The Final Phase of Drilling of the Hawaii Scientific Drilling Project

The principal goal of the Hawaii Scientific Drilling Project (HSDP) was to core continuously deep into the flank of a Hawaiian volcano and to investigate the petrology, geochemisty, geochronology, magnetics, etc. of the recovered samples. Drilling in Hilo, on the island of Hawaii proceeded in three phases. A 1.06 km pilot hole was core-drilled in 1993; a second hole was core-drilled to 3,098 meters below sea level (mbsl) in 1999, then deepened in 2004-2007 to 3,509 mbsl. Although the final phase of drilling was at times technically challenging, core recovery was close to 100%. All rocks from the final phase of drilling were emplaced below sea level and are from the Mauna Kea volcano. On-site core logging identified 45 separate units (the 1999 phase of drilling yielded 345 units). Five lithologies were identified: pillows (~60%); pillow breccias (~10%); massive lavas (~12%); hyaloclastites (~17%); intrusives (~1%; these are mostly multiple thin (down to cm scale) fingers of magma with identical lithologies occurring over narrow depth intervals). The rocks are primarily tholeiitic, ranging from aphyric to highly olivine-phyric lavas (up to ~25% olivine phenocrysts), with considerable fresh glass and olivine; clays and zeolites are present throughout the core. Forty whole-rock samples were collected as a reference suite and sent to multiple investigators for study. Additionally, glass was collected at roughly 3 m intervals for electron microprobe analysis. Although continuous and consistent with the shallower rocks from the previous phase of coring, there are several noteworthy features of the deepest core: (1) Glasses from shallower portions of the core exhibited bimodal silica contents, a low SiO2 group (~48-50 wt.%) and a high SiO2 group (~50.5- 52 wt.%). Glasses from the last phase of drilling are essentially all in the high-silica group and are somewhat more evolved than the high-silica glasses from the shallower portion of the core (5.1-7.6 vs. 5.1-10.4 wt.% MgO). (2) The expected sequence of lithologies with depth in the core is subaerial lava flows, hyaloclastites (formed by debris flows carrying glass and lithic fragments from the shoreline down the submarine flanks of the volcano), and finally pillow lavas. This sequence was generally observed in the earlier phases of drilling, and it appeared that the deepest rocks from the 1999 phase of drilling were essentially all formed from pillow lavas (i.e., there were no more hyaloclastites). However, thick hyaloclastites reflecting long distance transport from the ancient shoreline reappear in the bottom ~100 m of the drill hole. Although it may be coincidence, pillow breccias occur in the shallower parts of the core from the final phase of drilling, but not in the deeper parts in which the hyaloclastites reappear. (3) Intrusive rocks make up a lower fraction (~1%) of samples from the final phase of coring than in the deeper parts of the section from the 1999 phase of drilling (3.8%). It had been suggested that intrusives might become more common the deeper the drilling, but this is not the case at depths down to 3.5 km. (4) There are three units classified as "massive" including one relatively thick (~40 m), featureless (no internal boundaries, no evidence of mixing or internal differentiation), moderately olivine-phyric basalt.