Mathematics – Logic
Scientific paper
Mar 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44..387m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 3, pp.387-391
Mathematics
Logic
4
Scientific paper
The kinetics and mechanisms of reactions: 1. (I) Analcite + Quartz = Albite + H 2 O. 2. (II) Jadeite + Quartz = Albite. have been studied at 300-500°C. P(fluid) = 1 kbar, in aqueous solutions 0.1 M NaCl and 0.05 M Na 2 Si 2 O 5 (NaDS). Albite is the only phase to form, with the exception that at 300°C reaction II in NaCl forms metastable analcite with an Al 2 O 3 / SiO 2 ratio of 1:6. All reaction occurs by solution-precipitation mechanisms. The effects of the two solutions are opposed; reaction I in NaDS solution is faster than in NaCl, whereas reaction II proceeds more quickly in NaCl. The kinetics of reaction I can be substantially described by: X Ab = K · t 3/2 (X Ab = mole fraction of product albite, t = time of reaction and K is a rate constant). Reaction II appears to behave according to: X Ab = K ' · t 1/2 . In both reactions nucleation appears to be kinetically negligible. Oxygen isotope fractionation between albite and water at 350-500°C is described by: 1000 In Ab - H 2 O = 2.81 ± 0.11 × 10 6 T -2 -3.50 ± 0.23. The temperature dependencies of the rate constants for reaction I can be described by the functions: K NaCl = 1.2 × 10 11 ± 50 · exp (-44.5 ± 5.6 kcal / RT ) K NaDs = 7.1 × 10 17 ± 10 · exp (-60.8 ± 3.0 kcal / RT ). Basic problems in petrological applications of these rate equations are differing rate controlling mechanisms in natural and experimental situations and the complexities of metamorphic kinetic controls. Extrapolations of the Arrhenius equations suggest that the kinetics of reaction I would not perturb its occurrence at equilibrium in regional (burial) metamorphism [except, perhaps, when P(H 2 O) < P(load)]. However, in contact metamorphism `overshooting' of equilibrium may occur, leading to partial reaction in outermost zones of metamorphism and attenuation of thermal aureoles.
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