Downloaded By: [Hein, James R.][Hein, James R.] At: 16:54 25 October 2010 Hydrothermal Mineralization Along Submarine Rift Zones, Hawaii JAMES R. REIN ANNE. GIBBS U.S. Geological Sutvey, Menlo Park, California, USA DAVID A. CLAGUE U.S. Geological Sutvey, Hawaiian Volcano Obsetvatory Hila, Hawaii, USA MICHAEL TORRESAN U.S. Geological Sutvey Menlo Park, California, USA This is the first article to describe mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothennal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rockr and cement for clastic rockr; both deposit types. are composed of todorokite and birnessite. Thin Fe-Mn.crusts that coat some rockr formed by a combination of hydrogenetic and hydrothermal processes and are composed of 8-Mn0 2 • The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (FejMn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rockr leached by circulating hydrothermal fluids, clastic material (in Mn-cemented sandstone), and seawater that mixed with the hydrothermal fluids. Mineralization on Haleakala Ridge occurred sometime during the past 200 to 400 ka, when the summit was at a water depth of more than 1,000 m. Hydrothermal circulation was probably driven by heat produced by intrusion of dikes, magma reserooirs, and flow of magma through axial and lateral conduits. The supply of seawater to ridge interiors must be extensive because of their high porosity and Received 6 Aprill995; accepted 4 Januacy 1996. This article is not subject to U.S. copyright laws. We thank the following USGS scientists: R. Oscarson for help with SEM studies, P. Quintemo for determining microfossil assemblages, J. G. Moore for kindly providing the Su57 samples, and R. A Koski, R. T. Holcomb, and P. Halbach (Free University of Berlin) for helpful reviews of an earlier draft of the article; journal reviewers provided helpful comments. Chemical data were provided by A Bartel, J. H. Bullock, W. M. d'Angelo, L Espos, H. Kirschenbaum, M. J. Malcolm, J. W. Marinenko, C. L. Prosser, N. Rait, G. 0. Riddle, D. Siems, H. Smith, and J. Taggart. We thank the captain and crew of the R.V. Famella. The work was carried out in conjunction with IGCP Project 318. Address correspondence to James R. Hein, U.S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025-3591, USA E-mail: jhein@octopus.wr.usgs.gov. 177 Marine Georesources and Geotechnology, 14:177-203, 1996 1064-119X/96 $12.00 + .00