Discriminating lower mantle composition

Houser, Christine; Hernlund, J. W.; Valencia-Cardona, Juan; Wentzcovitch, Renata M.

doi:10.1016/j.pepi.2020.106552

Cited by 10 publications

(9 citation statements)

References 134 publications

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“…3E). Compared to temperature, the lower sensitivities of the 1,000 depth and seismic velocities to mantle composition as well as its trade-off with temperature hinder the possibility of distinguishing among composition models from seismic observations alone (62).…”

Section: Discussionmentioning

confidence: 99%

Seismic detection of a deep mantle discontinuity within Mars by InSight

Huang

Schmerr

King

et al. 2022

Proc. Natl. Acad. Sci. U.S.A.

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Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m 2 .

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Section: Discussionmentioning

confidence: 99%

Seismic detection of a deep mantle discontinuity within Mars by InSight

Huang

Schmerr

King

et al. 2022

Proc. Natl. Acad. Sci. U.S.A.

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“…Implications for lower mantle composition: A reduction in the mid-mantle fast velocity regions in P-wave models compared to S-wave models is consistent with the predictions of an iron spin crossover in Fp. Due to uncertainties in the average mantle geotherm 16 we compare our predicted P-and S-wave velocities to the Preliminary Reference Earth Model (PREM) 54 radial P-wave profile using a temperature profile that aligns the predicted S-wave velocity to PREM, Fig. 1d.…”

Section: Effect Of Iron Partitioningmentioning

confidence: 99%

“…The observed divergence in P-wave and S-wave models in the fastest and slowest regions of the mid-mantle indicates that the iron spin crossover signal is seismically detectable in mantle regions expected to have peridotitic-to-pyrolitic compositions. Variable composition, such as regions with less Fp than pyrolite or less iron in the Fp, could disrupt and reduce the spin crossover signal 10,16 . Lateral temperature variations introduce a depth dependence to the P-wave seismic velocity inflection, which could reduce the globally averaged signal.…”

Section: Effect Of Iron Partitioningmentioning

confidence: 99%

“…The Fe/Mg ratio of Fp in a pyrolitic model composition has been proposed as representative for 1-D seismic profiles 14,15 . However, the compressional velocity of a homogeneous pyrolitic mantle does not fit 1-D seismic models when the effects of the iron spin crossover are included in the predicted seismic velocity computations 16 . Our predicted velocity reductions are supported by experimental data 5 , especially from Brillouin scattering 11,17 .…”

Section: Introductionmentioning

confidence: 99%

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Seismological Expression of the Iron Spin Crossover in Ferropericlase in the Earth’s Lower Mantle

Shephard¹,

Houser²,

Hernlund³

et al. 2020

Preprint

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The two most abundant minerals on Earth which together make up over 90% of the Earth’s lower mantle are (Mg,Fe)O-ferropericlase (Fp) and (Mg,Fe)SiO3-bridgmanite (Bm). Iron in Fp undergoes a high-spin to low-spin (HS-LS) crossover that influences density, viscosity, elasticity, thermal conductivity, and elemental partitioning, however, the predicted effects of this transition are not apparent in global 1D seismic velocity profiles. This discrepancy suggests that the predictions are inaccurate, seismic resolution is insufficient to resolve the effects, or a substantial portion of the mid-lower mantle is relatively SiO2-rich (hence Fp poor) compared to the shallow mantle. The melt-depleted mantle lithosphere of subducted oceanic slabs that sink into the lower mantle contains 22% Fp, and thus offers the best opportunity to prospect for a spin change in Fp. Here we reveal a loss in the abundance of fast seismic velocity anomalies in compressional (P-wave) tomography models at 1,400-2,000 km depth that is opposite to the trend in shear (S-wave) models. This can be explained by the decreasing temperature sensitivity of P-velocity expected for the mixed spin state of iron in Fp at corresponding pressures7. We also observe a similar but subtle signal for seismically slow regions below 1,800 km, consistent with a pressure increase and broadening of the Fp spin transition at higher temperatures. Seismic wave raypath distribution is similar for both P- and S-waves in this depth range, therefore this signature cannot be attributed to substantial differences in data coverage. Our identification of the spin transition signal in seismically fast and slow regions indicates that the spin crossover can identify the presence of Fp in the lower mantle. The absence of a Fp spin crossover signal in global seismic profiles supports the notion that the lower mantle is chemically heterogeneous at large scales and contains SiO2-rich regions that suppress the average signature of these pressure-induced electron spin pairing transitions.

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“…The exact mineralogy of the lowermost mantle is still being debated (e.g., Cobden et al., 2009; Davies et al., 2015; Houser et al., 2020; Trønnes, 2010; Vilella et al., 2021) and seismological mapping of structures near the CMB has helped to test several possibilities of mantle mineralogy. While the mineral phase transition from bridgmanite to post‐perovskite seems to be a good explanation for regions of past subduction (e.g., Hernlund et al., 2005; Trønnes, 2010), the mineralogy in the LLSVPs where slower‐than‐average velocities exist is still not well understood (e.g., Deschamps et al., 2012, 2015; Koelemeijer, 2021; McNamara, 2019).…”

Section: Discussionmentioning

confidence: 99%

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

Thomas

Cobden

Jonkers

2022

Geochem Geophys Geosyst

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The lowermost mantle of the Earth, the D" region (Bullen, 1949), is characterized by a range of seismic structures that have been studied with a variety of seismic methods, in order to understand their formative processes and mineralogy in the deep Earth (for overviews, see Garnero, 2000;Lay, 2015). One prominent feature of the lowermost mantle is a seismic discontinuity at the top of the D" region that generates reflections for S and P waves (see

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Discriminating lower mantle composition

Cited by 10 publications

References 134 publications

Seismic detection of a deep mantle discontinuity within Mars by InSight

Seismic detection of a deep mantle discontinuity within Mars by InSight

Seismological Expression of the Iron Spin Crossover in Ferropericlase in the Earth’s Lower Mantle

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

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