The fundamental work performed during the development of MIGRATE was the characterization of the texture, geochemistry, isotope composition and U-Pb geochronology of zircon crystals (i.e. ZrSiO4). This tiny mineral (commonly < 300 µm in length) is a ubiquitous accessory phase in granites where it generally constitutes less than 0.1 vol% of the rock. Thanks to its chemical and physical resistivity, zircon represents a perfect archive of chemical and temporal information to trace geological processes in the past, utilizing the outstanding power and temporal resolution of the U–Pb decay schemes. In the project, significant efforts were devoted to the determination of zircon U-Pb radiometric ages at high-precision using a complex technique called chemical abrasion isotope-dilution thermal ionisation mass spectrometry (CA-ID-TIMS), which allows decreasing the uncertainties associated with U-Pb radiometric zircon dates.
In the project, the implementation of a multi-analytical workflow has allowed characterizing zircon crystals extracted from six granites from the Larderello–Travale system and from volcanic lavas which are genetically and spatially associated to the shallow-level granitoids. Geochemical, isotopic and geochronological data were combined with thermal, phase-equilibria and fluid-dynamics simulations. The most important results obtained are:
1. In the Larderello-Travale area, magmas were emplaced in the shallow crust and/or erupted in four pulses of magmatic activity at ∼3.6 3.2 2.7 and 1.6 million years.
2. High precision U–Pb zircon ages and isotope data point to a composite long-lived magmatic system in which most of zircon grains grew from physically separated and chemically heterogeneous small magma domains located in the middle crust (at 10-15 km); ca. 10 km below the level of final emplacement. Melt residence in the middle crust lasted for hundreds of thousands of years before magma ascent and emplacement/eruption in the upper crust, with magma delivered in pulses.
3. The Larderello-Travale granites form through mixing of magma batches derived by partial melting of different crustal source rocks, with this feature reflecting the inherent heterogeneous nature of the continental crust. Crustal heterogeneity is transferred to granitic magmas and preserved at the crystal-scale.
4. The distribution of zircon U-Pb dates is used to estimate the rate of assembly (influx rate) and total volume of the heterogeneous magmatic reservoir that fed the emplacement of the youngest magmatic pulse at Larderello-Travale. Our data indicate a magma flux of 0.002-0.008 km3/yr and an intrusive volume of ca. 1000 km3 with this magmas located at ca. 10-15 km depth.
5. Fluid dynamics numerical simulations show that convection and mixing of two granitic magmas can only be initiated under a limited range of density and viscosity contrasts. Therefore, shallow level granitic magmas are expected to form small non-eruptible intrusive bodies rather than large dynamic magma chambers.
Most of the results achieved over the life of the project were published in a high-ranking international scientific journal (i.e. Earth and Planetary Science Letter; Farina et al., 2018) as well as presented at national and international conferences. Two manuscripts are still in preparation.
