Organics and Microalteration textures in Olivines from a Martian Meteorite: Lessons for seeking life in ultramafic rocks and impact structures on Earth

Rover missions and remote sensing surveys seeking signs of life on Mars have documented abundant evidence for liquid water and located potential habitable environments, but the robust identification of traces of life remains elusive. Martian meteorites provide an alternative means of exploring the nature of aqueous alteration processes and tentative biosignatures from Mars. The Nakhla group of martian meteorites contain direct evidence of fluid-rock interaction in the form of pre-terrestrial iddingsite alteration veins that cross-cut the igneous minerals. Previous studies have reported microtunnels in the altered olivine grains that have been tentatively compared to microtextures found in seafloor volcanic rocks of inferred biological origin, and also spheres of organic carbon of uncertain origin embedded in the iddingsite veins.

Here I describe high-sensitivity in-situ chemical and morphological mapping to characterize microalteration textures in olivines of the martian meteorite Yamato 000593. High-resolution imaging shows that the altered olivine crystals have angular and micro-brecciated margins and are also highly strained due to impact induced fracturing. The shape of the olivine microalteration textures is in no way comparable to microtunnels of inferred biological origin found in terrestrial volcanic glasses and dunites, and rather I argue that the Yamato 000593 microtextures are abiotic in origin. Correlative electron microscopy and mass spectrometry data reveals amorphous organic carbon occurring as bands and sub-spherical concentrations <300 nm across in vein filling iddingsite that extends deep into the olivine microalteration textures. The distribution of carbon within early alteration products of olivine supports an origin from hydrothermal alteration on Mars.

It is postulated that a Martian impact event produced the micro-brecciated olivine crystal margins in Yamato Y000593 that reacted with subsurface hydrothermal fluids circulating in the impact crater to form the early organics. Both the olivine microalteration textures and organic matter found in Yamato 000593 record abiotic processes in an impact induced hydrothermal system on Mars and are not reliable biosignatures. The results of this study have implications for seeking life in ultramafic rocks and impact structures on Earth and Mars that will be explored.