Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TRUE DEPTHS (deTeRmine the trUe dEpth of DeEp subduction from PiezobaromeTry on Host –inclusions Systems)

Teaser

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...

Summary

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.

Work performed

2018:
Geometry of the host inclusion system (Mazzucchelli et al) IMPORTANT OBJECTIVE Numerical modelling

Initial test on raman barometry (Anzolini et al.)
Perovskite in diamond (Nestola et al.)
Geothermometry on synthesis (Murri et al)

Raman elastic geobarometry for anisotropic inclusions (Murri et al) IMPORTANT OBJECTIVE
Strainman to determine strain on inclusions from Raman spectroscopy (Angel et al) SOFTWARE DEVELOPMENT IMPORTANT OBJECTIVE
Raman barometry coupled withn FEM on anisotropic inclusions from Dora Maira (Campomenosi et al)

Final results

2018
Application of Finite element modelling to complex geometries to provide estimate for corrections from conventional measurements.
new method to determine strains on inclusions from Raman measurements.
Coupling FEM and Raman barometry

New method for anisotropic inclusions by FEM

Website & more info

More info: https://www.mineralogylab.com/.