Chemistry, petrography, and hydrothermal alteration of basalts from Hawaii Geothermal Project Well-A, Kilauea, Hawaii

Abstract

A successful 1962 m geothermal test well was drilled in the east rift zone of Kilauea volcano on the island of Hawaii in early 1976. Cores and cutting chips have been analysed chemically, petrographically, and by X-ray diffraction for hydrothermal alteration products. All of the lavas are quartz-normative tholeiitic basalts. Although scatter is pronounced, plots of oxides vs. depth in the well show that a decrease occurs in SiO2, FeO, Na2O, K2O, MnO, P2O5, and TiO2; an increase is noted in CaO, MgO, and Fe2O3. Cu is uniform below 500 m and Cr shows no trend. Most of these observed trends do not agree satisfactorily with leaching and enrichment patterns which should be seen if changes in chemical composition are due to basalt-sea water interaction. MgO variation diagrams show a negative correlation for most oxides. Total FeO and Al2O3 are invariant and CaO is covariant. These changes may possibly reflect differentiation of the source region with time, or differentiation of “batches” of magma periodically injected into the east rift zone which could account for the scatter. However, without probe analyses or normalizing the data to Al2O3, possible leaching and enrichment effects of groundwater and thermal fluids cannot be dismissed. Thin sections reveal basalts with phenocrysts of olivine, less than six per cent maximum, and clinopyroxene and plagioclase (An75-80), generally less than one per cent each. Texture is mainly intersertal; intergranular and glassy are not uncommon. Vesicularity ranges from moderate to high (30 per cent) in the upper 875 m of the well; and low to absent in the lower portion. Based on filled vs. unfilled fractures and vesicles, three zones of permeability can be identified: 0-875 m, highly permeable; 1113-1835 m, poorly permeable; core 10 at 1962 m, moderately permeable. Beneath a zone of unaltered lavas, high temperatures and thermal fluids have created three zones of altered lavas, each marked by the dominance of a particular mineral. The uppermost altered zone, 675-1300 m, is characterized by montmorillonite with minor calcite, quartz, zeolites, and chlorite. The second zone, 1350-1894 m, is dominated by chlorite with accessory quartz, actinolite, and montmorillonite. The third zone of alteration is observed beginning in core 9, 1984 m, and is pronounced in core 10, 1959 m. Actinolite is the dominant mineral; chlorite, quartz, and albite are accessory alteration products. The mineral assemblage in core 10 resembles that found in greenschist metamorphism. The changeover temperature from a montmorillonite to a chlorite zone is greater than that found in Icelandic geothermal wells for the same transition. Possibly more acidic water, rock permeability and/or glass content are more significant in the alteration of HGP-A lavas than they are in the alteration of Icelandic basalts.