Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise A.K. Barker1*, L.A. Coogan1, K.M. Gillis1, N.W. Hayman2, and D. Weis3 1
School of Earth and Ocean Sciences, University of Victoria, PO Box 3065 STN CSC, Victoria, British Columbia V8W 3V6, Canada Institute for Geophysics, University of Texas, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 787584445, USA 3 Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada 2
*Current address: Department of Earth Sciences, Uppsala University, Villavägen 16, Uppsala SE-752 36, Sweden; E-mail:
[email protected].
5500 20oN
Lava-dike transition zone
5490 5480
o
10 S
Pito Deep
5470
Y (m)
5460
120oW
5450
20 m
90oW
-12 0-4
5440
Fault breccia Veined basalt Wall-rock dike Fault Inferred fault
5430
J2
Drill core from the upper 125 m below the TAG vent field on the Mid-Atlantic Ridge provides evidence for extensive fluid-rock reaction and hydrothermal fluid-seawater mixing in the shallow subsurface area of an upflow zone. The stockwork zone here shows extensive early chloritization of basalts with Mg derived, most likely, from entrainment of seawater (Honnorez, 2003). Delaney et al. (1987) studied three quartz-cemented breccias from the MARK area of the slow-spreading Mid-Atlantic Ridge. They showed that these breccias formed in a region of high-temperature (200–300 °C) rapidly upflowing fluid. Saccocia and Gillis (1995) built on this by studying additional samples from the MARK area and a suite of samples from Hess Deep (formed at the fast-spreading East Pacific Rise). Using fluid inclusion data and chlorite chemistry, Saccocia and Gillis (1995) confirmed that these samples record upflow zones in which high-temperature hydrothermal fluids were focused. Here we present the first detailed study of the alteration associated with a fault formed in, or close to, the axial zone of a modern fast-spreading ridge. Geothermometry and geochemical data demonstrate that large fluxes (water/rock of 50–100) of high-temperature fluids migrated through fault strands. Fluid flow was sufficiently focused that dike samples
