Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

Abstract

Metamorphic transformations and fabric evolution are the consequence of thermodynamic processes, lasting from thousands to millions of years. Relative mineral percentages, their grain size distribution, grain orientation, and grain boundary geometries are first-order parameters for dynamic modeling of metamorphic processes. To quantify these parameters, we propose a multidisciplinary approach integrating X-ray computed microtomography (µ-CT) with X-ray chemical mapping obtained from an Electron MicroProbe Analyzer (EMPA). We used a metapelitic granulite sample collected from the Alpine HP-LT metamorphic rocks of the Mt. Mucrone (Eclogitic Micaschists Complex, Sesia-Lanzo Zone, Western Alps, Italy). The heterogeneous Alpine deformation and metamorphism allowed the preservation of pre-alpine structural and mineralogical features developed under granulite-facies conditions. The inferred granulitic mineral association is Grt + Bt + Sil + Pl + Qtz ± Ilm ± Kfs ± Wm. The subsequent pervasive static eclogite-facies re-equilibration occurred during the alpine evolution. The inferred alpine mineral association is Wm + Omp ± Ky + Qtz + Grt though local differences may occur, strongly controlled by chemistry of microdomains. X-ray µ-CT data extracted from centimeter-sized samples have been analyzed to quantify the volumetric percentage and shape preferred orientation (SPO) for each mineral phase. By combining tomographic phase separation with chemical variation and microstructures (i.e., different grain-size classes for the same phase and morphology of different pre-alpine microdomains) the pre-alpine mineralogical phases from the alpine overprint have been distinguished and quantified. Moreover, the sample preserves 100% of the pre-alpine granulite fabric, which surprisingly corresponds to less than 22% of the corresponding pre-alpine metamorphic assemblages, while the alpine eclogitic static assemblage corresponds to 78% though no new fabric is developed. This contribution

Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)

“…In the investigated metasediments, the first stage of Alpine garnet growth follows-after a temporal gap-the dissolution of the pre-Alpine cores due to the interaction with a hydrous fluid. At Monte Mucrone, re-hydration was not as pervasive as in the other investigated localities and abundant textural and mineralogical relics of HT/ LP metamorphism are still preserved (Oberhänsli et al 1985;Zucali et al 2002;Corti et al 2019). The first Alpine garnet generation grew at different P-T conditions from blueschist to eclogite facies in different areas and different lithologies, from T = 370-450 °C and P = 1.1-1.3 GPa in the Southern Sesia-Malone Valley area and in the Ivozio Complex (Pognante 1989b;Zucali et al 2004;Zucali and Spalla 2011), to T = 470-550 °C and P = 1.2-2.0 GPa for the Monte Mucrone-Mombarone and Chiusella Valley areas (Zucali et al 2002;Konrad-Schmolke et al 2011a;Regis et al 2014;Delleani et al 2018), and up to T = 600-650 °C and P = 1.5-1.6 GPa for the Central Sesia-Lys Valley area (Giuntoli et al 2018b) (Fig.…”

Metamorphic transformation rate over large spatial and temporal scales constrained by geophysical data and coupled modelling

“…In summary, in this model the large-scale, long-timeaveraged reaction rate depends on four factors, two of which we keep constant (ρ = 3,300 kg/m 3 , v = 17 mm/ year), one is varied systematically (r = 0.5-5 mm based on our field experience worldwide and on examples in the literature, e.g. Ahrens & Schubert, 1975;Corti et al, 2019), and L TR is taken from the simulations that fit the gravity data within uncertainty. This order-of-magnitude estimate for regional-scale average transformation rate is then compared to existing estimates from hand-sampleand outcrop-scale studies.…”

3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

“…The Sesia-Lanzo Zone, and in particular the Monte Mucrone area, has long been a training area for multiscale structural analysis and for studying in detail the correlation between deformation and metamorphism, thanks to the remarkable fabric and metamorphic partitioning from micro-to km scale (Hy 1984;Koons et al 1987;Ridley 1989;Ildefonse et al 1990;Zucali et al 2002;Babist et al 2006;Cenki-Tok et al 2011;Delleani et al 2013;Corti et al 2019). This area offers the opportunity to investigate the relationships between DFE and DRP.…”