The rate of Mg diffusion in labradorite may be faster under typical arc magmatic conditions than indicated by dry experiments.
Previous work determining DMg for plagioclase has not resulted in a consensus regarding the appropriate values or the variables that affect them (e.g., dependent on An content? of composition of surroundings?) These experiments were all performed under dry, 1-atm conditions, without a melt present.
We decided to see what Mg diffusion would look like under pressurized, hydrous conditions with natural magmatic melt present.
In collaboration with Julia Hammer and Tom Shea, we discovered that Mg diffusivities display a wide range of values in labradorite, revealing anisotropy never before noted and not directed along crystallographic axes. These startling results require further analysis and experimentation to confirm, but explanations may include the presence of water, providing H+ to the plagioclase structure, and the presence of an adhering melt, facilitating exchange between the crystal and its surroundings.
Mg concentration profiles for three different crystals in the same experiment (all different orientations). The profiles are fit with 1-D error function solutions to the diffusion equation.
Model of plagioclase (green) with ellipsoid showing the magnitude of DMg across crystallographic orientations. If only the crystallographic axis directions are examined, this anisotropy is not evident, perhaps explaining how it has “hidden” until now.
Publication: First, E., Hammer, J., Shea, T., Hellebrand, E., Tachera, D.* Magnesium diffusion in labradorite at hydrous magmatic conditions. Contributions to Mineralogy and Petrology, in prep.