Magnesium self-diffusion in orthoenstatite |
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| Authors: | Craig S Schwandt Randall T Cygan Henry R Westrich |
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| Institution: | (1) NASA, Johnson Space Center, SN4, Houston, Tx, 77058-3696, USA Tel: (281)483-1368; e-mail: cschwand@ems.jsc.nasa.gov, US;(2) Geochemistry Department, Sandia National Laboratories, Albuquerque, NM, 87185-0750, USA, US |
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| Abstract: | Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic
directions in natural orthoenstatite (En88Fs12). Experiments were conducted at 1 atm in CO-CO2 gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of 25Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched 25MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling.
The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E
a
, and frequency factors, D
o, where D = D
o exp (−E
a
/RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E
a
= 360 ± 52 kJ/mole and D
o = 1.10 × 10−4 m2/s; b-axis, E
a
= 339 ± 77 kJ/mole and D
o = 6.93 × 10−6 m2/s and c-axis, E
a
= 265 ± 66 kJ/mole and D
o = 4.34 × 10−9 m2/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for
diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite
provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to
previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/106 years) for the metal portions.
Received: 22 January 1997 / Accepted: 2 October 1997 |
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