Does the Fe2+ spin-crossover in Pv and Fp account for the properties and dynamics of the deep mantle?

The discovery [1]of the high pressure (HP) induced spin crossover in pure Wüstite (Fe0.94O) - a high-spin (HS) to low spin (LS) behavior led to a number of experimental and theoretical studies [2] regard-ing the implication of this electronic transition upon the structure and dynamics in the lower/upper man-tle boundaries. As a result of the collapse of the d-d electronic repulsion present in the high-spin state (S=2), it is expected that within the structural transition the FeLS - O bond-length in Pv Mg1-yFeySiO3) and Fp (Mg1-xFexO) will decrease, resulting in a volume contraction. Based on several HP XRD experimental studies by us and others in species containing Fe2+-O such as in Fp (1 > x > 0.15), Fe2+-Cl (FeCl2), and Fe2+-S (FeCr2S4), we conclude that the onset of any resulting volume contractions and their respective magnitudes are very small, well within the experimental errors of various physical parameters used in the theoretical simulation of the Earth dynamics. Another parameter attributed to the electronic transition, viz. the bulk modulus K0 of the LS phase is claimed to be larger than that of the HS phase suggesting a "hardening" of phonon density of states. However the enhanced lack of sample hydrostaticity with pressure increase could also lead an artificial larger K0. The several experimental inconsistencies together with insignificant crystallographic changes upon the spin transition may propose an alternative explanation of the seismological observations at upper-lower mantle boundary 1. M.P. Pasternak et al, PRL 79 5046 (1997) 2. K. Hirose, Science, 327, 151 (2010), J.F. Lin and T Tsuchya, Phys. Earth Planet. In. 170 248 (2008) and references therein.