MINERALOGY IN DRILL CORE FROM HAWAII: AN ANALOG FOR MARS

The upper surface of Mars is predominantly basaltic in composition, with large volcanic constructs that occur over presumably stable mantle hotspots or long-lived volcanic centers. Flood basalts and lava flows are widespread. Aqueous alteration of these basaltic precursors has been pervasive in the oldest (Noachian) terrains that are heavily cratered and linked to weathering, hydrothermal and diagenetic alteration. Hawaii has long been studied as a Mars analog, primarily through surface studies and relationship to mineralogy identified from orbit or in-situ.

The Humu’ula Groundwater Research Project (HGRP) drilled their first continuously- cored hole in the saddle region of the big island of Hawaii in March of 2013. The core traverses various sub aerial lava flows, representing the shield-building phase of the island and the lithology is dominantly basalt with varying amounts of plagioclase and olivine phenocrysts. Both pahoehoe and aa textures are identified as well as some explosive units and limited sedimentary units. Logging of HGRP-1 noted that discontinuous alteration became prevalent starting at ~ 1km depth. Over 780 spectra were collected on HGRP-1 from depths of 3190 to 5785 feet in May of 2014, primarily focused on alteration mineralogy. Based on that analysis, 25 cut sections were taken to explore the mineralogy in more detail, and eight of these were examined with the Ultra Compact Imaging Spectrometer (UCIS), yielding spectral mineralogy at ~ 80 μm/pixel.

The data show both notable changes in the primary mafic mineralogy and alteration suites that include a variety of Mg-Fe clay minerals and several types of zeolites, all phases that have been identified as crustal alteration products on Mars. This presentation will focus on the changes in mafic mineralogy identified in the bulk spectra and cut sections. A companion abstract focuses on the alteration products. Vertical variation in plagioclase and pyroxene composition is noted, and in the high resolution data, unaltered olivine and pyroxene phenocrysts occur in the groundmass adjacent to highly altered vugs, and are preserved throughout the section surveyed. Given the limited alteration and abundant preservation of olivine to depths of 1.5 km, the core may be representative of near- or sub-surface alteration in moderate pH environments on Mars.