CRYOGENIC OPAL-A DEPOSITION FROM YELLOWSTONE HOT SPRINGS

Abstract

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. © 2007 Elsevier B.V. All rights reserved.