Petrology, phase equilibria modelling, noble gas chronology and thermal constraints of the El Pozo L5 meteorite

1Pedro Corona-Chávez, 2María del Sol Hernández-Bernal, 3Pietro Vignola, 4Rufino Lozano-Santacruz, 5Juan Julio Morales-Contreras, 4Margarita Reyes-Salas, 5 Jesús Solé-Viñas, 6José F.Molina
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2017.12.003]
1Universidad Michoacana de San Nicolás de Hidalgo, Instituto de Investigaciones en Ciencias de la Tierra, Edificio U, Ciudad Universitaria, Morelia, 58020, Mexico
2Universidad Nacional Autónoma de México, Escuela Nacional de Estudios Superiores, Unidad Morelia, 58190, Mexico
3Consiglio Nazionale delle Ricerche (CNR) – Istituto per la dinamica dei processi ambientali, via Botticelli 23, 20133 Milan, Italy
4Universidad Nacional Autónoma de México, Instituto de Geología, Circuito interior Ciudad Universitaria, 04510, Mexico
5Universidad Nacional Autónoma de México, Instituto de Geofísica, Unidad Morelia, 58190, Mexico
6Departamento de Mineralogía y Petrología, Universidad de Granada, Spain
Copyright Elsevier

We present the results of physical properties, petrography, bulk chemistry, mineral compositions, phase relations modelling and Noble gases study of the meteorite El Pozo. The petrography and mineral compositions indicate that the meteorite is an L5 chondrite with a low shock stage of S2-S3. Heterogenous weathering was preferentially along shock structures. Thermobarometric calculations indicate thermal equilibrium conditions between 768 °C and 925 °C at ∼4 to 6 kb, which are substantially consistent with the petrological metamorphism type 5. A pseudosection phase diagram is relatively consistent with the mineral assemblage observed and PT conditions calculated. Temperature vs. fO2 diagram shows that plagioclase compositional stability is very sensitive to Tschermack substitution in orthopyroxene, clinopyroxene and XAn plagioclase during the high temperature metamorphic process. Based on noble gases He, Ne, Ar and K contents a cosmogenic exposure age CRE of 1.9 Myr was calculated. The 21Ne would be totally cosmogenic, with no primordial Ne. The 21Ne/22Ne value (0.97) is higher than solar value. According to the cosmogenic Ne content, we argue that El Pozo chondrite originally had a pre-atmospheric mass of 9–10 kg, which would have been produced by a later collision after the recognized collision of the L-chondrite parent body ∼470 Ma ago.

Chondritic Mn/Na ratio and limited post-nebular volatile loss of the Earth

1,2Julien Siebert, 1Paolo A. Sossi, 1Ingrid Blanchard, 1Brandon Mahan, 1,3James Badro, 1,2Frédéric Moynier
Earth and Planetary Science Letters 485, 130-139 Link to Article [https://doi.org/10.1016/j.epsl.2017.12.042]
1Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, 75005, Paris, France
2Institut Universitaire de France, France
3Earth and Planetary Science Laboratory, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
Copyright Elsevier

The depletion pattern of volatile elements on Earth and other differentiated terrestrial bodies provides a unique insight as to the nature and origin of planetary building blocks. The processes responsible for the depletion of volatile elements range from the early incomplete condensation in the solar nebula to the late de-volatilization induced by heating and impacting during planetary accretion after the dispersion of the H2-rich nebular gas. Furthermore, as many volatile elements are also siderophile (metal-loving), it is often difficult to deconvolve the effect of volatility from core formation. With the notable exception of the Earth, all the differentiated terrestrial bodies for which we have samples have non-chondritic Mn/Na ratios, taken as a signature of post-nebular volatilization. The bulk silicate Earth (BSE) is unique in that its Mn/Na ratio is chondritic, which points to a nebular origin for the depletion; unless the Mn/Na in the BSE is not that of the bulk Earth (BE), and has been affected by core formation through the partitioning of Mn in Earth’s core. Here we quantify the metal–silicate partitioning behavior of Mn at deep magma ocean pressure and temperature conditions directly applicable to core formation. The experiments show that Mn becomes more siderophile with increasing pressure and temperature. Modeling the partitioning of Mn during core formation by combining our results with previous data at lower P–T conditions, we show that the core likely contains a significant fraction (20 to 35%) of Earth’s Mn budget. However, we show that the derived Mn/Na value of the bulk Earth still lies on the volatile-depleted end of a trend defined by chondritic meteorites in a Mn/Na vs Mn/Mg plot, which tend to higher Mn/Na with increasing volatile depletion. This suggests that the material that formed the Earth recorded similar chemical fractionation processes for moderately volatile elements as chondrites in the solar nebula, and experienced limited post nebular volatilization.

Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

1,2Daisuke Nakashima,3,4Keisuke Nagao,5Anthony J. Irving
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13039]
1Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Sendai, Miyagi, Japan
2Geochemical Research Center, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
3Geochemical Research Center, Graduate School of Science, University of Tokyo, Bunkyo, Tokyo, Japan
4Division of Polar Earth-System Sciences, Korea Polar Research Institute, Incheon, Korea
5Department of Earth & Space Sciences, University of Washington, Seattle, Washington, USA
Published by arrangement with John Wiley & Sons

Noble gases in the five angrites Northwest Africa (NWA) 1296, 2999, 4590, 4801, and 4931 were analyzed with total melting and stepwise heating methods. The noble gases consist of in situ components: spallogenic, radiogenic, nucleogenic, and fission. Cosmic-ray exposure ages of the angrites (including literature data) spread uniformly from <0.2 to 56 Ma, and coarse-grained angrites have longer exposure ages than fine-grained angrites. It is implied that the parent bodies from which the two subgroups of angrites were ejected are different and have distinct orbital elements. The 244Pu-136Xe relative ages of the angrites obtained by using 244Pu/150Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain 244Pu-136Xe relative ages, using fission 136Xe, spallogenic 126Xe, and Ba/REE ratios, yields systematically older 244Pu-136Xe ages than those obtained by using 244Pu/150Nd ratios, which is explained by apparently high Ba/REE ratios caused by Ba contamination during terrestrial weathering. The 244Pu/238U ratio at 4.56 Ga of angrites is estimated as 0.0061 ± 0.0028, which is consistent with those for chondrules, chondrites, achondrites, and a terrestrial zircon. It is suggested that initial 244Pu/238U ratio has been spatially homogeneous at least in the inner part of the early solar system.

Shock history of the fossil ungrouped achondrite Österplana 065: Raman spectroscopy and TEM of relict chrome-spinel grains

1,2,3Surya S. Rout,1,2,4Philipp R. Heck,1,5Birger Schmitz
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13041]
1Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA
2Chicago Center for Cosmochemistry, Chicago, Illinois, USA
3Physikalisches Institut, Space Research and Planetary Sciences, Universität Bern, Bern, Switzerland
4Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
5Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
Published by arrangement with John Wiley & Sons

Chrome-spinel grains from the fossil ungrouped achondrite Österplana 065 (Öst 065) recovered from Middle Ordovician limestone in Sweden were studied using Raman spectroscopy and TEM. All the studied chrome-spinel grains have a high density of planar fractures and planar features, not seen in chromites from the other L chondritic Ordovician fossil meteorites. Raman spectra of the host chrome-spinel grain and its planar features are similar and no signatures of high-pressure phases of chromite were found. The planar features occur along planar fractures, are enriched in ZnO, and are most probably produced due to enhanced leaching during terrestrial weathering in the marine sediment. Dislocation densities within two FIB sections prepared from two chrome-spinel grains from Öst 065 are similar to the dislocation densities found within chromite grains from the matrix of Tenham L6 chondrite. Using this observation and taking into account the presence of significant fracturing in all the grains, we conclude that the Öst 065 chrome-spinel grains were subjected to moderate to very strong shock corresponding to shock stages of S4–S6. This makes Öst 065 fossil achondrite the highest shocked fossil meteorite studied so far. This is consistent with the hypothesis that Öst 065 is a piece of the impactor that led to the L chondrite parent body breakup.

A noble gas data collection of lunar meteorites

1,2Marianna Mészáros,2,3Beda A. Hofmann,1Ingo Leya
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13037]
1Space Research and Planetary Sciences, University of Bern, Bern, Switzerland
2Natural History Museum Bern, Bern, Switzerland
3Institute of Geological Sciences, University of Bern, Bern, Switzerland
Published by arrangement with John wiley & Sons

We collected the published noble gas data of altogether 35 lunar meteorites. This compilation includes the stable isotopes of He, Ne, Ar, Kr, and Xe. We also give a summary of cosmogenic, trapped, and radiogenic noble gas components of lunar meteorites for which data are available in the literature.