Volatile-induced transport of HFSE, REE, Th and U in arc magmas: evidence from zirconolite-bearing vesicles in potassic lavas of Lewotolo volcano (Indonesia)

Potassium-rich calc-alkaline lavas of Lewotolo volcano, situated in the Eas t Sunda Arc, Indonesia, contain the rare mineral zirconolite (CaZrTi2O7). S amples in which tiny grains of this mineral (3-25 mu m in size) were found span the entire range of lava compositions (47-62 wt% SiO2). To the best of our knowledge, this is the first record of primary zirconolite in juvenile are volcanics. The mineral forms part of a vesicle-filling assemblage cons isting of a network of quenched feldspar crystals and an SiO2 phase, probab ly cristobalite. High contents of Th, U and REE (up to 9.3, 4.3 and 15.6 wt % oxide respectively) and very high Fe contents (up to 13.5 wt% Fe2O3) dist inguish these zirconolites from those of other rock types. The extraction o f volatile-rich phases with changing compositions in successive stages is c onsidered to be responsible for the zirconolite formation. We hypothesise t hat a fluid capable of transporting HFSE, REE, Th and U was extracted from the magma and (partly) crystallised within voids which had formed earlier u pon saturation of an aqueous fluid. Assuming that zirconolite compositions largely reflect trace metal contents of the coexisting fluid phase, signifi cant amounts of 'immobile' elements must have been transported on a macrosc opic scale. Our findings thus point to a late-stage transfer of HFSE, REE, Th and U between different domains in a cooling magma body. Such a volatile -induced redistribution of trace elements at shallow levels of high-K volca nic systems may be significant for conventional geochemical modelling of ma gma evolution and for Th-U disequilibrium studies.