HR: 0800h
AN: T41E-1256    [Abstracts]
TI: Opal diagenesis and sediment properties in the Nankai Trough, Japan
AU: * Spinelli, G A
EM: spinelli@ees.nmt.edu
AF: Earth & Environmental Sciences Department, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AU: Mozley, P
EM: mozley@nmt.edu
AF: Earth & Environmental Sciences Department, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AU: Underwood, M
EM: underwoodm@missouri.edu
AF: Department of Geological Sciences, University of Missouri, 101 Geological Sciences Building, Columbia, MO 65211 United States
AU: Tobin, H
EM: tobin@nmt.edu
AF: Earth & Environmental Sciences Department, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AB: We have measured the opal content and modeled opal diagenesis for sediment from Ocean Drilling Program (ODP) Sites 1173, 1174, and 1177 in the Nankai Trough, Japan. The porosity of the Upper Shikoku Basin facies at 1173 is nearly constant with depth from $\sim$102 to 344 mbsf. There is a step decrease in porosity across the Upper Shikoku Basin / Lower Shikoku Basin facies boundary at Site 1173; porosity decreases with depth normally in the Lower Shikoku Basin facies. Sediment physical properties suggest that the cementing of grain contacts within the Upper Shikoku Basin facies inhibits sediment consolidation, however, the nature of the cement has not been previously identified. In addition, it has been suggested that the Upper Shikoku Basin / Lower Shikoku Basin facies boundary is at least partially a diagenetic boundary. Opal comprises $<$2 wt% of the sediment at all of the sites. At Site 1173, the Upper Shikoku Basin sediment contains $\sim$1.25 wt% opal. The opal content drops abruptly to $<$0.5 wt% across the Upper Shikoku Basin / Lower Shikoku Basin facies boundary - corresponding with the drop in porosity and the return to normal consolidation behavior in the Lower Shikoku Basin facies. Secondary and back-scattered electron image analysis of sediments at this site reveals a low density, Si-rich phase that is probably opal. This material occurs as a pore-filling cement, coating grains and grain-to-grain contacts. It appears to be more common in the Upper Shikoku Basin facies than in the Lower facies. SEM analyses suggest the presence of authigenic clays in the Lower Shikoku Basin facies. We model sediment accumulation and the thermal evolution of the sediment columns for the sites. Then, we use the thermal history of the sediment and laboratory derived kinetics for the opal-to-quartz diagenetic reaction to model the opal content in the sediment column. Modeled opal content at Site 1173 decreases rapidly below 250 mbsf, where temperatures exceed 50$^\circ$C. The model results indicate that the reaction runs to completion (i.e. the last of the opal is removed) approximately at the Upper Shikoku Basin / Lower Shikoku Basin facies boundary. The model results are in good agreement with the opal measurements and suggest that opal cement may be holding open the pore space in the Upper Shikoku Basin section. It appears that during opal diagenesis the strength of the cement is lost and the pore space collapses. This apparent loss of strength is consistent with previously described loss of shear rigidity reflected in anomalous seismic velocities measured across this zone. Both this loss of strength and high dissolved silica content in sediment pore water in the Upper Shikoku Basin facies may result from the loss of opal without immediate conversion into quartz. A sharp decrease in dissolved silica across the facies bounadry may be related to the uptake of silica into authigenic clays in the Lower Shikoku Basin facies.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3022 Marine sediments--processes and transport
DE: 1050 Marine geochemistry (4835, 4850)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting