ORCID Profile
0000-0002-9960-1484
Current Organisations
New Mexico State University
,
James Cook University
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Publisher: Springer Science and Business Media LLC
Date: 04-11-2021
Publisher: Wiley
Date: 24-11-2020
Publisher: Geological Society of America
Date: 2017
Publisher: Proceedings of the National Academy of Sciences
Date: 21-11-2022
Abstract: The formation and preservation of compositional heterogeneities inside the Earth affect mantle convection patterns globally and control the long-term evolution of geochemical reservoirs. However, the distribution, nature, and size of reservoirs in the Earth’s mantle are poorly constrained. Here, we invert measurements of travel times and litudes of seismic waves interacting with mineralogical phase transitions at 400–700-km depth to obtain global probabilistic maps of temperature and bulk composition. We find large basalt-rich pools (up to 60% basalt fraction) surrounding the Pacific Ocean, which we relate to the segregation of oceanic crust from slabs that have been subducted since the Mesozoic. Segregation of oceanic crust from initially cold and stiff slabs may be facilitated by the presence of a weak hydrated layer in the slab or by weakening upon mineralogical transition due to grain-size reduction.
Publisher: Elsevier BV
Date: 2018
Publisher: American Geophysical Union (AGU)
Date: 11-2017
DOI: 10.1002/2017GC007149
Publisher: American Geophysical Union (AGU)
Date: 04-2022
DOI: 10.1029/2021JB023540
Abstract: Investigations of the Earth's inner core (IC) using seismic body waves are limited by their volumetric s ling due to uneven global distribution of large earthquakes and receivers. The sparse coverage of the IC leads to uncertainties in its anisotropy, the directional dependence of seismic velocity. Yet, detailed constraints on anisotropy, such as its magnitude, and spatial distribution, are required to understand the crystallographic structure of IC's iron and its solidification and deformation processes. Here, we present a new method to investigate the IC's anisotropic properties based on Earth's coda‐correlation wavefield constructed from the late coda of large earthquakes. We perform a comprehensive travel time analysis of I2*, an IC‐sensitive correlation feature identified as a counterpart of the direct seismic wavefield's PKIKPPKIKP waves, yet fundamentally different. Namely, I2* is a mathematical manifestation of similarity among specific seismic phases with the same slowness detected in global correlograms in the short inter‐receiver distance range. Our new spatial s ling of the IC overcomes the shortage of direct seismic wavefield paths sensitive to the IC's central volume, also known as the innermost IC (IMIC). The observed I2*’s travel time variations relative to Earth's rotation axis (ERA) support a model of cylindrical anisotropy with 3.3% strength and a zonal pattern of slow axis oriented 55° from ERA. We thus find compelling evidence for a deep IC structure with distinct anisotropy, although we cannot resolve the depth at which the change occurs. This finding reinforces previous inference on the IMIC, with implications for Earth's evolution.
Publisher: Springer Science and Business Media LLC
Date: 26-01-2018
DOI: 10.1038/S41467-017-02709-4
Abstract: Seismic tomography indicates that flow is commonly deflected in the mid-mantle. However, without a candidate mineral phase change, causative mechanisms remain controversial. Deflection of flow has been linked to radial changes in viscosity and/or composition, but a lack of global observations precludes comprehensive tests by seismically detectable features. Here we perform a systematic global-scale interrogation of mid-mantle seismic reflectors with lateral size 500–2000 km and depths 800–1300 km. Reflectors are detected globally with variable depth, lateral extent and seismic polarity and identify three distinct seismic domains in the mid-mantle. Near-absence of reflectors in seismically fast regions may relate to dominantly subvertical heterogeneous slab material or small impedance contrasts. Seismically slow thermochemical piles beneath the Pacific generate numerous reflections. Large reflectors at multiple depths within neutral regions possibly signify a compositional or textural transition, potentially linked to long-term slab stagnation. This variety of reflector properties indicates widespread compositional heterogeneity at mid-mantle depths.
Publisher: American Geophysical Union (AGU)
Date: 07-2019
DOI: 10.1029/2019JB017307
Abstract: The mantle transition zone (MTZ) of Earth is demarcated by solid‐to‐solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660‐km depths. Mineral physics experiments predict that wadsleyite can have strong single‐crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice‐preferred orientation of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel‐times and litudes of SS precursors and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench‐parallel fast direction in the upper mantle and ~3% anisotropy with a trench‐perpendicular fast direction in the MTZ. The transition of fast directions indicates that the lattice‐preferred orientation of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle but negligible anisotropy ( %) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces ± 3 km difference and therefore has minor overall effects on global MTZ topography.
Publisher: Springer Science and Business Media LLC
Date: 27-09-2023
Publisher: Wiley
Date: 20-09-2020
Publisher: Elsevier BV
Date: 12-2019
Publisher: Oxford University Press (OUP)
Date: 04-01-2022
DOI: 10.1093/GJI/GGAB529
Abstract: The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve $\\ge $3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely s led areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ.
Publisher: Elsevier BV
Date: 09-2022
Publisher: American Geophysical Union (AGU)
Date: 21-09-2021
DOI: 10.1029/2020GL091658
Abstract: Typical seismic waveform data sets comprise hundreds of thousands to millions of records. Compilation is performed by time‐consuming handpicking of phase arrival times, or signal processing algorithms such as cross‐correlation. The latter generally underperform compared to handpicking. However, differences in picking methods creates variations in models and interpretation of Earth's structure. Here, we exploit the pattern recognition capabilities of Convolutional Neural Networks (CNN). Using a large handpicked data set, we train a CNN model to identify the seismic shear phase SS. This accelerates, automates, and makes consistent data compilation, a task usually completed by visual inspection and influenced by scientists' choices. The CNN model is employed to identify precursors to SS generated by mantle discontinuities. It identifies precursors in stacked and in idual seismograms, producing new measurements of the mantle transition zone with quality comparable to handpicked data. This rapid acquisition of high‐quality observations has implications for automation of future seismic tomography studies.
Publisher: Proceedings of the National Academy of Sciences
Date: 10-10-2022
Abstract: 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 .
Location: United States of America
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2019
End Date: 2022
Funder: Directorate for Geosciences
View Funded ActivityStart Date: 2017
End Date: 2018
Funder: Directorate for Geosciences
View Funded ActivityStart Date: 2017
End Date: 2019
Funder: Australian Research Council
View Funded Activity