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Sulfides in Mercury's Mantle: Implications for Mercury's Interior as Interpreted From Moment of Inertia

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Geophysical Research Letters
Authors:
L. H. Lark
L. H. Lark
L. H. Lark
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Stephen W. Parman at Brown University
Stephen W. Parman
Chris Huber at Brown University
Chris Huber
E.M. (Marc) Parmentier at Brown University
E.M. (Marc) Parmentier
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Abstract and Figures

Plain Language Summary Mercury's mantle is unusually rich in sulfur relative to the other terrestrial planets. That sulfur is not bound into the silicate rocks, but rather it is expected to take the form of calcium and magnesium‐rich sulfides. We estimate how much sulfide should be present in Mercury's mantle and what the presence of that sulfide means for the density of Mercury's mantle and core. We interpret recent measurements of the moment of inertia (MoI) of Mercury's mantle and of the whole planet in light of the possible sulfide content of Mercury's mantle. We find that if Mercury's mantle contains large amounts of sulfide, but not very much iron (which is what we expect), then the value of Mercury's polar MoI should be low. Alternatively, if Mercury's polar MoI is high, Mercury's mantle must not have much sulfide in it. If Mercury's mantle has little sulfide, either Mercury does not have much sulfur overall, or the sulfur is in Mercury's core. The latter implies that Mercury's interior chemistry (especially the amount of oxygen) is different from what is predicted from the abundance of elements and minerals measured at Mercury's surface.
Quantity of sulfide present in Mercury's outer solid shell (OSS) as a function of OSS sulfur mass fraction, represented as (a) mass fraction of sulfide, (b) thickness of pure sulfide layer between mantle and crust (extracted sulfide endmember), and (c) decrease in mantle density (dispersed sulfide endmember). Insets illustrate OSS density profiles for 9 wt.% S.
Quantity of sulfide present in Mercury's outer solid shell (OSS) as a function of OSS sulfur mass fraction, represented as (a) mass fraction of sulfide, (b) thickness of pure sulfide layer between mantle and crust (extracted sulfide endmember), and (c) decrease in mantle density (dispersed sulfide endmember). Insets illustrate OSS density profiles for 9 wt.% S.
… 
Core radius (a) and average density (b) calculated as a function of outer solid shell (OSS) sulfur content, as constrained by Cm ${C}_{m}$ (±1σ $\pm 1\sigma $ in shaded region). A more sulfide‐rich mantle requires a smaller, denser core. Location of the sulfides (extracted vs. dispersed) has a secondary effect.
Core radius (a) and average density (b) calculated as a function of outer solid shell (OSS) sulfur content, as constrained by Cm ${C}_{m}$ (±1σ $\pm 1\sigma $ in shaded region). A more sulfide‐rich mantle requires a smaller, denser core. Location of the sulfides (extracted vs. dispersed) has a secondary effect.
… 
(a) Core moment of inertia (normalized by core mass and radius) and average density calculated as a function of outer solid shell sulfur content for (C/MR2=0.343 $C/M{R}^{2}=0.343$) (blue) and (C/MR2=0.333 $C/M{R}^{2}=0.333$) (purple). Error bars at zero indicate results calculated with Cm±1σ ${C}_{m}\pm 1\sigma $. Alternate C $C$ measurements would shift results vertically, illustrated with the two example values. (b) Core MoI and average density as a function of Si content for Fe‐Si alloys.
(a) Core moment of inertia (normalized by core mass and radius) and average density calculated as a function of outer solid shell sulfur content for (C/MR2=0.343 $C/M{R}^{2}=0.343$) (blue) and (C/MR2=0.333 $C/M{R}^{2}=0.333$) (purple). Error bars at zero indicate results calculated with Cm±1σ ${C}_{m}\pm 1\sigma $. Alternate C $C$ measurements would shift results vertically, illustrated with the two example values. (b) Core MoI and average density as a function of Si content for Fe‐Si alloys.
… 
Core silicon content as a function of outer solid shell (OSS) sulfur content. Shaded region indicates uncertainty in Cm ${C}_{m}$, inner core size, and OSS sulfide distribution. Polar moment of inertia (MoI) determines location along the lines (extrapolation below zero permits comparison to predictions for high MoI). Less silicon is geodetically permissible if the core contains additional light elements. More silicon is geodetically impermissible in the context of the compositions and configurations we explore, requiring lower Cm ${C}_{m}$ or lower M $M$ than measured.
Core silicon content as a function of outer solid shell (OSS) sulfur content. Shaded region indicates uncertainty in Cm ${C}_{m}$, inner core size, and OSS sulfide distribution. Polar moment of inertia (MoI) determines location along the lines (extrapolation below zero permits comparison to predictions for high MoI). Less silicon is geodetically permissible if the core contains additional light elements. More silicon is geodetically impermissible in the context of the compositions and configurations we explore, requiring lower Cm ${C}_{m}$ or lower M $M$ than measured.
… 
Illustration of constraints on Mercury's composition derived from geodetic measurements (blue) and experimentally determined iron‐silicate equilibration (orange). On geochemical panel, scattered data are measurements of equilibrated silicate and metal composition (Namur, Charlier, et al., 2016) while lines are silicon partitioned into C‐free (solid) and C‐saturated (dashed) iron mapped to silicate sulfur content by oxygen fugacity (Steenstra & van Westrenen, 2020). Intersection of geodetic and geochemical data gives the sketched diagram. Refined moment of inertia from BepiColombo could place Mercury in I or II (existing measurements indicate III is unlikely). Placement in I suggests core‐mantle equilibration; refined geochemical constraints could distinguish degree of sulfur saturation. Placement in II suggests core and mantle did not equilibrate.
Illustration of constraints on Mercury's composition derived from geodetic measurements (blue) and experimentally determined iron‐silicate equilibration (orange). On geochemical panel, scattered data are measurements of equilibrated silicate and metal composition (Namur, Charlier, et al., 2016) while lines are silicon partitioned into C‐free (solid) and C‐saturated (dashed) iron mapped to silicate sulfur content by oxygen fugacity (Steenstra & van Westrenen, 2020). Intersection of geodetic and geochemical data gives the sketched diagram. Refined moment of inertia from BepiColombo could place Mercury in I or II (existing measurements indicate III is unlikely). Placement in I suggests core‐mantle equilibration; refined geochemical constraints could distinguish degree of sulfur saturation. Placement in II suggests core and mantle did not equilibrate.
… 
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1. Introduction
Compared to other terrestrial planets, Mercury's surface is highly enriched in sulfur, but low in iron (Nittler
etal.,2011; Weider etal.,2015). At low fO2, sulfur solubility increases in silicate melt (as much as 7–11 wt.%
at magma ocean conditions), whereas FeO solubility decreases to <0.5wt.% in both melts and silicate minerals
(McCubbin etal.,2012; Namur, Charlier, etal.,2016; Zolotov etal.,2013). Therefore, Mercury's surface compo-
sition implies that it differentiated and evolved under far more reducing conditions than the other terrestrial plan-
ets, similar to enstatite chondrites and the aubrite parent body (Cartier & Wood,2019; Malavergne etal.,2010).
Any Fe initially in Mercury's magma ocean would have existed as iron metal or, if sulfur was present in excess
of planetary saturation, iron sulfide (Malavergne etal.,2014). Both of these phases are far denser than plausible
magma ocean compositions and would have rapidly sunk to the core, leaving the silicate magma ocean with little
Fe but abundant dissolved S.
Under such conditions, nearly iron-free forsterite and enstatite would dominate the silicate portion of the magma
ocean cumulate (Cartier & Wood,2019; Namur, Collinet, etal.,2016). As Mercury's magma ocean crystallized,
dissolved sulfur would have been concentrated in the remaining melt. Once the melt reached sulfur saturation,
sulfides would exsolve. With little iron available, these sulfides would be dominantly magnesium (niningerite)
and calcium (oldhamite) sulfide, as found in enstatite chondrites (Anzures etal.,2020; Keil,1989). These Mg,
Ca-rich sulfides would be much less dense than FeS and even less dense than the crystallized silicate phases.
Sulfides exsolved from the magma ocean could have migrated to the top of the mantle or been trapped with
the cumulate. In either case, large quantities of low-density sulfide are likely to be present in Mercury's mantle
(Boukaré etal.,2019).
Abstract The partitioning of sulfur between Mercury's core and mantle reflects its formation conditions
and early evolution. If Mercury's core and mantle equilibrated under reducing conditions, and if Mercury is not
depleted in sulfur relative to chondrites, Mercury's mantle should contain large quantities (7–11 wt.%) of sulfur
in the form of Ca or Mg-rich sulfides. Using petrologic constraints, we estimate the quantity of these sulfides
and the implications of a sulfide-rich mantle for Mercury's radial density structure. We find that based on recent
measurements of Mercury's outer shell moment of inertia (MoI), a sulfide-rich, iron-poor mantle mineralogy
is consistent with a low value of Mercury's polar MoI (0.333MR
2). Alternatively, a higher value for Mercury's
MoI (0.343MR
2) would require a sulfide-poor mantle, indicating bulk sulfur depletion or more oxidizing
conditions than implied by surface composition.
Plain Language Summary Mercury's mantle is unusually rich in sulfur relative to the other
terrestrial planets. That sulfur is not bound into the silicate rocks, but rather it is expected to take the form of
calcium and magnesium-rich sulfides. We estimate how much sulfide should be present in Mercury's mantle
and what the presence of that sulfide means for the density of Mercury's mantle and core. We interpret recent
measurements of the moment of inertia (MoI) of Mercury's mantle and of the whole planet in light of the
possible sulfide content of Mercury's mantle. We find that if Mercury's mantle contains large amounts of
sulfide, but not very much iron (which is what we expect), then the value of Mercury's polar MoI should be low.
Alternatively, if Mercury's polar MoI is high, Mercury's mantle must not have much sulfide in it. If Mercury's
mantle has little sulfide, either Mercury does not have much sulfur overall, or the sulfur is in Mercury's core.
The latter implies that Mercury's interior chemistry (especially the amount of oxygen) is different from what is
predicted from the abundance of elements and minerals measured at Mercury's surface.
LARK ET AL.
© 2022. American Geophysical Union.
All Rights Reserved.
Sulfides in Mercury's Mantle: Implications for Mercury's
Interior as Interpreted From Moment of Inertia
L. H. Lark1 , S. Parman1 , C. Huber1 , E. M. Parmentier1 , and J. W. Head III1
1Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
Key Points:
• Petrologic constraints imply Mercury's
mantle may contain 13–20 wt.%
low-density sulfides; this is relevant
for geophysical studies
• A sulfide-rich mantle is consistent
with a low polar moment of inertia
(MoI), and implies a smaller, denser
core than does a pure silicate mantle
• If Mercury's polar MoI is closer to the
high end of published values, it rules
out a sulfur-rich mantle
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
L. H. Lark,
laura_lark@brown.edu
Citation:
Lark, L. H., Parman, S., Huber, C.,
Parmentier, E. M., & Head III, J.
W. (2022). Sulfides in Mercury's
mantle: Implications for Mercury's
interior as interpreted from moment of
inertia. Geophysical Research Letters,
49, e2021GL096713. https://doi.
org/10.1029/2021GL096713
Received 27 OCT 2021
Accepted 15 MAR 2022
10.1029/2021GL096713
RESEARCH LETTER
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Full-text available
  • Aug 2019
A compositional variety of planetary cores provides insight into their core/mantle evolution and chemistry in the early solar system. To infer core composition from geophysical data, a precise knowledge of elastic properties of core‐forming materials is of prime importance. Here, we measure the sound velocity and density of liquid Fe‐Ni‐S (17 and 30 at% S) and Fe‐Ni‐Si (29 and 38 at% Si) at high pressures and report the effects of pressure and composition on these properties. Our data show that the addition of sulfur to iron substantially reduces the sound velocity of the alloy and the bulk modulus in the conditions of this study, while adding silicon to iron increases its sound velocity but has almost no effect on the bulk modulus. Based on the obtained elastic properties combined with geodesy data, S or Si content in the core is estimated to 4.6 wt% S or 10.5 wt% Si for Mercury, 9.8 wt% S or 18.3 wt% Si for the Moon, and 32.4 wt% S or 30.3 wt% Si for Mars. In these core compositions, differences in sound velocity profiles between an Fe‐Ni‐S and Fe‐Ni‐Si core in Mercury are small, whereas for Mars and the Moon, the differences are substantially larger and could be detected by upcoming seismic sounding missions to those bodies.
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Effect of Sulfur Speciation on Chemical and Physical Properties of Very Reduced Mercurian Melts
Article
  • Jul 2020
  • GEOCHIM COSMOCHIM AC
The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO2 between IW-3 and IW-7). Due to decreasing O availability at these low ƒO2 conditions, and an abundance of S²⁻, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO2 between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ∼ IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.
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Geochemical constraints on core-mantle differentiation in Mercury and the Aubrite Parent Body
Article
  • Jan 2020
Differentiation of Mercury and the aubrite parent body (AuPB) at extremely reducing conditions is implied from the low FeO and high S contents of their mantles. Here, the mantle abundances of these elements as derived from remote sensing (in the case of Mercury) and aubrite meteorite analysis are used in conjunction with new experimentally determined metal-silicate partition coefficients at reducing conditions presented in Steenstra et al. (2020a) to quantify the geochemical consequences of core formation in highly reduced planetary bodies. Plausible core compositions of Mercury and the AuPB are assessed, and the distribution of Si and other elements during core formation are quantified. Combining the new results with previous remote sensing observations of the surface of Mercury it is found that its core is likely to be Si-rich (> 4.5–21 wt.%). The estimated core Si contents depend mostly on the assumption of C saturation in Mercury’s mantle and the type of bulk composition considered. Calculations for C-free conditions also yield significant quantities of Si in the AuPB core (> 6–20 wt.%, depending on bulk composition and redox state). The amount of Si in the AuPB core is greatly decreased if C-saturation is assumed. The new experimental data and related thermodynamic parameterization for predicting the C contents at graphite saturation of FeSi alloys (CCGS) shows that Mercury’s mantle was likely C-saturated following formation of a Si-rich core, allowing for the separation of a graphitic flotation crust. Due to the lower Si contents calculated for the AuPB core under C-saturated conditions, a graphitic flotation crust is unlikely to form in smaller-sized reduced asteroids such as the AuPB. The Si-rich nature of the AuPB core for C-undersaturated scenarios is expected to have resulted in preferential partitioning of the majority of the volatile siderophile elements (VSE) into the AuPB core. However, depletions for the most volatile VSE cannot be reconciled with core formation depletion only. Their depletions in aubrites require additional depletion from (I) segregating sulfide liquids during AuPB differentiation and/or (II) compatible behavior during mineral-melt fractionation and/or (III) loss of these elements in a vapor phase during and/or after AuPB differentiation.
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