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.
Understanding how halogens are distributed among usual hydrous silicates in the lithosphere is important to constrain their deep geochemical cycle and fluid-rock interactions in subduction zones. This article presents firstprinciples modelling of halogen (F -, Cl -, Br -) incorporation in hydrous silicates including mica, chlorite, serpentine, amphibole, epidote and carpholite. The approach allows studying the impact of crystal chemistry on halogen partitioning by quantification of the energetic cost of halogen incorporation in minerals. Calculations are carried out in large systems where halogens are in minor to trace concentrations. Estimations show that F-bearing defects must be separated at least 9 Å from one another to reproduce trace element behaviour, this value increasing to at least 10 Å for Cl and Br. Results highlight the competition between the effects of electrostatic interactions and steric hindrance for incorporation of halogens, where steric hindrance has greater importance for heavy halogens, in particular for Br. Interaction with alkalis is a major control for F incorporation, especially in mica. Other parameters such as octahedral site occupancy, Si/Al ratio of tetrahedral sites and the nature of alkalis in amphibole and mica (K or Na) appear to play subordinate roles. Partition coefficients have been estimated in mineral assemblages in an effort to be representative of subduction zone metamorphism. Results show that pargasite, biotite and lizardite are favoured hosts for all three halogens, followed by clinochlore, tremolite and carpholite. The energetic cost of incorporating halogens into dioctahedral phyllosilicates and epidote is comparatively higher, and partitioning is predicted as unfavourable to these minerals. Fractionation between halogens in subduction zones is predicted by the evolution of mineral assemblages and partition coefficients, a consequence of the influence of crystal chemistry over halogen incorporation in hydrous silicates.
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