Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications

Volcanic activity at Mt. Vulture lasted about 750 ka and produced SiO2-undersaturated volcanic rocks that can be classified as old (similar to 700 ka), intermediate (similar to 600-550 ka), and young (similar to 130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), hauyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mossbauer) methods. EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): SiO2 = 33.14-38.01, Al2O3 = 15.56-20.45, MgO = 13.02-20.81, FeOtot = 6.34-14.08, TiO2 = 2.34-6.02, K2O = 6.03-9.48, Na2O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded H2O = 2.86 +/- 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT191_2 and VUT191_19, which yielded values of 3.81 +/- 0.12 and 1.72 +/- 0.08 wt% H2O, respectively. Mossbauer investigation showed that all the iron in VUT191 mica is octahedral with Fe2+= 25.5% and Fe3+= 74.5%, confirming that Vulture micas are particularly Fe3+-rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 <= R <= 3.17, 2.09 <= R-w <= 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) angstrom. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [M-VI(2+) + 2(OH)(-) <-> Ti-VI(4+) + 2O(2-) + H-2] and M3+-oxy [M-VI(2+) + (OH)(-) <-> M-VI(3+) + O2- + 1/2H(2), with M3+ = Fe3+, Al3+] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 M-VI(2+) <-> Ti-VI(4+) + (VI)square and 3(VI)M(2+) <-> 2(VI)M(3+) + (VI)square occur.