Summer Project Description:

High Pressure Synthesis of beta-Mn 2 GeO 4 and of FeCr 2 O 4 spinel with iron isotopes and in situ synchrotron measurements .

Many of the phenomena observed at the surface of the Earth, including volcanism, and seismic activities have their origins much deeper in the Earth. The phenomena are driven by the mantle convection, a phenomenon driven by the internal heat of the Earth's interior. To fully understand the Earth's mantle dynamics, we must first understand the physical properties of the minerals in the mantle.

Although the upper mantle minerals have been and still are extensively studied, the mineral components the transition zone (from 410 to 660 km depth) are not directly accessible and their experimental studies can be difficult due to the high pressure that it has to be reached (14 to 24 GPa at 1400°C).

An issue of the rheology of the transition zone can be addressed through the mechanical properties of their volumetrically dominant phases, wadsleyite and ringwoodite, two high pressure polymorphs of (Mg, Fe) 2 SiO 4 . To work within the pressure limitation of the D-DIA, experiments on structural analog to (Mg, Fe) 2 SiO 4 will be used. In this case Mn 2 GeO 4 will be used to approximate (Mg, Fe) 2 SiO 4 . This analog possesses two high pressure phases, beta and gamma, analogs of wadsleyite and ringwoodite respectively. The pressure stability fields of beta- and gamma-Mn 2 GeO 4 (from 4GPa to 7GPa at 1000°C) fit well within the pressure range of the D-DIA press.

For this project, a shear deformation cell will be used in order to acquire the rheological parameters as well as to determine the crystallographic preferred orientation (CPO) of the studied phases. The shear deformation will be provided by two hard alumina pistons cut at 45° and placed at both ends of specimen slice. The first phase studied will be beta-Mn 2 GeO 4 . Before performing deformation experiments in the D-DIA press, the samples have to be synthesized and prepared. A cylindrical sample of beta-phase will be synthesized using "Kawai" type multianvil press. The first experiments using the D-DIA press will be dedicated to developing the shear deformation assembly. Those experiments can also be used to study the produced CPO. Once the cell is "ready", in situ experiments will be performed to determine the stress field in the shear configuration as well as the rheological properties of beta-Mn 2 GeO 4 .

Cosmochemical and meteoritic evidence indicates that surface rocks derived from Earth's crust and mantle are depleted in chromium. The missing chromium m ight therefore exist in significant amounts as chromite FeCr2O4 or its high pressure form at the base of the lower mantle. Recently a new mineral of high pressure polymorph of FeCr2O4 has been discovered in shock- metamorphosed Suizhou meteorite. It has also been reported that the iron in (Mg,Fe)SiO3 perovskite might experience a spin state transition in lower mantle.

I will use the experience I gained using the "Kawai" type press to synthesize chromite FeCr 2 O 4 with iron isotope (57Fe) . The sample will be examined using in-situ x-ray diffraction and Non-resonant inelastic X-ray scattering (NRIXS) at Advanced Photon Source (APS ) to study the spin state of iron at high pressures.

Joshua Nichols
Clarkson University, NY
Major: Physics & Mechanical Engineering
Mentor: Helene Couvy and Jiuhua Chen