Rock Coring Koolau Volcano, Hawai’i: Implications for Deep Mantle Recycling of the Crust
Michael O. Garcia University of Hawai’i
Mantle plumes produce basalts which provide fundamental information on the composition and history of the mantle.Ê The Hawaiian plume is the classic example of a mantle plume and its basalts are unquestionably the best studied suite from any plume. The subaerially exposed lavas of Koolau Volcano belong to the Enriched Mantle1 endmember of ocean island basalts and they define a geochemical endmember among Hawaiian shield lavas in major and trace elements and in isotopes. Koolau lavas are important to an understanding of the origin and evolution of the Hawaiian plume and the mantle. In particular, Koolau lavas appear to provide the strongest evidence for deep mantle recycling of crust (sediments and basalt), although this interpretation remain controversial and is based on sampling only the uppermost veneer of the volcano. It is important to establish the longevity of the distinctive Koolau geochemical signature by sampling deeper and older lavas from the volcano. If there were basaltic and sedimentary components in the Hawaiian plume, were they restricted to the final stages of Koolau volcano’s growth? Scientific drilling will allow us to answer this question by obtaining lavas from subsurface of this enigmatic volcano.
Figure 1 – Schematic drawing and photos of Kalihi Shaft Well drilling.
Funding in support of this project has been obtained from the U.S. National Science Foundation and from several U.S., German and Japanese research groups that will support collecting about 300 m of rock cores from the base of the Kalihi, a 345 m deep, water observation hole, which has just been drilled by the City of Honolulu. This coring will extend below any surface exposures of the volcano’s lavas and it is expected that the rocks obtained will have experienced less topical weathering than surface rocks. As part of this project, we will systematically log the chips obtained from the upper part of the well. The cores from the lower 300 m of the well will be logged following procedures we have developed for the Hawaii Scientific Drilling Program. We have assembled an international team of experts to make a thorough petrographic and geochemical characterization of the core and chips from the Kalihi hole as part of this one year project. This will include petrography, mineral and glass chemistry by microprobe, and whole-rock major and trace element (Rb, Sr, Y, Zr, Nb, V, Ni, Cr, Zn, by XRF , and all REE, Ta, Ba, Nb, Th, Hf, U, Pb, Rb, Cs by ICP-MS) compositions for approximately 50 lavas flows and O, Pb, Sr, Nd and Hf isotope ratios for select samples. Several of the flows will also be dated by Ar-Ar.
DOSECC has provided core drilling services for the Koolau Drillling Project that is under the direction of Dr. Michael Garcia of the University of Hawaii. Their CS1500 rig was used for deepen an existing well on the Kalihi Board of Water Supply Site (Figure 3.) The drilling project began on April 14 and drilling ended on May 24, 2000.
The Kalihi Shaft Well had previously been drilled to a depth of 1150 feet. 8-5/8″ casing extends from the surface to a depth to 150 feet; beneath that, the well has no casing. In order to core the bottom part of the well, a temporary liner is installed from the surface to 1150 feet. The core drilling is being done through this temporary liner.
The coring uses rod with an outer diameter of 3.65 inches and collects core of 2.4 inches. The coring assemblies are shown on the right. The rock is cut using a diamond core bit, and the core sample is fed into a 10 foot core barrel. When this barrel is full, it is retrieved to the surface using a wireline while the core bit and rods remain in the hole.
The major funding for the drilling project is from the University of Hawaii, NSF and ICDP, with additional funding from the following institutions: California Institute of Technology, Massachusetts Institute of Technology, University of California at Berkeley. Woods Hole Oceanographic Institute, Tokyo Institute of Technology, Carnegie Institute of Washington, Max-Plank-Institute fur Chemie.
Figure 2 – University of Hawaii geology and geophysics professor Mike Garcia, examines a core sample taken from the Koolau mountains. Graduate research assistant Eric Haskins works in the background. (George F. Lee, Star-Bulletin)
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Figure 3 – DOSECC’s CS1500 drilling rig at the Koolau drillsite.