MINERALOGICAL AND CHEMICAL TRENDS DURING ACID-SULFATE ALTERATION OF HIGH-FE BASALTS AND HYALOCLASTITE AT KRAFLA AND NAMAFJALL, ICELAND: IMPLICATIONS FOR MARS

Hydrothermal alteration in ancient volcanic settings may have formed a variety of secondary alteration minerals (e.g. sulfates) observed on Mars by both orbiters and rovers. We studied the acid-sulfate hydrothermal alteration of Mars-like high-Fe basalt (15.48-15.72 wt. %) and hyaloclastite at Krafla and Námafjall, Iceland, to help interpret the products of alteration in analogous environments on Mars. Both Icelandic geothermal fields feature intense surface alteration in gas- (fumarole) and fluid- (hot spring) dominated settings. The steam-derived acid-sulfate waters have pH values 1.5-4 with depleted Cl (<2 ppm) and enriched SO₄ (477-970 ppm) concentrations. We collected 41 altered rock samples and precipitates in transects with increasing distance from active fumaroles and hot springs to determine how mineralogical and geochemical trends vary. Preliminary XRD results indicate that secondary alteration mineralogy is largely controlled by the level of surface intensity, where high surface activity was defined by a high acid supply (H₂S + O₂ = H₂SO₄) and temperature, lower pH, but a variable redox state. Near steam vents and hot springs amorphous silica, anatase, pyrite, and abundant crystalline elemental sulfur were found. With increasing distance from the vent, sulfates dominate, especially Al-, Fe-, and Ca-sulfates. Phyllosilicates (along with Fe-(hydr)oxides) dominate at lower temperatures and decreasing activity along the margins. Chemical trends reveal that Ca, Mg, Mn, Na, and K were mobilized and leached out of the system, whereas Si and Ti were residually enriched (e.g. up to 89 and 23 wt. %, respectively). Fe and Al have comparative mobility, except in cases of Fe enrichment, and are largely retained in the alteration products. This study shows that redox conditions are a significant environmental control for secondary mineral assemblages, particularly, Fe- and S-bearing phases. A similar environment could have produced ferric sulfates and oxides on Mars.