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Journal of Petrology Advance Access published online on April 22, 2005
Journal of Petrology, doi:10.1093/petrology/egi034
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received July 28, 2004
Accepted March 7, 2005

Article

A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

JOHN M. FERRY 1*, DOUGLAS RUMBLE III2, BOSWELL A. WING 3, and SARAH C. PENNISTON-DORLAND 1

1 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, JOHNS HOPKINS UNIVERSITY, BALTIMORE, MD 21218, USA
2 GEOPHYSICAL LABORATORY, CARNEGIE INSTITUTION OF WASHINGTON, 5251 BROAD BRANCH ROAD, NW, WASHINGTON, DC 20015, USA
3 EARTH SYSTEM SCIENCE INTERDISCIPLINARY CENTER, UNIVERSITY OF MARYLAND, COLLEGE PARK, MD 20742, USA

* To whom correspondence should be addressed.
JOHN M. FERRY, E-mail: jferry{at}jhu.edu


  Abstract

Progress ({xi}) of the infiltration-driven reaction, 4olivine + 5CO2 + H2O = talc + 5magnesite, that occurred during Barrovian regional metamorphism, varies at the cm-scale by a factor of 3·5 within an {approx}3 m3 volume of rock. Mineral and stable isotope compositions record that XCOCO2, {delta}18Ofluid, and {delta}13Cfluid were uniform within error of measurement in the same rock volume. The conventional interpretation of small-scale variations in {xi} in terms of channelized fluid flow cannot explain the uniformity in fluid composition. Small-scale variations in {xi} resulted instead because (a) reactant olivine was a solid solution, (b) initially there were small-scale variations in the amount and composition of olivine, and (c) fluid composition was completely homogenized over the same scale by diffusion-dispersion during infiltration and subsequent reaction. Assuming isochemical reaction, spatial variations in {xi} image variations in the (Mg + Fe)/Si of the parent rock rather than the geometry of metamorphic fluid flow. If infiltration-driven reactions involve minerals fixed in composition, on the other hand, spatial variations in {xi} do directly image fluid flow paths. The geometry of fluid flow can never be determined from geochemical tracers over a distance smaller than the one over which fluid composition is completely homogenized by diffusion-dispersion.

Keywords: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress.
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