Insights from orthopyroxene reaction rims and layers (Invited)
Abstract
It is exactly 50 years since J.B. Thompson [1] published his ideas on reaction corona formation and made MgO-SiO2 his system of choice for their illustration. In hindsight this paper proved not only to be very influential for the theory of transport-controlled reactions, but also for the praxis of experimental petrology. Reaction rim growth in this system (with some minor, but substantial extensions) became explored in a depth unmatched by any other chemical system of geological relevance. We will review the achievements of the last few years up to the newest data. Starting from the question after the rate-controlling species in Opx rim growth between olivine and quartz, it became clear that the answer depends strongly on the presence of water, that transforms silicon from a very immobile to a highly mobile component. Several 100 ppm water (and where is the lower limit?) have practically the same effect as several wt%. New 3D-microimaging shows that this is in part due to fluid-filled nanopores at the propagating interfaces. Really dry experiments have until shortly been possible at 0.1 MPa, but not in solid medium high-P devices. This has now become feasible and was used to prove that the bulk diffusion coefficients governing the growth rates of Opx rims are in fact 3 to 5 orders of magnitude smaller under dry conditions than in the presence of traces of water, at P and T relevant to the lower crust and upper mantle. Opx rim growth has been used to study reaction-induced stress. The formation of Opx from Ol+Qtz implies negative volume change and thus requires the deformation of the surrounding matrix when the entire experimental system is under pressure. It was shown that Opx rims grow at different rates if they form around Ol grains in Qtz matrix, or vice versa. Reaction rim growth rates are apparently as well controlled by diffusional mobility of chemical species as by the rheological properties of the matrix. The stepwise extension of the system MgO-SiO2 by elements accepted to some degree in olivine (like Fe, Mn, Co, Ni, Ca, Cr) is now used to study kinetic fractionation of these elements during metasomatic exchange. Starting from Ol-Qtz couples, local equilibrium evolves at the Ol/Opx interface, but the bulk Opx may be far out of equilibrium with the bulk Ol. It is demonstrated that this kinetic fractionation mainly results from the interplay of equilibrium distribution at the Ol/Opx interface and limited grain boundary diffusion in the polycrystalline Opx rim. This type of fractionation is becoming a role model for reaction fronts in mantle metasomatism where the source of SiO2 is not quartz but silicic melts, thereby introducing a new level of complexity. [1] Thompson, J.B.jr., 1959, Local equilibrium in metasomatic processes: Researches in Geochemistry, v. I, p. 427-457
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V53C..03M
- Keywords:
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- 1012 GEOCHEMISTRY / Reactions and phase equilibria;
- 8412 VOLCANOLOGY / Reactions and phase equilibria