Hydrothermal Systems in the Early Solar System: The Record in the Rocks

Water ice has been present throughout the entire solar system, and it has melted in many environments. All of the most ancient hydrothermally-altered rocks from the earliest solar system appear to derive from asteroids, as sampled by meteorites. Asteroid classes are arranged in a concentrically zoned pattern. The most reduced asteroids are closest to the sun, and the most volatile rich are farthest away. Today, water ice is present on the surfaces of the P and D asteroids, which must delineate the original (°)snow line of the solar system. Despite this, members of all asteroid classes have at some time contained liquid water, and experienced aqueous alteration. The mineralogical products of aqueous alteration most commonly encountered within meteorites include phyllosilicates, hydroxides, tochilinites, sulfates, oxides and carbonates. Many organic compounds found within carbonaceous chondrites are believed to have been produced during aqueous and hydrothermal activity upon preexisting, perhaps interstellar, organic compounds. Clearly, liquid water was abundant on the parent asteroids of carbonaceous chondrites. Among the most challenging aspects of understanding aqueous alteration in asteroids is constraining the chemical and isotopic compositions of the altering fluids, in terms of both their chemistry and isotopic composition. A unique opportunity to study the composition of asteroidal fluids directly has come from the discovery of fluid inclusions in halite in two equilibrated ordinary chondrites and in Ca-carbonates in carbonaceous chondrites. Primary and secondary fluid inclusions are present in these meteorites. Vapor bubbles are rare in fluid inclusions in chondrites, suggesting that the trapping of the fluids occurred at temperatures 100 °C, perhaps in the range 25-50 °C. Halite in the ordinary chondrites appears to have formed by precipitation from concentrated brines. Analyses of the hydrogen isotopic composition of the fluid inclusion liquids will finally establish the origin and evolution of these fluids, and shed new light on the origin of water on the terrestrial planets. The oxygen isotopic systematics of aqueously-altered chondrites have been frequently discussed. Isotopic variability in minerals within individual carbonaceous chondrites appear to represent a record of changing fluid composition and varying temperatures with progressive alteration on the parent asteroids.