Hydrothermal circulation, Juan de Fuca Ridge eastern flank; factors controlling basement water composition

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doi: 10.1029/93JB01612
Author(s): Wheat, C. Geoffrey; Mottl, Michael J.
Author Affiliation(s): Primary:
University of Hawaii at Manoa, School of Ocean and Earth Science Technology, Manoa, HI, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 99(B2), p.3067-3080. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 31 refs.; illus., incl. 1 table, sketch map
Summary: Pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the Juan de Fuca Ridge as part of FlankFlux 90, a study of hydrothermal circulation through mid-ocean ridge flanks. Seismic reflection and heat flow surveys (Davis et al., 1992a) indicate that the three areas differ in sediment thickness, basement topography, abundance of outcrops, basement temperature, and fraction of heat lost by advection versus conduction. Area 1 is on 0.6 Ma crust with nearly continuous basement outcrop, area 2 is on 1.3 Ma crust over the first buried ridge parallel to the present ridge axis, and area 3 is on 3.5-3.8 Ma crust over two axis-parallel buried ridges that penetrate the sediment cover in three locations. Each area includes a hydrothermal system in which seawater flows into basement, reacts with crustal basalt, and then exits basement either through the sediment or directly into the overlying water column. As constrained by concentrations of sulfate and lithium in the pore waters, at least some seawater enters basement in all three areas without reacting fully with the overlying sediment, even where no outcrops are known nearby. Speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity, which behave conservatively in these areas, to numerical time-dependent transport models. The estimated velocities range from <0.1 to 7.4 cm/yr; faster flows probably occur but were not sampled. Upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness. The correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment. We have documented pore water upwelling through sediment up to 100 m thick. We estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick, corresponding to a heat flow of 0.44 W/m2 in area 2 and 0.3 W/m2 in area 3. Concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next. Both species remain at the bottom seawater concentration in area 1, where basement is cooled to <10°C at the base of the sediments mainly by advection. The concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2.5 mmol/kg at about 90°C in area 3. The transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement, as the uppermost basement is heated from < 10° to 40-50°C. Chlorinity of the basement fluid increases above the present-day bottom seawater concentration in areas 2 and 3 and in nearly all other mid-ocean ridge flanks studied to date, as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period. While chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems, it reaches a maximum in area 2 at only 40°C, probably because alteration there occurs at a lower water/rock ratio than elsewhere. For all mid-ocean ridge flanks studied to date, the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age. Copyright 1994 by the American Geophysical Union.
Year of Publication: 1994
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Advection; Chemical composition; Chlorine; Circulation; Crust; East Pacific; Geophysical methods; Geophysical surveys; Halogens; Heat flow; Hydrothermal conditions; Juan de Fuca Ridge; Leg 139; Mid-ocean ridges; North Pacific; Northeast Pacific; Ocean Drilling Program; Ocean floors; Oceanic crust; Pacific Ocean; Pore water; Reflection; Sea water; Sediments; Seismic methods; Solutions; Surveys; Upwelling; Water-rock interaction
Coordinates: N482600 N482730 W1283815 W1284250
Record ID: 1995037371
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.