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deTeRmine the trUe dEpth of DeEp subduction from PiezobaromeTry on Host –inclusions Systems

Periodic Reporting for period 2 - TRUE DEPTHS (deTeRmine the trUe dEpth of DeEp subduction from PiezobaromeTry on Host –inclusions Systems)

Reporting period: 2018-12-01 to 2020-05-31

Subduction zones located at convergent plate margins are major geodynamic sites that primarily affect plate tectonics and transport of crustal material deep into the Earth’s mantle. Subduction of one tectonic plate below another plate is accompanied by deformation processes enhancing seismicity and by dehydration/ melting of the subducting plate and overlying mantle, which affect magmatism and volatile (including greenhouse gases) recycling into magmas. Major earthquakes and explosive volcanism directly impact thousands of kilometers of coastal and mountain areas located at present-day and fossil convergent margins. However, the mechanisms attending the downwards transport of crustal material during subduction and its tectonic return back to the surface (exhumation) before/during continent collision, are still poorly understood and controversial. This is because the history of subduction cannot be obtained from real-time geophysical or seismic data which only provide static snapshots of subduction zones today. The study of paleo-geological processes is the key to understanding, modelling, predicting and ultimately ameliorating catastrophic geological events impacting people living near convergent margins.
Quantitative understanding the rates and true depths of burial, the thermal regimes of subduction zones and the tectonic processes allied with oceanic subduction and continental collision can only be achieved by determining the pressure-temperature-time-depth (P-T-t-h) histories of Ultra-High-Pressure Metamorphic (UHPM) rocks that have been subducted to pressures greater than 3 GPa and subsequently exhumed to the Earth’s surface.

The fundamental and fascinating challenge is not whether low-density crustal rocks can be subducted to extremely high pressures. Traditionally such pressures have been associated directly with depth h, by assuming that the stress state in the Earth is hydrostatic (so P = ρgh, leading to the simple correlation that 30 km depth = 1 GPa of pressure increase). If pressure in the collision zone environment is uniformly hydrostatic, then the pressures inferred from preserved index minerals indicate that the rocks reached depths of 90, 120 or even 300 km. It is then clear that the fundamental and fascinating problem is not that these dense high-pressure rocks exist. The question is how were they exhumed into the much less dense over-lying crustal units in which they are found?

We aim at using the fossil stresses preserved on inclusions to retrieve the stress state acting on the rocks when the inclusions has been encapsulated. This will enable to determine if the paradigm pressure equal depths is a good approximation or we have to considerably re/think our models for interpreting a large portion of deep geological processes. We will be able to determine for the first time the true depths reached by paleo-subduction processes. This has profound implications on the interpretation of many potentially catastrophic processed from the triggering mechanisms for Earthquakes to the magma genesis and pathways to much more common rock faulting mechanisms.

The project is articulated into three major work packages:

Theoretical development: We will establish the theoretical background for the application of elastic geobarometry on anisotropic mineral phases to enable full use of several phases to determine their P and T of encapsulation.
Validation: We will then validate the theoretical and practical aspects for measurements using laboratory prepared samples
Application to rocks: We will adopt the developed approaches to determine P T and depth of encapsulation for several host inclusion pairs on three terraines (Dora Maira, Lago di Cignana and Western Gneiss Region) selected because their similarities and controversial aspects.
2018:
• First key objective for the Geometry of the host inclusion system is the development of numerical modelling to work on mineral phases with complex shapes and geometries (Mazzucchelli et al 2018 Geology). KEY THEORETICAL DEVELOPMENT
• Initial test on Raman barometry to further test non hydrostatic deformation on inclusions computed ab initio using DFT and demonstrating the importance of a full analysis for anisotropic inclusions (Anzolini et al. 2018 Am Min, Nestola et al 2018 Nature)
• Raman barometry coupled withn FEM on anisotropic inclusions from Dora Maira demonstrating the importance of working with inclusion buried. We simulated proximity of the inclusion to the surface using progressive polishing and exposure of the inclusion (Campomenosi et al 2018 Contrib to Mineral and Petro and several abstracts to national and international conferences)
• Constraining temperature on synthesis and natural rocks with independent methods to assess P T estimates from elastic barometry (Murri et al 2018 Geochim Cosmochim)
• First full theoretical development of the concept of Gruneisen tensor that now leads the theoretical development for elastic barometry. This allows to reliably determine for the first time the state of strain and in turn stress of an inclusion trapped in another mineral. This is now established as the standard correct method in the scientific community for non-cubic inclusion in mineral host of any symmetry (Murri et al 2018 Am Min). THIS IS A KEY THEORETICAL DEVELOPMENT

2019
• This concept has been worked out and the full analysis tested for quartz inclusions in garnet (Murri et al 2018 Am Min and Bonazzi et al 2019 Lithos) and zircon in garnet (Stangarone et al 2019 Eur J Mineral).
• As a side output from the entire work on zircon inclusions we successfully investigated and unraveled the mechanism for zircon to reidite transformation discovering a new polymorph and its dynamic stability between the two phases that are fundamental for the investigation of meteorite impact processes (Stangarone et al 2019 Am Min). The computational study has been followed up by the HP micro-Raman experiments that confirmed out theoretical expectations (Mihailova et al. 2019 Phys Chem Mineral). IMPORTANT DEVELOPMENT CROSS-DISCIPLINARY
• After these test the entire procedure and calculation has been made available to the entire community with a simplified software STRAINMAN (Angel et al 2019 Z Krist) that allows determining the strain state of an inclusion using Raman spectroscopy. This is an IMPORTANT DEVELOPMENT OBJECTIVE
• We also kept carrying out measurements on cubic phases for which results can be immediately interpreted as in Anzolini et al 2019 Geology or Nimis et al 2019 Contrib Mineral Petro and where we could play combining all the knowledge acquired so far including the combination of x-ray tomography with Finite element numerical modeling on one side and the crystallographic orientation on the other side to unravel complex unexplored geological processes occurring at the micro scale. For example the latter proves the need for crystallographic analyses because processes of encapsulation of different inclusions can be very different.
• And at the same time we kept exploring the effect of deviatoric stresses on mineral structures that we expect will lead to fundamental new discoveries concerning mineral phase stability under non hydrostatic stress fields (e.g. Murri et al 2019 that assessed the evolution of quartz inclusions structure under deviatoric stress)
• Together with the development of the theory for determining strains on inclusion the elastic theory to calculated stresses from strains at any stress condition has been carried out extensively. We determined the equations of state parameters and model for stiff material such as for example zircon and rutile (Zafffiro et al 2019 Min Mag). This work has been followed by another paper on the model of equations of state for stiff materials that has bene published as a part of the volume in honor of Orson Anderson (Angel et al 2019 Minerals)
• Part of our development has been already used in incredibly different field of study such as meteorite and we have seen the implications for the zircon-reidite phase transformation that has been then pushed to the maximum extent within a single phase domain with the investigation of stacking disorder in diamonds (e.g. Murri et al 2019 SciRep). IMPORTANT DEVELOPMENT CROSS-DISCIPLINARY
• Direct applications of the developed methods on less complex terrains has been performed on UHP terraines in Papua New Guinea (Gonzalez et al 2019 J Metam Geol). We started exploring here correlations between strain components determined on a single inclusion to assess the reliability of the method to determine individual stresses for quartz inclusion in garnet
• We then carried out the development of the elasticity for axial compressions hat allows to back calculate the entrapment conditions not using volumetric isomekes but using axial isomekes. This allows to obtain from a single inclusion the simultaneous estimate for the P and T of encapsulation (Alvaro et al 2020). This is a KEY PUBLICATION that enables unique P and T estimate out of measurements of the remnant strains of a single inclusion. This method has been tested and validated on quartz inclusions trapped high T and high P garnet from mantle xenoliths where post entrapment non elastic deformations (e.g. brittle or plastic) are negligible (Alvaro et al 2020).
• The above mentioned development required also extensive theoretical development to account for the exact shape crystallographic orientation and state of strain for each of the inclusions. Such theoretical development has been included in the publication by Mazzucchelli et al 2019 in Lithos. KEY PUBLICATION for the development of numerical models for inclusions.
• All of these results and theoretical approaches have been validated on samples prepared ah hoc by means of laboratory experiment where quartz inclusions trapped in garnet have synthesized by piston cylinder at conditions similar to those expected for UHPM rocks (e.g. 3GPa and 800°C, Bonazzi et al Lithos). This is a KEY PUBLICATION that validates the entire methodology.
2018:
Application of Finite element modelling to complex geometries to provide estimate for corrections from conventional measurements.. First key objective for the Geometry of the host inclusion system is the development of numerical modelling to work on mineral phases with complex shapes and geometries (Mazzucchelli et al 2018 Geology). KEY THEORETICAL DEVELOPMENT
New method to determine strains on inclusions from Raman measurements. First full theoretical development of the concept of Gruneisen tensor that now leads the theoretical development for elastic barometry. This allows to reliably determine for the first time the state of strain and in turn stress of an inclusion trapped in another mineral. This is now established as the standard correct method in the scientific community for non-cubic inclusion in mineral host of any symmetry (Murri et al 2018 Am Min). THIS IS A KEY THEORETICAL DEVELOPMENT

Coupling FEM and Raman barometry
New method for anisotropic inclusions by FEM

2019
• This concept has been worked out and the full analysis tested for quartz inclusions in garnet (Murri et al 2018 Am Min and Bonazzi et al 2019 Lithos) and zircon in garnet (Stangarone et al 2019 Eur J Mineral).
• As a side output from the entire work on zircon inclusions we successfully investigated and unraveled the mechanism for zircon to reidite transformation discovering a new polymorph and its dynamic stability between the two phases that are fundamental for the investigation of meteorite impact processes (Stangarone et al 2019 Am Min). The computational study has been followed up by the HP micro-Raman experiments that confirmed out theoretical expectations (Mihailova et al. 2019 Phys Chem Mineral). IMPORTANT DEVELOPMENT CROSS-DISCIPLINARY
• After these test the entire procedure and calculation has been made available to the entire community with a simplified software STRAINMAN (Angel et al 2019 Z Krist) that allows determining the strain state of an inclusion using Raman spectroscopy. This is an IMPORTANT DEVELOPMENT OBJECTIVE
• Part of our development has been already used in incredibly different field of study such as meteorite and we have seen the implications for the zircon-reidite phase transformation that has been then pushed to the maximum extent within a single phase domain with the investigation of stacking disorder in diamonds (e.g. Murri et al 2019 SciRep). IMPORTANT DEVELOPMENT CROSS-DISCIPLINARY
• Theoretical development to account for the exact shape crystallographic orientation and state of strain for each of the inclusions. Such theoretical development has been included in the publication by Mazzucchelli et al 2019 in Lithos. KEY PUBLICATION for the development of numerical models for inclusions.
• All of these results and theoretical approaches have been validated on samples prepared ah hoc by means of laboratory experiment where quartz inclusions trapped in garnet have synthesized by piston cylinder at conditions similar to those expected for UHPM rocks (e.g. 3GPa and 800°C, Bonazzi et al Lithos). This is a KEY PUBLICATION that validates the entire methodology.