Alteration of Stromatolite Biosignatures: Mineralogic and Textural Consequences of Contact Metamorphism in Extraterrestrial Targets

The recognition of an extraterrestrial fossil record relies on both key life signatures and an understanding of the taphonomic history of the putative fossil deposit. Thus far, the focus of most astrobiology studies on macrofossils (stromatolites) has been defining biosignatures using early Earth examples. Less attention has been paid to the critical aspect of developing a taphonomic history and then decoding the diagenetic and metamorphic effects. Research on Paleoproterozoic biogenic stromatolites associated with a large igneous province intrusion in Minnesota and Ontario (Biwabik and Gunflint iron formations) has quantified significant mineralogic changes without substantial morphologic alteration. The original or early diagenetic oxidized minerals became reduced while organic carbon was oxidized and removed from the system. Closer to the gabbroic contact, more complex silicate minerals (e.g., cummingtonite) were formed while iron carbonates were removed and calcite formed in association with metamorphic magnetite. At highest temperatures at the contact, the original mineralogy and texture was obliterated and replaced with pyroxene and olivine (fayalite). Alteration in association with thinner dikes and sills of basaltic composition was minor in comparison though the carbon was systematically removed and replaced with hematite. Preservation of stromatolite microstructure, including microfossils, survived all but the most intense contact metamorphism. In other locations, stromatolites show deformation due to sill emplacement. On Earth, nearly all stromatolites are composed of carbonate cements or mud. Biogenic stromatolites on extraterrestrial targets would likewise be composed of metastable minerals, though not necessarily carbonate. In near surface conditions, sill emplacement can cause deformation that alters the original fabric. Gabbroic sills intruded stromatolites in the Nash Formation, Medicine Bow Mountains, Wyoming. The stromatolites are preserved in metadolomite though the metamorphism may be due to a regional tectonic overprint. Near the sills, stromatolite megastructures were elongated in the direction of sill emplacement and mesostructures are obliterated. The amount of deformation appears to be related to the sill thickness. In Minnesota, there was no measured alteration of meso- or megastructures and this is likely due to early silicification and the relative thinness of the sills. These results demonstrate that significant alteration can occur due to intrusion of magmatic bodies and the potential is likely in extraterrestrial targets. Our current technology focuses on robotic exploration of outcrops that are either in crater walls, fault scarps, or fluvial channels--likely to also be sites of significant volcanic activity. Investigation of sedimentary units in these locations should take into account a modeled taphonomic history. Critical to all studies of contact metamorphic alteration is the role of water and this will be a major issue in extraterrestrial targets. It is assumed that water is necessary for the original formation of biogenic stromatolites. However, volcanic activity can continue in the absence of appreciable surface water. The availability and distribution of crustal water most be known when modeling the taphonomic history.