Showing papers in "Geochemistry Geophysics Geosystems in 2018"

GPlates: Building a Virtual Earth Through Deep Time

Abstract: GPlates is an open‐source, cross‐platform plate tectonic geographic information system, enabling the interactive manipulation of plate‐tectonic reconstructions and the visualization of geodata through geological time. GPlates allows the building of topological plate models representing the mosaic of evolving plate boundary networks through time, useful for computing plate velocity fields as surface boundary conditions for mantle convection models and for investigating physical and chemical exchanges of material between the surface and the deep Earth along tectonic plate boundaries. The ability of GPlates to visualize subsurface 3‐D scalar fields together with traditional geological surface data enables researchers to analyze their relationships through geological time in a common plate tectonic reference frame. To achieve this, a hierarchical cube map framework is used for rendering reconstructed surface raster data to support the rendering of subsurface 3‐D scalar fields using graphics‐hardware‐accelerated ray‐tracing techniques. GPlates enables the construction of plate deformation zones—regions combining extension, compression, and shearing that accommodate the relative motion between rigid blocks. Users can explore how strain rates, stretching/shortening factors, and crustal thickness evolve through space and time and interactively update the kinematics associated with deformation. Where data sets described by geometries (points, lines, or polygons) fall within deformation regions, the deformation can be applied to these geometries. Together, these tools allow users to build virtual Earth models that quantitatively describe continental assembly, fragmentation and dispersal and are interoperable with many other mapping and modeling tools, enabling applications in tectonics, geodynamics, basin evolution, orogenesis, deep Earth resource exploration, paleobiology, paleoceanography, and paleoclimate.

Inversion of Surface Deformation Data for Rapid Estimates of Source Parameters and Uncertainties: A Bayesian Approach

Abstract: New satellite missions (e.g., the European Space Agency's Sentinel‐1 constellation), advances in data downlinking, and rapid product generation now provide us with the ability to access space‐geodetic data within hours of their acquisition. To truly take advantage of this opportunity, we need to be able to interpret geodetic data in a prompt and robust manner. Here we present a Bayesian approach for the inversion of multiple geodetic data sets that allows a rapid characterization of posterior probability density functions (PDFs) of source model parameters. The inversion algorithm efficiently samples posterior PDFs through a Markov chain Monte Carlo method, incorporating the Metropolis‐Hastings algorithm, with automatic step size selection. We apply our approach to synthetic geodetic data simulating deformation of magmatic origin and demonstrate its ability to retrieve known source parameters. We also apply the inversion algorithm to interferometric synthetic aperture radar data measuring co‐seismic displacements for a thrust‐faulting earthquake (2015 Mw 6.4 Pishan earthquake, China) and retrieve optimal source parameters and associated uncertainties. Given its robustness and rapidity in estimating deformation source parameters and uncertainties, our Bayesian framework is capable of taking advantage of real‐time geodetic measurements. Thus, our approach can be applied to geodetic data to study magmatic, tectonic, and other geophysical processes, especially in rapid‐response operational settings (e.g., volcano observatories). Our algorithm is fully implemented in a MATLAB®‐based software package (Geodetic Bayesian Inversion Software) that we make freely available to the scientific community.

Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate-Based Standardization

Abstract: About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long-standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long-standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories.

SHTools: Tools for Working with Spherical Harmonics

Abstract: Geophysical analyses are often performed in spherical geometry and require the use of spherical harmonic functions to express observables or physical quantities. When expanded to high degree, the accuracy and speed of the spherical harmonic transforms and reconstructions are of paramount importance. SHTools is a time and user-tested open-source archive of both Fortran 95 and Python routines for performing spherical harmonic analyses. The routines support all spherical-harmonic normalization conventions used in the geosciences, including 4p-normalized, Schmidt seminormalized, orthonormalized, and unnormalized harmonics, along with the option of employing the Condon-Shortley phase factor of ð21Þ m. Data on the sphere can be sampled on a variety of grid formats, including equally spaced cylindrical grids and grids appropriate for integration by Gauss-Legendre quadrature. The spherical-harmonic transforms are proven to be fast and accurate for spherical harmonic degrees up to 2800. Several tools are provided for the geoscientist, including routines for performing localized spectral analyses and basic operations related to global gravity and magnetic fields. In the Python environment, operations are very simple to perform as a result of three class structures that encompass all operations on grids, spherical harmonic coefficients, and spatiospectral localization windows. SHTools is released under the unrestrictive BSD 3-clause license.

Seismic Structure of the Upper Mantle Beneath Eastern Asia From Full Waveform Seismic Tomography

Abstract: To better understand the subsurface behavior of subducting slabs and their relation to the tectonic evolution of the overriding plate, we conduct a full waveform inversion on a large data set to determine a high-resolution seismic model, FWEA18 (Full Waveform inversion of East Asia in 2018), of the upper mantle beneath eastern Asia. FWEA18 reveals sharper, more intense high-velocity slabs in the upper mantle under the southern Kuril, Japan, and Ryukyu arcs, than previous studies have found. The subducting Pacific plate is imaged as a roughly 100 km thick high-velocity slab to near 550 km depth indicating relatively little deformation. Stagnation near 600 km depth is observed over horizontal distances of 600 km or less. The Pacific plate we image accounts for roughly 25 Myr of subduction with older slab likely located in the lower mantle. The Philippine plate, subducting beneath the Ryukyu Islands, has a clear termination at about 450 km depth. This may indicate a tearing event in the past or that less Philippine Sea plate has subducted than previously thought. We found a double-layer high-velocity anomaly above and below 660 km under the Yellow Sea and eastern coast of North China. This may correspond to parts of the Philippine Sea plate that detached in the past and Pacific plate that have intersected at depth or a complicated behavior of the Pacific plate at that depth. Slow cylindrical anomalies cross the entire upper mantle are imaged beneath major Holocene volcanoes, which are likely upwellings associated with the edges of deep slabs that are entering the lower mantle.

SubMachine: Web‐Based Tools for Exploring Seismic Tomography and Other Models of Earth's Deep Interior

Abstract: We present SubMachine, a collection of web-based tools for the interactive visualization, analysis, and quantitative comparison of global-scale data sets of the Earth's interior. SubMachine focuses on making regional and global-scale seismic tomography models easily accessible to the wider solid Earth community, in order to facilitate collaborative exploration. We have written software tools to visualize and explore over 30 tomography models-individually, side-by-side, or through statistical and averaging tools. SubMachine also serves various nontomographic data sets that are pertinent to the interpretation of mantle structure and complement the tomographies. These include plate reconstruction models, normal mode observations, global crustal structure, shear wave splitting, as well as geoid, marine gravity, vertical gravity gradients, and global topography in adjustable degrees of spherical harmonic resolution. By providing repository infrastructure, SubMachine encourages and supports community contributions via submission of data sets or feedback on the implemented toolkits.

Ultralow Velocity Zone Locations: A Global Assessment

Abstract: We have compiled all previous ultralow velocity zone (ULVZ) studies, and digitized their coremantle boundary (CMB) sampling locations. For studies that presented sampling locations based on infinite frequency ray theory, we approximated Fresnel zones onto a 0.58 3 0.58 grid. Results for these studies were separated according to wave type: (1) core-reflected phases, which have a single location of ULVZ sampling (ScS, ScP, PcP), (2) core waves that can sample ULVZs at the core entrance and exit locations of the wave (e.g., SPdKS, PKKP, and PKP), and (3) waves which have uncertainties of ULVZ location due to long CMB sampling paths, e.g., diffracted energy sampling over a broad region (Pdiff, Sdiff). For studies that presented specific modeled ULVZ geographical shapes or PKP scatter probability maps, we digitized the regions. We present summary maps of the ULVZ coverage, as well as published locations arguing against ULVZ presence. A key finding is that there is not a simple mapping between lowermost mantle reduced tomographic velocities and observed ULVZ locations, especially given the presence of ULVZs outside of lowermost mantle large low velocity provinces (LLVPs). Significant location uncertainty exists for some of the ULVZ imaging wave types. Nonetheless, this compilation supports a compositionally distinct origin for at least some ULVZs. ULVZs are more likely to be found near LLVP boundaries, however, their relationship to overlying surface locations of hot spots are less obvious. The new digital ULVZ database is freely available for download. Plain Language Summary Nearly half way to the center of Earth, small and thin regions of extremely anomalous mantle rock sit on top of Earth’s fluid core. The speeds of seismic waves are reduced by tens of percent in these tiny zones, and for over 20 years have been interpreted as being partially molten. Here, we summarize all the past studies and show that the geographical distribution of the sluggish patches is consistent with a requirement that they be compositionally distinct from the surrounding mantle. Their composition remains unknown. Dubbed ‘‘ultra-low velocity zones’’, they remain enigmatic less than 20% of Earth’s core mantle boundary has been explored in past investigations. However, this summary suggests they have a preference of being located near the margins of two much larger anomalies, continentalsized lowermost mantle low seismic wave speed provinces.

Measuring, Processing, and Analyzing Hysteresis Data

Abstract: G. A. P. acknowledges funding from a NERC Independent Research Fellowship (NE/P017266/1) and NSFC grants 41574063 and 41621004, and CAS project XDB18010203. M. J. acknowledges support of the Institute for Rock Magnetism, funded by the NSF Instruments and Facilities program and by the University of Minnesota. The data presented here are available with the HystLab software package (https:// github.com/greigpaterson/HystLab).

Tracking Formation of a Lava Lake From Ground and Space: Masaya Volcano (Nicaragua), 2014–2017

Abstract: A vigorously degassing lava lake appeared inside the Santiago pit crater of Masaya volcano (Nicaragua) in December 2015, after years of degassing with no (or minor) incandescence. Here, we present an unprecedented-long (3 years) and continuous volcanic gas record that instrumentally characterizes the (re)activation of the lava-lake. Our results show that, before appearance of the lake, the volcanic gas plume composition became unusually CO2-rich, as testified by high CO2/SO2 ratios (mean, 12.2±6.3) and low H2O/CO2 ratios (mean, 2.3±1.3). The volcanic CO2 flux also peaked in November 2015 (mean, 81.3±40.6 kg/s; maximum, 247 kg/s). Using results of magma degassing models and budgets, we interpret this elevated CO2 degassing as sourced by degassing of a volatile-rich fast-convecting (3.6-5.2 m3·s-1) magma, supplying CO2-rich gas bubbles from minimum equivalent depths of 0.36-1.4 km. We propose this elevated gas bubbles supply destabilized the shallow (<1 km) Masaya magma reservoir, leading to upward migration of vesicular (buoyant) resident magma, and ultimately to (re)formation of the lava lake. At onset of lava lake activity on 11 December 2015 (constrained by satellite-based (MODIS) thermal observations), the gas emissions transitioned to more SO2-rich composition, and the SO2 flux increased by a factor ~40 % (11.4±5.2 kg/s) relative to background degassing (8.0 kg/s), confirming faster than normal (4.4 vs. ~3 m3·s-1) shallow magma convection. Elevated shallow magma circulation is also supported by gradual increase in irradiated thermal energy, captured by MODIS, from which we calculate that 0.4-0.8 m3·s-1 of magma have been surface-emplaced since December 2015.

Macrostrat: A Platform for Geological Data Integration and Deep-Time Earth Crust Research

Abstract: Characterizing the lithology, age, and physical-chemical properties of rocks and sediments in the Earths upper crust is necessary to fully assess energy, water, and mineral resources and to address many fundamental questions in the Earth sciences. Although a large number of geological maps, regional geological syntheses, and sample-based measurements have been produced, there is no openly available database that integrates rock record-derived data while facilitating large-scale, quantitative characterization of the volume, age, and material properties of the upper crust. Here we describe Macrostrat, a relational geospatial database and supporting cyberinfrastructure that is designed to enable quantitative spatial and geochronological analyses of the entire assemblage of surface and subsurface sedimentary, igneous and metamorphic rocks. Macrostrat now contains general, comprehensive summaries of the age and properties of 33,903 lithologically and chronologically-defined geological units distributed across 1,474 regions in North and South America, the Caribbean, New Zealand, and the deep sea. Sample-derived data, including fossil occurrences in the Paleobiology Database, more than 180,000 geochemical and outcrop-derived measurements, and more than 2.1 million bedrock geologic map units from over 170 map sources, are linked to specific Macrostrat units and/or lithologies. Macrostrat has generated numerous quantitative results and its data infrastructure is being used in several independently developed mobile applications. However, it is necessary to expand geographic coverage and to refine age models and material properties to arrive at a more precise characterization of the upper crust globally.

An Improved Algorithm for Unmixing First-Order Reversal Curve Diagrams Using Principal Component Analysis

Abstract: This work was supported financially by the Australian Research Council through grant DP160100805 and by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC grant agreement 320750.

MERRILL: Micromagnetic Earth Related Robust Interpreted Language Laboratory

Abstract: Complex magnetic domain structures and the energy barriers between them are responsible for pseudo-single-domain phenomena in rock magnetism and contribute significantly to the magnetic remanence of paleomagnetic samples. This article introduces MERRILL, an open source software package for three-dimensional micromagnetics optimized and designed for the calculation of such complex structures. MERRILL has a simple scripting user interface that requires little computational knowledge to use but provides research strength algorithms to model complex, inhomogeneous domain structures in magnetic materials. It uses a finite element/boundary element numerical method, optimally suited for calculating magnetization structures of local energy minima (LEM) in irregular grain geometries that are of interest to the rock and paleomagnetic community. MERRILL is able to simulate the magnetic characteristics of LEM states in both single grains, and small assemblies of interacting grains, including saddle-point paths between nearby LEMs. Here the numerical model is briefly described, and an overview of the scripting language and available commands is provided. The open source nature of the code encourages future development of the model by the scientific community.

An image mapping approach to U-Pb LA-ICP-MS carbonate dating and applications to direct dating of carbonate sedimentation.

Abstract: We present a new approach to laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating of carbonates based on selection and pooling of pixels from 2‐D elemental and isotopic ratio maps. This image mapping technique is particularly useful for targeting subdomains in samples with complex geological histories. Key major and trace elements that are sensitive to detrital components, postformational fluid ingress, mineralogical changes, or diagenetic overprinting are measured along with the Pb and U isotopic data. Laser sampling is undertaken along successive linear rasters that are compiled into maps using the Monocle add‐on for Iolite, with one pixel in the map corresponding to one time slice of the time‐resolved signal. These element, element ratio, and isotope ratio maps can be overlain over photomicrographs or scanning electron microscopy images to spatially link compositional data to textural and structural features. The pixels corresponding to likely homogeneous age domains can be isolated by applying appropriate selection criteria (e.g., Th < 0.3 ppm, Mg/Ca < 0.004) and pooled into pseudo‐analyses using a proxy for the parent/daughter ratio (e.g., 207Pb/235U, 238U/208Pb) to retrieve the largest possible spread of the data points on isochron diagrams. The approach is best suited for analytical setups capable of rapidly or simultaneously scanning over a large mass range and can yield a precision of ±1% or better on quadrupole instruments depending on U concentration, 238U/204Pb, and age of the sample. The sample‐specific filtering criteria for selection and rejection of data and their rationale can be reported, resulting in more transparency with regard to data processing.

No Measurable Calcium Isotopic Fractionation During Crystallization of Kilauea Iki Lava Lake

Abstract: In order to investigate possible Ca isotopic fractionation during basaltic magma differentiation, wemeasured Ca isotopic compositions of lavas recovered from Kilauea Iki lava lake at Hawaii. This set of lavas record the whole crystal fractionation history of basaltic magma, ranging from olivine accumulation/fractionation to multiple phase crystallization, including plagioclase and clinopyroxene. Our results show no detectable Ca isotopic variation in all measured Kilauea lavas at a precision of ±0.07‰ for Ca/Ca (δCa = 0.80 ± 0.08, 2 SD, n = 19). Using such observation and published intermineral Ca isotopic fractionation factors, a Monte Carlo approach is used to estimate the mineral-melt Ca/Ca fractionation factors. We found that Ca isotopic fractionation between clinopyroxene and basaltic melt is small, with Δ Ca = 0.04 ± 0.03 at 1200 °C. To the best of our knowledge, this is the first estimated mineral-melt Ca isotopic fractionation factor reported. We use this estimated ΔCa and intermineral Ca isotopic fractionation factors to investigate Ca isotopic effects during mantle partial melting under 1–2 GPa. Our simulations show that the largest Ca/Ca effect, up to +0.3‰, is achieved in large degree melting residues during fractional and dynamic melting. In contrast, partial melts show negligible Ca/Ca isotopic effect, <0.07‰.

Multidisciplinary Constraints on the Abundance of Diamond and Eclogite in the Cratonic Lithosphere

Abstract: Author(s): Garber, JM; Maurya, S; Hernandez, JA; Duncan, MS; Zeng, L; Zhang, HL; Faul, U; McCammon, C; Montagner, JP; Moresi, L; Romanowicz, BA; Rudnick, RL; Stixrude, L | Abstract: Some seismic models derived from tomographic studies indicate elevated shear-wave velocities (≥4.7nkm/s) around 120–150nkm depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35nmW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high-Vs candidates to explain the observed velocities, but matching the high shear-wave velocities requires either a large proportion of eclogite (g50nvol.%) or the presence of up to 3nvol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ≤20nvol.% eclogite and ~2nvol.% diamond may account for high shear-wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.

Geochemical Discrimination and Characteristics of Magmatic Tectonic Settings: A Machine-Learning-Based Approach

Abstract: Geochemically discriminating between magmatism in different tectonic settings remains a fundamental part of understanding the processes of magma generation within the Earth's mantle. Here, we present an approach where machine-learning (ML) methods are used for quantitative tectonic discrimination and feature selection using global geochemical datasets containing data for volcanic rocks generated in eight different tectonic settings. This study uses support vector machine, random forest, and sparse multinomial regression (SMR) approaches. All these ML methods with data for 20 elements and 5 isotopic ratios allowed the successful geochemical discrimination between igneous rocks formed in eight different tectonic settings with a discriminant ratio better than 83% for all settings barring oceanic plateaus and back-arc basins. SMR is a particularly powerful and interpretable ML method because it quantitatively identifies geochemical signatures that characterize the tectonic settings of interest and the characteristics of each sample as a probability of the membership of the sample for each setting. We also present the most representative basalt composition for each tectonic setting. The new data provide reference points for future geochemical discussions. Our results indicate that at least 17 elements and isotopic ratios are required to characterize each tectonic setting, suggesting that geochemical tectonic discrimination cannot be achieved using only a small number of elemental compositions and/or isotopic ratios. The results show that volcanic rocks formed in different tectonic settings have unique geochemical signatures, indicating that both volcanic rock geochemistry and magma generation processes are closely connected to the tectonic setting.

How Subduction Interface Roughness Influences the Occurrence of Large Interplate Earthquakes

Abstract: The role of seafloor roughness on the seismogenic behavior of subduction zones has been increasingly addressed over the past years, although their exact relationship remains unclear. Do subducting features like seamounts, fracture zones, or submarine ridges act as barriers, preventing ruptures from propagating, or do they initiate megathrust earthquakes instead? We address this question using a global approach, taking into account all oceanic subduction zones and a 117-year time window of megathrust earthquake recording. We first compile a global database, SubQuake, that provides the location of a rupture epicenter, the overall rupture area, and the region where the largest displacement occurs (the seismic asperity) for M W ≥ 7.5 subduction interplate earthquakes. With these data, we made a quantitative comparison with the seafloor roughness seaward of the trench, which is assumed to be a reasonable proxy for the subduction interface roughness. We compare the spatial occurrence of megathrust ruptures, seismic asperities, and epicenters, with two roughness parameters: the short-wavelength roughness R SW (12-20 km) and the long-wavelength roughness R LW (80-100 km). We observe that ruptures with M W ≥ 7.5 tend to occur preferentially on smooth subducting seafloor at long wavelengths, which is especially clear for the M W > 8.5 events. At both short and long wavelengths, seismic asperities show a more amplified relation with smooth seafloor than rupture segments in general. For the epicenter correlation, we see a slight difference in roughness signal, which suggests that there might be a physical relationship between rupture nucleation and subduction interface roughness. Plain Language Summary Subduction zones are regions on Earth where an oceanic plate dives below another plate. Earthquakes that occur along the contact between plates in such regions are among the largest and most destructive on Earth. To better understand where these large earthquakes are most likely to occur, we look at the effect of seafloor roughness. A rough seafloor is often characterized by many topographic features, such as seamounts or ridges, while a smooth seafloor is generally more flat. On a global scale, we compared the roughness of the incoming seafloor of the downgoing plate, with the occurrence of large earthquakes in each subduction zone. We find that the seafloor in front of large earthquakes is generally smoother than in areas where no large earthquakes have occurred. This is the clearest for very large earthquakes, with magnitudes larger than 8.5. Investigating which parameters play a role in the location of earthquakes helps us to understand where future earthquakes are more likely to occur.

Crystallographic preferred orientation of olivine in sheared partially molten rocks : the source of the “a-c switch”

Abstract: To investigate the mechanism that produces the crystallographic preferred orientations (CPO) characteristic of sheared partially molten rocks of mantle composition, we analyzed the microstructures of samples of olivine plus 7% basaltic melt deformed in torsion to shear strains as large as urn:x-wiley:15252027:media:ggge21493:ggge21493-math-0001 13.3. Electron backscattered diffraction (EBSD) observations reveal a CPO characterized by a weak a‐c girdle in the shear plane that develops by urn:x-wiley:15252027:media:ggge21493:ggge21493-math-0002 4. This CPO, which exhibits a slightly stronger alignment of [001] than [100] axes in the shear direction, changes little in both strength and distribution with increasing stress and with increasing strain. Furthermore, it is significantly weaker than the CPO observed for dry, melt‐free olivine aggregates. Orientation maps correlated with grain shape measurements from tangential, radial, and transverse sections indicate that olivine grains are longer along [001] axes than along [100] axes and shortest along [010] axes. This morphology is similar to that of olivine grains in a mafic melt. We conclude that the weak a‐c girdle observed in sheared partially molten rocks reflects contributions from two processes. Due to their shape‐preferred orientation (SPO), grains rotate to align their [001] axes parallel to the flow direction. At the same time, dislocation glide on the (010)[100] slip system rotates [100] axes into the flow direction. The presence of this CPO in partially molten regions of the upper mantle significantly impacts the interpretation of seismic anisotropy and kinematics of flow.

PostSeafloor Spreading Volcanism in the Central East South China Sea and its Formation Through an Extremely Thin Oceanic Crust

Abstract: P-wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East sub-basin of the South China Sea (SCS). Four types of crust have been defined; a) thin oceanic crust ( 5 km), b) typical oceanic crust (5-6 km), c) thick oceanic crust hosting post-spreading volcanoes ( 6 km) with significant intrusive roots, and d) thick oceanic crust with enhanced spreading features ( 6 km) but without significant roots. Within the central East sub-basin, the thin oceanic crust, only identified inside a 80-km wide zone, is located within an overall 150-km wide domain characterized by N055° seafloor spreading trends. The post-spreading volcanoes were formed during a N-S tensional episode around 6-10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and post-spreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e. at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the post-spreading volcanism might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism.

Laboratory Insights Into the Effect of Sediment-Hosted Methane Hydrate Morphology on Elastic Wave Velocity From Time-Lapse 4-D Synchrotron X-Ray Computed Tomography

Abstract: A better understanding of the effect of methane hydrate morphology and saturation on elastic wave velocity of hydrate bearing sediments is needed for improved seafloor hydrate resource and geohazard assessment. We conducted X‐ray synchrotron time‐lapse 4D imaging of methane hydrate evolution in Leighton Buzzard sand, and compared the results to analogous hydrate formation and dissociation experiments in Berea sandstone, on which we measured ultrasonic P‐ and S‐wave velocity, and electrical resistivity. The imaging experiment showed that initially hydrate envelops gas bubbles and methane escapes from these bubbles via rupture of hydrate shells, leading to smaller bubbles. This process leads to a transition from pore‐floating to pore‐bridging hydrate morphology. Finally, pore‐bridging hydrate coalesces with that from adjacent pores creating an inter‐pore hydrate framework that interlocks the sand grains. We also observed isolated pockets of gas within hydrate. We observed distinct changes in gradient of P‐ and S‐wave velocity increase with hydrate saturation. Informed by a theoretical model of idealized hydrate morphology and its influence on elastic wave velocity, we were able to link velocity changes to hydrate morphology progression from initial pore‐floating, then pore‐bridging, to an inter‐pore hydrate framework. The latter observation is the first evidence of this type of hydrate morphology, and its measurable effect on velocity. We found anomalously low S‐wave velocity compared to the effective medium model, probably caused by the presence of a water film between hydrate and mineral grains.

Updated Iberian Archeomagnetic Catalogue: New Full Vector Paleosecular Variation Curve for the Last Three Millennia

Abstract: Plain language summary In this work, we present 16 directional and 27 intensity high-quality values from Iberia Moreover, we have updated the Iberian archeomagnetic catalogue published more than 10years ago with a considerable increase in the database This has led to a notable improvement of both temporal and spatial data distribution A full vector paleosecular variation curve from 1000 BC to 1900 AD has been developed using high-quality data within a radius of 900km from Madrid A hierarchical bootstrap method has been followed for the computation of the curves The most remarkable feature of the new curves is a notable intensity maximum of about 80T around 600 BC, which has not been previously reported for the Iberian Peninsula We have also analyzed the evolution of the paleofield in Europe for the last three thousand years and conclude that the high maximum intensity values observed around 600 BC in the Iberian Peninsula could respond to the same feature as the Levantine Iron Age Anomaly, after travelling westward through Europe Knowledge of the Earth's magnetic field plays an important role on the understanding of its dynamics By measuring certain rocks or archeological objects from around the world, we can determine the field's shape and intensity in former times Knowing its evolution is essential to understand how this field is generated, how it has varied through time and how it may behave in the future In this work, we present new measurements of the magnetic field from the Iberian Peninsula that provide useful constraints on the magnetic field for archeological times that currently lack information We have updated the compilation of Iberian data for the last 3,000years and calculated a new reference curve for the magnetic field for this region We have found that the magnetic field was particularly intense in the Iberian Peninsula about 2,600 years ago By comparing this result with data from Europe and the Middle East, we observe that this high intensity has been moving from east to west through southern Europe This feature is probably related with the rapid intensity change (the geomagnetic spike) recently discovered in the Levantine region