Phlogopite from the Ventaruolo subsynthem volcanics (Mt Vulture, Italy): a multi-method study

摘要

Volcanic activity at Mt Vulture lasted throughout the Middle Pleistocene and produced SiO_2-undersaturated volcanics. Deposits from the Monte Vulture stratovolcano have been classified into four subsynthems and clustered into the Barile Synthem. In the present investigation, trioctahedral micas from the uppermost units (the Ventaruolo Subsynthem) of the Barile Synthem are considered. The samples are labelled VUT187. The phlogopitic micas were separated from the host rock (an olivine-foidite) and underwent chemical (electron microprobe analysis - EMPA and C-H-N), structural (single-crystal X-ray diffraction) and spectroscopic (Mossbauer) investigations. The EMPA yielded: MgO (17.62-21.89 wt. percent ), FeO_(tot) (5.98-9.78 wt. percent ), TiO_2 (1.81-3.92 wt. percent ) and Al_2O_3 (14.47-17.98 wt. percent ), with H_2O contents = 2.86 (±0.42) wt. percent determined by C-H-N analyses. Mossbauer investigation provided ~([VI])Fe~(2+) = 12.6 percent ,~([VI])Fe~(3+) = 87.4 percent . The chemical and structural data are consistent with the occurrence of Ti-oxy,~([VI])M~(2+) + 2(OH)~-<=>~([VI])Ti~(2+) + 2O~(2-) + H_2, and M~(3+)-oxy substitutions, ~([VI])M~(2+) + (OH)~-<=> ~([VI])M~(3+) + O~(2-) + 1/2H_2. with M~(3+) = Fe~(3+) , Al~(3+) . In particular, Fe~(3+) -oxy substitution has affected the Fe~(2+) /Fe~(3+) ratio in the studied sample. This is probably due to the fact that interaction with underground water or a hydrotherrnal system may have altered the oxygen fugacity and raised the Fe~(3+) content of VUT187 phlogopite with respect to magmatic values.