HR: 15:00h
AN: V23C-05 INVITED     [Abstracts]
TI: Reaction Rates in Deformation and Hydrostatic Experiments in the Anhydrous System Anorthite - Forsterite
AU: * Stunitz, H
EM: holger.stuenitz@unibas.ch
AF: Dept. of Geosciences, Basel University, Bernoullistr. 32, Basel, 4056 Switzerland
AU: De Ronde, A
EM: almar.deronde@unibas.ch
AF: Dept. of Geosciences, Basel University, Bernoullistr. 32, Basel, 4056 Switzerland
AU: Tullis, J
EM: jan_tullis@brown.edu
AF: Dept. of Geological Sciences, Brown University, Providence, RI 02912 United States
AB: The reaction anorthite $+$ forsterite --$>$ cpx $+$ opx $+$ spinel $\pm$ gnt proceeds at high temperatures and elevated pressures in the lower crust and upper mantle. This solid-solid reaction was studied experimentally at 900$\deg$C in the pressure range of 1000 to 1600 MPa in both shearing deformation and hydrostatic experiments. Powder mixtures (1:1 by vol) of anorthite (An92) and forsterite (Fo93) are hot pressed at 970$\deg$C, 750 MPa for 48 hrs in a Griggs apparatus and deformed ($\dot\gamma$ = 5 $\times$ 10$^{5}$ sec$^{-1}$) after adjustment of P and T to run conditions. H$_{2}$O content of the samples has been measured by FTIR and is $<$ 30 ppm. At small pressure overstepping (ca. 200 to 300 MPa) undeformed samples show only 10 % reaction progress after 168 hrs, whereas reaction progress in deformed samples after 72 hrs is 60 %. At greater pressure overstepping (700 to 800 MPa) the difference between deformed and undeformed samples is less pronounced (95 % after 60 hrs deformed, 75 % after 168 hrs undeformed) but still present. At greater pressure overstepping, undeformed samples show an exponential reaction rate, whereas that of deformed samples is always linear. Samples initially deformed and then kept hydrostatically show a fast initial reaction rate (85 % of total reaction progress after 0.25 of total run time), followed by a slower reaction progress (15 % reaction after 0.75 of total time) under hydrostatic conditions. The difference in reaction progress is mainly attributed to different nucleation rates. In all experiments, enstatite rims form around olivine grains separating those from other reaction products. Such coronas are indicative of diffusion-controlled reactions. Plots of rim thickness vs time indicate a relative increase of the bulk diffusion coefficient by a factor 5 in the deformed samples compared to undeformed. However, as the grain size of reaction products of deformed samples is 10 times smaller than in undeformed ones, the nucleation rate in deformed samples is $\sim$ 5000 times higher. The increased nucleation rate is interpreted to result from higher defect densities and greater driving potential for the reaction in deformed samples. The increased nucleation rate enhances reaction rate and thus rim growth rate, so that a straightforward interpretation of rim growth in terms of diffusion parameters is impossible. Thus, nucleation and length scales of reaction processes in deforming rocks may be very different from those in hydrostatic cases, and for the purpose of extrapolation to nature rates of reaction from hydrostatic experiments provide only minimum values.
DE: 8159 Rheology--crust and lithosphere
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3902 Creep and deformation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting