Clinopyroxenes from Somma-Vesuvius: Implications of Crystal Chemistry and Site Configuration Parameters for Studies of Magma Genesis

A clinopyroxene suite from Somma-Vesuvius clinopyroxenite cjecta and mcgacrysts has been investigated by single crystal X-ray diffraction combined with electron probe microanalysis. The aim of the study was to characterize the intracrystalline configuration of the clinopyroxenes and delineate their possible parental liquid(s). Vesuvius megacrysts and nodule clinopyroxenes show the initial ‘signature’ of the leucitite (sensu lato) magma while Somma clinopyroxenes are characteristic of leucite phonolite magma. The crystal chemical variations demonstrate that the specimens investigated had originated by shallow-level crystallization of leucitite and its derivatives, under reducing conditions. T-site (Si + Al) deficiency, typical of clinopyroxene from lamproites is reported in some specimens from the Roman Volcanic Region for the first time. Intracrystalline Mg-Fe2+ ordering in M1-M2 appears to be configuration-dependent, yielding the largest KD variation in the clinopyroxene from phonolite. This is contrary to expectation.