Calcium diffusivity in alumino-silicate garnets: an experimental and ATEM study |
| |
| Authors: | D Vielzeuf A Baronnet A L Perchuk D Laporte M B Baker |
| |
| Institution: | (1) CRMCN-CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex 9, France;(2) IGEM, Russian Academy of Sciences, Staromonetny per 35, Moscow, 119017, Russia;(3) Laboratoire Magmas et Volcans, OPGC-UBP-CNRS, 5 rue Kessler, 63038 Clermont-Ferrand, France;(4) Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-125, Pasadena, CA 91125, USA |
| |
| Abstract: | Concentration gradients in calcium are common in metamorphic or magmatic garnets and can be used to determine the timescales
of geological processes. However, the kinetics of Ca diffusion in garnet is poorly constrained and experimental studies have
to date yielded widely varying diffusion coefficients. In this paper, we describe a new method for generating diffusion profiles
in garnet. We incorporated polished and compositionally homogeneous garnet seeds in a finely ground powder of clinopyroxene
and garnet. During the experiments (1.3 GPa, 1,050–1,250°C, and ƒO2 ≤ the graphite-O2 buffer), the mineral powder partially melted, recrystallized, and formed a 10–50 μm wide overgrowth zone of compositionally
distinct garnet around the seeds. Long duration experiments generated measurable relaxation profiles at these seed/overgrowth
interfaces. We performed analytical transmission electron microscope traverses across the interfaces in each experiment. Thirteen
usable compositional profiles were obtained with characteristic distances of diffusion ranging from 300 to 1,000 nm. From
these profiles, Ca–(Fe, Mg) interdiffusion coefficients were retrieved using an analytical solution for the diffusion equation
and the data were cast in an Arrhenius relation. Linear regression of the data yields an activation energy Q
Ca–(Fe, Mg) equal to 188 ± 48 kJ mol−1 and a frequency factor D
0 equal to 6.6 × 10−14 m2 s−1. Within the compositional range studied, the composition of garnet has no major effect on the Ca–(Fe, Mg) interdiffusion
coefficient. The very slow diffusion rate of Ca is in agreement with natural observations indicating that Ca diffuses more
slowly than Fe and Mg. The Ca diffusion coefficients derived from this study are not model-dependent and can be used to determine
the durations of geological events from Ca relaxation profiles in natural garnets.
|
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
