Interstitial Water Studies, Leg 66

Sediment pore fluids sampled by drilling in subduction zones are commonly depleted in chloride relative to seawater and highly enriched in methane. These features have been attributed to processes within the accretionary prism of sediment that is typical of most subduction zones. Sites 778 through 780 in the Mariana forearc yielded low-chlorinity pore fluids rich in methane, ethane, and propane from a subduction zone setting that lacks an accretionary prism. They were drilled on the flanks and summit of Conical Seamount, an active serpentine mud volcano located 80 km behind the Mariana Trench and 120 km in front of the island arc. Alvin dives on this summit in 1987 found chimneys made of aragonite, calcite, and amorphous Mg-silicate from which cold water was seeping. During drilling at the summit Site 780 we recovered these fluids from serpentine silts at depths up to 130 mbsf and demonstrated that the fluids are upwelling through the seamount at velocities greater than a few millimeters per year. The fluids have less than one-half the chloride and bromide of seawater, pH up to 12.6, methane up to 37 mmol/kg along with ethane and propane, H 2 S up to 2 mmol/ kg, and ammonia up to 270µ mol/kg. Relative to seawater, they are enriched in alkalinity (×26), sulfate (×1.7), K (xl.5), Rb (×5.6), and B (×IO). They are highly depleted in Li, Mg, Ca, Sr, and 34 S, and have low concentrations of Si, Ba, and Mn. Although they are also depleted in Na, their Na/ Cl ratio increases to nearly twice that in seawater. These pore fluids from the summit of Conical Seamount contrast greatly with those recovered from Sites 783 and 784 on Torishima Seamount, an inactive serpentinite seamount in the Izu-Bonin forearc. The Torishima pore fluids were produced by the reaction of seawater with harzburgite at low temperatures. They have pH up to 10, very low Si, Mn, and methane, and no ethane or propane. Relative to seawater, they have low alkalinity, sulfate, Mg, K, and B; slightly lower Li and Rb; little-changed chloride, Br, Na, and Na/Cl; and high Ca, Sr, Ba, and 34 S. Pore waters from Sites 778 and 779 on the flanks of Conical Seamount are complex mixtures of these two types of fluids. The absence of an accretionary prism in the Mariana forearc severely constrains the origin of the fluids upwelling through Conical Seamount. Their freshening relative to seawater does not result from uptake of chloride into solids during serpentinization, but it requires instead a source of H 2 O deeper than was drilled. Whereas the presence of gas hydrates cannot be excluded, there is no direct evidence of them, and they probably are not a major source of H 2 O. The fluids at Conical Seamount probably originate at the top of the downgoing slab, 30 km below the seafloor, by heating of the sediments and basalt of the subducted oceanic crust. The water driven off serpentinizes the overlying mantle wedge. Serpentinite seamounts subsequently act as conduits through which H 2 O, CO 2 , hydrocarbons, and sulfur pass from the slab into the oceans.

Section: Evidence From Drilling In Other Subduction Zonesmentioning

confidence: 90%

Pore Waters from Serpentinite Seamounts in the Mariana and Izu-Bonin Forearcs, Leg 125: Evidence for Volatiles from the Subducting Slab

Mottl¹

1992

“…At these sites there was no evi dence for the occurrence of gas hydrates, even though organic gases were present at Site 438. In other areas, however, espe cially in drill sites across the Middle American Trench (e.g., Harrison et al, 1982;Hesse et al, 1985;Gieskes et al, 1985), low chloride concentrations have been explained in terms of di lution artifacts caused by the decomposition of gas hydrates during the pore-water retrieval process. Similar observations have been made on the Blake Bahamas Plateau during DSDP Leg 76 (Jenden and Gieskes, 1983).…”

Section: Chloride Anomaliesmentioning

confidence: 99%

Interstitial Water Chemistry—Major Constituents

Gieskes¹,

Blanc²,

Vrolijk³

et al. 1990

Self Cite

Major-element compositions (Cl", SO 2 .", Ca 2+ , Mg 2 + , Li + , K + , Na + , Sr 2+ ) of interstitial waters obtained from sediment cores along the ODP Leg 110 transect across the Northern Barbados accretionary prism have shown that a complex set of geochemical processes are of importance in this area. In the volcanic ash-rich Pleistocene-Pliocene sedi ments, alteration reactions involving volcanic ash lead to depletions of Mg 2+ and K + . This process is confirmed by the much lower than contemporaneous seawater values of the 87 Sr/ 86 Sr ratios of dissolved strontium. In the deeper sedi ments recovered below the zone of decollement (Sites 671 and 672) large increases in Ca 2+ and gradual decreases in Mg 2 + , Na + , and 6 18 0 (H 2 0) indicate a potential contribution to the interstitial water chemistry by exchange with un derlying basement rocks. This process has been hard to confirm because the drill holes were terminated well short of reaching basement. However, the concentration gradient pattern is consistent with observations in a large number of DSDP drill holes. Finally, but most importantly, low Cl" concentrations in the decollement zone and underlying sand layers, as well as in fault zones at Sites 673 and 674, indicate dilution of interstitial waters. The potential origins of the low Cl" concentrations are discussed, though we are not able to distinguish any mechanism in particular. Our evidence supports the concept of water migration along the decollement and through the underlying sandstones as well as along recent fault zones in the accretionary complex. Interstitial water concentration depth profiles are affected by faulting, thrusting, and overturn processes in the accretionary prism. These processes have caused a diminished diffusive ex change with the overlying ocean, thus explaining increased depletions in Mg 2+ and SO 2 " in sites farther onto the accre tionary prism.

“…The next problem is to explain the occurrence of low chloride concentrations. Previous studies on interstitial waters in accretionary prisms in the area of the Middle America Trench during DSDP Legs 66, 67, and 84 have revealed large decreases in the concentrations of dissolved chloride (Harrison et al, 1982;Hesse et al, 1985;Gieskes et al, 1985). In these settings, however, the observed decreases in chloride were accompanied by increases in the 5 18 0 (H 2 0) of the interstitial waters and could be considered as artefacts caused by the decomposition of methane hydrates during retrieval of the cores.…”

Section: Chloride and Methanementioning

confidence: 93%

Hydrogeochemistry, ODP Leg 110: an Overview

Gieskes¹,

Vrolijk²,

Blanc³

1990

Self Cite

Drilling of the northern Barbados accretionary complex transect during Leg 110 of the Ocean Drilling Program provided a unique opportunity to study the effects of sedimentary thickening, overthrusting, and faulting on the evolution of the chemical composition of interstitial waters. The production of low-chloride fluids farther in the complex has enabled evaluation of the main pathways of fluid expulsion from the complex. These are (1) along faults in the accretionary prism itself and (2) along the decollement and through underlying sandstones. In the latter case, methane anomalies, presumably caused by methane production as a result of thermal decomposition of organic matter farther into the complex, accompany the chloride anomalies. Major cation distributions (Ca 2+ , Mg 2+ , K + , Na +) are primarily affected by alteration of volcanic ash and exchange with formation waters in underlying basement rocks. However, as a result of tectonic activity in the complex, exchange with the overlying ocean is diminished and concentration-decreases in Mg 2+ , S0 4 2_ , and K + become larger in an arcward direction. Major cation distributions rule out the importance of pervasive advection through the sediments, thus confirming that the main pathways for fluid expulsion are along faults, the decollement, and underlying sandstones.