3D analytical investigation of melting at lower mantle conditions in the laser-heated diamond anvil cel

Abstract:

The diamond anvil cell is a unique tool to study materials under static pressures up to several hundreds of GPa. It is possible to generate temperatures as high as several thousand degrees in the diamond anvil cell by laser heating. This allows us to achieve deep mantle conditions in the laser-heated diamond anvil cell (LHDAC). The small heated volume is surrounded by thermally conductive diamond anvils results in high temperature gradients which affect phase transformation and chemical distribution in the LH-DAC.

Analytical characterization of samples in three dimensions is essential to fully understand phase assemblages and equilibrium in LHDAC. In this study we used San Carlos olivine as a starting material as a simple proxy to deep mantle composition. Three samples were melted at ~3000 K and at ~45 GPa for three different durations ranging from 1 to 6 minutes; two other samples were melted at 30 GPa and 70 GPa. All samples were then sliced by focused ion beam (FIB). From each slice, an electron image and energy dispersive X-ray (EDX) map were acquired by scanning electron microscope (SEM) in the dual beam FIB instrument. These slices were collected on one half of the heated area in each sample, from which we obtained 3D elemental and phase distribution. The other half of the heated area was used to extract a 100 nm thick section for subsequent analysis by analytical transmission electron microscopy (TEM) to obtain diffraction patterns and high resolution EDX maps.

3D reconstruction of SEM EDX results shows at least four differentiated regions in the heated area for all samples. The exact Fe and Mg compositions mentioned below are an example of the sample melted at 45 GPa for 6 minutes. The bulk of the heated are is surrounded by ferropericlase (Mg_0.92, Fe_0.08)O shell (Fp). Inside this shell we find a thick region of (Mg,Fe)SiO₃ perovskite-structured bridgmanite (Brg) coexisting with Fp. In the center lies a Fe-rich core which is surrounded by magnesiowüstite (Mg_0.65, Fe_0.35)O (Mw). Moreover TEM EDX results show that we have not only 2 oxide phases (outer Fp shell and inner Mw layer) but rather 4 different compositions of oxide phases. The average composition of third oxide is (Mg_0.77, Fe_0.23)O which is the phase coexisting with Brg and the fourth one has a (Mg_0.46, Fe_0.54)O composition and is a 70 nm layer sitting between the core and the Mw phase.