Genetic implications, composition and structure of trioctahedral micas in xenoliths related to Plinian eruptions of the Somma-Vesuvius volcano (Italy)

The present work is part of a systematic mineralogical and petrographic characterization of mica-bearing xenoliths from Somma-Vesuvius volcano (Roman Magmatic Province, southern Italy). Skarns, composite skarns-marbles and cumulates from Pompeii Plinian eruption (AD 79), and skarns and syenite from Avellino eruption (3945 ± 10 cal yr BP) were investigated in order to define the crystal chemistry of the Somma-Vesuvius trioctahedral micas and to draw inferences on petrogenetic processes they were subjected. Xenoliths were characterized by means of polarized optical microscopy, scanning electron microscopy (SEM-EDS), X-ray powder diffraction (XRPD) and bulk-rock geochemical analyses. Mica crystals were studied through electron microprobe analysis (EMPA) and single crystal X-ray diffraction (SCXRD). Micas from skarns and composite skarn-marble rocks are associated with calcite, dolomite, and Mg+Ca silicates, and silicate minerals (clinopyroxene, vesuvianite, humite, clinohumite, sodalite, forsterite, leucite and garnet), respectively. Cumulate samples consist of mica and clinopyroxene whereas syenite is mainly composed of mica, K-feldspar, feldspathoids and clinopyroxene. Trace element arrays are scattered for skarn and composite skarn-marble samples. The REE patterns show a general enrichment in light (La, Ce, Pr, Nd) and medium (Sm, Eu, Gd, Tb, Dy) rare earth elements, in some case with slight positive Gd anomaly. Cumulate samples generally show low amounts of Ba, Sr, Zr and Th, while syenite exhibits low concentrations of trace elements, except for Rb, Cs and Tl. Mica crystals occurring in the studied xenoliths are phlogopite with variable Al and Mg content at the octahedral site, a negligible tetraferriphlogopite component and different dehydrogenation degree. All samples belong to the 1M polytype (C2/m and C2 space group) and show a wide range of unit cell parameters, especially of the c axis [5.3055(1) ≤ a ≤ 5.3218(1) Å, 9.1893(1) ≤ b ≤ 9.2188(4) Å, 10.1803(2) ≤ c ≤ 10.2951(2) Å]. The shortest c cell parameter pertains to de-hydrogenated phlogopite from Avellino skarn whereas OH-rich phlogopite from Pompeii composite skarns-marbles has a c cell parameter which approximates that of the endmember phlogopite. Overall, it is observed that the crystal chemistry of the micas of the present study extend the known range of the other Vesuvian micas from literature. The Ti-depletion and the wide degree of dehydrogenation of phlogopites from skarns and composite skarns-marbles suggest that the studied samples originated under variable pressure conditions. In addition, the presence of humite in the mineral assemblage seems to indicate the occurrence of devolatilization reactions. The scarce mica occurrence in cumulate and mainly in syenite, instead, may depend on pressure conditions exceeding the mica stability.