HR: 08:45h
AN: V11E-04    [Abstracts]
TI: Chrysotile Nanotubes: Potential Host of Insoluble Chlorine in Serpentinitized Oceanic Crust.
AU: * Brearley, A J
EM: brearley@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States
AU: Barnes, J D
EM: jdbarnes@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States
AU: Sharp, Z D
EM: zsharp@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States
AB: Numerous studies have shown that seafloor serpentinites contain significant concentrations of chlorine (up to 1 wt% Cl) and hence represent an important reservoir of Cl that can potentially be subducted into the mantle. Chlorine in serpentinites is clearly present in both water-soluble and insoluble phases, but a longstanding and as yet unresolved question is the exact identity of the mineralogic hosts of the chlorine. One possibility is that insoluble Cl may be present in the serpentine structure substituting for hydroxyl. To address this question we have used EPMA and TEM to study an ocean floor serpentinite sampled by the Ocean Drilling program (Kane Fracture Zone - Mid Atlantic Ridge). The ODP sample 153-920D-8R-2 is a serpentinized harzburgite, containing water-soluble and insoluble Cl contents of 0.09 wt% and 38 ppm, respectively. This sample has undergone extensive serpentinization (~85%) resulting in the development of both mesh and bastitic replacement textures. A significant portion of the serpentine consists of chrysotile nanotubes with diameters of 20-35 nm. Using a combination of FEG HRTEM and energy filtered TEM, we have examined the spatial distribution of chlorine at the nanoscale within regions of these nanotubes. This process is challenging because the nanotubes damage quite rapidly in the electron beam and so acquisition of HRTEM and EFTEM images must be carried out quickly in order to avoid significant degradation of the sample. Our data show that Cl is clearly concentrated in both the cores of the nanotubes and in amorphous or poorly ordered materials around the periphery of the nanotubes. This observation has been confirmed from images taken both normal and parallel to the nanotube axes. The contrast in HRTEM images indicates that the nanotube cores may be partially filled by an amorphous material that is the carrier of the chlorine. We cannot rule out the possibility that some Cl is present substituting for hydroxyl in the chrysotile structure. However, it is clear that the levels of structurally-bound Cl are below the detection limits for EFTEM analysis and are significantly lower than in the nanotube cores. The amorphous material that is pervasive surrounding the chrysotile nanotubes generally contains lower apparent concentrations of Cl than the nanotube cores. These data suggest that the cores of chrysotile nanotubes maybe an important and previously unrecognized carrier of Cl in serpentinized oceanic crust. During subduction, Cl contained within the nanotube cores may be released into fluids at relatively shallow levels, prior to complete dehydration of chrysotile.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3630 Experimental mineralogy and petrology
DE: 3653 Fluid flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting