Kinetics of high-pressure phase transformations: Implications to the evolution of the olivine → spinel transition in the downgoing lithosphere and its consequences on the dynamics of the mantle

摘要: Abstract The rate of a high-pressure phase transition increases exponentially with temperature (T) and overpressure or pressure beyond equilibrium (ΔP). It is also greatly promoted by introducing shear stress, diminishing grain size, adding water other catalysts to the reactants. For an isothermal isobaric no compositional change, if steady state nucleation on surfaces attained, equation can be expressed: (1) before site saturation by: X = 1 − exp(−Kt4), where K C T 4 exp[− 2 (C 3 + ΔP) 5 ][exp( −C 6 )-exp −(C 7 8 ] (2) after exp(−K′T), K/t 9 T[exp( )-exp{t-(C ΔP })] , volume fraction completion transformation, t time, C's are characteristic constants. C1 C9 functions C3 C6 stress. All almost independent pressure. Thus, as function T, ΔP, over narrow P-T range experimentally determined, calculated effect size stress transformation evaluated. equations for given composition, then determinable. non-isothermal non-isobaric from ones penetration into metastability field known. important feature kinetics transitions predicted these that field, there defined temperature, Tch, below which virtually zero matter how metastable material is. olivine → spinel in mantle, this may high 700° C. fast moving downgoing slab, at its cold center remain Tch even down depths excess 600 km, thereby depressing olivine—spinel boundary. At early stage development plunging speed slow. This allows interior slab heat up proceed rapidly near equilibrium. As result, boundary will distorted upwards. rising denser provides additional driving force accelerates plate. Since upper portion pulled lower pushed above, earthquakes down-dip extension occur mantle while those compression originate zone. Because occurs close equilibrium, aseismic region separating two seismic zones. When plate velocity exceeds certain limit, becomes low enough depress transition. downwards. buoyant created reduce force, approach state. In addition, compress result throughout length slab. Now so far reaction implosive. A rise frequency deep towards implosion zone thus predicted. Therefore, well stabilizing velocity, control earthquake distributions