The oxidation state of the Earth's interior is the main variable controlling the speciation and cycle of volatile elements like carbon in the reduced form of diamond, or oxidized carbonate minerals and CO2-rich melts. The release of carbon into the atmosphere as CO2 is likely to have occurred as consequence of sudden changes of the mantle redox state promoted by increase of oxygenation (Kasting et al. 1993, Stagno and Fei 2020). Such increase is known to cause changes in the ferrous/ferric iron ratio of the surrounded minerals that, at depths below 400 km are represented by majoritic garnet, ferropericlase and bridgmanite (Frost and McCammon 2008; Stagno 2019).
The main goal of this project is twofold: 1) to determine the ferric/ferrous iron ratio of synthetic majoritic garnet, ferropericlase and bridgmanite as function of pressure (P), temperature (T) and oxygen fugacity (fo2) through a series of experiments performed using the multi anvil press to simulate the interior of Earth; 2) to measure the ferric/ferrous iron ratio of natural majoritic garnet, ferropericlase and bridgmanite found as mineral inclusions in diamonds from Sao Luiz (Brazil). The aims of this ambitious research project will be achieved by performing experiments at pressures of 14-28 GPa and temperatures between 800-1700 °C using several multi anvil presses installed in Rome and abroad taking benefits of ongoing collaborative projects and agreements. Natural diamonds will be provided by international and national collaborators. Measurements of ferric/ferrous iron will be performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). All expected results are novel and of high impact to the international community of geoscience with the chance to be published on high-IF journals. The collected data will have important implications for the redox state of the deep Earth's interior, the transport of volatiles through time and the origin of diamonds.
To date it has been accepted that the fo2 of the lower mantle is very reducing at conditions where FeNi metal alloy is stable. At such low fo2s volatiles would likely be dissolved into FeNi alloy (Frost et al.2004-Nature) or, in case of carbon, being stable as diamond or carbide, not carbonate. However, FeNi metal alloy are rarely found as inclusion in sublithospheric (also known as superdeep) diamonds and, when observed, their Ni content (63 wt% Fe-37 wt% Ni; Wirth et al.2014-EPSL) is in disagreement with thermodynamic predictions (88 wt% Fe-20 wt% Ni plus 1 wt% S). In addition, some deep diamonds show inclusions of carbonates either solid or liquid (Brenker et al. 2007-EPSL) suggesting that the lower mantle might be more oxidized than thought at similar levels than the upper mantle (Kaminsky et al. 2015-EPSL). This is a fundamental question to address since high oxygenation of the lower mantle, or the interior of Earth in general, would cause extensive outgassing and depletion in volatile elements. If this was the case, all CO2 degassed from volcanoes would be recycled at subduction zones, and not of primordial mantle origin.
To date, existing theories on the fo2 of the lower mantle arise from experiments that were limited to pressures representative of shallow regions of the lower mantle (~24-25 GPa). No data are available to assess the effect of pressure and temperature on the oxidation state of redox sensitive minerals like majorite, ferropericlase and bridgmanite, the latter being the dominant phases in the lower mantle.
Results from these experiments will allow to reproduce experimentally conditions for the formation of diamonds by redox interaction with Fe-bearing minerals and apply the model to mineral inclusions from a unique set of natural sublithospheric diamonds.
Importantly, we highlight the importance of this project in performing experiments supported by renown scientists in the field of experimental petrology. The use of multi anvil presses will benefit of active collaborations with the INGV session of Rome and the Geodynamics Research Center of Ehime University. Importantly, some experiments will be performed using the recently purchased rotating multi anvil press at the DST of Sapienza, a unique apparatus in Italy, the second in the world. The methodology proposed here consisting of high pressure generation by multi anvil + SEM/microprobe + Mössbauer spectroscopy must be considered absolutely innovative in the field of experimental petrology and mineral physics within the national paramount as well as highly competitive within the international scientific community.
In case of unexpected issues with respect to the original aims, results from this project represent an invaluable source of data that can also be used for further purposes such as to determine 1) phase equilibria and partitioning of major elements as function of pressure and temperature, 2) thermodynamic data of solid solutions for high pressure phases, 3) the crystal structure of the quenched phases by X-ray diffraction techniques and relative equation of state (EOS); 4) inelastic X-ray scattering for precise determination of the elastic constants of the recovered phases.
Additionally, the future data obtained from this research project will be of interest both for outreach activity in terms of visualization of the interior of Earth, other than become matter of study at teaching level. A class was activated a couple of years ago named Geology of Diamonds that anticipates issues here proposed to be addressed experimentally.