Measurement of natural anisotropic diffusion profiles of 18O in Adirondack diopsides: preliminary data
Abstract
Anisotropic 18O diffusion has been recently observed in diopside from diffusion experiments performed in single crystals at high temperatures (D_b <<Da,c, 1050-1370^oC; Ingrin et al., 2001). Such anisotropic behaviour has not been yet confirmed in nature, rising questions about the validity of extrapolating oxygen diffusion laws to low temperature natural conditions. However, the occurrence of oxygen diffusion profiles in natural diopsides has already been demonstrated and the data used to estimate the cooling history of rocks (Edwards and Valley, 1998). We performed δ18O analyses on a diopside single crystal from a granulite facies marble of Adirondack Mountains (sample 95ADK-1) using an IMS1270 ion probe (CRPG, Nancy). A slice of the crystal (1mm x 1mm) was cut and polished perpendicular to the c-axis in order to measure oxygen profiles along [010] and [100]^* directions. A spatial resolution of 10 microns and a sensitivity of 0.2-0.3 ppm (2 δ) on relative δ18O measurements were achieved with success. Preliminary measurements show an isotopic signature decrease by 2 ppm in the outermost 100 microns toward the sample edge along the [100]^* direction. This result is in good agreement with early predictions of Edwards and Valley (1998), which found a core to rim difference of 2.4±1.5 ppm from ion probe analyses on non-orientated samples from the same origin. Measurements along the [010] direction are currently in progress. Direct measurement of the true δ18O profiles as a function of the crystallographic direction in single crystals will significantly increase our understanding of the cooling paths undertaken by metamorphic rocks. We will be able to check possible modifications of the cooling rate during retrogression, the effect of fluid activity on diffusion (Johnson et al., 2002) and generate further data for the use of the most evolved geospeedometers (Jaoul and Bejina, 2001). References: Ingrin, J., Pacaud, L., Jaoul, O. (2001) Anisotropy of oxygen diffusion in diopside. Earth Planet. Sci. Lett. 192, 347-361. Edwards K.J., Valley, J.W. (1998) Oxygen isotope diffusion and zoning in diopside: The importance of water fugacity during cooling. Geochim. Cosmochim. Acta., 62, 2265-2277. Jaoul,O., Bejina, F. (2001) Diffusion in Geophysics: Geospeedometry and compensation rule, diffusion in Materials (DIMAT 2000). Trans. Tech. Publications, p. 983-1000. Johnson,E.A., Rossman, G.R., Dyar, M.D., Valley, J.W. (2002) Correlation between OH concentration and oxygen isotope diffusion rate in diopside from the Adirondack Mountains, New York. Am. Mineral., 87, 899-908.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8301I