Cryogenic opal-A deposition from Yellowstone hot springs

Sub-zero winter temperatures on the Yellowstone Plateau alter the opal-A precipitation pathway of fluid erupting from hot springs and geysers. Frozen fluid, often only meters from boiling pools, contains abundant opal-A particles, comprising sheet and filament-like aggregations of opal-A microspheres which are formed by opal-A precipitation in brine pockets, channels and veins by natural cryogelling. Unconsolidated cryogenic opal-A sediment accumulates in and below water-ice where it is locked until spring thaw conditions. Sediment is then either remobilized, contributing large volumes of opal-A particulate to geothermally influenced wetlands, or becomes adhered, in situ, by dehydration and cementation. This strongly seasonal opal-A precipitation regime has been overlooked in investigations of sinter deposition, accretion rates and microbe/mineral interactions. Natural opal-A textures recorded from Yellowstone may be replicated simply by freezing and thawing synthetic silica–salt solution in the laboratory. Cryogenic process may have influenced mineral precipitation and sediment accumulation in many other geothermal areas. Particularly, active terrestrial springs located at high altitude/latitude, fossil systems influenced by ancient glaciations, plus potential astrobiological targets e.g. Mars and Europa.