¹T.E. Johnson, ^1,2G.K. Benedix, ¹P.A. Bland
¹Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²Department of Earth and Planetary Sciences, Western Australia Museum, 49 Kew Street, Welshpool, WA 6986, Australia

Constraining the metamorphic pressures (P) and temperatures (T) recorded by meteorites is key to understanding the size and thermal history of their asteroid parent bodies. New thermodynamic models calibrated to very low P for minerals and melt in terrestrial mantle peridotite permit quantitative investigation of high-T metamorphism in ordinary chondrites using phase equilibria modelling. Isochemical P–T phase diagrams based on the average composition of H, L and LL chondrite falls and contoured for the composition and abundance of olivine, ortho- and clinopyroxene, plagioclase and chromite provide a good match with values measured in so-called equilibrated (petrologic type 4–6) samples. Some compositional variables, in particular Al in orthopyroxene and Na in clinopyroxene, exhibit a strong pressure dependence when considered over a range of several kilobars, providing a means of recognising meteorites derived from the cores of asteroids with radii of several hundred kilometres, if such bodies existed at that time. At the low pressures (<1 kbar) that typify thermal metamorphism, several compositional variables are good thermometers. Although those based on Fe–Mg exchange are likely to have been reset during slow cooling, those based on coupled substitution, in particular Ca and Al in orthopyroxene and Na in clinopyroxene, are less susceptible to retrograde diffusion and are potentially more faithful recorders of peak conditions. The intersection of isopleths of these variables may allow pressures to be quantified, even at low P, permitting constraints on the minimum size of parent asteroid bodies. The phase diagrams predict the onset of partial melting at 1050–1100 °C by incongruent reactions consuming plagioclase, clinopyroxene and orthopyroxene, whose compositions change abruptly as melting proceeds. These predictions match natural observations well and support the view that type 7 chondrites represent a suprasolidus continuation of the established petrologic types at the extremes of thermal metamorphism. The results suggest phase equilibria modelling has potential as a powerful quantitative tool in investigating, for example, progressive oxidation during metamorphism, the degree of melting and melt loss or accumulation required to produce the spectrum of differentiated meteorites, and whether the onion shell or rubble pile model best explains the metamorphic evolution of asteroid parent bodies in the early solar System.

Reference
Johnson TE, Benedix GK, Bland PA (2015) Metamorphism and partial melting of ordinary chondrites: Calculated phase equilibria. Earth and Planetary Science Letters 433, 21–30
Link to Article [doi:10.1016/j.epsl.2015.10.035]
Copyright Elsevier

¹Audrey Bouvier, ²Janne Blichert-Toft, ³Maud Boyet,²Francis Albarède
¹Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, ON, Canada
²Laboratoire de Géologie de Lyon, CNRS UMR 5276, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, Lyon, France
³Laboratoire Magmas et Volcans, CNRS UMR 6524, Université Blaise Pascal, Clermont-Ferrand, France

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.

Reference
Bouvier A, Blichert-Toft J, Boyet M, Albarède F (2015) 147Sm-143Nd and 176Lu-176Hf systematics of eucrite and angrite meteorites. Meteoritics & Planetary Science (in Press).
Link to Article [DOI: 10.1111/maps.12553]
Published by arrangement with John Wiley & Sons

¹Piero D’Incecco et al. (>10)*
¹Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, D-12489 Berlin, Germany
*Find the extensive, full author and affiliation list on the publishers website

We have performed a combined geological and spectral analysis of two impact craters on Mercury: the 15 km diameter Waters crater (106 °W; 9 °S) and the 62.3 km diameter Kuiper crater (30 °W; 11 °S). Using the Mercury Dual Imaging System (MDIS) Narrow Angle Camera (NAC) dataset we defined and mapped several units for each crater and for an external reference area far from any impact related deposits. For each of these units we extracted all spectra from the MESSENGER Atmosphere and Surface Composition Spectrometer (MASCS) Visible-InfraRed Spectrograph (VIRS) applying a first order photometric correction. For all the mapped units, we analyzed the spectral slope in two wavelength ranges, 350–450 nm and 450–650 nm, and the absolute reflectance in the 700-750 nm range. Normalized spectra of Waters crater display a generally bluer spectral slope than the external reference area over both wavelength windows. Normalized spectra of Kuiper crater generally display a redder slope than the external reference area in the 350–450 nm window, while they display a bluer slope than the external reference area in the 450–650 nm wavelength range. The combined use of geological and spectral analyses enables reconstruction of the local scale stratigraphy beneath the two craters, providing insight into the properties of the shallower crust of Mercury. Kuiper crater, being ~4 times larger than Waters crater, exposes deeper layers with distinctive composition, while the result for Waters crater might indicate substantial compositional homogeneity with the surrounding intercrater plains, though we can’t exclude the occurrence of horizontal compositional heterogeneities in the shallow sub-surface.

Reference
D’Incecco P. et al. (2015) Shallow crustal composition of Mercury as revealed by spectral properties and geological units of two impact craters. Icarus (in Press)
Link to Article [doi:10.1016/j.pss.2015.10.007]

Copyright Elsevier

The most popular papers on Cosmochemistry Papers in October were:

1.Renne PR, Sprain CJ, Richards MA, Self S, Vanderkluysen L, Pande K (2015) State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science 6256:76-78. Link to Article [doi:10.1126/science.aac7549]

2.Defouilloy C, Cartigny P, Assayag N, Moynier F, Barrat J-A (2015) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.10.009]

3.Becker M, Hezel DC, Schulz T, Elfers B-M, Münker C (2015) Formation timescales of CV chondrites from component specific Hf–W systematics. Earth and Planetary Science Letters (in Press) Link to Article [doi:10.1016/j.epsl.2015.09.049]

4.Laurent B, Roskosz M, Remusat L, Robert F, Leroux H, Vezin H, Depecker C, Nuns N, Lefebvre J-M (2015) The deuterium/hydrogen distribution in chondritic organic matter attests to early ionizing Irradiation. Nature Communications 6, 8567 Link to Article [doi:10.1038/ncomms9567]

5. Goderis S, Brandon AD, Mayer B, Humayun M (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118 Link to Article [doi:10.1016/j.epsl.2015.09.021]