CHARACTERIZATION OF ALTERATION OF LACUSTRINE PILLOW BASALTS: INSIGHTS FROM HYPERSPECTRAL IMAGING AND IMPLICATIONS FOR WATER-ROCK INTERACTIONS ON MARS

Hydrothermal alteration generates mineral assemblages that are dependent upon degree of water-rock interaction and fluid chemistry. Understanding remotely-sensed signatures of these systems on Earth provides insight into past environments on Mars. We are studying altered ~187 Ma Hartford Basin lacustrine pillow basalts exposed in Meriden, CT, as a potential analog for volcanically resurfaced paleolakes on Mars. We present signatures of hydrothermal alteration at high spatial resolution with hyperspectral imagers similar to those proposed for future landed planetary missions, correlate spectral measurements with mineralogy and chemistry from other microscale measurements, and scale these analyses to hand samples and outcrops.

A suite of samples including a thick section across an alteration rind and corresponding hand sample were imaged at Headwall Photonics with their hyperspectral imagers (0.4 to 2.5 µm), and outcrops were imaged with Channel Systems imagers (0.42-1.1 µm). Electron microprobe and µXRD analyses of the thick section further characterize the alteration and correlate with imaging data. Bulk major element chemistry (ICP-AES and CN analyses) and mineralogy (inXitu Terra XRD) analyses were done on a variety of samples.

Hyperspectral analyses of the thick section show a hydrated Fe/Mg-bearing phase and Fe³⁺_­­­-bearing high-Ca pyroxene in the rind and an oxidized, anhydrous, mafic interior with calcite amygdules throughout. Microprobe analyses show that glass, albite, and calcite dominate the sample mineralogy. The glassy matrix of the rind is enriched in Fe, Mg, Mn, and Ca and depleted in Al, Na, and Si relative to the interior. µXRD results show varying proportions of albite and diopside throughout the sample along with hematite in the interior only. Locations of chemical, mineralogical, and spectral boundaries differ, providing important complementary information concerning the nature of altering fluids and the timing of alteration. The samples were likely altered initially after emplacement and metasomatized later. These results scale to hand samples and outcrops.

Other types of alteration present at the outcrop include baking of the underlying sediments as well as red coatings and alteration at the base of the basalt above the sediment contact. Analyses of these units are ongoing.