Jones, Genge, Carmody Carbonate Melts and Carbonatites 293 structures to be predicted, suggest that radical transformation of carbonate mineral structures occur at lower mantle pressures (Oganov et al. 2008, 2013). Simulations predict that the stable MgCO 3 and CaCO 3 phases at pressures >82 GPa and >19 GPa respectively are dominated by corner-sharing CO 4 tetrahedra networks, with those of CaCO 3 adopting a β-cristobalite structure, and MgCO 3 adopting a pyroxene-like structure at pressures >110 GPa. Such predictions are supported experimentally by recovered Mg-Fe carbonates with polymerized structures (Boulard et al. 2011). The formation of CO 4 4− at high-pressure was predicted (Genge 1994) because it is compatible with the similar electronic configurations of C and Si, which satisfy the requirements of both sp 2 (trigonal) and sp 3 (tetrahedral) hybridization. Transformation from carbonate structures to tetrahedral CO 4 groups, however, requires the breaking of pp bonds and the formation of antibonding orbitals and is likely to be associated with significant activation energy. Metastable carbonate phases, therefore, are likely to be present across the transition region of the mantle at which such transformations occur. Indeed ab initio models predict that numerous energetically similar metastable phases exist for MgCO 3 (Oganov et al. 2013).

Hydrothermal environments driven by volcanism are prime astrobiological targets on Mars, due to their ability to support and preserve microbial ecosystems. Volcano -ice interactions on On Earth, volcano -ice interactions produce many hydrothermal habitats available to microbial colonisation, and thus provide an analogue to past environments on Mars, where many landforms have been attributed to volcano -cryospheric interaction. However, Mars exploration urgently requires a framework for identifying such environments on a range of scales and with a range of geological criteria. In this paper rRemote sensing data were combined with sub-mm environmental mapping and sample analysis that included (X-ray diffraction, Raman spectroscopy, thin section petrography, scanning electron microscopy, electron dispersive spectrometer analysis, and dissolved ion water chemistry,) to characterise samples from two areas of basaltic volcano -ice interaction: namely Askja and Kverkfjöll volcanoes in Iceland. Askja was erupted subglacially during the Pleistocene, and is now exposed within a volcanic desert. Kverkfjöll is a subglacial volcano beneath on the northern margin of Vatnajökull ice cap, and hosts active hydrothermal systems. NE-trending fissure swarm ridges extend between these two volcanic systems. Multiple Holocene glacial outburst (jökulhlaup) sedimentary deposits lie to the north of Kverkfjöll. Hydrothermal environments at Kverkfjöll were found to be predominantly acidic, with dissolved sulphate dominating the water chemistry. These hydrothermal environments vary across a small (<100 m) spatial scale, and include hot springs, anoxic pools, meltwater lakes, and sulphur-and iron-depositing fumaroles. Biomats, two in association with individual goethite and pyrite mineral terraces, were common at Kverkfjöll. In-situ and laboratory VNIR (440 -1000 nm) reflectance spectra representative of Mars rover multispectral imaging show spectral profiles to be influenced by Fe 2+/3+ -bearing minerals. Overall, sediments and lavas display two types of hydrothermal alteration: a low-temperature (<120°C) assemblage dominated by palagonite, sulfates, and iron oxides; and a high-temperature (>120°C) assemblage signified by heulandite and quartz. This Overall, this work provides a framework for identifying such environments during future exploration of Mars, given their high astrobiological potential, and Claire Cousins -BBK 14/08/12 3 provides a descriptive reference for the two prominent active hydrothermal environments at Kverkfjöllwhich can be used as analogues for those on Mars.

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