Convergent margins are areas of high tectonic activity and dynamic hydrology, making them important regions for geochemical cycling between major reservoirs such as seawater, oceanic sediment and crust, continental crust, and the mantle. The distillation and loss of some volatiles and fluid-soluble elements from the shallow subduction zone changes the composition of the slab (sediments and igneous basement) delivered to the depths of magmatism beneath volcanic arcs and, ultimately, the mantle. The escape of fluids from the downgoing and over-riding plates at depth may affect seawater chemistry for select elements and isotope ratios.
Barium is a large-ion lithophile element (LILE) that is incompatible and enriched in oceanic sediments and the continental crust. Ba is strongly partitioned into the fluid phase at moderate to high temperatures and is leached from the oceanic crust at depths of magma generation and incorporated into the melt. As a result, Ba is enriched in arc volcanics worldwide and volcanic outputs reflect the sediment inputs (Morris, 1991; Plank and Langmuir, 1993), assuming there is insignificant loss of Ba with fluids that escape from the downgoing slab. Using bulk sediment input fluxes of Ba and other incompatible elements, coupled with the volcanic output fluxes, the amount of sediment contributed to the arcs as well as the elemental fluxes to the mantle have been estimated (e.g., Plank and Langmuir, 1993; Patino et al., 2000). The incoming sediment section offshore the Nicoya Peninsula of Costa Rica contains 152 m of diatom-rich hemipelagic sediments overlying 226 m of pelagic calcareous nannofossil oozes and chalks. The subducting sediment section offshore Guatemala (Deep Sea Drilling Project [DSDP] Site 495) is nearly identical to that drilled offshore Costa Rica (Fig. F1). Ba/La and Ba/Th ratios (Carr et al., 1990; Leeman et al., 1994; Patino et al., 2000), coupled with 10Be isotopic concentrations of the Central American lavas (Tera et al., 1986; Morris et al., 1990, 2002), suggest that the entire sediment section is subducting to depths of magma generation beneath Nicaragua, whereas there is little contribution from the uppermost hemipelagic sediments and a proportionally larger contribution from the pelagic carbonate section at Costa Rica. Since the entire sediment section is subducted at Costa Rica, the low 10Be in the Costa Rican volcanics must be due to sediment dynamics under the forearc, such as sediment underplating or subduction erosion (Morris et al., 2002).
The distribution of Ba in deep-sea sediments is variable. High concentrations are typically found in sediments underlying high-productivity waters (Dymond et al., 1992, 1996; Paytan et al., 1996; Eagle et al., 2003) and are thought to result from the rapid release of dissolved Ba from labile particulate Ba during the early stages of plankton decomposition either by cell lysis or decay of labile organic matter in surface waters (Ganeshram et al., 2003). A fraction of the released Ba precipitates abiotically as barite (BaSO4) within supersaturated microenvironments, where it is deposited on the seafloor (Dehairs et al., 1980; Bishop, 1988; Ganeshram et al., 2003). Ba is also contained in other biogenically related phases such as refractory organic matter and biogenic carbonate, as well as inorganic phases like detrital silicates and Fe-Mn oxides and oxyhydroxides (e.g., Dymond et al., 1992; McManus et al., 1998; Plank and Langmuir, 1998; Eagle et al., 2003). The concentration of these biogenic phases, as well as the detrital phases, may represent a significant fraction of the total Ba in the sediment column, especially at continental margin settings. Ba concentrations are ~600 ppm in Post-Archaean Australian Shale (PAAS) (Taylor and McLennan, 1985) and North American Shale Composite (NASC) (Condie, 1993; Plank and Langmuir, 1998), ~690 ppm in green clay (Plank and Langmuir, 1998), and generally <200 ppm in some volcaniclastic sediments (Elliot et al., 1997; Plank and Langmuir, 1998). In general, sediment samples with total Ba concentrations >1000 ppm contain up to 70% Ba associated with the mineral barite (Eagle et al., 2003).
The Ba in aluminosilicates is typically immobile during sediment diagenesis; however, the Ba in barite is affected by variations in pore fluid sulfate concentrations (Dymond et al., 1992; McManus et al., 1998; Torres et al., 1996a). In organic-rich sediments, microbial degradation of organic matter leads to sulfate reduction and methanogenesis; the pore water SO42– is consumed by oxidation of organic carbon and, at some locations, also by methane oxidation. When pore fluid sulfate becomes depleted, the solubility of barite increases greatly and dissolved Ba2+ concentrations can rise by several orders of magnitude (Brumsack and Gieskes, 1983; Torres et al., 1996b; Dickens, 2001). In tectonically active regions, like convergent margins, the dissolved Ba can be transported by compaction-induced fluid expulsion in the underthrust sediments and fluid advection along higher permeability conduits in the décollement and upper fault zones where it is reprecipitated as barite when it reaches SO42–-rich water (Torres et al., 2003). Recently, cold seep authigenic barite deposits have been discovered in a wide variety of continental margin environments. Torres et al. (1996a) sampled barite chimneys as high as 15 cm along a scarp failure at the Peru convergent margin. Other deposits have been discovered in the San Clemente Basin (Lonsdale, 1979; Torres et al., 2002), Monterey Bay (Naehr et al., 2000), the Sea of Okhotsk (Greinert et al., 2002), and the Gulf of Mexico (Fu et al., 1994).
Ocean Drilling Program (ODP) Legs 170 and 205 drilled a transect of three boreholes across the Middle America Trench with a reference site seaward of the trench in the incoming sediments and igneous basement (Sites 1039/1253) and two sites landward of the trench that drilled through the margin wedge, the décollement, and the underthrust sediments (Sites 1040/1254 and 1043/1255) (Fig. F2). At Sites 1039/1253, sulfate concentrations reach a minimum of ~13 mM within the uppermost 20 m of the hemipelagic sediment section and are near seawater value within the pelagic carbonate section (Kimura, Silver, Blum, et al., 1997) (Fig. F3C). At Sites 1040/1254, ~1.6 km landward of the trench, sulfate is totally depleted within the prism sediments and the zero-sulfate zone is also observed in the underthrust hemipelagic sediments to a depth of ~30 m below the décollement (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003), despite the fact that minimum SO42– concentrations at the reference site were ~13 mM in the uppermost hemipelagic section. This suggests that upon underthrusting, the supply of sulfate from seawater by diffusion ceased and the sulfate reducing bacteria in the underthrust sediments utilized the remaining SO42– at the top of the section (Fig. F3D). Once this sulfate was depleted, barite would be undersaturated and significant release of dissolved Ba2+ would occur. As the incoming sediment section is further underthrust, sulfate depletion would reach deeper levels in the sediment section, further liberating Ba2+ from barite. This progressive barite distillation could have a profound impact on the amount of Ba that originally was present within the sedimentary section at Sites 1039/1253 reaching depths of magma generation under the Costa Rican arc volcanoes. This process should operate in all convergent margins and may reduce estimates of Ba input flux to the subduction factory in margins subducting sediments with an appreciable amount of biogenic barite (i.e., bulk sediment Ba > ~1000 ppm).