Matching Items (6)

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Extreme Seismic Anomalies near Earth’s Core Mantle Boundary

Description

The interior of Earth is stratified due to gravity. Therefore, the lateral heterogeneities observed as seismic anomalies by seismologists are extremely interesting: they hold the key to understand the composition,

The interior of Earth is stratified due to gravity. Therefore, the lateral heterogeneities observed as seismic anomalies by seismologists are extremely interesting: they hold the key to understand the composition, thermal status and evolution of the Earth. This work investigates seismic anomalies inside Earth’s lowermost mantle and focuses on patch-like ultra-low velocity zones (ULVZs) found on Earth’s core-mantle boundary (CMB). Firstly, all previous ULVZ studies are compiled and ULVZ locations on the CMB are digitized. The result is a database, which is publicly available online. A key finding is that there is not a simple mapping between the locations of the observed ULVZs and the large low velocities provinces (LLVPs). Instead, ULVZs are more likely to occur near LLVP boundaries. This spatial correlation study supports a compositionally distinct origin for at least some ULVZs. Next, the seismic structure of the basal mantle beneath the Central America is investigated. This region hosts present and past subducted slabs, which could have brought compositionally distinct oceanic basalt all the way down to the CMB. The waveform distortions of a core-reflected seismic phase and a forward modeling method are used to constrain the causes of the CMB structures. In addition to ULVZ structures, isolated patches of thin zones with shear velocity increased by over 10% relative to background mantle are found for the first time. Ultra-high velocity zones (UHVZs) are interspersed with ULVZs and could be caused by subducted mid-ocean ridge basalt (MORB) that undergoes partial melting and melt segregation. Fe-rich partial melt of MORB can form ULVZs, and silica polymorphs (SiO2) and calcium-perovskite (CaPv) rich solid residue can explain the UHVZs. Finally, large-scale heterogeneities in the lowermost mantle are investigated using S waveform broadening observations. Several basal layer models are case-studied via synthetic calculations. S wave arrivals received at a distance larger than 80˚ in a global dataset from large earthquakes between the years 1994 and 2017 are examined and S waveform broadenings are documented. This approach exploits large distance data for the first time, and therefore is complementary to previous studies in terms of sampling locations. One possible explanation of S waveform broadening is velocity discontinuity inside the D″ layer due to the temperature controlled Bm-pPv phase transition.

Contributors

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Created

Date Created
  • 2020

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High-Resolution Imaging of Earth's Lowermost Mantle

Description

This research investigates the fine scale structure in Earth's mantle, especially for the lowermost mantle, where strong heterogeneity exists. Recent seismic tomography models have resolved large-scale features in the lower

This research investigates the fine scale structure in Earth's mantle, especially for the lowermost mantle, where strong heterogeneity exists. Recent seismic tomography models have resolved large-scale features in the lower mantle, such as the large low shear velocity provinces (LLSVPs). However, differences are present between different models, especially at shorter length scales. Fine scale structures both within and outside LLSVPs are still poorly constrained. The drastic growth of global seismic networks presents densely sampled seismic data in unprecedented quality and quantity. In this work, the Empirical Wavelet construction method has been developed to document seismic travel time and waveform information for a global shear wave seismic dataset. A dataset of 250K high-quality seismic records with comprehensive measurements is documented and made publicly available. To more accurately classify high quality seismic signal from the noise, 1.4 million manually labeled seismic records have been used to train a supervised classification model. The constructed model performed better than the empirical model deployed in the Empirical Wavelet method, with 87% in precision and 83% in recall. To utilize lower amplitude phases such as higher multiples of S and ScS waves, we have developed a geographic bin stacking method to improve signal-to-noise ratio. It is then applied to Sn waves up to n=6 and ScSn wave up to n=5 for both minor and major arc phases. The virtual stations constructed provide unique path sampling and coverage, vastly improving sampling in the Southern Hemisphere. With the high-quality dataset we have gathered, ray-based layer stripping iterative forward tomography is implemented to update a starting tomography model by mapping the travel time residuals along the ray from the surface down to the core mantle boundary. Final updated models with different starting tomography models show consistent updates, suggesting a convergent solution. The final updated models show higher resolution results than the starting tomography models, especially on intermediate-scale structures. The combined analyses and results in this work provide new tools and new datasets to image the fine-scale heterogeneous structures in the lower mantle, which advances our understanding of the dynamics and evolution of the Earth's mantle.

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Created

Date Created
  • 2019

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Thermochemical structure and dynamics of Earth's lowermost mantle

Description

Seismic observations have revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Pacific and Africa. One hypothesis for the origin of LLSVPs is that they are

Seismic observations have revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Pacific and Africa. One hypothesis for the origin of LLSVPs is that they are caused by accumulation of subducted oceanic crust on the core-mantle boundary (CMB). Here, I perform high resolution geodynamical calculations to test this hypothesis. The result shows that it is difficult for a thin (~ 6 km) subducted oceanic crust to accumulate on the CMB, and the major part of it is viscously stirred into the surrounding mantle. Another hypothesis for the origin of LLSVPs is that they are caused by thermochemical piles of more-primitive material which is remnant of Earth's early differentiation. In such case, a significant part of the subducted oceanic crust would enter the more-primitive reservoir, while other parts are either directly entrained into mantle plumes forming on top of the more-primitive reservoir or stirred into the background mantle. As a result, mantle plumes entrain a variable combination of compositional components including more-primitive material, old oceanic crust which first enters the more-primitive reservoir and is later entrained into mantle plumes with the more-primitive material, young oceanic crust which is directly entrained into mantle plumes without contacting the more-primitive reservoir, and depleted background mantle material. The result reconciles geochemical observation of multiple compositional components and varying ages of oceanic crust in the source of ocean-island basalts. Seismic studies have detected ultra-low velocity zones (ULVZs) in some localized regions on the CMB. Here, I present 3D thermochemical calculations to show that the distribution of ULVZs provides important information about their origin. ULVZs with a distinct composition tend to be located at the edges of LLSVPs, while ULVZs solely caused by partial melting tend to be located inboard from the edges of LLSVPs. This indicates that ULVZs at the edges of LLSVPs are best explained by distinct compositional heterogeneity, while ULVZs located insider of LLSVPs are better explained by partial melting. The results provide additional constraints for the origin of ULVZs.

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Agent

Created

Date Created
  • 2015

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High-resolution imaging of structure and dynamics of the lowermost mantle

Description

This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized

This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized by two nearly antipodal large low shear velocity provinces (LLSVPs), one beneath the Pacific Ocean the other beneath Africa and the southern Atlantic Ocean. However, fine-scale LLSVP structure as well as its relationship with plate tectonics, mantle convection, hotspot volcanism, and Earth's outer core remains poorly understood. The recent dramatic increase in seismic data coverage due to the EarthScope experiment presents an unprecedented opportunity to utilize large concentrated datasets of seismic data to improve resolution of lowermost mantle structures. I developed an algorithm that identifies anomalously broadened seismic waveforms to locate sharp contrasts in shear velocity properties across the margins of the LLSVP beneath the Pacific. The result suggests that a nearly vertical mantle plume underlies Hawaii that originates from a peak of a chemically distinct reservoir at the base of the mantle, some 600-900 km above the CMB. Additionally, acute horizontal Vs variations across and within the northern margin of the LLSVP beneath the central Pacific Ocean are inferred from forward modeling of differential travel times between S (and Sdiff) and SKS, and also between ScS and S. I developed a new approach to expand the geographic detection of ultra-low velocity zones (ULVZs) with a new ScS stacking approach that simultaneously utilizes the pre- and post-cursor wavefield.. Strong lateral variations in ULVZ thicknesses and properties are found across the LLSVP margins, where ULVZs are thicker and stronger within the LLSVP than outside of it, consistent with convection model predictions. Differential travel times, amplitude ratios, and waveshapes of core waves SKKS and SKS are used to investigate CMB topography and outermost core velocity structure. 1D and 2D wavefield simulations suggest that the complicated geographic distribution of observed SKKS waveform anomalies might be a result of CMB topography and a higher velocity outermost core. These combined analyses depict a lowermost mantle that is rich in fine-scale structural complexity, which advances our understanding of its integral role in mantle circulation, mixing, and evolution.

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Agent

Created

Date Created
  • 2012

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Early solar system to deep mantle: the geochemistry of planetary systems

Description

The origin of the solar system and formation of planets such as Earth are among the most fascinating, outstanding scientific problems. From theoretical models to natural observations, it is possible

The origin of the solar system and formation of planets such as Earth are among the most fascinating, outstanding scientific problems. From theoretical models to natural observations, it is possible to infer a general way of how the solar system evolved from the gravitational collapse of the molecular cloud to accretion and differentiation of planetary-sized bodies. This dissertation attempts to place additional constraints on the source, distribution, and evolution of chemical variability in the early solar system, Mars, and Earth.

A new method was developed for the measurement of titanium isotopes in calcium-aluminum-rich inclusions (CAIs) by laser ablation multi-collector inductively coupled plasma mass spectrometry. The isotopic compositions of 17 Allende CAIs define a narrow range with clearly resolved excesses in 46Ti and 50Ti and suggests that "normal" CAIs formed from a relatively uniform reservoir. Petrologic and isotopic analysis of a new FUN (Fractionated and Unknown Nuclear effects) CAI suggests that normal and FUN CAIs condensed in similar environments, but subsequently evolved under vastly different conditions.

Volatiles may have influenced the formation and evolution of basaltic magmas on Mars. Light lithophile element (LLE) and fluorine (F) concentrations and isotopic compositions of pyroxene determined in situ in several Martian meteorites suggests that the primary magmatic signature of LLE and F zonation in Shergottite pyroxene has been disturbed by post-crystallization diffusive equilibration. Using relevant crystal-melt partition coefficients the F contents for Martian meteorite parental melts are ~910 and ~220 ppm. Estimates of the F content in the Shergottite and Nakhlite source regions are similar to that of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), respectively, here on Earth.

Noble gas systematics of OIBs relative to MORBs, suggests OIBs preferentially sample a primordial reservoir located within Earth's mantle. Geodynamic calculations were performed to investigate the time-dependent rate of material entrained into plumes from these primordial reservoirs. These models predict melts rising to the surface will contain variable proportions of primordial material. The results demonstrate that although high 3He/4He ratios may mandate a mantle plume that samples a primordial reservoir, more MORB-like 3He/4He ratios in OIBs do not preclude a deep plume source.

Contributors

Agent

Created

Date Created
  • 2014

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Impact-related processes on Mercury and the Moon

Description

Impact craters are ubiquitous throughout the Solar System, formed by one of the principal processes responsible for surface modification of terrestrial planets and solid bodies (i.e., asteroids, icy moons). The

Impact craters are ubiquitous throughout the Solar System, formed by one of the principal processes responsible for surface modification of terrestrial planets and solid bodies (i.e., asteroids, icy moons). The impact cratering process is well studied, particularly on the Moon and Mercury, where the results remain uncomplicated by atmospheric effects, plate tectonics, or interactions with water and ices. Crater measurements, used to determine relative and absolute ages for geologic units by relating the cumulative crater frequency per unit area to radiometrically-determined ages from returned samples, are sensitive to the solar incidence angle of images used for counts. Earlier work is quantitatively improved by investigating this important effect and showing that absolute model ages are most accurately determined using images with incidence angles between 65° and 80°, and equilibrium crater diameter estimates are most accurate at ~80° incidence angle. A statistical method is developed using crater size-frequencies to distinguish lunar mare age units in the absence of spectral differences. Applied to the Moon, the resulting areal crater densities confidently identify expansive units with >300–500 my age differences, distinguish non-obvious secondaries, and determine that an area >1×104 km2 provides statistically robust crater measurements. This areal crater density method is also applied to the spectrally-homogeneous volcanic northern smooth plains (NSP) on Mercury. Although crater counts and observations of embayed craters indicate that the NSP experienced at least two resurfacing episodes, no observable age units are observed using areal crater density measurements, so smooth plains emplacement occurred over a relatively short timescale (<500 my). For the first time, the distribution of impact melt on Mercury and the Moon are compared at high resolution. Mercurian craters with diameters ≥30 km have a greater areal extent of interior melt deposits than similarly sized lunar craters, a result consistent with melt-generation model predictions. The effects of shaking on compositional sorting within a granular regolith are experimentally tested, demonstrating the possibility of mechanical segregation of particles in the lunar regolith. These results provide at least one explanation toward understanding the inconsistencies between lunar remote sensing datasets and are important for future spacecraft sample return missions.

Contributors

Agent

Created

Date Created
  • 2013