Elastic thermobarometry on Zircon-in-Garnet (ZiG) from the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps)

Here we studied metapelites from the ultrahigh-pressure (UHP) Brossasco-Isasca unit in the Dora-Maira Massif, Western Alps, combining zircon-in-garnet elastic geo-thermobarometry and phase equilibria modelling. We determined the residual strain and pressure of zircon inclusions via micro-Raman spectroscopy and the dedicated softwares available online such as stRAinMAN [1] and EntraPT [2]. The entrapment isomekes obtained for 28 zircon inclusions in garnet from metapelites (Alm_67-79-Py_9-30-Grs_1-6-Sps_0-6) were combined with thermodynamic modelling to constrain the P-T range of garnet growth, assuming purely elastic behaviour.

The presence of chloritoid and/or staurolite inclusions at the garnet core-mantle and the presence of coesite inclusions only at the garnet rim suggest that most of the garnet volume formed during an early prograde path and only a small portion under UHP conditions. Most of the selected inclusions, however, come from the rim of the garnet. Since the rim is limpid, we could localize and target those inclusions that are spaced enough to be used reliably for elastic thermobarometry without corrections. The entrapment pressures obtained for most zircon inclusions do not match the previously published results obtained from conventional petrologic methods [3]. For example, combining our results with the available retrograde P-T paths of the UHP unit [3], we bracket the apparent entrapment conditions of zircon inclusions at 0.5 GPa and 600-650 °C, below the expected conditions in the coesite stability field. The same discrepancy between the elastic and chemical barometric methods has been documented for the pyrope-bearing whiteschists from the same metamorphic unit [4]. The observed misfit has been tentatively attributed to post-entrapment viscous relaxation of the garnet–zircon inclusion system, which cannot be accounted for by purely elastic models. These results provide further evidence of a general post-entrapment elastic resetting of the zircon-in-garnet pairs along the retrograde path at temperatures near 600-650°C.

This work was supported by ERC-StG TRUE DEPTHS (grant number 714936) to Matteo Alvaro. Nicola Campomenosi and Mattia L. Mazzucchelli are supported by the SIMP PhD Thesis Award and by the Alexander von Humboldt research fellowship. [1] Angel et al. (2019) Zeitschrift für Kristallographie, 234, 219. [2] Mazzucchelli et al. (2021) American Mineralogist, 106, 830. [3] Groppo et al. (2019) European Journal of Mineralogy, 31, 665. [4] Campomenosi et al. (2021) Contrib Mineral Petrol 176, 36.