Fluid kinematics, fluid residence times, and rock degassing in oceanic crust determined from noble gas contents of deep sea drilling project pore waters

Abstract

The 3He, 4He, Ne, and Ar in sedimentary pore fluids, extracted in situ, were measured on samples from Deep Sea Drilling Project (DSDP) sites 398, 410, 419, 420, 424, 436, and 438. Earlier results from DSDP leg 15 are also discussed. Where regional sediment cover prevents direct penetration of seawater into basement rocks at sites 149, 436, and 438, He concentration profiles indicate slow vertical and/or horizontal advection of sedimentary pore fluids, suggesting that fluids can ventilate through the sediment cover either in diffuse, bulk flow or in “aquifers” confined to regions of relatively high permeability such as ash beds, sand/silt layers, or fracture channels in lithified sediments. At site 410, where basement outcrops are prevalent, no advection was detectable in the sediments; however, the basement was well flushed by seawater. On <2 m.y. crust near the East Pacific Rise and the Galapagos Spreading Center, the collection of only one sample per hole precluded Calculation of sedimentary fluid velocities. However, most juvenile helium is degassed from the crust in <6 × 105 years, except for the deep crust or upper mantle, where juvenile helium may be released locally to fluids advecting in deep fissures or faults. Fluid residence times in upper basement rocks increased from ∼5 × 103 years in 10 m.y. crust open to direct seawater penetration to ∼105–106 years in Cretaceous basement sealed by hundreds of meters of sediment, to >1–5 × 108 years (apparent age) in sediments on the active continental margin off NE Japan.

Helium Isotope Ratios of Pore Gases in Deep-Sea Sediments, Leg 128

“…1) He and low 4 He/ 20 Ne ratios were ascribed to air-saturated seawater and air-saturated water with a slight addition of mantle-derived 3 He, respectively. Existence of the latter component was suggested in the GEOSECS Expedition Atlas (1987) and confirmed by Barnes and Clarke (1987) and Igarashi et al (1987). The third and fourth end members were attributed to in-situ radiogenic helium and trapped helium from the degassing of the basement rock beneath the sediments, respectively.…”

“…The third and fourth end members were attributed to in-situ radiogenic helium and trapped helium from the degassing of the basement rock beneath the sediments, respectively. The contribution from cosmo- genie helium in the solid phase of pelagic sediments was negligibly small (Barnes and Clarke, 1987;Sano and Wakita, 1987). Thus, the mixing ratio of the radiogenic and trapped helium as well as the variable He ratio in the trapped helium could define the slope of the samples in Fig.…”

“…The observed 3 He/ 4 He and 4 He/ 20 Ne ratios, and sampling depth are listed in Table 1 together with reported data of Site 584 in the Japan Trench and Site 583 in the Nankai Trough (Sano and Wakita, 1985). Also indicated are data of bottom water in the eastern Japan Sea , Pacific bottom water in the Nankai Trough (Igarashi et al, 1987), that in the Japan Trench (Barnes and Clarke, 1987), and air-saturated water at 20° C (Benson andKrouse, 1980: Top et al, 1987). Errors assigned to the isotope ratio determinations are one standard deviation.…”

“…Assuming that the 4 He/ 20 Ne ratios of the radiogenic and trapped helium are significantly higher than those of the first and second components, it is possible to correct atmospheric helium components as follows: (1) He ratios are listed in Table 3 together with those of Sites 436 and 438 in the Japan Trench (Barnes and Clarke, 1987). These values could be a mixture of the radiogenic and the trapped helium, as the atmospheric component is now eliminated.…”

Elemental and Isotopic Abundance of Noble Gases in Formation Fluids Recovered In Situ from the Chile Triple Junction

“…In pore fluids and deep sea sediments it can be used to obtain insights into fluid kinematics and fluid residence times in the ocean crust (Barnes and Clarke, 1987). Helium is a chemically inert gas that is steadily produced by radioactive decay within the sediments; this radiogenic helium is enriched in 4 He and thus it can be used as a tracer to derive information on fluid sources and flow patterns not available through the study of the distribution of diagenetically active chemical species.…”