2002 Scientific Objectives, Hilo Ridge

P. Lipman & B. Eakins, U.S. Geological Survey

Hilo Ridge is one of the most morphologically striking submarine rift zones in Hawaii Islands (Fig. Z). Prominent slope breaks at ~1100 and ~400 m wd mbsl, inferred to mark the end of tholeiitic shield building at Kohala and Mauna Kea respectively, are preserved high on the crest of the Hilo Ridge. Dives on distal Hilo Ridge, previously interpreted as the submarine extension of an east rift zone of Mauna Kea, provide material to test the alternative that it may be the east rift zone of a highly elongate large Kohala edifice [Holcomb et al., 2000; Kauahikaua et al., 2000]; as such it could also provide important samples of early Kohala growth. An exploratory Kaiko dive in 2001, on the distal south flank of Hilo Ridge (K215) recovered alkalic basalt from the lowest sample sites, overlain by picritic tholeiite, that probably sample early growth stages of Kohala, analogous to those recently found along the lower scarp of the mid-slope bench offshore of Kilauea volcano. Preliminary interpretation of shipboard magnetic data, acquired in previous JAMSTEC cruises, indicates that the bulk volume of Hilo Ridge has a reverse magnetic polarity [Y. Kanamatsu, unpubl. data]; as such, much of the growth of this ridge is older than 780 ka, significantly older than subaerial shield-stage tholeiitic lavas of Kohala (~350 ka) or the deepest submarine-emplaced tholeiitic pillow lavas and hyaloclastite recovered from the Hilo scientific drill hole (~450 ka; R. Sharp, oral commun., 2001).

Another important problem is the degree of similarity/differences among tholeiites from the successive Kea-trend volcanoes on Hawaii Island (Kilauea, Mauna Kea, Kohala) that have been potential sources of volcaniclastic deposits sampled in Kilauea’s mid-slope bench, Hilo scientific drill hole, and distal turbidites. Reliable compositional data are now available for Kilauea (young lavas, marine samples) and Mauna Kea (drill hole), but not from Kohala because of the ubiquitous weathering of its subaerial tholeiite. Basalt samples from the deep-water distal slopes of Hilo Ridge, if determined to be a Kohala rift zone, will have far better quality for petrologic study than subaerial tholeiitic material from Kohala, and should provide some of the first reliable materials to characterize this large Kea-trend volcano.

Year 2002 dive targets: The attached map (Fig. Z) shows two recommended dive sites on the north flank of Hilo Ridge, based mainly on the available JAMSTEC bathymetry, geologic interpretation [Smith et al., 2002], and analogies with successful sampling strategies along the lower south flank of Hawaii Island. Emphasis is on deep parts of the distal north flank, to complement last year’s exploratory Kaiko dive on the south flank, where steep-sided terraces and spurs will permit sampling of even deeper flanks of the rift zone. Prior to last year, this ridge had not previously been studied by diving, and compositional date for only a few (4?) rock samples have been reported from dredging.

Work plan: The new dives on Hilo Ridge will closely interface with the abundant existing subaerial data and several other in-progress studies on Hawaiian volcanoes. Successful interpretation of the stratigraphic, structural, and petrologic complexities of Hilo Ridge, for which questions far outnumber answers, has critical implications for structural evolution of Kohala and Mauna Kea, and also for understanding the primarily constructional growth of the submarine ridges of oceanic volcanic islands.

We will use submersible visual/video data and gravity, magnetic, and marine seismic profiling data to interpret the structure of Hilo Ridge. Samples will be analyzed chemically and petrographically in order to determine compositions and eruption depths of pillow lavas and the fragments in volcaniclastic rocks. Analytical methods will include major and trace elements for bulk-rock samples by XRF, INAA, and solution-based ICP-MS methods, glass compositions by electron-probe (EPMA), laser-ablation ICP-MS, and ion-probe analyses, and volatile contents by EPMA and FTIR measurements. Radiogenic and stable isotopic compositions of pillow- glass, glass-sand, and whole-rock samples will be compared to analogous data from other Hawaiian volcanoes, especially with assured submarine samples of Kohala volcano from the Laupahoehoe slump, and the detailed data emerging for Mauna Kea from the Hawaii Scientific Drilling Project. We will use a combination of dating techniques, including K-Ar, and ⁴⁰Ar/³⁹Ar methods, to determine the eruption ages of any high-K basalt samples.

References:

Holcomb, R.T., B.K. Nelson, P.W. Reiners, and N.-L. Sawyer, Overlapping volcanoes: The origin of Hilo Ridge, Hawaii, Geology, 28, 547-550, 2000.

Kauahikaua, J., T. Hildenbrand, and M. Webring, Deep magmatic structures of Hawaiian volcanoes, imaged by three-dimensional gravity models, Geology, 28, 883-886, 2000.

Smith, J.R., K. Satake, J.K. Morgan, and P.W. Lipman, Submarine landslides and volcanic features on Kohala and Mauna Kea Volcanoes and the Hana Ridge, Hawaii, in Hawaiian Volcanoes: Deep Underwater Perspectives, edited by E. Takahashi, P.W. Lipman, M.O. Garcia, J. Naka, and S. Aramaki, pp. 11-28, American Geophysical Union, Washington, DC, 2002.