^1,2Joseph R. Michalski, ¹Javier Cuadros, ³Janice L. Bishop, ⁴M. Darby Dyar, ⁵Vesselin Dekov, ⁶Saverio Fiore
¹Dept. of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
²Planetary Science Institute, Tucson, AZ, USA
³SETI Institute, Mountain View, CA, USA
⁴Mount Holyoke College, South Hadley, MA, USA
⁵Département Géosciences Marines, IFREMER, Plouzané, France
⁶University of Bari, Bari, Italy

Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal–OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27–2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal–chemical substitution and mixed layering affect the position of the diagnostic metal–OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified di–trioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates.

Reference
Michalski JR, Cuadros J, Bishop JL, Dyar MD, Dekov V, Fiore S (2015) Constraints on the crystal-chemistry of Fe/Mg-rich smectitic clays on Mars and links to global alteration trends. Earth and Planetary Science Letters 427, 215–225.
Link to Article [doi:10.1016/j.epsl.2015.06.020]

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¹Nicolas Dauphas, ²Franck Poitrasson, ¹Christoph Burkhardt, ³Hiroshi Kobayashi, ⁴Kosuke Kurosawa
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA
²Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, avenue Edouard Belin, 31400 Toulouse, France
³Department of Physics, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
⁴Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth’s mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe–Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at ∼1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of ∼1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth’s core of 3.6 (+6.0/−3.6) wt%(+6.0/−3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

Reference
Dauphas N, Poitrasson F, Burkhardt C, Kobayashi H, Kurosawa K (2015) Planetary and meteoritic Mg/Si and δ30Siδ30Si variations inherited from solar nebula chemistry. Earth and Planetary Science Letters 427, 236–248
Link to Article [doi:10.1016/j.epsl.2015.07.008]
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¹Maximilian Matthes, ¹Mario Fischer-Gödde, ¹Thomas S. Kruijer, ²Ingo Leya, ¹Thorsten Kleine
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Space Research and Planetology, University of Bern, Bern, Switzerland

The short-lived 107Pd-107Ag system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of 107Ag/109Ag, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth (IIAB) and Carbo (IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107Pd-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic 107Ag excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions.
Reference
Matthes M, Fischer-Gödde M, Kruijer TS, Leya I, Kleine T (2015) Pd-Ag chronometry of iron meteorites: correction of neutron capture-effects and application to the cooling history of differentiated protoplanets. Geochimica et Cosmochimica Acta (in Press).
Link to Article [doi:10.1016/j.gca.2015.07.027]
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