Mg tracer diffusion in aluminosilicate garnets at 750-850°C, 1 atm. and 1300°C, 8.5 GPa

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doi: 10.1007/s004100050136
Authors:Chakraborty, S.; Rubie, D. C.
Author Affiliations:Primary:
Universitat Bayreuth, Bayreuth, Federal Republic of Germany
Other:
University Cologne, Federal Republic of Germany
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 122(4), p.406-414. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1996
Note:In English. 19 refs.; illus., incl. 1 table
Summary:Tracer diffusion coefficients of Mg in natural aluminosilicate garnets of composition Alm38Pyr50Gr10Sp2 and Alm73Pyr21Gr5Sp1 were measured at 1 bar, 750-850°C and at 8.5 GPa, 1300°C by chemically depositing a salt layer enriched in 26Mg on the specially prepared surface of a garnet single crystal. Diffusion anneals at 1 atm were carried out at a controlled fO2 of ∼ 10-17 bar maintained by a flowing gas mix of CO-CO2. Annealing conditions were carefully chosen to avoid decomposition of garnet by redox reactions. High-P anneals were carried out in a multi-anvil apparatus. Induced diffusion profiles (0.1-0.6 µm) were measured by an ion-microprobe with SIMS attachment. Diffusion coefficients at 1 atm are in agreement with extrapolations of data from high P-T experiments, and also with the low T (750-900°C) dataset of earlier workers, if the diffusion coefficients are assumed to be proportional to fO21/6. Such an fO2 dependence, however, makes this dataset inconsistent with the recent work of Schwandt et al. (1995) on garnets of composition (Alm15Pyr72Gr13Sp0) unless a strong compositional dependence is weak to non-existent for garnets with >><[dollar]I> 38 mol.% almandine component. It is emphasized that the T dependence of diffusion coefficients at constant fO2 (activation energy 54 kcal/mol.) are different from that along an fO2 buffer (activation energy ∼ 64.5 kcal/mol.) as already pointed out by Chakraborty & Ganguly (1991). This distinction is of importance for modelling natural processes. The measurements at low T either eliminate the need for, or greatly reduce the uncertainty of, extrapolation of laboratory data for modelling metamorphic processes. The high P results, combined with those from earlier workers, indicate that P dependence of Mg tracer diffusivity in garnets is much stronger than that in forsterite. This difference may be caused by the difference in compressibility of the coordination polyhedra of Mg between olivines and garnets. Activation volumes of Mg tracer diffusion as high as 8 cm3/mol. may be estimated, using the present data in combination with earlier results. These data suggest that at 1300°C Mg diffusion rates in garnets will decrease by an order of magnitude for every 100 km depth. The P effect will be stronger for every 100 km depth. [P.Br.]
Subjects:Alkaline earth metals; Aluminosilicates; Crystal chemistry; Diffusion; Fugacity; Garnet group; Geochemistry; Magnesium; Metals; Nesosilicates; Orthosilicates; P-T conditions; Silicates
Abstract Numbers:96M/2962
Record ID:1996066690
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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