HR: 17:45h
AN: V12J-08    [PDF]
TI: An Estimate of Fluid-Flux in Fast-Spreading East Pacific Rise Crust Exposed at Hess Deep
AU: * Gillis, K
EM: kgillis@uvic.ca
AF: SEOS, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 Canada
AU: Coogan, L
EM: lac8@leicester.ac.uk
AF: Dept. of Geology, University of Leicester, University Road, Leicester, LE1 7RH United Kingdom
AU: Pedersen, R
EM: rolf.pedersen@geol.uib.no
AF: Dept. of Geology, University of Bergen, Allegaten 41, Bergen, N-5007 Norway
AB: Hydrothermal circulation at mid-ocean ridges (MORs) has a profound influence on the heat and chemical budgets of the oceanic lithosphere. A geochemical approach for examining the hydrothermal fluid flux at MORs is based on the exchange of Sr-isotopes during fluid-rock interaction recorded by hydrothermally altered rocks. We present a new Sr isotopic dataset for sheeted dykes recovered from tectonic scarps at Hess Deep that in order to evaluate the controls on Sr mobility and Sr-isotopic exchange and calculate the time-integrated fluid flux of this section of fast-spreading East Pacific Rise crust. Similar to hydrothermal alteration patterns evident from mineral assemblages and O-isotope data, Sr-isotope data are spatially complex and do not systematically change with depth. All samples have $^{8}$$^{7}$Sr/$^{8}$$^{6}$Sr ratios that are shifted towards higher values (0.70251 to 0.70391) than fresh mid-ocean ridge basalt (MORB)(0.70245), due to interaction with seawater-derived, hydrothermal fluids. Strontium concentrations vary from 72 to 121ppm; some dykes are Sr-depleted, others are Sr-enriched, whereas others have Sr contents typical of fresh MORB. Sr mobility is mineralogically controlled such that Sr is liberated where plagioclase is replaced by chlorite plus quartz and is enriched where a small amount ($<$2%) of epidote is present. Interestingly, the degree of albitization was found to have little control on Sr mobility even though the albite structure favours Sr partitioning. Sr-isotopic exchange was far less influenced by secondary mineral assemblages, except for epidote-bearing samples. In order to gain an estimate of the time-integrated fluid flux we have applied a linear-kinetic fluid-rock exchange model applied by Teagle et al. [2003] for sheeted dykes recovered at ODP Hole 504B. The fluid flux of 1.1 x 10$^{6}$ kg$^{2}$/m for Hess Deep is lower than that calculated for Hole 504B dykes and far lower than fluid flux estimates based on thermal constraints. We explore the significance of these low fluid fluxes and suggest that the Sr-isotopic budget of hydrothermally altered sheeted dykes is not set during vigorous flow associated with black smokers. Instead, we predict that Sr isotopes are exchanged during less vigorous flow, at or near a ridge axis.
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 8135 Hydrothermal systems (8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting