Endolithic Colonization of Fluid Inclusion Trails in Mineral Grains

Endoliths in sandstones: Many scenarios for the colonization of planetary surfaces by microbial life involve endoliths [1, 2]. The need to avoid the damaging effects of ultraviolet radiation makes colonization of the immediate sub-surface desirable, particularly at an early stage of evolution before screening pigments have been developed [3, 4]. Minimal (millimetreto sub-millimetre-scale) penetration of the surface may filter ultraviolet radiation but admit enough light for photosynthesis. Endolithic borings in minerals are found through the terrestrial geological record at least back to the Proterozoic [5], and are an important potential trace fossil that may be sought in extraterrestrial materials [6]. Endolithic communities in sandstones have been studied in detail, particularly in the quartzose Beacon Sandstone in the dry valleys of southern Victoria Land, Antarctica [1, 2, 7], where they inhabit a hostile (cold, dry) terrestrial environment. These rocks also contain permineralized fossil endoliths [1]. Cyanobacteria inbetween sand grains in the Beacon Sandstone can photosynthesize because the quartz grains are highly translucent. The present study records that some of the microbial mass colonizes microfractures in the quartz grains, i.e. it is intragranular as well as intergranular. Fluid inclusions: Fluid inclusions are micronscale volumes of fluid trapped during mineral growth. In most cases the fluids represent the ambient aqueous fluid present during growth, although entrapment of oils and gases is also possible. Aqueous fluids are not pure water, but contain dissolved salts: most aqueous inclusions are more saline than sea water, in some cases substantially so. Some fluid inclusions are formed during mineral deposition; others are formed during healing of microfractures. Microfractures are ubiquitous in quartz grains in sandstones, although commonly only evident under cathodoluminescence [8] because they are very rapidly healed by new quartz growth. Microfractures are even developed in incompletely consolidated sands [9], so that sand that has only experienced shallow burial can be microfractured. On Earth, much sand has experienced multiple cycles of deposition, burial, uplift and erosion, and contains multiple sets of microfractures. On planetary surfaces that have been subject to repeated, widespread impact events, microfracturing should be more intense: planar deformation features (pdfs) in quartz sand grains are an expression of this [10]. The microfractures are marked by linear trails of fluid inclusions, and polished surfaces of grains show trails of pits representing the exhumed inclusions (Fig. 1).