Effect of Ca content on equilibrium Ca isotope fractionation between orthopyroxene and clinopyroxene

Concentration effect on equilibrium inter-mineral isotope fractionation is ubiquitous in solid solution systems, but it is not clear in which concentration range such effect is prominent. Using first-principles calculations, we examine the effect of Ca and Fe contents in orthopyroxene (opx) on its average Ca-O bond length and the equilibrium Ca isotope fractionation factor (10(3)ln alpha) between opx and clinopyroxene (cpx). Our results reveal that the average Ca-O bond length in opx is much smaller than that in cpx and it does not change with variable Ca content x (x and y are mole ratios in CaxFeyMg1-x-ySiO3 thereafter here) when x <= 1/48. Incorporation of Fe (y >= 1/32) into opx with a fixed Ca content can only slightly increase the average Ca-O bond length. 10(3)ln alpha(opx-cpx) of Ca-44/Ca-40 is linearly correlated with the average Ca-O bond length in opx, suggesting that 10(3)ln alpha(opx-cpx) of Ca-44/Ca-40 is controlled by opx Ca-O bond strength. Our calculations indicate that the Ca concentration effect on 10(3)ln alpha(opx-cpx) is significant when x in opx ranges from 2/16 to 1/48, while Fe in natural opx only causes a slight decrease in 10(3)ln alpha(opx-cpx). Our results provide insights into Ca isotope fractionation in high-temperature geochemical processes. Given that Ca content x in opx from natural peridotites is usually lower than 1/32 and Fe content y is generally similar to 10 mol%, Ca and Fe concentration effects on 10(3)ln alpha(opx-cpx) in natural samples are negligible. Rather, 10(3)ln alpha(opx-cpx) is mainly controlled by temperature. 10(3)ln alpha(opx-cpx) of Ca-44/Ca-40 decreases from 0.50% to 0.26% when temperature increases from 1000 K to 1400 K if the Fe effect is taken into account. Therefore, if Ca isotope fractionation between opx and cpx (Delta Ca-44/40(opx-cpx)) in natural peridotites is greater than 0.50% or lower than 0.26%, it may indicate disequilibrium of Ca isotopes. Finally, the large 10(3)ln alpha(opx-cpx) relative to our current analytical precision suggests that Delta Ca-44/40(opx-cpx) can be used as an independent thermometer with a precision comparable to elemental thermometers. Because most naturally occurring minerals are solid solutions with variable chemical compositions, this study presents a guideline to explore the concentration effect on equilibrium isotope fractionation among minerals. (C) 2017 Elsevier Ltd. All rights reserved.