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The mechanisms governing a commonly observed seismic velocity drop in the cratonic lithosphere, referred to as the mid‐lithospheric discontinuity (MLD), have been widely debated. To identify the composition and seismic structure of MLDs, we have analyzed Sp receiver functions (SRF) and mantle xenocrysts for six regions across Australia. We utilize locations where seismic stations and kimberlite‐hosted mantle xenocrysts are both available, allowing for comparison between seismological and petrological constraints. Our results show negative SRF phases indicative of the MLD coincide with clinopyroxene‐depleted zones at 60–140 km depth. Clinopyroxenes with different chemical compositions across the MLD define a litho‐chemical discontinuity. Modeling and experimental data show that MLDs may be explained by modified lherzolite with 10%–20% modal pargasite. Pargasite MLDs may form when rising H2O‐bearing melts cross the amphibole dehydration curve and react with clinopyroxene in lherzolite. Because the amphibole dehydration curve is isobaric at 80–120 km, pargasite will be precipitated as horizontal channels. Plain Language Summary Seismic imaging of Earth's lithosphere has revealed a seismic velocity drop at 60–120 km depth, termed the mid‐lithosphere discontinuity (MLD). The mechanisms governing the MLD have been extensively debated. To advance the understanding of the MLD we carried out an interdisciplinary study to investigate the seismic structure and composition of MLDs identified in the cratonic lithosphere of Australia. We integrated analysis of seismic converted signals from discontinuities recorded by permanent seismometers with geochemical data for kimberlite‐hosted mantle xenocrysts collected in the vicinity. The method allows direct comparison between seismological and petrological constraints. Our results suggest that the MLD comprises anomalously low abundances of clinopyroxene and separates geochemically distinct layers within the lithospheric mantle. Experimental results and modeling suggests that the observed decrease in seismic velocity and absence of clinopyroxene may relate to the formation of pargasite channels in modified lherzolite. Key Points Sp receiver functions and mantle xenocrysts are used to study mid‐lithosphere discontinuities beneath Australian cratons Mid‐lithosphere discontinuity at 60–120 km depth corresponds with mantle xenocryst populations that are depleted in clinopyroxene Modeling and experimental data suggests the mid‐lithosphere discontinuity is caused by pargasite‐bearing lherzolite

Cratons are generally thought to be characterized by stable, long‐lived mantle roots. However, recent studies have suggested that the lithospheric mantle may be prone to removal, implying that it may be denser than the asthenosphere. To address these suggestions, we use a global data set of mantle xenoliths to estimate the density structure of the cratonic lithospheric mantle (CLM), which we then compare with the density of the asthenospheric mantle at equivalent depths. Most of the CLM is either neutrally buoyant or slightly positively buoyant relative to the asthenosphere. The exception is where pyroxenite has accumulated due to melt infiltration. For these pyroxenites to remain within the CLM over geologically significant timescales they must be relatively small, that is, ≤1–20 km in size for a CLM viscosity of 1021–1023 Pa·s. Our results suggest that the majority of cratonic lithospheric roots are long‐lived due to their neutral‐to‐positive buoyancy.

Heterogeneities present within the structure of our planet cause seismic waves to attenuate, especially when they are on the order of the seismic wavelength. Cracks, fluids, and patches of different temperature or composition are only a few examples of such inhomogeneities, all of which can produce complex wavefield fluctuations in time and amplitude and affect the signals recorded at the surface. Seismic source and velocity inversions, the discrimination and yield of a chemical or nuclear explosion, or peak ground velocity and acceleration are only a few examples of calculations directly derived from seismic data which require accurate amplitude measurements. However, while seismic amplitudes are particularly affected by scattering and absorption, many of the models used for these and other estimations are laterally homogeneous or smoothly varying, potentially biasing the results obtained from them. In this thesis, I combine both single- and multi-layer energy flux models (EFMs) with a Bayesian inference algorithm to rigorously and probabilistically characterise the small-scale heterogeneity and attenuation structure of the lithosphere beneath seismic stations and arrays. The single-layer energy flux model, or EFM, characterizes the energy losses to the ballistic arrivals by means of the intrinsic, scattering and diffusion quality factors. I then use these values to compare the strength of these different attenuation mechanisms and their effects on the recorded signals. I implemented two main versions of the multi-layer EFM. The first of these, called here the Depth Dependent Energy Flux Model (EFMD), uses the intrinsic quality factor obtained from the EFM and a new Bayesian inversion algorithm to compute synthetic coda envelopes. By comparing synthetic and data envelopes, I can then obtain the scattering parameters (correlation length (a) and RMS velocity fluctuations (ε)) in each layer of the model. The second, expanded, version of the EFMD, called the E-EFMD, does not rely on the EFM and can simultaneously invert for both the scattering and intrinsic attenuation (intrinsic quality factor at 1 Hz (Qi0) and its frequency dependence coefficient (α)) parameters in each layer of the model. Both the EFMD and E-EFMD use the Metropolis-Hastings algorithm to sample the likelihood space and obtain posterior probability distributions for each parameter and layer in the model. My thorough testing of these methods reveals the specific effect each of these parameters has on the seismic codas, with initial coda amplitudes being more affected by the scattering parameters and decay rates controlled mostly by intrinsic attenuation. Independent calculation of these parameters in multi-layer models using the EFMD or E-EFMD remains challenging due to complex and strong trade-offs between them and to solutions being extremely non-unique in most cases. This issue is accentuated by an apparent bias of the E-EFMD towards extreme values of the intrinsic quality factor at 1 Hz. Overall, my results highlight the importance and usefulness of the Bayesian inference framework in this kind of study, since it provides detailed information about the uncertainty and uniqueness of the solutions. I applied these approaches to large, high quality, datasets of teleseismic events recorded by the Pilbara (PSA), Alice Springs (ASAR), Warramunga (WRA), Eielson (ILAR), Lajitas (TXAR), Pinedale (PDAR), Yellowknife (YKA) and Boshof (BOSA) seismic arrays or stations. For PSA, ASAR and WRA, my EFM and EFMD results suggest scattering is the main driver of attenuation, with the crust beneath them presenting different heterogeneity strengths and the lithospheric mantle being mostly homogeneous. Data inversions of ILAR, PDAR, TXAR, YKA and BOSA data using the EFMD and E-EFMD point to the algorithm being unable to fit the data in many cases, likely because of the assumed power law frequency dependence for Qi not being good enough to explain the complex coda behaviours shown in their datasets but also due to the aforementioned bias of the algorithm towards extreme values of some parameters, which is also observed in PSA, ASAR and WRA E-EFMD data inversions. Relating these inversion results to the physical structure beneath the stations is, therefore, not possible. In general, my results suggest that parameter trade-offs and solution non-uniqueness in the E-EFMD are too extreme to allow for successful simultaneous recovery of all the parameters, while the combination of the EFM and EFMD can yield stable and reliable results for 1- and 2-layer models and also allow us to compare between different attenuation mechanisms.

The lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m 2 . Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr 2 O 3 garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H 2 O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO 2 melts from the asthenosphere. The Kimberley Craton has retained a thick (>220 km) thermally stable lithospheric root since the Mesoproterozoic The lithospheric mantle comprises a depleted shallow layer (<100 km) and metasomatized deep layer (>150 km) separated by a diopside‐depleted mid‐lithosphere discontinuity Diamondiferous lamproites at the craton margin are associated with eclogite and high levels of K 2 O metasomatism of the lithospheric mantle

To improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on >3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (>160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere. Key Points The South Australian Craton comprises three litho‐chemical layers within the shallow, middle and deep lithospheric mantle Xenocrysts define a heterogeneously depleted‐kimberlite melt metasomatized shallow sub‐continental lithospheric mantle (SCLM) and fertile‐melt metasomatized deep SCLM The mid‐lithosphere is a seismic and geochemical discontinuity marked by negative Vp and lower modal proportions of garnet and diopside

Comprehending the temperature distribution within Earth's lithospheric mantle is of paramount importance for understanding the dynamics of Earth's interior. Traditional mineral‐based thermobarometers effectively constrain temperature and pressure for particular compositions, but their application is limited at the global scale. Here, we trained machine‐learning (ML) algorithms on 985 published high‐temperature and high‐pressure experiments for use as thermometers and barometers to overcome the limitations of classic methods. We compared our ML models to classic thermobarometers to assess the accuracies of predicted pressures and temperatures. The comparison shows that the ML models outperform classic methods and better fit various mineral pairs. Global application of the ML models unveils mantle conditions beneath cratons. Furthermore, depths to the lithosphere‐asthenosphere boundary (LAB) calibrated based on the ML thermobarometry results are generally deeper by ∼40 km than those derived geophysically, implying the existence of melt‐bearing or hydrated mineral zones at the LAB. Plain Language Summary The temperature of the lithospheric mantle is pivotal to processes such as the production of magmas and the structural stability of cratons. Researchers have traditionally employed mineral‐based thermobarometers to lithospheric mantle temperatures, but those conventional techniques have limited applications and are susceptible to inaccuracies. Here, we explored a new approach using machine learning, a powerful tool for identifying complex patterns in high‐dimensional data space. We trained machine‐learning models using data from high‐pressure and high‐temperature experiments, then compared our machine‐learning models to traditional methods. Our results show that the machine‐learning models better predict pressure and temperature for specific mineral combinations. We also used our optimized model to predict mantle temperatures and pressures based on a global data set of xenolith analyses, revealing insights about the temperature of the lithospheric mantle and the depth to the lithosphere‐asthenosphere boundary beneath continents. This research shows that machine learning can greatly improve our understanding of Earth's deep processes, providing more accurate insights into its dynamics and evolution. Key Points Thermometers and barometers calibrated and validated for xenolithic mantle minerals by machine learning Global lithospheric mantle temperatures predicted based on xenolithic mineral compositions calculated by machine‐learning thermobarometry Thermal continental lithosphere‐asthenosphere boundarys refined by machine‐learning thermobarometry are consistently ∼40 km deeper than those determined geophysically

The formation of large igneous provinces (LIPs) has been widely believed to be linked to mantle plume activity. However, how the plume modifies the overlying lithosphere, particularly its compositional structure, remains uncertain. Here, we characterize the deep thermochemical structure beneath the Emeishan LIP (ELIP), which is a well‐known Permian plume‐related LIP in China, by taking a multi‐observable probabilistic inversion. Our results find a clear correlation between the lithospheric composition with the ELIP's concentric zones. We infer that the fertile feature of the lithospheric mantle in the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into and chemically refertilized the ambient depleted lithosphere. This plume‐modified lithospheric compositional structure is likely to be preserved after the plume event, while the present lithospheric thermal structure has been mainly influenced by the subsequent thermal‐tectonic activity. Our results improve our understanding of the physicochemical interactions between the lithosphere and ancient plume. Plain Language Summary Gaining insights into the nature of large igneous provinces (LIPs) helps understand mass extinction and climate change in the past, since the outpouring of large accumulations of igneous rocks associated with LIPs could alter ancient climates and environments. Here, we focus on a well‐known plume‐related LIP during the Permian in China, Emeishan LIP (ELIP), to construct its deep thermochemical structure based on a multi‐observable probabilistic inversion method. Our results suggest that the bulk fertile feature (not depleted by melt extraction) of the lithospheric mantle in the vicinity of the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into the ambient depleted (deficient in minerals extracted by partial melting of the rock) lithospheric mantle and chemically refertilized it by melt‐rock interaction. However, the imaged thermal structure shows a large ongoing asthenospheric upwelling and small‐scale thermal convection, implying that the present‐day lithospheric thickness has been mainly influenced by the subsequent tectonic events. Our results improve the understanding of the physicochemical interactions between the lithosphere and ancient plume and contribute to the knowledge of the nature of LIPs. Key Points Image the thermochemical structure beneath the Emeishan Large Igneous Province via novel joint inversions Reveal plume refertilization of the lithosphere beneath the Emeishan Large Igneous Province's inner zone Image complex mantle circulation patterns beneath the Emeishan Large Igneous Province region

Oceanic lithosphere constitutes the bulk of Earth's tectonic plates and also likely represents the building blocks of the continental lithosphere. The depth and nature of the oceanic lithosphere‐asthenosphere boundary are central to our understanding of the definition of the tectonic plates and lithospheric evolution. Although it is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to conductive cooling models, this relatively simple realization of the tectonic plates is not completely understood. Old (>70 Ma) ocean depths are shallower than predicted. Furthermore, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we directly map the depth and nature of a seismic discontinuity that is likely the lithosphere‐asthenosphere boundary across the Pacific plate using a new method that models variations in the shapes of stacked SS waveforms from 17 years of seismic data. The depth to the discontinuity varies from 25 to 130 km and correlates with distance from the ridge along mantle flow lines. This implies that the depth of the oceanic lithosphere‐asthenosphere boundary depends on the temperature of the underlying asthenosphere, defined by a best fitting isotherm at 930°C with a 95% confidence region of 820–1020°C, although the sharpness of the observations in some locations implies a mechanism besides temperature may also be required. Key Points We develop a method for imaging lithospheric discontinuities using SS precursors We image the lithosphere‐asthenosphere boundary across the Pacific Depth is correlated with square root of distance from the ridge along flow lines

The Mariana subduction zone, with its simple oceanic structure and water‐rich environment, provides a natural laboratory for studying mantle hydration and the Lithosphere–Asthenosphere Boundary (LAB). Here, we analyze Ocean Bottom Seismometer (OBS) deployed across both the forearc and incoming plate regions to investigate the S‐wave velocity structure beneath the central Mariana subduction zone. Using multi‐frequency receiver functions and surface‐wave dispersion data, we apply a transdimensional Bayesian joint inversion that accounts for water‐layer effects to obtain high‐resolution images of the subsurface structure. Our results confirm significant mantle hydration and reveal a distinct low‐velocity zone at the LAB. Unlike previous studies that depict the LAB as a single boundary, our results indicate a rapid velocity decrease followed by an equally sharp increase, delineating a ∼15 km thick melt‐rich zone. The existence of such a melt‐rich zone may reduce mantle viscosity and facilitate decoupling between the lithosphere and asthenosphere.

What happened to the Indian mantle lithosphere (IML) during the Indian–Eurasian collision and what role it has played on the plateau growth are fundamental questions that remain unanswered. Here, we show clear images of the IML from high-resolution P and S tomography, which suggest that the subducted IML is torn into at least four pieces with different angles and northern limits, shallower and extending further in the west and east sides while steeper in the middle. Intermediate-depth earthquakes in the lower crust and mantle are located almost exclusively in the high-velocity (and presumably strong) part of the Indian lithosphere. The tearing of the IML provides a unified mechanism for Late Miocene and Quaternary rifting, current crustal deformation, and intermediate-depth earthquakes in the southern and central Tibetan Plateau and suggests that the deformations of the crust and the mantle lithosphere are strongly coupled.