Chemistry of Hot Waters Sampled from Basaltic Basement in Hole 504B, Deep Sea Drilling Project Leg 83, Costa Rica Rift

Abstract

Seawater that has been altered by reaction with basaltic basement has been sampled from Deep Sea Drilling Project Hole 504B, located on 5.9-m.y.-old crust on the southern flank of the Costa Rica Rift. Fourteen water samples have been collected on Legs 69, 70, and 83, both before and after renewed drilling on the latter two legs, at temperatures from 69 to 133°C and pressures from 390 to 425 bars.The water sampled prior to renewed drilling on Leg 83 had occupied the hole for nearly 2 yr. since it was last flushed with surface seawater at the end of Leg 70. Despite some contamination by seawater during sampling, the composition of two of these waters has been determined by using nitrate as a tag for the contaminant. Both the 80 and 115°C waters have seawater chlorinity, but have lost considerable Mg, Na, K, sulfate, and 0 2 , and have gained Ca, alkalinity, Si, NH 3 and H 2 S. The loss of sulfate is due to anhydrite precipitation, as indicated by the δ 34 S value of the remaining dissolved sulfate. The 87 Sr/ 86 Sr ratio has been lowered to 0.7086 for the 80°C water and 0.7078 for the 115°C water, whereas the Sr concentration is nearly unchanged. The changes in major element composition relative to seawater are also larger for the 115°C water, indicating that the basement formation water at this site probably varies in composition with depth. Based on their direction relative to seawater, the compositional changes for the 80 and 115°C waters do not complement the changes inferred for the altered rocks from Hole 504B, suggesting that the bulk composition of the altered rocks, like their mineralogy, is largely unrelated to the present thermal and alteration regime in the hole.The exact nature of the reacted seawaters cannot be determined yet, however. During its 2 yr. residence in the hole, the surface seawater remaining at the end of Leg 70 would have reacted with the wall rocks and exchanged with their interstitial formation waters by diffusion and possibly convection. How far these processes have proceeded is not yet certain, although calculations suggest that diffusion alone could have largely exchanged the surface seawater for interstitial water. The δ 18 θ of the samples is indistinguishable from seawater, however, and the Δ 14 C of the 80°C sample is similar to that of ocean bottom water. Although the interpretation of these species is ambiguous, that of tritium should not be. Tritium analyses, which are in progress, should clarify the nature of the reacted seawaters obtained from the hole.

Geochemical Studies in Hole 504B, Leg 92

“…More direct inferences about the nature of formation waters were made possible during Legs 69, 70, and 83, when water samples were acquired with a go-devil-packer sampler working in both active and inactive modes (Mottl, Anderson, et al, 1983;Mottl et al, 1985). The studies carried out during Leg 83 indicated that large changes in the composition of the borehole waters had taken place after surface seawater was introduced into the hole during Leg 70 (about 2 yr. before).…”

Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous

“…While experimental evidence suggests that hot-spring Sr is released only during high-temperature diabase alteration (Berndt, Seyfried, and Beck, 1988), numerous studies on sub-seafloor pore waters show that basaltic Sr is also released during low-temperature alteration (Mottl and others, 1985;Mottl and Gieskes, 1990;Hess, Bender, and Schilling, 1991). Moreover, there is evidence that warm (ϳ60°C) and intermediate-temperature (ϳ250°C) hot springs may be important along oceanic ridges (Mottl and others, 1998;Teagle, Alt, and Halliday, 1998); these may also carry considerable amounts of Sr.…”

A Guide to ODP Tools for Downhole Measurements

“…As no obstructions were encountered, no rotation or circulation was required, and the fluid samples and temperature measurements were generally good (Mottl et al, 1983(Mottl et al, , 1985Gieskes et al, 1986;Becker et al, 1983aBecker et al, , 1983b. When Leg 111 reentered Hole 504B, no hole problems were expected, and the initial reentry was made with a lightweight French temperature logging tool, which was run to within 55 m of hole bottom, followed by several runs of Kuster water samplers and a Schlumberger water sampler.…”

“…The WSTP and Kuster tools are described as "passive" samplers (Mottl et al, 1983(Mottl et al, , 1985 was run in Hole 504B as the primary sampling tool to extract a pore-fluid sample from the wall of the borehole. The RFT is a "doughnut" sampler run on the logging line, with a circular sealing port that is pressed against the side of the borehole using a mechanical arm.…”