which were deformed at subsolidus temperatures. However, subsolidus deformation temperatures for both studied ureilites are inconsistent with a “hot disruption” model for the ureilite parent body (UPB). The further lack of correlation between 2°–10° misorientation metrics and olivine core Fo content argues against deformation temperature as the main control on olivine slip systems in ureilites. Our findings highlight the use of olivine petrofabric to gain insights into ureilite deformation, as well as complexities in interpreting olivine deformation data with respect to the history of the UPB.

¹A. N. Krot,¹K. Nagashima,²M. I. Petaev,¹E. Dobrică,³C. Ma,⁴B. Jacobsen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70036]
¹Hawai‘i Institute of Geophysics & Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA
²Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
⁴Lawrence Livermore National Laboratory, Livermore, California, USA
Published by arrangement with John Wiley & Sons

We report on the mineralogy, petrology, oxygen, and aluminum–magnesium isotopic systematics of the secondary corundum-bearing assemblages in type B CAIs 3529Z and 3529G and fluffy type A (FTA) CAI ALH-2 from Allende (CV > 3.6). In 3529Z and 3529G, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary alumoåkermanite [(Ca,Na)2AlSi2O7], grossular, spinel, grossite, celsian, kushiroite, and wadalite. In ALH-2, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary grossular, nepheline, spinel, and kushiroite. In 3529Z and 3529G, corundum and associated secondary grossite, spinel, alumoåkermanite, grossular, and kushiroite have similar 16O-poor compositions (Δ17O = −2.2 ± 1.5‰); primary spinel is 16O-rich (Δ17O ~ −23‰); Al,Ti-diopside shows a range of Δ17O (from ~ −24‰ to ~ −15‰); anorthite and melilite are 16O-depleted to various degrees (−6.5‰ ≤ Δ17O ≤ −4.5‰ and Δ17O = −2.7 ± 0.8‰, respectively). In ALH-2, corundum shows a range of Δ17O, from ~ −9‰ to ~ −1‰; primary hibonite and spinel are 16O-rich (Δ17O ~ −23‰); melilite and perovskite are 16O-poor (Δ17O = −2.6 ± 1.5‰ and −3.1 ± 1.3‰, respectively). On the Al-Mg isotope diagram (26Mg* versus 27Al/24Mg), primary Al,Ti-diopside, hibonite, melilite, and spinel in the Allende CAIs studied along the canonical isochron with inferred initial 26Al/27Al ratio [(26Al/27Al)0] of ~5 × 10−5. All secondary minerals have resolved excesses of 26Mg*: alumoåkermanite, corundum, and grossite plot below the canonical isochron, whereas most spinel analyses plot above it. An internal isochron defined by the coexisting secondary corundum and alumoåkermanite in 3529Z has (26Al/27Al)0 = (7.5 ± 2.6) × 10−7. We conclude that the corundum-bearing assemblages in Allende CAIs resulted from metasomatic alteration of primary melilite and anorthite, ~4–5 Ma after their crystallization. Metasomatic alteration of CAIs in the Allende parent asteroid by an aqueous fluid having Δ17O of ~ −3 ± 2‰ modified the O-isotope composition of their primary melilite, anorthite, and Ti-rich pyroxene; O-isotope compositions of primary hibonite, spinel, and low-Ti pyroxene escaped this modification.