Chemical and K–Ar systematics of a mylonitic mica after natural and experimental interactions with varied fluids

Flow-through and batch-leaching experiments combining mineralogical, chemical and K–Ar isotopic analyses of mica separates from a mylonitic sample of the Tyndrum faulting system in Scotland (U.K.) were conducted to evaluate and simulate the natural interaction with H2SO4-loaded river water at pHs of 3–4. The flow-through experiments with H2SO4-loaded and pure deionized water completed by a batch-leaching experiment with 1 M HCl at room temperature had varied effects on soluble mineral phases, such as Fe-sulfates and Ca/Mg-carbonates, that were present in the varied mica size fractions, but none caused the mineralogical and K–Ar characteristics of the mica to differ from separates of the natural environment.Despite the limited number of K–Ar ages, size fractionation of the mylonitic mica identified two generations of mica with different REE patterns. The coarser (2–8 μm) fraction yielded a high La/Yb ratio of 4.3 and crystallized at 359 ± 6 Ma at a temperature of 250–300 °C, probably during a major tectonic-thermal activity. The smaller (< 0.5 μm) fraction yielded a La/Yb ratio of 2.1, a younger K–Ar age at 315 ± 5 Ma and a lower crystallization temperature of about 200 °C.

► Fluid-flow and batch-leaching experiments on mica fractions from a mylonite of Tyndrum fault in Scotland (U.K.). ► Fluid-flow experiments designed to simulate natural interaction with H2SO4-loaded water at pH ≈ 3. ► Experiments affected the Fe-sulfides and Ca-carbonates, not the K–Ar ages of the mica. ► Experiments provided the same K–Ar results than the mica fractions in natural conditions. ► Two generations of mica of 359 ± 6 and 315 ± 5 Ma with different REE patterns occur in the size fractions.