CHARACTERIZING THE CHANGES IN MAFIC MINEROLOGY OF HYDROTHERMALLY ALTERED BASALTIC DRILL CORE FROM THE HUMU'ULA SADDLE, HI

The Hawaii Groundwater and Geothermal Resources Center (HGGRC) drilled investigative boreholes in the Humu’ula Saddle, Hawaii with the goal to provide new insight into the groundwater and geothermal resources in Hawaii. A sequence of shallow perched aquifers were encountered during drilling; however, they were underlain by the regional water table that was observed to have higher temperatures, that increased with depth at ~160 C/km. One of the drill cores recovered, PTA-2, shows moderate hydrothermal alteration at depth within pore spaces and fractures. These minerals included Fe-Mg smectites and zeolites, which are indicative of low to moderate temperature alteration in neutral to sub basic pH conditions, within the subsurface - removed from atmospheric interaction. This study provides an examination of changes in mafic mineralogy in both spectroscopic and petrographic analysis, complementing prior studies of alteration minerals from mainly pahoehoe units in the drill core. Using long-wave infrared spectra and linear mixing models, we are able to determine semi-quantitative abundance of minerals within the pyroxene, olivine, feldspar, Fe-Mg phyllosilicates, and zeolite mineral groups. SEM EDS work on cut thick sections, confirmed the endmembers of mafic minerals identified in linear mixing models, and provided elemental distribution. Olivine phenocrysts, dominant in upper portions of the drill core, disappear at depth, leaving higher concentrations of pyroxene, spinel, oxide and volcanic glass minerals. Most notably, heavier elements such as Ti, Cr, and As become more abundant within mafic compositions in lower units. Although conclusions drawn from the study are consistent with prior observations of the PTA-2 drill core, the geochemical and textural variations between different lava flows are still unknown. Therefore, having a definitive characterization of mafic minerals in the PTA-2 drill core is incomplete. Future work will obtain additional measurements and samples from the upper portion of PTA-2 and the second drill core, KMA-1.