HR: 0800h
AN: V31C-0626    [Abstracts]
TI: Progressive Barite Distillation at the Costa Rica Convergent Margin: Implications for Ba Contribution to the Arc Volcanoes
AU: * Solomon, E
EM: esolomon@ucsd.edu
AF: Scripps Institution of Oceanography, 8615 Discovery Way, La Jolla, CA 92037-0212 United States
AU: Kastner, M
V31C-0626 AF: Scripps Institution of Oceanography, 8615 Discovery Way, La Jolla, CA 92037-0212 United States
AU: Robertson, G
V31C-0626 AF: Scripps Institution of Oceanography, 8615 Discovery Way, La Jolla, CA 92037-0212 United States
AB: Convergent margins are areas of intense tectonic activity and dynamic hydrology, thus they are important regions for geochemical cycling between the major reservoirs seawater, oceanic sediment and igneous basement, continental crust, and the mantle. The distillation and loss of some volatiles and fluid-soluble elements, such as Ba, 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. In margin sediments, Ba is primarily contained in barite that precipitates in water column microenvironments, and is present in biogenic phases such as refractory organic matter and biogenic carbonate, detrital silicates, and Fe-Mn oxyhydroxides. The Ba in silicates is typically immobile during diagenesis; however, in sediments where pore fluid sulfate becomes depleted, barite (BaSO₄) dissolves and Ba concentrations rise by several orders of magnitude. In tectonically active regions, like convergent margins, the dissolved Ba can be transported seaward through fluid flow conduits where it is reprecipitated as barite when it reaches sulfate-rich water. The incoming sediment section offshore Costa Rica contains 152 m of hemipelagic sediments (HS) overlying 226 m of pelagic carbonates (PC). This entire sediment section is being underthrust, providing an ideal setting to examine Ba cycling in the shallow levels of the subduction zone. ODP Legs 170 and 205 drilled a transect of 3 boreholes across the Middle America Trench with a reference site seaward of the trench (Site 1039/1253) and two sites landward of the trench that drilled through the margin wedge, the décollement, and the underthrust sediments (Sites 1040/1254, 1043/1255). At Site 1039/1253, sulfate concentrations reach a minimum of ~13 mM within the upper 20 m of the HS and are near seawater value within the PC. At Site 1040/1254, ~1.6 km landward of the trench, sulfate is totally depleted within the underthrust HS to a depth of ~30 m below the décollement. This suggests that upon underthrusting the supply of sulfate from seawater by diffusion ceased, and the sulfate reducing bacteria utilized the remaining SO₄ at the top of the underthrust section. In this SO₄-depleted zone, barite dissolves releasing Ba to the pore fluid. The dissolved Ba concentrations range from 0.378 to 4.257 μM in the upper 146 m of Site 1039 to 17.26 to 209.57 μM in the equivalent depth interval at Site 1040/1254. The highest concentration, of 209.57 μM, occurs at the base of the décollement and is ~53 times higher than the Ba concentration of the equivalent sample at Site 1039 and ~1400 times higher than the bottom water value. In order to quantify the Ba loss from Site 1039 to Site 1040, average bulk sediment Ba compositions in the HS and the PC at Sites 1039/1253 and 1040/1254 were determined. The upper HS exhibit an 18% loss in bulk sedimentary Ba, whereas the lower HS and PC show little change across the trench. The extremely high pore fluid Ba concentrations and the large % bulk sediment Ba lost in the upper 30 m of the HS at Site 1040 indicate intense Ba distillation from barite due to dissolution in the zero sulfate zone. As the subducting sediments move arcward the sulfate depletion zone becomes thicker, eventually consuming the pore water SO₄ in the lower HS and PC, thus also liberating Ba from barite in these sections. This process should operate in all convergent margins and may have a profound impact on the amount of bulk sedimentary Ba originally present seaward of the trench subducting to depths of magma generation.
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1051 Sedimentary geochemistry
DE: 3653 Fluid flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005