Non-traditional stable isotope fractionation at high temperatures

The last ten years have seen big progress and wide applications of a novel field,non-traditional stable isotope(NTSI)geochemistry,to high temperature geo-science studies.Invention of multi-collector-inductively coupled plasma-mass spectrometry(MC-ICP-MS)led to the big breakthrough of analytical methods for heavy stable isotopes.This contribution summarizes Li,Fe,and Mg isotope studies on igneous rocks and minerals,as representative of NTSI geochemistry.Li isotopes have been widely applied to the studies of mantle geochemistry,recycling of subducted materials,and metamorphism to constrain the source of magma and kinetic diffusion process.Fe isotope fractionation is related to partitioning of multi-valent Fe between Fe-bearing phases,which can occur in the course of mantle metasomatism,partial melting,and fractional crystallization.Mg isotopic compositions of igneous rocks most likely reflect the source signatures.Variation of Mg isotopic ratios of mantle peridotites is trivial and this provides a homogenous background for Mg isotope fractionation in low temperature processes.Furthermore,Cl,Si,Cu,Ca,and U isotopes are also promising in the future geochemical studies.Experimental studies and theoretical simulation for the mechanisms of isotope fractionation provide important guidances for understanding the NTIS data.Experimental studies show that light and heavy isotopes have different migration velocity at high temperature processes such as chemical diffusion,evaporation,and desublimation,which could produce significant kinetic isotope fractionation.Equilibrium isotopic fractionation could occur among mineral,melt,and fluid when chemical environment of the isotopes are different between the phases.Recent thermal diffusion and migration experiments on silicate material reveal a new mechanism of magma differentiation and isotope fractionation.Along a temperature gradient in silicate magma,large elemental variation and isotopic fractionation can occur,by which a wet andesite can even be differentiated to granite.This suggests that thermal migration could be important for continental crustal formation and evolution.If temperature gradient exists long enough during magma differentiation,thermal diffusion can produce significant stable isotope fractionation,which is contrast to the mechanism of traditional kinetic and equilibrium isotope fractionations.Such process can be fingerprinted by positive correlations among multi-stable isotopic systems.Due to thermal diffusion,concentration of material loaded or dissolved in the fluid is a function of Soret coefficient(ST).However,because ST is highly variable and sensitive to lots of factors,the basic physics of thermal diffusion is still poorly understood.As shown by Mg,Ca,and Fe isotope measurement of thermal diffusion experiments,isotope fractionation driven by temperature gradient is independent to the bulk composition and temperature of the system,suggesting that the difference of ST between two isotopes of the same element can be considered as a constant.This can simplify and help the studies on thermal diffusion and ST.