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An empirical estimate of the diffusion rate of oxygen in diopside
Author(s) -
SHARP Z. D.,
JENKIN G. R. T.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00005.x
Subject(s) - diopside , calcite , geology , mineralogy , diffusion , grain size , isotopes of oxygen , analytical chemistry (journal) , oxygen , geochemistry , thermodynamics , chemistry , environmental chemistry , physics , organic chemistry , geomorphology
The intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ 18 O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ 18 O(calcite‐diopside) are 800d̀C, in agreement with the highest previous thermometric estimates for these rocks. The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates ( D 800d̀C parallel to the c ‐axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy ( Q ) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10 ‐20 cm 2 /s is two orders of magnitude faster than from the FRM; at Q =500kJ/mol, the D 800d̀C estimate for both methods is c. 5.6 times 10 ‐20 cm 2 /s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct. The δ 18 O(SMOW) and δ 13 C(PDB) values of the calcite, measured in situ with a CO 2 laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, ‐0.6±0.1‰ abutting diopside grains. The δ 18 O and δ 13 δC values measured conventionally are: crystal cores, 22.96, ‐0.95‰; abutting diopside grains, 22.38, ‐0.93‰; bulk, 22.79, ‐0.95%. Use of the bulk δ 18 O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ 18 O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.