Lower mantle hydrogen partitioning between periclase and perovskite: A quantum chemical modelling

Fe-periclase reactivity at Earth’s lower mantle conditions: Ab-initio geochemical modelling

“…The resulting bulk chemical composition (Table 1) has a Mg/Si molar ratio of 1.20, which is lower than the pyrolitic value ($1.27) but higher than the chondritic one ($1.1). Such a ratio is in agreement with a lower bulk chemical composition slightly enriched in SiO 2 (and FeO) and depleted of MgO, with respect to the pristine pyrolitic model Matas et al, 2007;Merli et al, 2016). The fractions by weight of Fepericlase and bridgmanite in the lower mantle were estimated 0.13 and 0.87 (Table 1) respectively, by means of least-square mass balance calculations (Paktunc, 1998;Kobayashi et al, 2005) under the constraint of the LM-LLM bulk chemical composition.…”

“…Such phases occur in proportions that are a function of both geochemical and geophysical models. The former constrains the bulk chemical composition (McDonough and Sun, 1995;Lyubetskay and Korenaga, 2007;Stixrude and Lithgow-Bertelloni, 2007;Merli et al, 2016); the latter accounts for density-pressure-temperature profiles that are consistent with observations (i.e. : seismic velocities, density and bulk sound speed; Jackson, 1998;Mattern et al, 2005;de Wit and Trampert, 2015).…”

“…Assuming the mantle to be representable by the CMASF system (CaO-MgO-Al 2 O 3 -SiO 2 -FeO), the pyrolitic model's phase composition of the lower mantle consists of $75-78 wt% (Mg,Fe)SiO 3 , $6-10 wt% CaSiO 3 , $15 wt% (Mg,Fe) O and $2 wt% high-pressure Al-rich phase (Ono and Oganov, 2005;Mainprice, 2007;Merli et al, 2016). Given that Ca-perovskite and bridgmanite exhibit similar thermoelastic responses at the reference lower mantle P-T conditions Li et al, 2005;Matas et al, 2007), a simplified MgO-SiO 2 -FeO system (MSF) was used in the present work.…”

“…1) with a potential temperature of 1920 K at 24 GPa and T-vertical-gradient (dT/dz) of $0.5 K/km (Matas et al, 2007;Ono, 2008;Merli et al, 2016) was assumed for LM ($700-1900 km depth). A geotherm (>80 GPa; >2000 km) with a potential temperature of 2615 K at 96 GPa and a higher T-gradient than LM's, i.e.…”

First Principles Thermodynamics of Minerals at HP–HT Conditions: MgO as a Prototypical Material

“…The slight differences with respect to the values of Isaak et al [57] and Srivastava [102] at low temperature are due to the constraint P th (300 K, 1 bar) = 0, assumed to be valid in those works; this assumption lacks a theoretical justification, because thermal vibrations obviously occur already at T = 0 K (zero-point motions), and their effects cannot be disregarded; (iii) Although the lower mantle consists primarily of bridgmanite, the thermal pressure of MgO seems to be representative of that of the lower mantle [103,104] owing to the peculiar thermoelastic and transport properties of periclase. Lower mantle thermodynamics and rheology are thus largely controlled by periclase and its Fe-bearing analogue (ferropericlase) [16,[107][108][109][110]. (14) and (15) Calculated results of this work are compared to selected experimental data obtained by hydrostatic or quasi-hydrostatic compression in the diamond-anvil cell [90][91][92][93] and ultrasonic measurements [94][95][96][97].…”