Hydrothermal acid-sulfate alteration at Krafla and Namafjall, NE Iceland: Implications for Gusev Crater and Meridiani Planum, Mars.

摘要

Opaline silica, sulfate, and phyllosilicate-bearing deposits have been detected on Mars from both obiters and rovers, indicating extensive aqueous alteration of the basaltic surface, some of which likely occurred under hydrothermal conditions. Ongoing hydrothermal acid-sulfate alteration of Mars-like high-Fe (15.48-16.27 wt. % Fe2O3T) basalts and hyaloclastites in northeastern Iceland was studied to help interpret the products of alteration in analogous environments on ancient Mars. The Krafla and Namafjall areas feature intense surface alteration in gas- (fumarole) and fluid- (hot spring/mud pot) dominated settings. Influx of H2S gas (H2S + 2O2 = H 2SO4) produced steam-derived acid-sulfate waters with pH values 1.96-2.50 at measured temperatures between 15-92°C, enriched in SO4, Fe, and Al. During alteration, primary igneous phases (plagioclase, olivine, augite, and basaltic glass) reacted with acid-sulfate waters to produce secondary mineral assemblages dominated by amorphous silica, iron-sulfides, Ca/Fe/Mg/Al-sulfates, phyllosilicates (kaolin and smectite groups), and Fe-(hydr)oxides. Bulk compositions of alteration products were controlled by the leaching and mobilization of major elements (e.g. Ca, Mg, Na, K) out of the deposits, while Si, Ti, and Zr were residually enriched. Fe and Al mobility varied significantly, but these elements are largely retained in the products of alteration. These diverse environmental and geochemical processes in the Krafla and Namafjall areas may provide insight into Martian hydrothermal systems, specifically, the sulfate- and silica-rich deposits near Home Plate, Gusev Crater and in the layered sulfate and hematite deposits at Meridiani Planum.