The Geochemistry and Mineralogy of Surface Hydrothermal Alteration at Nesjavellir, SW Iceland

Abstract

Abundant sulfates have been detected by the Mars Exploration Rover Spirit in the Columbia Hills of Mars, consistent with extensive alteration of basalt by hydrothermal processes. This study uses Iceland’s Nesjavellir geothermal system as an analogue for Columbia Hills hydrothermal alteration. This terrestrial site is home to a variety of acidic and near-neutral waters that are actively altering the Mars-like basalt of host volcano Mt. Hengill. Hydrothermal features heated by H2S gas and phase-segregated steam created oxidizing acid-sulfate conditions at the surface with pH values varying between 3.0 and 2.0 and near-boiling temperatures. Mobilization of cations (FeO, MgO, CaO, Na2O, and K2O) at these sites is due to the extensive leaching. The resulting alteration products include amorphous silica, anatase, native sulfur, iron sulfides, Ca-, Fe-, Mg-, and Al-sulfates, kaolinite, and montmorillonite, and likely nanophase Fe-oxides. Fe-sulfates were the most common sulfates due to the Fe-rich substrate, and several likely formed from the oxidation of the iron sulfide phases. Due to the ubiquitous presence of iron sulfides and native sulfur in the hydrothermal sites of the Nesjavellir field, it is inferred that reducing conditions are dominant at depth, and conditions become oxidizing either at or near the surface due to interaction with atmospheric oxygen. Two hydrothermal streams at Nesjavellir exhibited a white, pure sulfur coating and an iron-rich red biofilm, respectively. The sulfur coating is attributed to the oxidation of H2S under near neutral conditions, while the red biofilm forms under acidic, oxidizing conditions. A nearby travertine spring precipitates travertine deposits that preserve microscopic evidence for microbial activity. The diverse variety of environments present in the Nesjavellir geothermal field have formed distinct deposits that resemble several Martian sites in the Columbia Hills, especially the Paso Robles and Arad localities.