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

Periodic Reporting for period 4 - TRUE DEPTHS (deTeRmine the trUe dEpth of DeEp subduction from PiezobaromeTry on Host –inclusions Systems)

Reporting period: 2021-12-01 to 2022-05-31

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.
• First key objective for the Geometry of the host inclusion system: Development of numerical modelling to work on mineral phases with complex shapes and geometries (Mazzucchelli et al 2018 Geology).
• 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 (simulated proximity of the inclusion to the surface using progressive polishing and exposure of the inclusion, Campomenosi et al 2018 CMP)
• Temperature determination with independent methods (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. (Murri et al 2018 Am Min).
• Full analysis on quartz and zircon inclusions in garnet (Murri et al 2018 Am Min and Bonazzi et al 2019 Lithos, Stangarone et al 2019 Eur J Mineral).
• Unraveled the mechanism for zircon to reidite transformation discovering a new polymorph with implications for meteorite impact processes (Stangarone et al 2019 Am Min, Mihailova et al. 2019 Phys Chem Mineral).
• software STRAINMAN released (Angel et al 2019 Z Krist): determine the strain state of an inclusion using Raman spectroscopy.
• Measurements on cubic inclusions in cubic hosts (Anzolini et al 2019 Geology or Nimis et al 2019 Contrib Mineral Petro) combining 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.
• New discoveries on mineral phase stability under non-hydrostatic stress fields (e.g. Murri et al 2019)
• Equations of state for stiff material (e.g. zircon and rutile Zafffiro et al 2019 Min Mag, Angel et al 2019 Minerals, Musyiachenko et al 2021)
• Applications on UHP terraines in Papua New Guinea (Gonzalez et al 2019 J Metam Geol).
• Elasticity theory for axial compressions to obtain from a single inclusion the simultaneous estimate for the P and T of encapsulation (Alvaro et al 2020, Mazzucchelli et al 2019 in Lithos).
• Validation of the results with synthesis of host inclusion systems at conditions similar to those expected for UHPM rocks (Bonazzi et al Lithos).
• Theoretical development for non ideal host inclusions (Morganti et al 2020)
• Establishing protocols for Zircon inclusions in garnet with implications for the Western Alps (Campomenosi et al 2020 Am Min and Campomenosi 2021 CMP)
• Elastic geothermobarometry and stresses on rocks for lago di Cignana Unit (van Schrojenstein Lantman et al 2021 JMG)
• Release of EntraPT software (Mazzucchelli et al 2021 Am Min)
• Theoretical development for host inclusion with low symmetry (Gonzalez et al 2021 JGR)
• HP vs UHP in Western Gneiss Region Norway (Gilio et al 2022 JMG)
• Symmetry breaking for host inclusion systems (Murri et al 2022 Lithos)
We open the way for the development of a new branch of research and novel tools in the field of metamorphic petrology that has implications for other fields in geology including magmatic petrology and volcanology, rock mechanics and even for material science.
We developed the relevant theoretical tools, validated them with experiments and applied these tools to several localities worldwide.
strained mineral inclusion in garnet megablast from dora maira