Mid-lithospheric Discontinuity Research Articles

Thermoelasticity of tremolite amphibole: Geophysical implications

AbstractWe investigated the structure, equation of state, thermodynamics, and elastic properties of tremolite amphibole [Ca2Mg5Si8O22(OH)2] up to 10 GPa and 2000 K, using first principles simulations based on density functional perturbation theory. We found that at 300 K, the pressure-volume results can be adequately described by a third-order Birch-Murnaghan equation of state with bulk moduli K0 of 78.5 and 66.3 GPa based on local density approximation (LDA) and generalized gradient approximation (GGA), respectively. We also derived its coefficients of the elastic tensor based on LDA and GGA and found that the LDA result is in good agreement with the experimental results. At 300 K, the shear modulus G0 is 58.0 GPa based on LDA. The pressure derivative of the bulk modulus K′ is 5.9, while that of the shear modulus G′ is 1.3. The second Grüneisen parameter, or δT = [–1/(αKT)](∂KT/∂T)P, is 3.3 based on LDA. We found that at ambient conditions, tremolite is elastically anisotropic with the compressional wave velocity anisotropy AVP being 34.6% and the shear wave velocity anisotropy AVS being 27.5%. At higher pressure corresponding to the thermodynamic stability of tremolite, i.e., ~3 GPa, the AVP reduces to 29.5%, whereas AVS increases to 30.8%. To evaluate whether the presence of hydrous phases such as amphibole and phlogopite could account for the observed shear wave velocity (VS) anomaly at the mid-lithospheric discontinuity (MLD), we used the thermoelasticities of tremolite (as a proxy for other amphiboles), phlogopite, and major mantle minerals to construct synthetic velocity profiles. We noted that at depths corresponding to the mid-lithosphere, the presence of 25 vol% amphibole and 1 vol% phlogopite could account for a VS reduction of 2.3%. Thus based on our thermoelasticity results on tremolite amphibole, it seems that mantle metasomatism could partly explain the MLD.

Diamondiferous and barren eclogites and pyroxenites from the western Kaapvaal craton record subduction processes and mantle metasomatism, respectively

Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO = 7.3 to 14.5 wt%; n = 43) with accessory diamond ± corundum and kyanite, high garnet grossular content (median Ca# = 0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29 ppm) and have low median Cr2O3 (0.06 wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO = 14.0 to 20.0 wt%; n = 29), which are barren and enriched in LREE (median La 1.39 ppm), Cr2O3 (0.25 wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr > 0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3 Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton.Based on the proportions of MgO-poor and –rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135 ± 15 km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ≤20 km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenites, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.

New insights into the structural elements of the upper mantle beneath the contiguous United States fromS-to-Pconverted seismic waves

SUMMARYThe S-receiver function (SRF) technique is an effective tool to study seismic discontinuities in the upper mantle such as the mid-lithospheric discontinuity (MLD) and the lithosphere–asthenosphere boundary (LAB). This technique uses deconvolution and aligns traces along the maximum of the deconvolved SV signal. Both of these steps lead to acausal signals, which may cause interference with real signals from below the Moho. Here we go back to the origin of the SRF method and process S-to-P converted waves using S-onset times as the reference time and waveform summation without any filter like deconvolution or bandpass. We apply this ‘causal’ SRF (C-SRF) method to data of the USArray and obtain partially different results in comparison with previous studies using the traditional acausal SRF method. The new method does not confirm the existence of an MLD beneath large regions of the cratonic US. The shallow LAB in the western US is, however, confirmed with the new method. The elimination of the MLD signal below much of the cratonic US reveals lower amplitude but highly significant phases that previously had been overwhelmed by the apparent MLD signals. Along the northern part of the area with data coverage we see relics of Archean or younger northwest directed low-angle subduction below the entire Superior Craton. In the cratonic part of the US we see indications of the cratonic LAB near 200 km depth. In the Gulf Coast of the southern US, we image relics of southeast directed shallow subduction, likely of mid-Palaeozoic age.

Craton destruction links to the interaction between subduction and mid-lithospheric discontinuity: Implications for the eastern North China Craton

The continental craton is generally considered to be stable, due to its low-density and high viscosity; however, the thinning and destruction of cratonic lithosphere have been observed at various parts of the globe, for example, the eastern North China Craton (NCC). Although a large number of geological and geophysical data have been collected to study the NCC, the mechanisms and dynamic processes are still widely debated. In this study, using 2-D high-resolution thermo-mechanical models, we systematically explore the key constraints on the destruction of cratonic lithosphere. The model results indicate that the craton destruction processes can be strongly influenced by the presence of the so-called mid-lithosphere discontinuity (MLD), and its interaction with subduction. The properties of the MLD layer and the density contrast between the lithospheric mantle and asthenosphere play significant roles in the destruction processes. Specifically, the presence of a deep and low-viscosity MLD layer within the cratonic lithosphere tends to enhance instability of the craton, making it easier for lithosphere destruction. In addition, a relatively thick oceanic crust, high convergence rate, and large initial subduction angles favor the craton destruction. Finally, we compare the model results with the observations of NCC, which indicate that the interaction between the Paleo-Pacific subduction and the MLD layer in the cratonic lithosphere has played an important role in the observed large-scale lithospheric removal of the eastern North China Craton.

Sp Receiver-Function Images of African and Arabian Lithosphere: Survey of Newly Available Broadband Data

Abstract The growing quality and improving spatial coverage of broadband seismic stations on the African (Nubian + Somalian) and Arabian plates motivate us to present a catalog of S-to-P receiver functions (SRFs) from southern Africa to northern Arabia. As in North America where the ability to compare data from cratons to modern rift provinces has led to new insights about lithospheric discontinuities, so, too, in Africa and Arabia can we begin to track and study the Moho, midlithospheric discontinuities, and the lithosphere–asthenosphere boundary (LAB) between tectonothermal provinces and beneath plate boundaries. We utilized 1508 seismic stations recording 9349 teleseismic events to calculate 103,878 SRFs that we stacked in 1° circular bins. We find a robust positive arrival due to a seismic-wavespeed increase downward across the Moho in virtually all our stacked SRF traces at 15–55 km depth, and we verify this is in good agreement with previously published Ps results, thereby validating the quality of our data set. Our stacked SRF traces also show a sub-Moho negative arrival at a delay time equivalent to 50–132 km depth that should correspond to a negative velocity discontinuity at or above the LAB. Our continent-wide, plate-scale database offers the opportunity to explore for spatial and temporal evolution of lithospheric parameters.

Phase Relations of a Depleted Peridotite Fluxed by a CO2‐H2O Fluid—Implications for the Stability of Partial Melts Versus Volatile‐Bearing Mineral Phases in the Cratonic Mantle

AbstractWe present phase‐equilibria experiments of a K‐bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2‐H2O fluid (0.5 wt.% CO2 and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile‐bearing partial melts versus volatile‐bearing mineral phases in a depleted peridotite system. Experiments were performed at 850–1150 °C and 2–4 GPa using a piston‐cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900–1000 °C at 2 GPa, 850–950 °C at 3 GPa, and 950–1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900–1000 °C at 2 GPa, 950–1000 °C at 3 GPa, and 1000–1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2‐H2O content introduced via a silicic melt show that introduction of CO2‐H2O as fluid lowers the temperature of phlogopite breakdown by 150–200 °C at 2–4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile‐bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile‐bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.

Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

SummaryTo elucidate the nature and extent of the lithospheric modification in the central and western North China Craton (NCC) and adjacent regions, we used the wave equation–based migration technique of S-receiver function on teleseismic data collected from 314 broadband stations in this region to image the lithospheric structure. Incorporating data from previous lithospheric structure studies, we obtained unprecedented high-resolution depth maps of the lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuity (MLD) in the NCC. Our results show more detailed variations of the lithospheric thickness in the central and western NCC and adjacent regions, which ranges from 100 to >170 km, in marked contrast to the thinned lithosphere (60–100 km) in the eastern NCC. Despite its generally thick lithosphere (>130 km), the Ordos Block shows a concordant N–S difference from the surface to deep lithosphere with a boundary at the latitude of 37–38°N. The central NCC is laterally heterogeneous in the lithospheric structure, and the thick lithosphere (∼160 km) in the south is interpreted as a remnant cratonic mantle root. The central Qinling Orogenic Belt preserves a thick lithosphere (∼150 km), which may block the asthenospheric flow driven by extrusion of the Tibetan Plateau to the west of the NCC. Moreover, a negative MLD is widely identified at the depth of 80–110 km within the thick lithosphere, which corroborates the global existence of the MLD in continental regions. The consistence in the depth of the MLD and the shallow LAB in the eastern NCC supports the conjecture that the MLD may have played an important role in the lithospheric modification of the eastern NCC.

The origins and geodynamic implications of mid-lithospheric discontinuities

Geophysical, petrological and geochemical studies have shown that the lithospheric mantle is highly heterogeneous in its physical properties and chemical compositions. They provide critical information on the evolution of the lithosphere-asthenosphere boundary (LAB). Considerable efforts by geophysical approaches have demonstrated that LAB is characterized by profound changes in physical properties such as seismic velocities and electrical resistivity at an average depth of ~70 km in the oceanic areas. In the continental regions, however, the variations in the physical properties at LABs are not that significant and their depths vary greatly with tectonic environments (e.g., orogens vs. cratons). Mid-lithospheric discontinuities (MLDs; ~60 to 120 km depths) have been equally well documented by various techniques, including deep seismic reflections and refractions, receiver functions analyses of teleseismic Ps and Sp phases, teleseismic and ambient-noise based surface-waves tomography. Except in some regional cases, MLDs are usually featured by reduced velocities (i.e., negative velocity gradients). It appears that MLDs are statistically correlated with the low-resistivity layers, as imaged for the European continent. Many hypotheses have been proposed to explain the origin of MLDs, including the accumulation of volatile-bearing assemblages due to metasomatism of magmas/fluids and/or metamorphism; the change of deformation patterns of rocks in the lithospheric mantle and finally a “frozen” LAB. Here we review the related advances and try to reconcile available data and models. We first provide a brief introduction about the discovery of MLDs, by summarizing their velocity, radial and azimuthal anisotropies as well as their internal structures across Australian, North American and Euro-Asian continents. Then we discuss the possible origin of MLDs, by considering mantle heterogeneities in mineralogy and petrology and in particular the potential role of volatile-rich minerals such as phlogopite, amphibole and carbonates. Finally, we conclude that MLDs may be frozen LABs. It sheds light on the potential importance of MLDs in Earth’s evolution and especially the global tectonic and the changes over time. We present a working hypothesis using a dynamically-evolved LAB model. MLDs began as oceanic LABs, with their depths increasing gradually from the mid-ocean ridges to subduction zones due to change in temperature and age. During continental accretion, the “inherited” oceanic LAB merged with the continental lithosphere and became deepened along with the cooling of the new continental lithosphere. In brief, the old LAB was “frozen” into the continental lithosphere and produced MLDs. Several possible mechanisms supporting this hypothesis are proposed, and the conditions that are tenable from recent results of geodynamic modeling studies in Africa are presented. The advantages of this hypothesis are highlighted. Its potential in explanation of scientific puzzles of solid earth is evaluated.

K-rich hydrous mantle lithosphere beneath the Ontong Java Plateau: Significance for the genesis of oceanic basalts and Archean continents

Olivine-free metasomatic mantle-derived xenoliths, frequently recovered from kimberlite and lamprophyre intrusions on the continents, are seldom described from oceanic settings. We report the mineralogy, geochemistry and Sr-Nd-Hf-Pb-Os isotopic compositions of a unique K-rich, hydrous mantle nodule sourced from a 34 Ma ultramafic lamprophyre (alnöite) pipe on the island of Malaita, Ontong Java Plateau, SW Pacific. The Ontong Java Plateau, the most voluminous oceanic large igeous province, has a >100 km thick lithospheric root reminiscent of thick lithospheres that characterize cratonic settings. The phlogopite, amphibole, clinopyroxene and ilmenite-rich nature of the Malaita metasomatic nodule bears striking similarity to cratonic mica-amphibole-rutile-ilmenite-diopside (MARID) suite xenoliths, and it provides a tangible example of heavily overprinted oceanic mantle lithosphere. The nodule phlogopite 40Ar/39Ar age of 44.7 ± 1.8 Ma (95% confidence level) predates the 34 Ma alnöites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnöite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnöite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnöite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (87Sr/86Sr45Ma = 0.70419–0.70423; εNd45Ma = +3.5; εHf45Ma = +5.3; 206Pb/204Pb45Ma = 18.66–18.71; 207Pb/204Pb45Ma = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high 187Os/188Os45Ma (0.1824–0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. K-rich hydrous metasomatized mantle lithosphere could represent an enriched source component that is required for the formation of many oceanic intraplate basaltic provinces. As such, strongly metasomatized oceanic lithosphere provides an alternative to recycled crust components within the convecting mantle to explain enriched mantle signatures observed in oceanic basalts worldwide. However, constraining the volumetric abundance of strongly metasomatized oceanic lithosphere, possibly revealed by seismic mid-lithospheric discontinuities, is required before its importance can be established. The existence of K-rich metasomatic components within oceanic plateau lithosphere may also have implications for Archean continental crust formation models. The K-rich mantle component required to explain sanukitoid magma formation associated with continental crust building TTG magmatism may not be restricted to a subduction channel environment, but it can equally reside within overthickened oceanic lithosphere. Thus, oceanic plateaus away from subduction zones could have served as nucleation points for growth of continental crust during the Archean.

Craton Destruction 2: Evolution of Cratonic Lithosphere After a Rapid Keel Delamination Event

AbstractCratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190 Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200 km to <60 km. This change was associated with a peak time (approximately 120 Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two‐stage evolutionary process. First, approximately 100 km of cratonic “keel” underlying a weak midlithospheric discontinuity layer (approximately 80–100 km) was rapidly removed in <10–20 Ma. This keel delamination stage was followed by a protracted (approximately 50–100 Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on numerical exploration of the well‐recorded second stage of the eastern North China Craton's lithospheric evolution. We find that (1) lithospheric mantle capped by thick crust can be locally replaced by deeper mantle material in 100 Ma due to small‐scale convective erosion; (2) asthenospheric upwelling and related extension can replace lithospheric mantle over horizontal length scales of ~50–150 km, and account for observed “mushroom‐shaped” low‐velocity structures; (3) modeling shows conditions that could lead to the multiple eastern North China Craton magmatic pulses between 190 and 115 Ma that are associated with temporal and spatial changes in magma source petrology and a magmatic hiatus; and (4) a “wet” midlithospheric discontinuity layer provides a potential source material for on‐craton magmatism.

Craton Destruction 1: Cratonic Keel Delamination Along a Weak Midlithospheric Discontinuity Layer

AbstractCratons are generally observed to retain thick (>180 km) conductive keels for billions of years. However, some cratons have undergone keel removal, with well‐documented examples being the eastern North China Craton (NCC) and the Wyoming Craton (WC). These keelless subregions appear to have kept a lithospheric bottom at ~80–100‐km depths. This is also the depth range where modern cratons, including the remaining portions of the NCC and the WC, have seismically visible midlithospheric discontinuity layers (MLDLs). MLDLs are proposed to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1,000 ppm) peridotites, both of which would lead to a relatively weak rheology. We propose that the cratonic keels of the eastern NCC (ENCC) and the western WC (WWC) utilized this weak MLDL layer to delaminate from overlying lithosphere. We first explore this hypothesis with a lubrication‐theory‐based analytical model. This model suggests a close relationship between a cratonic keel's long‐term stability and the strength of the MLDL's edge. We further test this prediction with less idealized 2‐D numerical experiments which reveal that (a) dense lower keels beneath MLDL‐bearing cratons can persist for billions of years as long as the MLDL's edges abut relatively cold and strong lithosphere; (b) MLDL edge failure can induce rapid intramantle lower keel delamination; and (c) the predicted rates of keel delamination along a ~10‐km‐thick MLDL with a hydrous olivine or metasomatic mineral‐dominated rheology are consistent with observations for the removal speeds of the WWC and the ENCC.

Seismic Imaging of the North American Midcontinent Rift Using S-to-P Receiver Functions.

North America's ~1.1‐Ga failed Midcontinent Rift (MCR) is a striking feature of gravity and magnetic anomaly maps across the continent. However, how the rift affected the underlying lithosphere is not well understood. With data from the Superior Province Rifting Earthscope Experiment and the USArray Transportable Array, we constrain three‐dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the MCR using S‐to‐P receiver functions. We image a velocity increase with depth associated with the Moho at depths of 33–40 ± 4 km, generally deepening toward the east. The Moho amplitude decreases beneath the rift axis in Minnesota and Wisconsin, where the velocity gradient is more gradual, possibly due to crustal underplating. We see hints of a deeper velocity increase at 61 ± 4‐km depth that may be the base of underplating. Beneath the rift axis further south in Iowa, we image two distinct positive phases at 34–39 ± 4 and 62–65 ± 4 km likely related to an altered Moho and an underplated layer. We image velocity decreases with depth at depths of 90–190 ± 7 km in some locations that do not geographically correlate with the rift. These include a discontinuity at depths of 90–120 ± 7 km with a northerly dip in the south that abruptly deepens to 150–190 ± 7 km across the Spirit Lake Tectonic Zone provincial suture. The negative phases may represent a patchy, frozen‐in midlithosphere discontinuity feature that likely predates the MCR and/or be related to lithospheric thickness.

Lithospheric Architecture and Mantle Metasomatism Linked to Iron Oxide Cu‐Au Ore Formation: Multidisciplinary Evidence from the Olympic Dam Region, South Australia

AbstractThe role of lithospheric architecture and the mantle in the genesis of iron oxide copper‐gold (IOCG) deposits is controversial. Using the example of the Precambrian Gawler Craton (South Australia), which hosts the giant Olympic Dam IOCG deposit, we integrate geophysical data (seismic tomography and magnetotellurics) with geological and geochemical data to develop a new interpretation of the lithospheric setting of these deposits. Spatially, IOCG deposits are located above the margin of a mantle lithospheric zone with anomalously high electrical conductivities (resistivity <10 ohm.m, top at ~100–150 km depth), low seismic shear‐wave velocities (horizontal component, Vsh < 4.6 km/s), and unusually high ratios of compressional‐ to shear‐wave velocities (Vp/Vsh > 1.80). The high conductivity cannot be explained by water‐bearing olivine‐rich rock alone. Relatively fertile and metasomatized peridotitic mantle with additional high‐Vp/Vs phases, for example, clinohumite, hydrous garnet, and/or phlogopite, could explain the anomalous velocity and conductivity. The top of this high‐Vp/Vsh zone marks a midlithospheric discontinuity at ~100–130 km depth that is interpreted to reflect locally orthopyroxene‐rich mantle. A sub‐Moho zone with high Vp/Vsh at ~40–80 km depth correlates spatially with primitive Nd isotope signatures and arc‐related ~1,620–1,610 Ma magmatism and is interpreted as the eclogitic root of a magmatic arc. Mafic volcanics contemporaneous with ~1,590 Ma IOCG mineralization have geochemistry suggesting derivation from subduction‐modified lithospheric mantle. We suggest that Olympic Dam formed inboard of a continental margin in a postsubduction setting, related to foundering of previously refertilized and metasomatized lithospheric mantle. Deposits formed during the switch from compression to extension, following delamination‐related uplift and exhumation.

Lithospheric discontinuities in Central Australia

Lithospheric discontinuities are elusive, with properties that are strongly frequency dependent. Results from a temporary deployment of broadband stations along a north-south transect through Central Australia, and the permanent arrays ASAR and WRA, are used to evaluate the spatial coherence of lithospheric features, particularly mid-lithospheric discontinuities. We exploit stacked station autocorrelograms that provide an estimate of P-wave reflectivity beneath stations, with imaging methods exploiting teleseismic arrivals. We use both common conversion point (CCP) stacking from Ps receiver functions and reflection point imaging using the autocorrelation of the P wavetrain. The results tie well for the Moho and have a good general correspondence for deeper levels. Although indications of mid-lithospheric discontinuities from changes in the frequency of reflectivity occur at similar depths, the spatial continuity of specific features at high frequency (around 2 Hz) is of the order of 10–15 km. Broader trends can be tracked across the profile, but no strong lithospheric interfaces can be mapped, except for a south dipping feature traversing the lithosphere on the southern part of the profile that is likely to be a former mantle detachment zone.

Publisher Correction: Modification of the Western Gondwana craton by plume–lithosphere interaction

In the version of this Article originally published, a thick, white line indicating the mid-lithosphere discontinuity was missing from Fig. 5a. Additionally, in Fig. 5c, where two foundered lithosphere piles are depicted, the colours of the pile on the right were inverted. These errors have now been corrected in all online versions of the Article.

On the Role of Lower Crust and Midlithosphere Discontinuity for Cratonic Lithosphere Delamination and Recycling

AbstractDelamination is an important mechanism of continental lithosphere removal and recycling. It has been invoked as a likely cause for the observed thinning in the eastern North China (North China Craton) and several other cratons, returning thick depleted cratonic roots to the deep mantle. However, how delamination occurred, its lateral extent, and detachment depth of delamination are still debated. Here we test this process by means of 2‐D numerical modeling and find that delamination below lower crustal depths can only initiate near cratonic margins, where thickening‐induced eclogitization can significantly destabilize the lower part of the lithosphere, allowing it to detach and leading to intraplate orogeny. When a weak midlithosphere discontinuity is present, widespread delamination can initiate near cratonic margins and easily propagate to cratonic interiors, yet without obvious intraplate deformation. We illustrate how this delamination mechanism can recycle large volumes of depleted Archean subcontinental lithospheric mantle using numerical models. Our models explain the large‐scale delamination in the North China Craton, where a midlithosphere discontinuity is inferred by geophysical studies and shows that large portions of subcontinental lithospheric mantle can be recycled in the deep mantle potentially affecting mantle heterogeneity.

Seismic evidence for depth-dependent metasomatism in cratons

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100–150 km. The metasomatic component can consist of 0.5–1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100–200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Volatile-bearing partial melts beneath oceans and continents–Where, how much, and of what compositions?

Besides depth and temperature, CO2 and H2O, are the two most important variables in stabilizing partial melts in the Earth9s mantle. However, despite decades of experimental studies on the roles of these two volatile species in affecting mantle melting, ambiguity remains in terms of the stability, composition, and proportion of volatile-bearing partial melts at depths. Furthermore, the difference in the influence of H2O versus CO2 in production of mantle melts is often inadequately discussed. Here I first discuss how as a function of depth and concentration of volatiles, the peridotite + H2O versus peridotite + CO2 near-solidus melting conditions differ - discussing specifically the concepts of saturation of volatile-bearing phases and how the mode of storage of ‘water’ and carbon affects the near solidus melting relations. This analysis shows that for the Earth9s mantle beneath oceans and continents, deep, volatile-induced melting is influenced mostly by carbon, with water-bearing carbonated silicate melt being the key agent. A quantitative framework that uses the existing experimental data, allows calculation of the loci, extent of melting, and major element compositions of volatile-bearing partial melts beneath oceans and continents. How the domains of volatile-bearing melt stability are affected when possible oxygen fugacity variation at depths in the mantle is taken into account is also discussed. I show that trace amount hydrous carbonated silicate melt is likely stabilized at two or more distinct depths in the continental lithospheric mantle, at depths ranges similar to where mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have been estimated from seismology. Whereas beneath oceans, hydrous carbonated silicate melt likely remain continuously stable from the base of the thermal boundary layer to at least 200 km or deeper depending on the prevailing oxygen fugacity at depths. Hotter mantles, such as those beneath oceans, prevent sampling strongly silica-undersaturated, carbonated melts such as kimberlites as shallower basaltic melt generation dominates. Thick thermal boundary layers, such as those in cratonic regions, on the other hand allow production of kimberlitic to carbonatitic melt only. Therefore, the increasing frequency of occurrence of kimberlites starting at the Proterozoic may be causally linked to cooling and growth of sub-continental mantles through time.

Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities

The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ∼100–170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene–spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ∼90–110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590–550 Ma extended to >210 km depth, and suggests loss of ∼40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (∼90–110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.

On the Implications of A Priori Constraints in Transdimensional Bayesian Inversion for Continental Lithospheric Layering

AbstractMonte Carlo methods are powerful approaches to solve nonlinear problems and are becoming very popular in Earth sciences. One reason being that, at first glance, no constraints or explicit regularization of model parameters are required. At second glance, one might realize that regularization is done through a prior. The choice of this prior, however, is subjective, and with its choice, unintended or undesired extra information can be injected into the problem. The principal criticism of Bayesian methods is that the prior can be “tuned” in order to get the expected solution. Consequently, detractors of the Bayesian method could easily argue that the solution is influenced by the form of the prior distribution, which choice is subjective. Hence, models obtained with Monte Carlo methods are still highly debated. Here we investigate the influence of a priori constraints (i.e., fixed crustal discontinuities) on the posterior probability distributions of estimated parameters, that is, vertical polarized shear velocity VSV and radial anisotropy ξ, in a transdimensional Bayesian inversion for continental lithospheric structure. We follow upon the work of Calò et al. (2016), who jointly inverted converted phases (P to S) without deconvolution and surface wave dispersion data, to obtain 1‐D radial anisotropic shear wave velocity profiles in the North American craton. We aim at verifying whether the strong lithospheric layering found in the stable part of the craton is robust with respect to artifacts that might be caused by the methodology used. We test the hypothesis that the observed midlithospheric discontinuities result from (1) fixed crustal discontinuities in the reference model and (2) a fixed Vp/Vs ratio. The synthetic tests on two Earth models show that a fixed Vp/Vs ratio does not introduce artificial layering, even if the assumed value is slightly wrong. This is an important finding for real data inversion where the true value is not always available or accurate. However, fixing crustal discontinuities can lead to the introduction of spurious layering, and this is not recommended. Additionally, allowing the Vp/Vs ratio to vary does not help preventing that. Applying the modified approach resulting from these tests to two stations (FRB and FCC) in the North American craton, we confirm the presence of at least one midlithospheric low‐velocity layer. We also confirm the difficulty of consistently detecting the lithosphere‐asthenosphere boundary in the craton.