Alexander N. Krot^a,b. Kazuhide Nagashima^a, Krisztián Fintor^c, ElemérPál-Molnár^c
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.002]
^aSchool of Ocean, Earth Science and Technology, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, HI 96822, USA
^bGoethe University Frankfurt Altenhoeferallee 1, 60438 Frankfurt am Main, Germany
^c’Vulcano’ Petrology and Geochemistry Research Group, Department of Mineralogy Geochemistry and Petrology, Faculty of Science and Informatics, University of Szeged, Hungary
Copyright Elsevier

We report on the mineralogy, petrology, and in situ measured oxygen-isotope compositions of three Fluffy Type A Ca,Al-rich inclusions (FTA CAIs) and two amoeboid olivine aggregates (AOAs) from the CV3.1 carbonaceous chondrite Kaba. The FTA CAIs are aggregates of several inclusions composed of spinel, Al,Ti-diopside, and gehlenitic melilite replaced to various degrees by anorthite; they are surrounded by the Wark-Lovering rim layers made of spinel, anorthite, Al-diopside, and forsterite. One of FTA CAIs contains a relict ultrarefractory inclusion composed of Sc-rich Al,Ti-pyroxene, spinel, and Zr-rich oxides. The AOAs are aggregates of Ca- and/or Al-rich minerals (spinel, anorthite, and Al,Ti-diopside) surrounded by forsterite ± Fe,Ni-metal condensates; Fe,Ni-metal is almost entirely replaced by magnetite and Fe,Ni-sulfides. Neither the FTA CAIs nor the AOAs show evidence for being melted after aggregation, and contain very minor secondary alteration minerals resulted from fluid-rock interaction on the CV parent asteroid. These include magnetite, fayalite, hedenbergite, phyllosilicates, and Fe-bearing Ti-free Al-diopside; secondary anorthite of asteroidal origin is absent in Kaba CAIs and AOAs. There are large variations in Δ¹⁷O (deviation from the terrestrial fractionation line = δ¹⁷O – 0.52×δ¹⁸O) within the individual FTA CAIs and AOAs: anorthite and melilite are systematically ¹⁶O-depleted (Δ¹⁷O range from ∼ −14 to ∼ −2‰) relative to the uniformly ¹⁶O-rich forsterite and Al,Ti-diopside (Δ¹⁷O ∼ −25 to −20±2‰, 2σ). Scandium-rich Al,Ti-pyroxene has ¹⁶O-poor composition (Δ¹⁷O ∼ −4‰). Many anorthite and melilite analyses plot close to or along mass-dependent fractionation line with Δ¹⁷O of −1.5±1‰ (average ± 2SD) defined by the aqueously-formed magnetite and fayalite from Kaba, and, therefore, corresponding to Δ¹⁷O of an aqueous fluid that operated on the CV parent asteroid. We conclude that anorthite and probably melilite in the Kaba FTA CAIs and AOAs experienced postcrystallization oxygen-isotope exchange with this fluid. The similar process must have affected plagioclase/plagioclase mesostasis and probably melilite in refractory inclusions and chondrules from CV3 chondrites of higher petrologic types [e.g., Allende (CV_oxA3.6) and Efremovka (CV_red3.1–3.4)], which appear to have experienced higher temperature metasomatic alteration than Kaba and were subsequently metamorphosed.

We conclude that the carbonaceous chondrite anhydrous mineral (CCAM) line defined by oxygen-isotope compositions of whole-rock and mineral separates of Allende CAIs and having a slope of 0.94 is not the primary nebular line. Instead this line results from superposition of the nebular slope-1 line recorded by the primitive chondrule mineral (PCM) line, the mass-dependent fractionation line with slope of ∼0.5 defined by the secondary minerals, and the minerals which experienced oxygen-isotope exchange with an aqueous fluid.

C. C. Bedford^a, J. C. Bridges^b, S. P. Schwenzer^c, R. C. Wiens^d, E. B. Rampe^e, J. Frydenvang^f, P.J.Gasda^d
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.031]
^aSchool of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^bLeicester Institute for Space and Earth Observation, University of Leicester, Leicester, LE1 7RH, UK
^cSchool of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^dLos Alamos National Laboratory, Los Alamos, New Mexico, USA
^eNASA Johnson Space Centre, Houston, TX, USA
^fNatural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
Copyright Elsevier

The Mars Science Laboratory’s Chemistry and Camera (ChemCam) instrument suite on-board the Curiosity rover has analysed ∼1200 sedimentary targets during the mission up to sol 1482. These targets have included sedimentary rock, diagenetic features (e.g., fracture-associated alteration halos, mineral veins, nodules, and erosion resistant raised ridges), active aeolian fines, soils and float. We have isolated ChemCam geochemical trends relating to diagenetic features and alteration products from those of the sedimentary rock in order to identify the compositional characteristics of Gale crater’s sediment source regions. The effects of grain size variation on sedimentary unit geochemistry have been taken into account by grouping and analysing geological units according to grain size. With obvious diagenetic features removed from the database, and predominately isochemical aqueous alteration inferred for the Mt Sharp Group samples, we propose that source region composition is a stronger source of geochemical change between the Bradbury and Mt Sharp Groups than open-system alteration. Additionally, a lack of correlation between the Chemical Index of Alteration (CIA) values and SiO₂, MgO or FeO_T indicates that the slight increase in chemical weathering of the Mt Sharp Group sediments was insufficient to overprint sediment source compositional signatures. This has led to the identification of five unique igneous endmember compositions which we hypothesise to have contributed to Gale crater’s stratigraphic record. These endmembers are: (1) a subalkaline basalt, compositionally similar to the tholeiitic Adirondack Class basalts of Gusev crater, and dominant within the finer grained units up to the base of Mt Sharp; (2) a trachybasalt, mostly identified within conglomerate units from the Darwin waypoint to the base of Mt Sharp; (3) a potassium-rich volcanic source, determined from strong potassium enrichment and a high abundance of sanidine that is most dominant in the fluvial sandstones and conglomerates of the Kimberley formation; (4) a highly evolved, silica-rich igneous source that correlates with the presence of tridymite, and is recorded in the lacustrine mudstone of Mt Sharp’s Marias Pass locality; and, (5) a fractionated, relatively SiO₂-rich subalkaline basalt, seen to have influenced the composition of mudstone deposited in the lower part of the Mt Sharp Group. Endmembers (1), (2), (3), and (4) have previously been identified at specific waypoints along the rover’s traverse, but we show that their influence extends throughout Gale’s stratigraphic record. The occurrence of detected endmembers is also strongly correlated with stratigraphic position, which suggests changing sediment source regions with time. We conclude that Gale sediment provenances were much more varied than suggested by the largely homogenous, globally-distributed Martian basalt inferred from orbit, showing that complex magmatic assemblages exist within the ancient highland crust surrounding Gale.

¹N. Ysard, ¹A. P. Jones, ²K. Demyk, ¹T. Boutéraon, ¹M. Koehler
Astronomy & Astrophysics 617, A124 Link to Article [https://doi.org/10.1051/0004-6361/201833386]
¹Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay cedex, France
e-mail: nathalie.ysard@ias.u-psud.fr
²Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

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