Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars

Mari, N., Riches, A.J.V. , Hallis, L.J. , Marrocchi, Y., Villeneuve, J., Gleissner, P., Becker, H. and Lee, M.R. (2019) Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars. Geochimica et Cosmochimica Acta, 266, pp. 416-434. (doi: 10.1016/j.gca.2019.05.025)

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Martian lava flows likely acquired S-rich material from the regolith during their emplacement on the planet’s surface. We investigated five of the twenty known nakhlites (Nakhla, Lafayette, Miller Range (MIL) 090032, Yamato 000593, and Yamato 000749) to determine whether these lavas show evidence of regolith assimilation, and to constrain the potential implications that this process has on chemical tracing of martian mantle source(s). To establish the proportionate influence of atmospheric, hydrothermal, and volcanic processes on nakhlite isotopic systematics we obtained in situ sulphur isotope data (Δ33S and δ34S) for sulphide grains (pyrrhotite and pyrite) in all five nakhlite samples. For Nakhla, Lafayette, and MIL 090032, these data are integrated with highly siderophile element (HSE) abundances and Os-isotope compositions, as well as textural information constrained prior to isotopic analysis. This work thereby provides the first Re-Os isotope systematics for two different nakhlites, and also the first Re-Os isotope data for martian sample for which detailed petrographic information was constrained prior to digestion. We report the largest variation in δ34S yet found in martian meteorites (−13.20‰ to +15.16‰). The relatively positive Δ33S and δ34S values of MIL 090032 (δ34S = +10.54 ± 0.09‰; Δ33S = −0.67 ± 0.10‰) indicate this meteorite assimilated sulphur affected by UV-photochemistry. In contrast, the strongly negative values of Lafayette (δ34S = −10.76 ± 0.14‰; Δ33S = −0.09 ± 0.12‰) are indicative of hydrothermal processes on Mars. Nakhla, Yamato 000593, and Yamato 000749 sulphides have a narrower range of sulphur isotope compositions (Δ33S and δ34S ∼ 0) that is consistent with no assimilation of martian surface materials during lava flow emplacement. Consequently we used this second group of Δ33S values to approximate the Δ33S of the nakhlite source, yielding a Δ33S value of −0.1‰. Nakhlite HSE patterns result from a sulphide-saturated melt where Ru-Os-Ir alloys/sulphide were likely crystallized during earlier phases of magmatic processing in Mars to result in the fractionated HSE patterns of the nakhlites. Our data, alongside a synthesis of previously published data, suggest assimilation of an enriched component to the primary nakhlite melt, potentially a late-stage crystallization cumulate from the martian magma ocean stage. In the context of this model, and within large uncertainties, our data hint at perturbation and potential decoupling of nakhlite Re-Os isotope systematics from other isotopic systems as a result of small degrees of assimilation of a regolith component with highly radiogenic 187Os/188Os.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Riches, Dr Amy and Lee, Professor Martin and Hallis, Dr Lydia
Authors: Mari, N., Riches, A.J.V., Hallis, L.J., Marrocchi, Y., Villeneuve, J., Gleissner, P., Becker, H., and Lee, M.R.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):1872-9533
Published Online:24 May 2019
Copyright Holders:Copyright © 2019 The Authors
First Published:First published in Geochimica et Cosmochimica Acta 266:416-434
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
697411A journey from the solar nebula to planetary bodies: cycling of heat, water and organicsMartin LeeScience & Technology Facilities Council (STFC)ST/N000846/1GES - GES ADMINISTRATION
519741Follow the water: insights into the martian hydrosphere from nakhlitesMartin LeeScience & Technology Facilities Council (STFC)ST/H002960/1GES - GES ADMINISTRATION