Mineral grain size in the mantle affects fluid migration by controlling mantle permeability; the smaller the grain size, the less permeable the mantle is. Mantle shear viscosity also affects fluid migration by controlling compaction pressure; high mantle shear viscosity can act as a barrier to fluid flow. Here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2-D numerical fluid migration model. Our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged downdip by the mantle flow due to small grain size (<1 mm) and high mantle shear viscosity that develop along the base of the mantle wedge. Increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at subarc depths. Fluids introduced into the mantle wedge at postarc depths migrate upward due to relatively large grain size in the deep mantle wedge, forming secondary fluid pathways behind the arc. Fluids that reach the shallow part of the mantle wedge spread trench-ward due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55-65 km depths. These results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc.

The migration pathways of hydrous fluids in the mantle wedge are influenced by the compaction of the porous mantle matrix, which depends on the matrix permeability, fluid viscosity, and fluid density. Experimental studies show that when fluids are interconnected, the permeability depends on mineral grain size and porosity, the latter of which depends on the amount of fluids introduced into the system (fluid influx). Here, we investigate the role of fluid influx, fluid viscosity, and fluid density in controlling fluid migration in the mantle wedge, using a 2-D numerical model accounting for the effects of grain-size variation and matrix compaction. Our models predict that fluid influx and fluid viscosity are key controls on fluid pathways, while fluid density plays a secondary role. Temperature dependence of fluid viscosity promotes downdip drag of fluids at the base of the forearc mantle toward the subarc region. High fluid influx at postarc depths promotes updip flow near the base of the mantle wedge, guiding the fluids arcward. The model that is applied to northern Cascadia predicts upward fluid migration focused beneath the arc but cannot explain high electrical conductivity observed slightly west of the upward fluid migration. We estimate the amount of hydrous melt that can be produced in the mantle wedge using calculated fluid distributions. Up to a few percent partial melting is predicted in a relatively small region in the core part of the subarc mantle wedge in most subduction settings, including northern Cascadia, and beneath the backarc in old-slab subduction zones.

Earth's surface topography is a direct physical expression of our planet's dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a substantial proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth's deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth's oceanic residual topography power spectrum. This indicates water-loaded power of 0.5 +/- 0.35 km(2) and peak amplitudes of up to similar to 0.8 +/- 0.1km at long wavelengths (similar to 10(4) km), decreasing by roughly one order of magnitude at shorter wavelengths (similar to 10(3) km). We show that geodynamical simulations can be reconciled with observational constraints only if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-wavelength) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth's lithosphere.

At mid-ocean ridges, oceanic crust is emplaced in a narrow neovolcanic region on the seafloor, whereas basaltic melt that forms this oceanic crust is generated in a wide region beneath as suggested by a few geophysical surveys. The combined observations suggest that melt generated in a wide region at depths has to be transported horizontally to a small region at the surface. We present results from a suite of two-phase models applied to the mid-ocean ridges, varying half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the litho-sphere-asthenosphere boundary (LAB). Three distinct melt focusing mechanisms are recognized in these models: 1) melting pressure focusing, 2) decompaction layers and 3) ridge suction, of which the first two play dominant roles in focusing melt. All three of these mechanisms exist in the fundamental two phase flow formulation but the manifestation depends largely on the choice of rheological model. The models also show that regardless of spreading rates, the amount of melt and melt transport patterns are sensitive to changes in intrinsic permeability, K-0. In these models, the LAB is delineated by the melt-rich decompaction layers, which are essentially defined by the temperature dependent rheological and freezing boundaries. Geophysical observations place the LAB at a steeper incline as compared to the gentler profile suggested by most of our models. The models suggest that one way to reconcile this discrepancy is to have stronger melting pressure focusing mechanism as it is the only mechanism in these models that can focus melt before reaching the typical model thermal LAB. The apparent lack of observable decompaction layers in the geophysical observations hints at the possibility that melting pressure focusing could be significant. These models help improve our understanding of melt focusing beneath mid-ocean ridges and could provide new constraints for mantle rheology and permeability.

Computational models of mantle convection must accurately represent curved boundaries and the associated boundary conditions of a 3-D spherical shell, bounded by Earth's surface and the core-mantle boundary. This is also true for comparable models in a simplified 2-D cylindrical geometry. It is of fundamental importance that the codes underlying these models are carefully verified prior to their application in a geodynamical context, for which comparisons against analytical solutions are an indispensable tool. However, analytical solutions for the Stokes equations in these geometries, based upon simple source terms that adhere to physically realistic boundary conditions, are often complex and difficult to derive. In this paper, we present the analytical solutions for a smooth polynomial source and a delta-function forcing, in combination with free-slip and zero-slip boundary conditions, for both 2-D cylindrical- and 3D spherical-shell domains. We study the convergence of the Taylor-Hood (P2-P1) discretisation with respect to these solutions, within the finite element computational modelling framework Fluidity, and discuss an issue of suboptimal convergence in the presence of discontinuities. To facilitate the verification of numerical codes across the wider community, we provide a Python package, Assess, that evaluates the analytical solutions at arbitrary points of the domain.

We present the first results of a comprehensive investigation aimed at testing the hypothesis of chondrule-matrix complementarity and the four-component model for the compositions of the carbonaceous chondrites and their components. Combining point-counting with electron microprobe analyses, we have determined the bulk compositions of thin sections, as well as the average abundances and compositions of the major chondritic components (chondrules, matrix, refractory inclusions, isolated silicate grains and isolated opaque grains). To minimize the potential for element exchange between components during parent body processing, the two most primitive COs, DOM 08006 and ALH 77307, and the primitive ungrouped CO/CM-like Acfer 094 were selected for this study. To verify our method, we also examined one section of the well-studied CO3.2 Kainsaz, a fall that is free of weathering. We were able to reproduce all major and many minor elemental concentrations reported in the literature for average bulk COs and Kainsaz to better than 10%. The elements most commonly cited as displaying evidence for complementarity are Mg, Si, Al, Ca, Fe and Ti. Iron, however, can be easily affected by chondrule metal-silicate fractionation, redistribution in the parent body and weathering, and our Ti data for matrix are likely compromised by an analytical artifact. Hence, we focused on Mg, Al, Si and Ca - four elements that we can determine very accurately - and show that their relative abundances in chondrules are on average CI-like within the uncertainties of the method. The matrix is not CI-like, but its composition can be explained by the loss of 10-15 wt.% of forsterite from an initially CI-like material prior to or during parent body accretion. These results are inconsistent with chondrule-matrix complementarity. Our average CO chondrule compositions, as well as chondrule and matrix abundances, are in line with the predictions of the four-component model. However, the four-component model assumes a CI-like composition for matrix, and also predicts refractory inclusion abundances that are higher and compositions that are less refractory than we observe. While similar studies of the other carbonaceous chondrite groups are needed, these differences may indicate the limitations of the simplifying assumptions made in the four-component model. (C) 2021 Elsevier Ltd. All rights reserved.

Chondrites are meteorites from undifferentiated parent bodies that provide fundamental information about early Solar System evolution and planet formation. The element Cr is highly suitable for deciphering both the timing of formation and the origin of planetary building blocks because it records both radiogenic contributions from Mn-53-Cr-53 decay and variable nucleosynthetic contributions from the stable Cr-54 nuclide. Here, we report high-precision measurements of the massindependent Cr isotope compositions (epsilon Cr-53 and epsilon Cr-54) of chondrites (including all carbonaceous chondrites groups) and terrestrial samples using for the first time a multi-collection inductively-coupled-plasma mass-spectrometer to better understand the formation histories and genetic relationships between chondrite parent bodies. With our comprehensive dataset, the order of decreasing epsilon Cr-54 (per ten thousand deviation of the Cr-54/Cr-52 ratio relative to a terrestrial standard) values amongst the carbonaceous chondrites is updated to CI = CH >= CB >= CR >= CM approximate to CV >= CO >= CK > EC > OC. Chondrites from CO, CV, CR, CM and CB groups show intra-group epsilon Cr-54 heterogeneities that may result from sample heterogeneity and/or heterogeneous accretion of their parent bodies. Resolvable epsilon Cr-54 (with 2SE uncertainty) differences between CV and CK chondrites rule out an origin from a common parent body or reservoir as has previously been suggested. The CM and CO chondrites share common epsilon Cr-54 characteristics, which suggests their parent bodies may have accreted their components in similar proportions. The CB and CH chondrites have low-Mn/Cr ratios and similar epsilon Cr-53 values to the CI chondrites, invalidating them as anchors for a bulk Mn-53-Cr-53 isochron for carbonaceous chondrites. Bulk Earth has a epsilon Cr-53 value that is lower than the average of chondrites, including enstatite chondrites. This depletion may constrain the timing of volatile loss from the Earth or its precursors to be within the first million years of Solar System formation and is incompatible with Earth's accretion via any of the known chondrite groups as main contributors, including enstatite chondrites. (C) 2021 Elsevier Ltd. All rights reserved.

Like most primitive carbonaceous chondrites, the CM chondrites experienced varying degrees of asteroidal aqueous alteration, which may have overprinted pre-accretionary processing. Several aqueous alteration scales for CM chondrites (and other carbonaceous chondrites) have been proposed based on alteration-dependent changes in various petrological and geochemical characteristics. Given the possibility that the intensity of aqueous alteration could be recorded in the primordial noble gas compositions, we test potential correlations between petrologic, geochemical and noble gas characteristics in a detailed study on 39 CM chondrites, including some of the most pristine CM chondrites identified to date, and 4 CM related carbonaceous chondrites. We mainly compare our noble gas data with the alteration schemes proposed by Alexander et al. (2013) and Howard et al. (2015). In addition to the noble gas analyses, we determined the phyllosilicate fractions of 17 of the CM chondrites using X-ray diffraction (XRD) to complement missing data points in the Howard alteration scheme. The influence of post-hydration thermal modification on noble gases in CM chondrites is investigated by comparison of heated and unheated samples. Cosmic-ray exposure (CRE) ages are determined for all samples in this study as well as for 26 more samples based on CM chondrite literature noble gas data.