Fumiya Maeda

Diamond formation in the deep lower mantle: a high-pressure reaction of MgCO 3 and SiO 2

Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO3-SiO2 system up to 152 GPa and 3,100 K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms from magnesite to the high-pressure polymorph of MgCO3, phase II, above 80 GPa. A reaction between MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.

Spin state and electronic environment of iron in basaltic glass in the lower mantle

Abstract The spin states of iron in deep magmas are one of the most important properties that affect the partitioning of iron between magmas and minerals and, thus, the gravitational stability of magmas in the Earth. We investigated the spin state and electronic environments of iron in a basaltic glass containing ~70 Fe3+/ΣFe at room temperature and pressures from 1 bar to 130 GPa using a diamond-anvil cell combined with energy domain synchrotron 57Fe Mössbauer source spectroscopy. The basaltic glass represents an analog of a multi-component magma typical for the Earth. The Mössbauer spectra could be fitted by a two pseudo-Voigt doublet model including a high quadrupole splitting (QS) doublet and a low QS doublet, which were assigned to high-spin Fe2+ and high-spin Fe3+, respectively. The high-spin states of Fe2+ and Fe3+ remained up to 130 GPa corresponding to the pressure in the lowermost mantle. The center shift values of high-spin Fe2+ and Fe3+ did not show large changes with pressure, ruling out sharp electronic changes in the basaltic glass. Therefore, a sharp and complete spin crossover of Fe2+ from the high-spin to the low-spin state does not appear to occur in the basaltic glass although the possibility of a partial spin transition cannot be fully excluded. The QS values of Fe2+ increased slightly at 0–20 GPa and above 100 GPa, and the higher value was preserved after decompression to ambient conditions. This behavior may be related to distortion of Fe2+ polyhedra due to short-range ordering on compression. Such a distortion of Fe2+ polyhedra could gradually stabilize Fe2+ in the basaltic glass with pressure compared to bridgmanite according to the Jahn-Teller effect, and thus could gradually enhance the partitioning of iron into deep magmas in the lower mantle.

Complete agreement of the post-spinel transition with the 660-km seismic discontinuity

The 660-km seismic discontinuity, which is a significant structure in the Earth’s mantle, is generally interpreted as the post-spinel transition, as indicated by the decomposition of ringwoodite to bridgmanite + ferropericlase. All precise high-pressure and high-temperature experiments nevertheless report 0.5–2 GPa lower transition pressures than those expected at the discontinuity depth (i.e. 23.4 GPa). These results are inconsistent with the post-spinel transition hypothesis and, therefore, do not support widely accepted models of mantle composition such as the pyrolite and CI chondrite models. Here, we present new experimental data showing post-spinel transition pressures in complete agreement with the 660-km discontinuity depth obtained by high-resolution in situ X-ray diffraction in a large-volume high-pressure apparatus with a tightly controlled sample pressure. These data affirm the applicability of the prevailing mantle models. We infer that the apparently lower pressures reported by previous studies are experimental artefacts due to the pressure drop upon heating. The present results indicate the necessity of reinvestigating the position of mantle mineral phase boundaries previously obtained by in situ X-ray diffraction in high-pressure–temperature apparatuses.