SUMMARY The presence of seismic anisotropy at the base of the Earth's mantle is well established, but there is no consensus on the deformation mechanisms in lower mantle minerals that could explain it. Strong anisotropy in magnesium post-perovskite (pPv) has been invoked, but different studies disagree on the dominant slip systems at play. Here, we aim to further constrain this by implementing the most recent results from atomistic models and high-pressure deformation experiments, coupled with a realistic composition and a 3-D geodynamic model, to compare the resulting deformation-induced anisotropy with seismic observations of the lowermost mantle. We account for forward and reverse phase transitions from bridgmanite (Pv) to pPv. We find that pPv with either dominant (001) or (010) slip can both explain the seismically observed anisotropy in colder regions where downwellings turn to horizontal flow, but only a model with dominant (001) slip matches seismic observations at the root of hotter large-scale upwellings. Allowing for partial melt does not change these conclusions, while it significantly increases the strength of anisotropy and reduces shear and compressional velocities at the base of upwellings.
more »
« less
Forward modeling seismic anisotropy in the lower mantle through 3-phase aggregate deformation
In recent years there have been several attempts to make the link between mineral properties and seismic anisotropy in the D’’ region but have yet to reach consensus with regards to the dynamics in lower mantle minerals that could give rise to the observed seismic anisotropy. Here, we aim to provide further constraints on the observed long wavelength shear velocity patterns seen in seismic tomography studies. We introduce a forward model of deformation in a subducting slab as it impacts the core mantle boundary (D’’ layer) and proceeds to upwelling at the edge of a simulated LLSVP. By implementing the most recent results from atomistic modeling and high-pressure deformation experiments coupled with a 3-dimensional geodynamic model, we compare the microstructural evolution of an aggregate with a pyrolytic composition to the macroscopically observed seismic anisotropy of the lowermost mantle. We account for topotaxial relations in the forward and reverse phase transitions of MgSiO3-perovskite (Pv) to post-perovskite (pPv) within the slab as well as explore the effects introduced by partial melting near the CMB. Comparisons in the two leading candidate deformation mechanisms in the post-perovskite phase, (001) and (010), are compared. In this study we find that the reverse transition (pPv to Pv) occurs at a depth which is ~ 150 km deeper than that of the forward transition due to increasing temperature near the CMB providing a varying topography of the D’’ discontinuity. Our model also produces good fits with the isotropic velocities of PREM for the bulk lower mantle. When coupled with temperature and pressure dependent forward and reverse phase transitions, a pPv system with dominant (001) slip provides good correlation with the currently observed VSH fast horizontal (~ 1 – 6%) in D’’ and with VSV consistently fast in upwelling areas. Azimuthal variations along the streamline are also investigated showing a symmetry lower than that of the assumed VTI in D’’ introduced by ‘rolling’ effects near the slab’s edge. The addition of 1% partial melting at the CMB is shown to increase S and P wave anisotropy beneath the slab at the base of upwelling with up to ~2.5 & 4.0% P and S wave reductions respectively compared to the global reference.
more »
« less
- Award ID(s):
- 2054951
- NSF-PAR ID:
- 10329114
- Date Published:
- Journal Name:
- Geophysical journal international
- Volume:
- 227
- Issue:
- 3
- ISSN:
- 0956-540X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
SUMMARY It is well known that regions of the lowermost mantle—D″—exhibit significant seismic anisotropy. Identifying a unique mechanism for seismic anisotropy in D″ and interpreting results in terms of mantle flow has proved challenging. In an attempt to address this, we outline a method for the direct inversion of shear wave waveform data for the orientation and strength of seismic anisotropy. We demonstrate our method by jointly inverting SKS, SKKS and ScS shear wave data for seismic anisotropy in a fast shear wave velocity anomaly beneath the Eastern Pacific Ocean. Using our inversion method we evaluate four candidate mechanisms for seismic anisotropy in D″: elliptical transverse isotropy (representing layering or inclusions), bridgmanite and post-perovskite (for fabrics dominated by either [100](001) or [100](010) slip). We find that all candidate mechanisms can reasonably explain our input data, with synthetic inversions demonstrating that improved backazimuthal coverage is required to identity a single best-fitting mechanism. By inverting for orientation and anisotropic strength parameters we are able to discount bridgmanite as a candidate mechanism as less plausible solution, as our inversion requires an unreasonable ca. 40 per cent of D″ to consist of aligned bridgmanite crystals. The inversion results for the 4 candidate mechanisms predict two different mantle flow regimes, near vertical upwelling (or downwelling) or predominantly horizontal Southwesterly (or Northwesterly) deformation, both of which are inconsistent with recent mantle flow models. These results show that our new inversion method gives seismologists a powerful new tool to constrain lowermost mantle anisotropy, allowing us to test predictions of lowermost mantle flow.
-
more » « less
SUMMARY Iceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.
-
more » « less
Abstract Observations of seismic anisotropy just above the core‐mantle boundary are a powerful way to understand the dynamics of the lowermost mantle. Here we present models of seismic anisotropy in the lowermost mantle beneath Siberia based on new and previously published body wave observations. We compile a set of measurements based on a variety of data types, including the differential splitting of SK(K)S‐PKS and S‐ScS phases and polarities of PdP and SdS reflections off the D″ discontinuity. We carry out ray theoretical forward modeling of these data sets in combination, using a novel approach that allows for tighter constraints on the geometry of seismic anisotropy than would be possible using a single data type. Observations for each seismic phase alone only provide limited information; by combining different data types into one modeling approach, we can constrain D″ seismic anisotropy more tightly. We test a range of plausible mechanisms for seismic anisotropy, including a variety of candidate minerals (bridgmanite, post‐perovskite, and ferropericlase), dominant slip systems, and orientations, and consider both single‐crystal elasticity and elastic tensors based on polycrystalline texture modeling. In general, we find that post‐perovskite (with either a [100](010) or [100](001) dominant slip system) or bridgmanite models provide the best fit to the observations. The best‐fitting models suggest a dominant shear direction to the southwest or northeast direction at the base of the mantle. This is consistent with flow directed toward the Perm anomaly, potentially driven by slab remnants impinging on the core‐mantle boundary.
-
more » « less
Abstract The primary phase of the Earth’s lower mantle, (Al, Fe)‐bearing bridgmanite, transitions to the post‐perovskite (PPv) phase at Earth’s deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)‐, (Al3+)‐, (Fe2+)‐, and (Fe3+)‐bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two‐phase region depend distinctly on the chemical composition: (a) Fe3+‐, Al3+‐, or (Al3+, Fe3+)‐alloying increases the transition pressure, while Fe2+‐alloying has the opposite effect; (b) in the absence of coexisting phases, the pressure‐depth range of the Pv‐PPv transition is likely too broad to cause a sharp D” discontinuity (<30 km); (c) the average Clapeyron slope of the two‐phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, (d) we observe a softening of the bulk modulus in the two‐phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D region by analyses of tomographic images.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi.org/10.1093/gji/ggab278
