HR: 17:35h
AN: V24A-07    [Abstracts]
TI: Diffusion of Ca in San Carlos Olivine at 800 to 1200 $\deg$C
AU: * Gaetani, G A
EM: ggaetani@whoi.edu
AF: Dept Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Dept Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Cherniak, D J
EM: chernd@rpi.edu
AF: Dept Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180 United States
AB: Calcium is a minor constituent in magnesian olivine that has potential as both a geothermobarometer [1] and a geospeedometer [2]. Fully exploiting the potential of Ca in olivine requires knowledge of it's diffusivity as a function of temperature. Here we present results from new experiments in which the diffusivity of Ca in San Carlos olivine has been experimentally determined over a temperature range of 400 $\deg$C. These experiments demonstrate that a significant anisotropy develops at temperatures below 950 $\deg$C, with diffusion parallel to the {\it c} crystallographic axis becoming significantly faster than parallel to either the {\it a} or {\it b} axes. We conclude that this anisotropy is due to the influence of low angle subgrain boundaries. Experiments were carried out on oriented pieces of gem-quality San Carlos olivine at 1 bar and temperatures of 800 $\deg$ to 1200 $\deg$C using the powder-source technique. The fugacity of oxygen was controlled at the NiNiO buffer. Diffusion profiles generated at 800 $\deg$ to 1000 $\deg$C were analyzed using Rutherford Backscattering Spectroscopy. Analyses of run products from higher temperature experiments were carried out using either Secondary Ion Mass Spectrometry or electron microprobe. At temperatures of 950 $\deg$C or greater, our experiments show no evidence for significant anisotropy with respect to diffusion. Activation energies for diffusion parallel to the {\it a} (440$\pm$60 kJ/mol), {\it b} (457$\pm$14 kJ/mol), and {\it c} (520$\pm$90 kJ/mol) crystallographic axes are indistinguishable. However, at temperatures below 950 $\deg$C the activation energy for diffusion parallel to the {\it c} axis decreases to only 270$\pm$40 kJ/mol. This change occurs only for diffusion parallel to {\it c}, resulting in considerable anisotropy. At 800 $\deg$C diffusion parallel to the {\it c} axis is faster than diffusion parallel to either {\it a} or {\it b} by more than an order of magnitude. The change in diffusivity parallel to {\it c} is attributed to the presence of (100) tilt boundaries, producing a regime in which type B kinetics dominate. Diffusion is anisotropic owing to pipe diffusion along the cores of (010)[100] edge dislocations within the low angle boundaries. The cores of edge dislocations in the (010)[100] system are parallel to the c (i.e., [001]) direction. References: [1] K\"{o}hler and Brey (1990) Geochim Cosmochim Acta 54:2375-2388. [2] Pan and Batiza (2002) J Geophys Res DOI 10.1029/2000JB000435.
DE: 3630 Experimental mineralogy and petrology
DE: 3904 Defects
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting