T172 Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

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

Paper number: T172-14
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308674

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