Diopside (CaMgSi2O6) + merwinite (Ca3MgSi2O8) reaction rims were experimentally grown at contacts between monticellite (CaMgSiO4) single crystals and isotopically labelled wollastonite (44Ca29SiO3) powder at 900 °C and 1.2 GPa with trace amounts of H2O present. The rim is comprised of three monomineralic layers forming the sequence merwinite – diopside – merwinite and has a symmetrical internal microstructure. NanoSIMS analyses revealed that both 44Ca stemming from the wollastonite and 40Ca stemming from the monticellite are distributed across the entire rim. In contrast, 28Si and 29Si are retained in those regions of the reaction rim that stem from the monticellite and from the isotopically doped wollastonite, respectively. This and the internal microstructure indicate that MgO was the only component that was transferred across the entire reaction rim with an effective bulk diffusion coefficient of Dbulk, MgODi+Mer = 10−16.3±0.2 m2 s−1. In addition to MgO, CaO was relatively mobile during reaction rim growth, whereas SiO2 had a significantly lower mobility compared to MgO and CaO at least in the diopside layer. The observed multilayer-type rim microstructure can either be explained with a two-step model starting with the development of a cellular-type microstructure comprised of alternating diopside–merwinite lamellae oriented perpendicular to the original interface, which transformed into the multilayer-type microstructure through mobility of CaO, or with a one-step model, which implies that SiO2 was transferred across the two merwinite layers on either side of the central diopside layer. The first model does not require any SiO2 mobility across any layer of the rim, the latter model requires neither transfer of SiO2 across the central diopside layer nor any re-distribution of CaO. The potential finite mobility of SiO2 and the observed comparatively high mobility of CaO, which are both notoriously immobile at very dry conditions, are ascribed to the presence of minute amounts of water. The profound effect of trace amounts of water on relative component mobilities and their effect on rim microstructures therefore implies that reaction rims may be used to infer the availability of water during their formation.

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