Thermodynamics and kinetics of Fe²⁺-Mg exchange between orthopyroxene and spinel: Experimental calibrations and applications

摘要

The variation of equilibrium Fe²⁺-Mg fractionation between orthopyroxene and spinel as a function of temperature was experimentally determined in the range from 1250-850°C and 0.8-1.3 GPa. The effect of chromium on the equilibrium fractionation was constrained by limited experiments at 1000°C and 1.2 GPa. The experimental data can be cast into a thermometric expression as: T(K) = [(1373±155)+121· P(GPa)+(2558±181)·Xˢᵖ(Cr3+)]/[ln K(D) + (0.55± 0.13)] where K(D) = (Fe²⁺/Mg)ˢᵖ/(Fe²⁺/Mg)ᴼᵖˣ. The self-diffusion of Fe²⁺ and Mg in spinel was determined from binary diffusion couple experiments at 1325-950°C and 2.0 GPa. These experiments yield for Fe²⁺: Dₒ = (1.8 ± 2.8) x 10⁻⁵ cm²/sec and Q = (198 ± 19) kJ/mol; and for Mg: Dₒ = (1.9 ± 1.4) x 10-5 cm²/sec and Q = (202 ± 8) kJ/mol. The above data were applied to compositional profiles of coexisting orthopyroxenes and spinels in diogenite samples from the Antarctic meteorite collection to constrain their thermal histories. The modeling suggests that diogenites cooled to a temperature of ≥ 885°C at a rate of ≤ 3800°C/Ma at 1000°C. Modeling of the compositional zoning of spinel indicates that it cooled at ∼(6-20) x 10³ °C/Ma at 700°C corresponding to a burial of ∼65-130 m within a 100-200 m thick regolith blanket. This supports the idea that the samples were ejected and buried within a blanket of regolith material after initial cooling from the magmatic stage. Application of the diffusion data for spinel to the cooling rates of terrestrial ultrabasic rocks suggests that Ozawa (1984) cooling rates derived from the modeling of compositional profiles between olivine and spinel should be revised downwards by approximately two orders of magnitude.