Skip to main content

MINERAL-SCALE HETEROGENEITY OF GRANITES: A NEW APPROACH INTEGRATING MICROCHEMICAL ANALYSIS AND THERMO-MECHANICAL MODELLING TO DISCLOSE THE THERMOCHEMICAL EVOLUTION OF THE CONTINENTAL CRUST

Periodic Reporting for period 1 - MIGRATE (MINERAL-SCALE HETEROGENEITY OF GRANITES: A NEW APPROACH INTEGRATING MICROCHEMICAL ANALYSIS AND THERMO-MECHANICAL MODELLING TO DISCLOSE THE THERMOCHEMICAL EVOLUTION OF THE CONTINENTAL CRUST)

Reporting period: 2016-05-01 to 2018-04-30

Earth is unique among known terrestrial planets in having a continental crust; i.e. the life-sustaining interface between our planet’s deep interior and surface. The withdrawal of large volumes of granitic magmas from the middle and lower crust and their emplacement at higher structural levels caused the chemical differentiation of the continental crust, with its upper part enriched in incompatible elements that are fundamental for the establishment and maintenance of a habitable planet. The fundamental objective of the MIGRATE project was to establish the timescale and mechanism of heat and mass transfer within the continental crust, determining the physicochemical evolution of upper crustal magma reservoirs. In this frame, three main issues were addressed. The main goal of the project was to get a deeper understanding on the main petrogenetic processes that, controlling the composition of granitic magmas generated by partial melting of crustal rocks, ultimately regulate the composition of the outermost layer of our Planet. The second issue addressed in this project has been the thermal evolution and thermal structure of the continental crust in response to the formation, migration and emplacement of granitic magmas. This topic is the one that has the most immediate impact for society as heat transfer induced by magma ascent and storage in the shallow crust generates transient thermal anomalies that, under specific conditions of water availability and permeability, gives rise to geothermal fields, thus potentially producing cost-effective, renewable energy. MIGRATE targeted non-exposed young granitic rocks associated to the large-scale steam-dominated Larderello-Travale geothermal field in Tuscany (Italy). Today, this field produces ca. 6000 GWh of electricity, corresponding to ca. 2% of the total Italian national demand. Finally, the third goal was to retrieve the long-term thermal history of potentially eruptible magma reservoirs to constrain the physical conditions under which these magmas are retained in the shallow crust. This issue is relevant to get a better understanding on the fundamental processes that control storage and crystallization of magmas at depth vs. volcanic eruptions.
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.
In the Larderello-Travale area, zircon crystals from individual samples of young plutonic and volcanic rocks exhibit an age spread of 300–500 thousands of years. The most innovative part of the project is related to the discussion of the meaning of the U-Pb age scatter observed in the population of zircon crystals from the Larderello-Travale system as well as from many different igneous rocks worldwide. In the project, the distribution of precise U-Pb dates was examined and different interpretations were explored based on sets of different assumptions leading to the use of two types of numerical simulations: thermal modelling and phase equilibria modelling. The goal of these numerical simulations was to link the distribution of dates of a population of zircons with the time-integrated evolution of temperature in the magmatic reservoir and in the continental crust as a whole.
Sketch illustrating the formation of the Larderello-Travale magmas.