Extreme mineral-scale Sr isotope heterogeneity in granites by disequilibrium melting of the crust

The broadest ranges of initial Sr isotopic ratios (87Sr/86Sri) ever reported within a single igneous rock ( ≈ 2 × 10 − 2 ) are preserved within the late Miocene laccolith-pluton-dyke felsic complex of Elba Island (Italy). For these units, the integration of textural and crystal-scale isotope data allows tracing the evolution of the 87Sr/86Sri of the melt from the emplacement level back to the earliest pre-emplacement crystallization stage. The rock matrix minerals record the 87Sr/86Sri composition of the magma at the emplacement level (0.715–0.716). K-feldspar megacrysts, representing an earlier phase crystallized at depth, record a rim-to-core increase of Sr-isotopic ratios from values similar to those of the matrix to significantly higher ones (≈0.719). Remarkably, biotites hosted within megacrysts, representing the first crystallization stage, have extreme and contrasting 87Sr/86Sri values in the different intrusive units: biotites within megacrysts from the laccolith record the lowest ratio in the intrusive complex (≈0.710), while those in the megacrysts from the pluton and associated felsic dyke have the highest 87Sr/86Sri ( ≈ 0.732 ) . This time-transgressive record of isotopic variation in the magma reflects episodic recharge and mixing of magma batches formed by disequilibrium melting of crustal sources that produced melts through different reactions as temperature was increasing. The progression from muscovite- to biotite-dominated fluid-absent melting generates melts with increasing 87Sr/86Sr, while at higher temperatures, the progression from biotite- to hornblende-dominated melting reactions results in a decrease in the 87Sr/86Sr of the melt.