Aqueous Alteration of Two Drill Core from Hawaii: A Geochemical Analog to Alteration in the Martian Subsurface

The discovery of aqueous alteration minerals found in exposures of ancient Martian crust supports the role of groundwater or some sort of subsurface fluid interaction in their formation. This work focuses on the contribution of subsurface terrestrial analogs from Hawaii to understand potential formation pathways for crustal and stratigraphic alteration minerals observed on Mars. In 2013 and 2015, two drill cores, KMA and PTA were made in the Humu'ula Saddle region of Hawaii for groundwater and geothermal resource exploration. Both drill cores reached depths of over 1 km below the surface and are horizontally separated by about 10 km. Using infrared spectroscopy, and higher resolution methods such as x-ray diffraction, scanning electron microscope, and x-ray fluorescence, rocks recovered from depths between ~600 m to ~1.5 km below the surface are observed to have altered with the regional groundwater, with preservation of primary mafics. The PTA2 core was recovered from a relatively warm well, and has alteration indicative of a near neutral pH environment, with primarily olivine altering to phyllosilicates such as saponite and zeolites. The KMA1 core was recovered from a much cooler well, with evidence that the rocks experienced acid-sulfate alteration, resulting in heavily oxidized basalt, and zonation of sulfate, carbonate and clay minerals. Alteration in KMA1 correlates with drilling data suggesting perched aquifers at specific depths. The contrast in alteration mineralogy, and regional ground water of the two drill cores offer a promising Earth analog environment for localized Martian subsurface aqueous geochemistry. This suggests that the subsurface alteration pathways for basalt can vary dramatically in a relatively small spatial context. This has important implications for future subsurface exploration and the scale of spatial heterogeneity that could be encountered.