skip to main content

Attention:

The NSF Public Access Repository (PAR) system and access will be unavailable from 11:00 PM ET on Thursday, February 13 until 2:00 AM ET on Friday, February 14 due to maintenance. We apologize for the inconvenience.


Title: Implications for the Melting Phase Relations in the MgO ‐ FeO System at Core‐Mantle Boundary Conditions
Abstract

At nearly 2,900‐km depth, the core‐mantle boundary (CMB) represents the largest density increase within the Earth going from a rocky mantle into an iron‐alloy core. This compositional change sets up steep temperature gradients, which in turn influences mantle flow, structure, and seismic velocities. Here we resolve the thermodynamic parameters of (Mg,Fe)O and compute the melting phase relations of the MgO‐FeO binary system at CMB conditions. Based on this phase diagram, we revisit iron infiltration into solid ferropericlase along the CMB by morphological instability and find that the length scale of infiltration is comparable with the high electrical conductivity layer inferred from core nutations. We also compute the (Mg,Fe)O‐SiO2pseudo‐binary system and find that the solidus melting temperatures near the CMB decrease with FeO and SiO2content, becoming potentially important for ultralow velocity zones. Therefore, an ultralow velocity zone composed of solid‐state bridgmanite and ferropericlase may be relatively enriched in MgO and depleted in SiO2and FeO along a hot CMB.

 
more » « less
PAR ID:
10455921
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Volume:
124
Issue:
2
ISSN:
2169-9313
Page Range / eLocation ID:
p. 1294-1304
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Pyrite‐type FeO2Hx(P phase) has recently been suggested as a possible alternative to explain ultralow‐velocity zones due to its low seismic velocity and high density. Here we report the results on the congruent melting temperature and melt properties of P phase at high pressures from first‐principles molecular dynamics simulations. The results show that P phase would likely be melted near the core–mantle boundary. Liquid FeO2Hxhas smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO2Hxcould account for the observed seismic anomaly of ultralow‐velocity zones. However, to maintain the liquid FeO2Hxwithin the ultralow‐velocity zones against compaction requires special physical conditions, such as relatively high viscosity of the solid matrix and/or vigorous convection of the overlying mantle.

     
    more » « less
  2. Abstract

    High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O3, and (Mg, Fe, Al, Si)O3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox. Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.

     
    more » « less
  3. Abstract

    The carbon and water cycles in the Earth's interior are linked to key planetary processes, such as mantle melting, degassing, chemical differentiation, and advection. However, the role of water in the carbon exchange between the mantle and core is not well known. Here, we show experimental results of a reaction between Fe3C and H2O at pressures and temperatures of the deep mantle and core‐mantle boundary (CMB). The reaction produces diamond, FeO, and FeHx, suggesting that water can liberate carbon from the core in the form of diamond (“core carbon extraction”) while the core gains hydrogen, if subducted water reaches to the CMB. Therefore, Earth's deep water and carbon cycles can be linked. The extracted core carbon can explain a significant amount of the present‐day mantle carbon. Also, if diamond can be collected by mantle flow in the region, it can result in unusually high seismic‐velocity structures.

     
    more » « less
  4. Abstract

    We present a new method to construct an internally consistent thermodynamic model using a compilation of high‐pressure melting experiments. The steepest descent method and Monte Carlo sampling are combined to constrain all model parameters simultaneously instead of determining each parameter sequentially from relevant experiments. Our approach is applied to the published melting experiments on mantle materials to obtain the thermodynamic parameters of the MgO‐FeO‐SiO2ternary system. Inversion with the subsets of experimental data is conducted as well to investigate the source of discrepancy among existing studies, and the key parameters are found to be the thermal expansivity of SiO2and the excess volume of mixing between MgO and SiO2. Mixing between FeO and SiO2is only constrained with large uncertainty, which could also imply that oxides with low concentrations have minimal effects on melting. Constraining the thermodynamics of MgO and SiO2will be important for a better understanding of mantle melting at high pressures.

     
    more » « less
  5. Abstract

    The Martian mantle is considered to have a higher Fe/Mg ratio than the Earth's mantle. Ringwoodite, γ‐(Mg,Fe)2SiO4, is likely the dominant polymorph of olivine in the core‐mantle boundary (CMB) region of Mars. We synthesized anhydrous iron‐rich ringwoodite with molar Mg/(Mg + Fe) = 0.44 and determined its thermal equation of state up to 35 GPa and 750 K by synchrotron X‐ray diffraction. Using a third order Birch‐Murnaghan equation of state, we obtainKT0 = 182 (3) GPa, K′ = 4.6 (2), andα0 = 3.18 (6) × 10−5 K−1. Using these results and an updated mineralogical model with an iron‐rich composition of Mg/(Mg + Fe) = 0.75 for the Martian mantle, we estimate ∼1900 K for the temperature of the D1000 seismic discontinuity inside Mars. The resulting adiabat predicts a warm aerotherm, which could explain the presence of partial melt at the CMB of Mars recently detected with seismic data from the 2019 InSight mission.

     
    more » « less