CHEMICAL FINGERPRINT OF HYDROTHERMAL QUARTZ CRYSTALS SAMPLED ALONG A TRAVERSE ACROSS THE SWISS ALPS

Quartz crystals record the evolution of hydrothermal fluid systems that accompanied the formation of the Swiss Alps. Crystals from twenty localities were collected along a geologic traverse across four distinct growth regimes that define the Alpine Mountain Range in central Switzerland: the low temperature Helvetic zone in the north, the Aar Massif, the Gotthard Massif, and the high temperature Lepontine in the south (Mullis et al., 1994). Spectroscopic measurements offer insights into changing fluid compositions, crystal growth timescales, and post-crystallization thermal histories characteristic of each regime. We present results on crystals analyzed using micro-FTIR spectroscopy, with emphasis placed upon concentrations of the three abundant hydrous species (AlOH, LiOH, and molecular water). Individual crystals were sectioned into 1 mm thick wafers cut perpendicular to the c-axis. FTIR traverses were completed with 100 µm increments to achieve high spatial resolution of impurity concentrations.

All crystals show measurable abundances of the three hydrous impurities with maximum impurity concentrations widely variable across the four metamorphic zones. AlOH was the most abundant impurity in three of the four zones, with the Aar Massif containing higher overall impurity concentrations and relatively higher LiOH abundances. Significant differences in impurity concentrations across r and z sector zones allow for mapping the morphologic evolution of individual crystals from each of the four growth regimes. Our results also show pronounced diffusion profiles that document syn and post-crystallization diffusive loss from the edges of crystals, offering insights into their thermal histories.