Comprehensive characterization and geochemical alteration pathways of drill core from the Humu'ula Groundwater Research Project, Hawaii, USA: I. Pohakuloa Training Area

Abstract

Scientific analysis of the PTA-2 drill core focuses on the investigation of mafic mineralogy through long-wave infrared (LWIR) spectroscopy, laboratory scanning electron microscopy with energy-dispersive spectroscopy (SEM EDS), and bulk X-ray fluorescence (XRF) geochemical analysis. We performed both LWIR and short-wave infrared (SWIR) imaging of thin sections and cut sections at high spatial resolution. These analyses complement prior work and provide additional information in order to understand the mineralogical transformations and geochemical processes that have occurred in the subsurface. LWIR spectroscopy allowed us to quantify specific mineral phases prevalent in shield phase basalts – which are largely comprised of Ca-feldspars, pyroxenes, and olivine. Collected spectra also showed the influence of alteration components on the spectral signatures. New analyses using SEM EDS and bulk XRF geochemistry, augment the spectroscopy and offer valuable insights into the mineralogy and fluid-rock interactions. Petrographic images confirmed the varying mineral textures of basalt, ranging from fine-grained, to more prominent phenocrysts in the form of olivine cumulates. Data acquired from whole rock geochemistry was instrumental in determining fluid-rock interactions by showing that elemental components are not lost with alteration. This study provides the first comprehensive analysis of subsurface drill core away from active margins, and a foundation for further subsurface core logging and research.