fluid-rock interaction; thermodynamics; chlorine; SIMS; experimental petrology; contact metamorphism; hydrous minerals
Lafay Romain, Baumgartner Lukas P., Putlitz Benita, Siron Guillaume (2019), Oxygen isotope disequilibrium during serpentinite dehydration, in Terra Nova
Siron Guillaume, Baumgartner Lukas P., Bouvier Anne-Sophie, Vennemann Torsten (2018), Accurate Measurements of H 2 O, F and Cl Contents in Biotite Using Secondary Ion Mass Spectrometry, in Geostandards and Geoanalytical Research
, 42(4), 523-537.
Siron Guillaume, Baumgartner Lukas, Bouvier Anne-Sophie (2018), Significance of OH, F and Cl content in biotite during metamorphism of the Western Adamello contact aureole, in Contributions to Mineralogy and Petrology
, 173(8), 63-63.
We study the F, Cl and hydrogen contents of biotite, white mica, apatite, and tourmaline in meta-pelites from the Little Cottonwood aureole (Utah, USA). This field-based study will be complemented by 1 atm diffusion experiments and hydrothermal experiments investigating the response of apatite to H2O-HCl-HF fluids. We have developed measurement techniques and standards for OH-, Cl-, and F- in biotite, and published papers on oxygen isotope standards for biotite and quartz; furthermore, standards are currently being developed for white mica. Our current work highlights the possibility that the above-mentioned minerals might have significant amounts of an oxy-component on the hydroxyl site. Our work on biotite suggests that the reaction 2 OH-biotite => oxy-biotite +H2O reduces the H2O activity in biotite, and hence that the water content is indeed a sensor for water fugacity. The exchange in biotite might be masked by a coupled Ti-oxy biotite substitution. Apatite, on the other hand, does not suffer from this and might thus prove to be an easier candidate for which to develop a practical water fugacity sensor.Our work on biotite seems to suggest that this mineral is capable of retaining chlorine incorporated into its crystal structure during prograde biotite formation. This is surprising, since biotite is known for fast diffusion of cations such as Fe + Mg. A similar discrepancy has been found for apatite: while field studies suggest that Cl in apatite is resilient to re-equilibration even at high temperatures, the existing experimental diffusion data suggest it would readily exchange Cl with adjacent phases down to temperatures as low as 400°C. Brenan (1993a) suggested that significant O2- in apatite might slow down diffusion considerably. Since water fugacity decreases upon cooling due to hydration of high-T phases (Markl and Bucher 1998) this could lead to sluggish diffusion of halogens in many natural apatites. The Little Cottonwood granodiorite stock (Utah, USA) intruded into a thick sequence of late Proterozoic anchizone metamorphic quartzites and pelites. The highest-grade rocks are magnetite-cordierite-sillimanite-biotite rocks showing abundant partial melting. Intrusion depth is estimated at ca. 10-12km. Crystallization of partial melts resulted in locally abundant retrograde muscovite. The overall aim is to establish 1) a comprehensive inventory of halogens in contact metamorphic pelites; 2) develop and test the water fugacity sensors; 3) provide the petrologic community with robust experimental data on apatite-fluid equilibrium and kinetic exchange parameters.