¹Chi Ma, ²Oliver Tschauner, ¹John R. Beckett, ³Yang Liu, ¹George R. Rossman, ⁴Kirill Zhuravlev, ⁴Vitali Prakapenka, ⁵Przemyslaw Dera, ⁶Lawrence A. Taylor

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
²High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁴GSECARS, University of Chicago, Argonne National Laboratory, Argonne, IL 60439, USA
⁵e Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
⁶Planetary Geosciences Institute, Department of Earth and Planetary Science, University of Tennessee, Knoxville, TN 37996, USA

Tissintite is a new vacancy-rich, high-pressure clinopyroxene, with a composition essentially equivalent to plagioclase. It was discovered in maskelynite (shocked plagioclase) and is commonly observed included within, or in contact with, shock-melt pockets in the Tissint meteorite, a depleted olivine-phyric shergottite fall from Mars. The simple composition of tissintite (An58–69) and its precursor plagioclase (An59–69) together with the limited occurrence, both spatially (only in maskelynite less than ∼25 μm of a shock melt pocket) and in terms of bulk composition, make tissintite a “goldilocks” phase. It formed during a shock event severe enough to allow nucleation and growth of vacancy-rich clinopyroxene from a melt of not too calcic and not too sodic plagioclase composition that was neither too hot nor too cold. With experimental calibration, these limitations on occurrence can be used to place strong constraints on the thermal history of a shock event. The kinetics for nucleation and growth of tissintite are probably slower for more-sodic plagioclase precursors, so tissintite is most likely to occur in depleted olivine-phyric shergottites like Tissint and other highly shocked meteorites and lunar and terrestrial rocks that consistently contained calcic plagioclase precursors in the appropriate compositional range for a shock of given intensity.
Tissintite, (Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6(Ca0.45Na0.31□0.24)(Al0.97Fe0.03Mg0.01)(Si1.80Al0.20)O6, is a C2/cC2/c clinopyroxene, containing 42–60 mol% of the Ca-Eskola component, by far the highest known. The cell parameters are a=9.21a=9.21 (17) Å, b=9.09b=9.09 (4) Å, c=5.20c=5.20 (2) Å, β=109.6β=109.6 (9)°, V=410V=410 (8) Å3, Z=4Z=4. The density is 3.32 g/cm3 and we estimate a cell volume for the Ca-Eskola end-member pyroxene of 411±13 Å3411±13 Å3, which is consistent with a previous estimate and, therefore, supports the importance of this component in clinopyroxenes from ultra-high pressure metamorphic rocks from the Earth’s upper mantle. At least in C2/cC2/c clinopyroxenes as sodic as tissintite, the a- and b-cell parameters as a function of vacancy concentration intersect at ∼0.3 vacancies pfu, much lower than the Ca-Eskola end-member (0.5), an inversion of anisotropy suggesting an elastic instability that drives clinopyroxene toward a disordered trigonal structure closely related to that of wadeite; it may mark the boundary beyond which the breakdown of vacancy-rich clinopyroxene to a wadeite-structured phase + stishovite becomes stable, although this was not observed in Tissint.

Reference
Ma C, Tschauner O, Beckett JR, Liu Y, Rossman GR, Zhuravlev K, Prakapenka V, Dera P, Taylor LA (2015) Tissintite, (Ca, Na, □)AlSi2O6, a highly-defective, shock-induced, high-pressure clinopyroxene in the Tissint martian Meteorite. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.03.057]

Copyright Elsevier

^1,2,3Han, J., ¹Brearley, A. J.
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²USRA Lunar and Planetary Institute, Houston, Texas, USA
³NASA Johnson Space Center, Houston, Texas, USA

The microstructures and compositions of olivine and refractory components in six amoeboid olivine aggregates (AOAs) in the Allan Hills A77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique with transmission electron microscopy. In the AOAs, refractory components (perovskite, melilite, spinel, anorthite, and Al-Ti-bearing diopside) provide evidence of a high degree of textural and compositional heterogeneity, suggesting that these phases have formed by disequilibrium gas–solid condensation at high temperatures under highly dynamic conditions. We infer different possible reactions of early-condensed solid minerals (perovskite and spinel) with a nebular gas, forming diopside with wide ranges of Al and Ti contents and/or anorthite. The progressive, incomplete consumption of spinel in these reactions may have resulted in the Cr enrichment in the remaining, unreacted spinel in the AOAs. In contrast to the refractory components, olivines in the AOAs have equilibrated textures with 120° triple junctions, indicating that the AOAs were subjected to high-temperature annealing after agglomeration of olivine and refractory components. Because the AOAs consist of fine-grained olivine grains with numerous pores, the annealing is constrained by experimental data to have occurred for a short duration of the order of a few hours to tens of hours depending on the annealing temperature. In comparison, the effects of annealing on the refractory components are minimal, probably due to pinning of grain boundaries in the multiphase assemblages that inhibited grain growth.

Reference
Han J, Brearley AJ (2015) Microstructural evidence for complex formation histories of amoeboid olivine aggregates from the ALHA77307 CO3.0 chondrite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12439]

Published by arrangement with John Wiley & Sons

¹Zack Gainsforth et al. (>10)*
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA
*Find the extensive, full author and affiliation list on the publishers website

Using chemical and petrologic evidence and modeling, we deduce that two chondrule-like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤100 °C h-1 and within ≈2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈103. There was no detectable metamorphic, nebular, or aqueous alteration. In previous work, Ogliore et al. (2012) reported that Iris formed late, >3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar, but not identical to Iris.

Reference
Gainsforth Z et al. (2015) Constraints on the formation environment of two chondrule-like igneous particles from comet 81P/Wild 2. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12445]

Published by arrangemént with John Wiley & Sons