¹J. Semprich,²S. P. Schwenzer,¹ A. H. Treiman,¹ J. Filiberto
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005869]
¹Lunar and Planetary Institute, USRA, Houston, TX, USA
²School of Environment, Technology, Engineering and Mathematics, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Hydrous phases have been identified to be a significant component of martian mineralogy. Particularly prehnite, zeolites, and serpentine are evidence for low‐grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the martian crust is sufficiently hydrated for low‐grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability and rock properties in the martian crust. In this study, we use phase equilibria models to explore low‐grade metamorphic reactions covering a pressure‐temperature range of 0‐0.5 GPa and 150‐450 °C for several martian protolith compositions and varying fluid content. Our models replicate the detected low‐grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower‐grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole rock composition, we can estimate the amount of water required to precipitate low‐grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, either suggesting a geotherm of ~ 20 °C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity.

^1,2Mingming Zhang,^1,2Yangting Lin,³Ingo Leya,⁴ Guoqiang Tang,⁴Yu Liu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13260]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
²University of Chinese Academy of Sciences, Beijing, 100049 China
³Physical Institute, Space Sciences and Planetology, University of Bern, 3012 Bern, Switzerland
⁴State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

Palisade bodies, mineral assemblages with spinel shells, in coarse‐grained Ca‐, Al‐rich inclusions (CAIs) have been considered either as exotic “mini‐CAIs” captured by their host inclusions (Wark and Lovering 1982) or as in situ crystallization products of a bubble‐rich melt (Simon and Grossman 1997). In order to clarify their origins, we conducted a comprehensive study of palisade bodies in an Allende Type B CAI (BBA‐7), using electron backscatter diffraction (EBSD), micro‐computed tomography (Micro‐CT), electron probe microanalysis (EPMA), and secondary ion mass spectrometry (SIMS). New observations support the in situ crystallization mechanism: early/residual melt infiltrated into spinel‐shelled bubbles and crystallized inside. Evidence includes (1) continuous crystallography of anorthite from the interior of the palisade body to the surrounding host; (2) partial consolidation of two individual palisade bodies revealed by micro‐CT; (3) a palisade body was entirely enclosed in a large anorthite crystal, and the anorthite within the palisade body shows the same crystallographic orientation as the anorthite host; and (4) identical chemical and oxygen isotopic compositions of the constituent minerals between the palisade bodies and the surrounding host. Oxygen isotopic compositions of the major minerals in BBA‐7 are bimodal‐distributed. Spinel and fassaite are uniformly 16O‐rich with ∆17O = −23.3 ± 1.5‰ (2SD), and melilite and anorthite are homogeneously 16O‐poor with ∆17O = −3.2 ± 0.7‰ (2SD). The latter ∆17O value overlaps with that of the Allende matrix (∆17O ~ −2.87‰) (Clayton and Mayeda 1999), which could be explained by secondary alteration with a 16O‐poor fluid in the parent body. The mobility of fluid could be facilitated by the high porosity (1.56–2.56 vol%) and connectivity (~0.17–0.55 vol%) of this inclusion.

^1,2Gregory J. Archer,¹ Richard J. Walker,¹Anthony J. Irving
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13261]
¹Department of Geology, University of Maryland, College Park, Maryland, 20742 USA
²Institut für Planetologie, University of Münster, Münster, 48149 Germany
³Department of Earth and Space Sciences, University of Washington, Seattle, Washington, 98195 USA
Published by arrangement with John Wiley & Sons

The abundances of highly siderophile elements (HSE; including Re, Os, Ir, Ru, Pt, and Pd) and ¹⁸⁷Re‐¹⁸⁷Os isotopic systematics were determined for two fragments from ungrouped achondrite NWA 7325. Rhenium‐Os systematics are consistent with closed‐system behavior since formation or soon after. The abundances of the HSE were therefore largely unaffected by late‐stage secondary processes such as shock or terrestrial weathering. As an olivine gabbro cumulate, this meteorite has a bulk composition consistent with derivation from a body that produced a core, mantle, and crust. Also consistent with derivation from a body that produced a core, both fragments of NWA 7325 have HSE abundances that are highly depleted compared to bulk chondrites. One fragment has ~0.002× CI chondrite Ir and relative HSE abundances similar to bulk chondrites. The other fragment has ~0.0002× CI chondrite Ir and relative HSE abundances that are fractionated compared to bulk chondrites. The chondritic relative HSE abundances of the fragment characterized by higher HSE abundances most likely reflect the addition of exogenous chondritic material during or after crystallization by surface impacts. The HSE in the other fragment is likely more representative of the parent body crust. One formation model that can broadly account for the HSE abundances in this fragment is multiple episodes of low‐pressure metal‐silicate equilibration, followed by limited late accretion and mantle homogenization. Given the different HSE compositions of the two adjoining fragments, this meteorite provides an example of the overprint of global processes (differentiation and late accretion) by localized impact contamination.