HR: 0830h
AN: V21D-0552    [PDF]
TI: Linking Basement Vein Compositions to Porewater Geochemistry Across the Eastern Flank of the Juan de Fuca Ridge, ODP Leg 168
AU: * Coggon, R M
EM: rmc01@soc.soton.ac.uk
AF: Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Teagle, D A
EM: dat@soc.soton.ac.uk
AF: Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Cooper, M J
EM: mjco@soc.soton.ac.uk
AF: Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, European Way, Southampton, SO14 3ZH United Kingdom
AU: Vanko, D A
EM: dvanko@towson.edu
AF: Department of Physics, Astronomy and Geosciences, Towson University, 8000 York Road, Towson, MD 21252-0001 United States
AB: Leg 168 of the Ocean Drilling Program (ODP) investigated the heat flow, fluid chemistry and crustal alteration associated with ridge flank hydrothermal systems. 10 sites were drilled on the eastern flank of the Juan de Fuca Ridge, along an 80 km transect, between 20 and 100 km east of the spreading centre. Recovered cores consisted of 100-500 m of sediment with shallow penetration (1.7-48.1 m) into the underlying igneous basement (0.8 to 3.6 Ma). Here we use the composition of calcium carbonate minerals, from veins within the upper basement, to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age and sediment cover thickness. We show for the first time, a clear link between the alteration of the basement rocks as recorded by secondary minerals, and the near basement sedimentary pore fluids which are often assumed to be representative of the basement fluids responsible for low temperature alteration of the upper crust. Our results show that the carbonates precipitated from basement fluids that ranged in strontium isotopic composition from near modern seawater ($^{87}$Sr/$^{86}$Sr $\sim$ 0.70918) to the near-basement pore fluid values at any one site. $^{87}$Sr/$^{86}$Sr ratios are independent of mineralogy with both aragonite and calcite precipitating from variably evolved fluids with the range in carbonate $^{87}$Sr/$^{86}$Sr increasing with crustal age. A parallel geochemical evolution of basement fluids and sediment pore-waters is shown since $^{87}$Sr/$^{86}$Sr ratios of near-basement pore fluids decrease from 0.709013 to 0.707108 away from the ridge axis. A correlation exists between $^{87}$Sr/$^{86}$Sr ratios and $\delta$$^{18}$O-calculated fluid temperatures, with more geochemically evolved carbonates having precipitated from warmer fluids. Given an observed increase in basement temperature with age, from $16\deg$C to $64\deg$C along the transect, a progressive chemical development of basement fluid is demonstrated.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting