Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars


The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear1. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts1,2 or in mixing between enriched and depleted mantle reservoirs3. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet’s history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth4.

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Figure 1: Sulphur isotopic compositions of shergottite AVS.
Figure 2: Sulphur isotopic compositions of nakhlites and ALH 84001.
Figure 3: Reflected-light images of MIL 090030, paired with MIL 03346.
Figure 4: Covariation between Δ33S and Δ36S in different groups of Martian meteorites.


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We acknowledge the Meteorite Working Group, L. Welzenbach, T. McCoy, S. Ralew, M. N. Rao, L. Nyquist, J. Zipfel, C. Smith, H. Kojima, A. Treiman, T. Bunch and B. Zanda for providing meteorite samples analysed in this study. We also thank P. Piccoli for assistance with electron microprobe analyses. The manuscript benefited from independent reviews by M. Thiemens, P. Cartigny, S. Ono, D. Johnston and B. Wing during revision. The UCLA ion microprobe facility is partly supported by a grant from the US National Science Foundation Instrumentation and Facilities Program. This work was supported by NASA Cosmochemistry grants NNX09AF72G and NNX13AL13G to J.F.

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H.B.F. and J.F. designed the study and wrote the manuscript. All authors contributed to data collection, data interpretation and editing the manuscript.

Corresponding author

Correspondence to Heather B. Franz.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Sulphur concentrations for shergottites and Chassigny.

Asterisks indicate samples for which not all fractions were recovered. Uncertainty (1 s.d.) is estimated as ±2% of values.

Extended Data Figure 2 Covariation between Δ33S and Δ36S in different groups of Martian meteorites, with comparison to iron meteorite FeS, Archaean samples and products of laboratory photochemical experiments accessing different experimental conditions and ultraviolet wavelengths.

Diamonds, nakhlites; circles, ALH 84001; triangles, shergottites; square, Chassigny. Includes data from other studies: Martian meteorites5,6, iron meteorites11, Archaean data4 and photochemistry data (refs 5, 34 and ref. 89 in Supplementary Information). Source data

Extended Data Figure 3 Plot of Δ33S versus δ34S showing arrays for mixing between compound A and compound B (solid black line), and Rayleigh fractionation with formation of compound B at the expense of compound A.

The dotted black line is the array of compositions formed for residual reactant (A). The dotted grey line is the array of compositions formed for the instantaneous product (B). The solid grey line is the array of compositions formed for the accumulated product (B). The calculations assumed fractionation factors between compound B and compound A of 0.9739409 for 34S/32S and 0.9864855 for 33S/32S, which are similar to those observed between sulphide and sulphate at 250 °C. This plot illustrates how non-zero Δ33S can be produced by mixing and Rayleigh fractionation involving fractionated endmembers. The scarcity of evidence for highly fractionated δ34S in the data collected for Martian meteorites argues against this process as an origin for the non-zero Δ33S in the nakhlites, and by extension, the other Martian meteorites.

Extended Data Figure 4 Back-scattered electron (BSE) image and Ca, Ti, P, S, Fe, Cl and K X-ray maps of MIL 03346, section 118.

Note the fine filaments of phosphate-rich materials and pervasive high-K content of the intercumulus matrix. Sulphides are restricted to the intercumulus matrix and range in size from ≥20 μm to <1 μm. Scale bars, 100 μm.

Extended Data Figure 5 Reflected-light images of representative sulphide grains in MIL 03346, sections 6, 93, 104 and 132.

All images, except k and l (mag., ×20), are at ×50 magnification. Images a, b, c, f, i and l are from MIL 03346, section 6, and images d and e, h and k, and g are from MIL 03346, sections 104, 93 and 132, respectively.

Extended Data Figure 6 Back-scattered electron images of sulphides in MIL 03346, section 6 (c) and MIL 03346, section 93 (a, b).

Note the fractured appearance of the sulphides and occurrence of Fe(OH) in the cracks and along the edges of matrix-exposed sulphides, as well as the presence of a sulphate ‘plume’ in b, giving a smoky appearance to the enclosing matrix material. Scale bars, 100 μm.

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Franz, H., Kim, S., Farquhar, J. et al. Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars. Nature 508, 364–368 (2014).

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