Mg diffusion in forsterite from 1250-1600 °c

Michael C. Jollands, Irina Zhukova, Hugh St C. O'Neill, Jörg Hermann

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Abstract

26Mg tracer diffusion coefficients were determined in single crystals of pure synthetic forsterite (Mg2SiO4). Isotopically enriched powder sources both acted as the 26Mg source and buffered the activities of silica (aSiO2) at forsterite + protoenstatite (Mg2Si2O6) (high aSiO2) and forsterite + periclase (MgO) (low aSiO2). Experiments were conducted at atmospheric pressure between 1250 and 1600 °C, and at oxygen fugacities (fO2s) between 10-12 bars (CO-CO2 mix) and 10-0.7 bars (air). The resulting diffusion profiles were measured along the three principal crystallographic axes (a, b, and c; ∥[100], ∥ [010], ∥ [001]) using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), with a quadrupole mass spectrometer. These measurements were corroborated by ion microprobe using the sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) instrument. Mg tracer diffusion is anisotropic, with D[001] > D[010] > D[100], the difference in diffusion coefficients varying by about one order of magnitude at a given temperature with crystallographic orientation. Diffusion is faster in protoenstatite-buffered than periclase-buffered conditions, again with around one order of magnitude difference in diffusivity between buffering conditions. There is no apparent effect of fO2 on diffusion. A global fit to all data, including data from Chakraborty et al. (1994) and Morioka (1981) yields the relationship: log10D=log10D0(m2s-1)+0.61(±0.03)log10 αSiO2+-359(±10)kJ/mol/2.303RT where log10D0 is -3.15 (±0.08), -3.61 (±0.02), and -4.01 (± 0.05) m2 s-1 for the [001], [010], and [100] directions, respectively (1 s.d.). The LA-ICP-MS technique reproduces diffusion coefficients determined by SHRIMP-RG, albeit with slightly different absolute values of isotope ratios. This shows that LA-ICP-MS, which is both accessible and rapid, is a robust analytical method for such tracer diffusion studies.

Original languageEnglish
Pages (from-to)525-537
Number of pages13
JournalAmerican Mineralogist
Volume105
Issue number4
DOIs
Publication statusPublished - 28 Apr 2020
Externally publishedYes

Keywords

  • Diffusion, olivine
  • experimental petrology
  • forsterite
  • magnesium

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