Zirconolite and calzirtite in banded forsterite-spinel-calcite skarn ejecta from the 1631 eruption of Vesuvius: inferences for magma-wallrock interactions

摘要

Two Ca-Zr-Ti oxides, zirconolite CaZrTi_2O_7 and calzirtite Ca_2Zr_5Ti_2O_(16), occur as minute interstitial crystals in skarn (forsterite-spinel-calcite, with rhythmic banding) ejecta from the 1631 eruption of Vesuvius. The substitutions in zirconolite observed here mainly include Nb-for-Ti (typical for zirconolites in alkaline magmatic surroundings) and (Th,U)-for-Ca, and produce a crystal-chemical formula Ca_(0.9-1)Th_(0.01-0.12)U_(0.04-0.10)ZrrTi_(1.36-1.61)Nb_(0.09-0.22)(Fe,Mg,Al)_(0.29-0.47). The skarn, which occurs in contact with a pyroxenite of magmatic origin, displays a mineralogical zoning with Zr-, Ti-, Nb- and (U,Th)-rich oxides (e.g. Nb-perovskite and zirconolite) close to the pyroxenite (<2 mm), whereas those oxides observed further from the pyroxenite (>1 cm) are richer still in Zr but (Ti, Nb, U, Th)-poor or free (e.g. calzirtite and baddeleyite ZrO_2). Textural relationships between minerals provide evidence for a metasomatic development of the skarn at the expense of the pyroxenite, through drastic leaching of Na, K, Si, Fe. The same process is responsible for the zoning in the skarn (leaching of Fe, Si, Ti, Nb, U and Th), in which Zr was less mobilized than other HFSE. This process, related to the circulation of fluids equilibrated with carbonates, is responsible for those forsterite-spinel (± calcite) skarns which can be observed as remnants in a large part of the 1631 ejecta. Such endoskarns probably formed repeatedly during at least the last millennia of Vesuvius' history, and existed prior to the emplacement at shallow depth of the 1631 magma whose chamber walls were different from the limestone/dolostone classically assumed to host the Vesuvius magmas (Fulignati et al., 2005).