Abstract
Abstract. Understanding the geochemical behaviour of trace and minor elements in mineral assemblages is of primary importance to study small- and large-scale geological processes. Partition coefficients are frequently used to model the chemical evolution of minerals and fluids during melting and in metamorphic rocks of all grades. However, kinetic effects hampering equilibrium partitioning may invalidate the modelling. This study aims at calculating partition coefficients and testing their applicability in natural mineral assemblages, choosing Cr in garnet and clinopyroxene via exchange with Al as a case study. First-principle modelling has been combined with measurements and element mapping to estimate partition coefficients for Cr and the deviation from equilibrium. Results highlight the role of crystal chemistry over the strain field around point defects, controlling the dynamics of the Cr3+ = Al3+ exchange between clinopyroxene and garnet. Ab initio calculations allowed estimation of Cr partition coefficients between garnet and clinopyroxene, using a thermodynamic approach based on endmembers and mixing models simplified for trace element behaviour. The Cr3+ = Al3+ exchange reaction between garnet and the jadeite component of clinopyroxene depends on the grossular and pyrope content, with Cr preferentially incorporated into grossular over jadeite but preferentially incorporated into jadeite over pyrope. Comparison of predicted partition coefficients to measured concentrations in natural samples, together with element mapping, shows large disequilibrium. Cr-rich and Cr-poor sectors exhibit disequilibrium attributed to slow diffusivity of Cr during crystal growth and interface-coupled dissolution–precipitation, even for garnet–clinopyroxene assemblages crystallized around 850 ∘C.
Highlights
Trace and minor elements have been used as tracers of many geological processes and understanding their geochemical behaviour is highly important to petrology
This study aims at calculating partition coefficients and testing their applicability in natural mineral assemblages, choosing Cr in garnet and clinopyroxene via exchange with Al as a case study
Ab initio calculations allowed estimation of Cr partition coefficients between garnet and clinopyroxene, using a thermodynamic approach based on endmembers and mixing models simplified for trace element behaviour
Summary
Trace and minor elements have been used as tracers of many geological processes and understanding their geochemical behaviour is highly important to petrology. The partitioning of trace and minor elements between minerals and/or fluids remains poorly constrained in many cases, on both theoretical and applied grounds (Albarède and Bottinga, 1972; Dubacq and Plunder, 2018). Modelling approaches for the incorporation of trace elements in mineral structures are challenged by the necessary large number of atoms in simulations, as for melts Wagner et al, 2017) Despite this complexity, theoretical modelling provides in-depth understanding of defect incorporation in mineral structures, linking strain to energy differences independently of experimental petrology and measurements. Interfacecoupled dissolution–precipitation appears to control the distribution of Cr due to its very low effective mobility
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call