Calcium diffusion in enstatite, with application to closure temperature of the Ca-in-opx thermometer

Abstract

Chemical diffusion of Ca has been characterized in natural enstatite under buffered conditions (IW, NNO) and in air. Experiments were conducted using synthesized powder sources, with Ca diffusion profiles measured with Rutherford Backscattering Spectrometry. A variety of sources of diffusant were used to investigate the effects of silica activity and Ca concentration on diffusion. Calcium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions, and also appears little influenced by differences in silica activity and Ca concentration in the diffusant source. For Ca diffusion in a natural enstatite, we obtain the following Arrhenius relation for diffusion over the temperature range 750–1150 °C,D = 1.17 × 10−10exp(−240 ± 10 kJ mol−1/RT) m2sec−1.Calcium diffusion is slower than Mg, Cr and Fe in enstatite, but faster than larger divalent cations Pb and Eu that also are likely to preferentially occupy the M2 site. Calcium diffusion is also faster than that for trivalent REE and tetravalent Ti in enstatite. Calcium diffusivities in enstatite are intermediate between Ca diffusivities in clinopyroxene and olivine, with Ca diffusion in enstatite about 2 orders of magnitude faster than Ca self-diffusion in diopside and 2 orders of magnitude slower than Ca diffusion in olivine.Diffusion parameters obtained from this study were used to develop a simple model for closure temperature of the Ca-in-opx thermometer of Brey and Köhler (1990). By coupling closure temperature and the Ca-in-opx thermometer, it is possible to constrain cooling rates of peridotites and pyroxenites. Applications to peridotites from the Lanzo Massif and the Oman ophiolite are presented.