Compositional variation and zoning of epidote supergroup minerals in the Campi Flegrei geothermal field, Naples, Italy

Abstract. Authigenic epidote supergroups are an abundant accessory mineral in the calcium–aluminum silicate and thermometamorphic hydrothermal zones of Campi Flegrei (Phlegraean Fields) geothermal field located west Naples, Italy. Geothermal exploration for high-enthalpy fluid produced drill core cuttings to ∼ 3 km depth Mofete (MF1, MF2, MF5) San Vito (SV1, SV3) wells, where measured down-hole temperatures epidote-bearing samples range from 270–350 ∘C 285–390 areas, respectively. Two group (epidote, clinozoisite), some rare earth element (REE)-bearing, two allanite (allanite-(Ce), ferriallanite-(Ce)) minerals were identified by electron microprobe. The is light (LREE, La–Gd) enriched, Ce dominant, with REE + Y that varies 30.59 wt %–14.32 %. Complex compositional variation such as oscillatory, sector, complex (mixed) zoning a ubiquitous feature observed group, which occurs veins, vugs, various size masses, isolated single crystals. Compositional caused variable Fe ↔ Al3+ substitution XFe [(Fe3+) / (Fe3++ Al)] ranges 0.06–0.33 (Fe3+=0.185–0.967 apfu). tends decrease increasing temperature but its distribution more records recent fault displacement. variety complexity supergroup suggest rapid composition and/or intensive parameter fluctuations local system.