Volcanism on Mars controlled by early oxidation of the upper mantle

• Published in: Nature (London) Vol. 498; no. 7454; pp. 342 - 345
• Main Authors: Tuff, J., Wade, J., Wood, B. J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 20-06-2013; Nature Publishing Group
• Subjects: 704/2151/209; 704/445/845; Basalt; Chlorine; Craters; Crystallization; Earth; Humanities and Social Sciences; Igneous rocks; Iron; letter; Magma; Manganese; Mantle; Mars; Mars (Planet); Mathematical models; Meteorites; Meteors & meteorites; multidisciplinary; Nickel; Observations; Oxidation; Recycling; Rocks; Science; Space exploration; United Kingdom; Upper mantle; Volcanic rocks; Volcanoes; United Kingdom
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12225

The compositions of the 3.7-billion-year-old surface rocks on Mars — as observed by the Spirit rover at Gusev crater — are shown to be consistent with early mixing of oxidized surface material into the uppermost Martian mantle: such oxidation appears to have had less influence on more recent volcanic rocks, which are sampled as Martian meteorites.
The composition of the Martian mantle (Wood JVD)
Much of our understanding of the chemical composition and evolution of Mars stems from analysis of the 120 or so known Martian meteorites — particularly from the 'SNC' group, a subset consisting of related igneous rocks. When NASA's Spirit rover found that 3.7-billion-year-old surface rocks from the Gusev crater exhibit marked compositional differences from SNC meteorites, which are thought to have formed more recently, questions were raised about the relevance of meteorite-derived data for constraining the composition of the Martian interior. James Tuff
et al
. present a model that can resolve these difficulties in terms of differences in the oxygen fugacity of melting of the sulphur-rich mantle. The composition of the 3.7-billion-year-old surface rocks, they suggest, resulted from early oxidation of the uppermost mantle of Mars. This oxidation may have had less influence on deeper regions of the mantle, which produced the more recent volcanic rocks, sampled by Martian meteorites.
Detailed information about the chemical composition and evolution of Mars has been derived principally from the SNC (shergottite–nakhlite–chassignite) meteorites, which are genetically related igneous rocks of Martian origin
1
,
2
. They are chemically and texturally similar to terrestrial basalts and cumulates, except that they have higher concentrations of iron and volatile elements such as phosphorus and chlorine and lower concentrations of nickel and other chalcophile (sulphur-loving) elements
3
. Most Martian meteorites have relatively young crystallization ages (1.4 billion years to 180 million years ago
4
) and are considered to be derived from young, lightly cratered volcanic regions, such as the Tharsis plateau
4
,
5
. Surface rocks from the Gusev crater analysed by the Spirit rover are much older (about 3.7 billion years old
6
) and exhibit marked compositional differences from the meteorites
7
. Although also basaltic in composition, the surface rocks are richer in nickel and sulphur and have lower manganese/iron ratios than the meteorites. This has led to doubts that Mars can be described adequately using the ‘SNC model’. Here we show, however, that the differences between the compositions of meteorites and surface rocks can be explained by differences in the oxygen fugacity during melting of the same sulphur-rich mantle. This ties the sources of Martian meteorites to those of the surface rocks through an early (>3.7 billion years ago) oxidation of the uppermost mantle that had less influence on the deeper regions, which produce the more recent volcanic rocks.