Trace element diffusion during basaltic andesite-dacite equilibration: Effects of coupled dissolution and diffusion

Magmatic systems from the mantle source to the surface experience a plethora of physicochemical processes that alter their heat and mass budget and drive compositional diversity. Magma recharge and mixing of distinct magma batches is recognized as one of the dominant processes occurring throughout the magmatic system. Mixing occurs at the large scale mostly through convective transport, while hybridization ultimately also requires diffusive exchange on the micrometer to centimeter scale. In natural systems, this small-scale exchange is not limited to simple melt-melt diffusive equilibration, but also includes local phase changes of crystals as well as of the melt and vapor phase. As a result, diffusive exchange during magma-magma mixing may be altered relative to melt-melt diffusion. We present extensive trace element zoning profiles (LA-ICPMS data) from an experimental study that investigates the local mass transfer during the juxtaposition of natural basaltic andesite and dacite. Our time series experiments (up to 80 hours) were performed at sub-liquidus conditions (T = 950 to 1000 °C, P = 1.5 kbar, nearly H2O-saturated) to document the complex interplay of mineral dissolution and diffusive exchange. While dissolution is mostly limited to the basaltic andesite, diffusion is operating throughout the entire experimental charge. As a result, we can compare and contrast the observed dissolution-diffusion interplay against determined diffusivities in dacitic melt. Most major elements (e.g., Fe) show diffusion at rates comparable to simple diffusion experiments. Trace elements show a more diverse behavior where some are slowed down relative to known diffusivities (e.g., Pb), and others show uphill diffusion (e.g., La, Nd) that may be a complex interplay of changing partition due to an evolving melt composition near the interface and multi-element coupled diffusion.