THE DISTRIBUTION OF HYDROXYL SPECIES IN HYDROTHERMAL QUARTZ CRYSTALS: THE DOMINANT ROLE OF POST-CRYSTALLIZATION DIFFUSION

Multiple factors control the amount of contaminant species that are incorporated into growing hydrothermal quartz crystals, including the self-evident thermodynamic variables (P, T, and fluid composition) responsible for equilibrium growth. Non-equilibrium factors associated with sector growth can also affect impurity uptake during growth. However, in our extensive study of hydrothermal quartz sampled from a variety of geologic environments, we observe that diffusion associated with post-crystallization thermal soaking exerts the most dominant control on the abundance and distribution of contaminants within natural crystals. Impurity concentration gradients provide important information regarding the post-crystallization thermal histories of their host crystals and offer valuable new insights into the evolution of metamorphic hydrothermal systems.

Here, we focus on a gemmy quartz crystal collected in a vein from the high-temperature Lepontine Zone of the Swiss Alps. We utilize high-resolution (200µm spot size) infrared spectroscopic analyses conducted on traverses across multiple thin (~750µm) wafers cut perpendicular to the c-axis of the crystal. HOH, LiOH and AlOH contaminant species concentrations vary vertically and horizontally through the crystal both as a function of uptake during growth and diffusion following growth, with diffusive loss ranging from near 0% in the core of the crystal to near 100% toward both its terminus and base. We show that all concentration profiles reveal identical relative diffusion rates with LiOH > HOH > AlOH, and that diffusion toward terminus faces (parallel to c) = diffusion toward the base (also parallel to c) >> diffusion toward prism faces (perpendicular to c). We show that Dauphiné twinning can increase diffusion rates by a factor of two within the same sector, and that multiple phases of growth on individual crystals can be discerned with our technique.