Experimental studies of iron-magnesium order-disorder in orthopyroxene: Equilibrium, kinetics, and applications

Abstract

The thermodynamic and the kinetics of the Fe-Mg order-disorder process in orthopyroxene were studied by means of thermal annealing experiments at fix fO₂ condition, and single crystal X-raydiffraction to determine the site occupancies. The behavior of Mn on the equilibrium fractionation of Fe-Mg was studied by a series of annealing experiments using a naturally occurring Mn-rich orthopyroxene (donpeacorite). It was shown that, although Mn and Fe preferentially order to the M2 site relative to Mg, Mn has a significantly stronger M2 site preference than Fe. This result implies that when computing the site fractionation for Fe-Mg in Fe-rich Mn-poor orthopyroxene, the small amount of Mn present in the structure should always be totally ordered in the M2 site. The kinetics of the order-disorder reaction was studied as a function of temperature, composition, and fO₂. The results are compatible with the theoretical predicted variation of the rate constant as (fO₂)¹/⁶. The temperature and compositional dependence of the disordering rate constant, K⁺, can be expressed as: ln K⁺(Ord) = - [(36420)/T(K)] min⁻¹ + 29.03 + 4162(XFe) where XFe is the Fe molar fraction of the sample. The thermodynamic and kinetics data of Fe-Mg order-disorder in orthopyroxene permit retirval of cooling rates natural orthopyroxene crystals from their quenched ordering states around the closure temperature (T(C)) of ordering. Thus, I have applied the data to Central Gneiss Complex, British Columbia, and to a diogenite-meteorite which is believed to have originated on Vesta, to constrain their cooling rates. For the Central Gneiss Complex the cooling rate was found to be ∼10-15°C/My at T(C) ∼290°C. This cooling rate is in excellent agreement with that constrained by geochronological data, and implies an exhumation velocity of ∼0.2mm/y. The thermal history inferred for the diogenite-meteorite GRO95555 suggests that the sample underwent very fast cooling at two different rates: a faster cooling at ∼400°C/year as retrieved from modeling of the compositional zoning the orthopyroxene-spinel couple, followed by a slower cooling at ∼5°C/1000y as obtained from the modeling of the observed quenched state in the orthopyroxene. The implication of these rapid but constraining cooling rates on the excavation and burial of the sample in its parent body has been discussed.