Skiff Bank is a bathymetric and gravimetric high located ~350 km southwest of the Kerguelen Islands in the northern portion of the Kerguelen Igneous Province (KIP). Ocean Drilling Program Site 1139 was drilled at a water depth of 1415 m on Skiff Bank's western edge. Hole 1139A penetrated to a depth of 695 meters below seafloor (mbsf) with sediment recovered from the upper 462 m and igneous basement recovered from the lower 233 m. A total of 19 basement units were identified, including variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks (Units 1–5) and 14 lava flows with intermediate to mafic compositions (Units 6–19) (Shipboard Scientific Party, 2000).
Core observations indicate that the basement units recovered from Hole 1139A display unique alteration patterns, in terms of both the intensity of alteration and secondary mineralogy, when compared to other drill holes from the Kerguelen Plateau. Basement Units 6–17 contain highly altered breccias, commonly corresponding to flow tops, and more massive and somewhat less altered flow interiors. The breccias are highly oxidized and cemented by calcite and clay minerals. Calcite veins and stringers are common throughout, and the groundmass has a bleached appearance due to the replacement of primary igneous minerals by secondary calcite. The lowermost basement Units 18 and 19 are highly to completely altered by intense oxidation to hematite or by nearly complete bleaching to produce a final mineral assemblage that includes quartz, sanidine, and siderite. Veins within the bleached zones contain hematite, quartz, siderite, and calcite.
To better assess the mass transfer of major elements associated with these unique alteration patterns, we have conducted a detailed chemical analysis of both major and trace elements in samples from the basement units of Hole 1139A. These data are reported here, along with whole-rock analyses for carbon, sulfur, and ferrous/ferric ratios (Table T1). Downhole trends for SiO2, C, Fe3+/Fe total, and loss on ignition (LOI) are shown in Figures F1, F2, F3, and F4. The trends serve to quantify the distinct alteration observed in some basement units within Hole 1139A. For example, carbon increases dramatically below 570 mbsf and shows large enrichments in basement Units 5–19 (Fig. F2). The data also show that iron becomes more reducing with depth overall, with Fe3+/Fe total ratios dropping to as low as 0.13 in basement Unit 18 (Fig. F3). The large variations in this ratio with depth probably reflect alteration zones with variable amounts of hematite and siderite. These unique alteration assemblages and whole-rock compositions were likely produced by CO2-rich hydrothermal fluids interacting with intermediate to felsic volcanic rocks at high water-to-rock ratios. The fluids involved were probably unlike hydrothermal fluids that affected other parts of the KIP.
1Saccocia, P.J., Teagle, D.A.H., Telford, R.H., Eastley, N., and Brewer, T.S., 2004. Data report: Downhole major and trace element chemistry of volcanic rocks from Skiff Bank, Kerguelen Plateau (ODP Site 1139). In Frey, F.A., Coffin, M.F., Wallace, P.J., and Quilty, P.G. (Eds.), Proc. ODP, Sci. Results, 183 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/183_SR/018/018.htm>. [Cited YYYY-MM-DD]
2Department of Earth Sciences and Geography, Bridgewater State College, Bridgewater MA 02325, USA. Correspondence author: psaccocia@bridgew.edu
3Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton SO14-3ZH, UK.
4Department of Geology, University of Leicester LE1 7RH, UK.
Initial receipt: 21 May 2004
Acceptance: 26 October 2004
Web publication:
20 December 2004
Ms 183SR-018