The Deccan nephelinite conundrum - derivation from a metasomatized sub-continental lithospheric mantle or a mantle transition zone?

Fluid-fluxed melting of mantle versus decompression melting of hydrous mantle plume as the cause of intraplate magmatism over a stagnant slab: Implications from Fukue Volcano Group, SW Japan

Upper mantle scale enrichment of Cenozoic intraplate magmatism in Northeast Asia: He-Sr-Nd-Pb-O isotope geochemistry of the basalts around the Korean peninsula

Intraplate basaltic volcanism commonly exhibits wide compositional ranges from silica-undersaturated alkaline basalts to silica-saturated tholeiitic basalts. Possible mechanisms for the compositional transition involve variable degrees of partial melting of a same source, decompression melting at different mantle depths (so-called ‘lid effect’), and melt-peridotite interaction. To discriminate between these mechanisms, here we investigated major-trace elemental and Sr–Nd–Mg–Zn isotopic compositions of a suite of intraplate alkaline and tholeiitic basalts from the Datong volcanic field in eastern China. Specifically, we employed Mg and Zn isotope systematics to assess whether the silica-undersaturated melts originated from a carbonated mantle source. The alkaline basalts have young HIMU-like Sr and Nd isotopic compositions, lower δ26Mg (-0·42‰ to -0·38‰) and higher δ66Zn (0·40‰ to 0·46‰) values relative to the mantle. These characteristics were attributable to an asthenospheric mantle source hybridized by carbonated melts derived from the stagnant Pacific slab in the mantle transition zone. From alkaline to tholeiitic basalts, δ26Mg gradually increases from -0·42‰ to -0·28‰ and δ66Zn decreases from 0·46‰ to 0·28‰ with decreasing alkalinity and incompatible trace element abundances (e.g. Rb, Nb, Th and Zr). The Mg and Zn isotopic variations are significantly beyond the magnitude (<0·1‰) induced by different degrees of fractional crystallization and partial melting of a same mantle source, excluding magmatic differentiation, different degrees of partial melting and the ‘lid effect’ as possible mechanisms accounting for the compositional variations in the Datong basalts. There are strong, near-linear correlations of δ26Mg and δ66Zn with 87Sr/86Sr (R2=0·75 − 0·81) and 143Nd/144Nd (R2=0·83 − 0·90), suggesting an additional source for the Datong basalts. This source is characterized by pristine mantle-like δ26Mg and δ66Zn values as well as EM1-like Sr–Nd isotopic ratios, pointing towards a metasomatized subcontinental lithospheric mantle (SCLM). Isotope mixing models show that mingling between alkaline basaltic melts and partial melts from the SCLM imparts all the above correlations, which means that the SCLM must have been partially melted during melt-SCLM reaction. Our results underline that interaction between carbonated silica-undersaturated basaltic melts and the SCLM acts as one of major processes leading to the compositional diversity in intracontinental basaltic volcanism.

Uvarovite, an important garnet component in the Earth’s mantle, plays a critical role in interpreting seismic signatures of the deep Earth. In this study, we employed first-principles calculations based on density functional theory (DFT) to examine the thermodynamic and elastic properties of uvarovite under high-pressure and high-temperature conditions (up to 30 GPa and 3000 K). Our results show that elastic moduli and seismic velocities increase with pressure but decrease with temperature, displaying a slightly nonlinear response. The relatively low AVP of uvarovite suggests that its contribution to the observed seismic anisotropy in the upper mantle is likely negligible. We simulated the variations in density and wave velocity of uvarovite along the geotherm, and investigated the potential geophysical effects of its enrichment in different geological bodies. At depths of 50–250 km, uvarovite exhibits higher density while maintaining shear wave velocities (VS) comparable to the reference model, implying that minor compositional variations may drive density heterogeneities within the subcontinental lithospheric mantle (SCLM) and potentially promote craton delamination. At depths of 250–660 km, uvarovite shows higher density and lower seismic velocities (VP, VS) relative to the upper mantle (UM) and mantle transition zone (MTZ). Our results demonstrate that at 660 km depth, the addition of 20 vol.% uvarovite to pyrolite increases density by +4.0%, while reducing compressional wave (VP) and shear wave (VS) velocities by −2.0% to −2.9%. If uvarovite becomes locally enriched in the mantle transition zone, it is likely to generate seismically detectable low-velocity layers, which could significantly influence Earth's mantle dynamics.

Geochemistry of ultrapotassic volcanic rocks in Xiaogulihe NE China: Implications for the role of ancient subducted sediments

The Eifel volcanic area in western Germany has been active for tens of million years. Geodetic observations, geochemical analysis and seismological studies all indicate that the source of these long-term volcanic activities is a mantle plume. However, it still remains controversial whether the Eifel plume has a deep origin. This is because the tomography images do not consistently show a continuous plume conduit between the surface and the core-mantle boundary. The Eifel plume is also not associated with any flood basalt province or a clearly age-progressive hotspot track, which is regarded as a key surface feature indicating a deep plume origin. In addition, it has been proposed that the Alpine subduction zone, south-east of the volcanic area, has created a stagnant slab in the mantle transition zone, which might be interacting with the Eifel plume. Based on the previous studies and observations, our two contrasting hypotheses are as follows: (1) The Eifel plume is not rooted in the lower mantle. In this case, subduction beneath the Alps might trigger a return asthenospheric flow and the ascent of an upper-mantle plume, leading to the formation of intraplate volcanism beneath the European plate. (2) The Eifel plume is assisted from an upwelling in the lower mantle. In this case, the subducting plate might tilt the plume conduit and influence the position where volcanism takes place. In this study, we apply the Finite-Element-Method geodynamic modeling code ASPECT to model the ascent of the Eifel plume and its interaction with the subducting slab. We design both slab advancing and slab retreating model set-ups, with and without a plume from the lower mantle beneath the subducting plate. We check whether and under what conditions the Eifel plume will be triggered behind the slab due to slab overturn. Preliminary results show that the return flow induced by subduction can help generate an upper-mantle plume and lead to the formation of volcanism. A series of models are also performed to investigate the effects of mantle viscosity and plume temperature on the plume-slab interaction.

<p>Seismic topography models reveal that both upwelling plumes and downgoing slabs are deflected or stagnate at various depths in Earth’s mantle transition zone (MTZ) and mid-mantle (MM). Deflection within the MTZ is associated with the mineral physics phase changes at 410 and 660-km depth, however the cause of deflection in the MM remains debated. There are no candidate mineral transformations to explain the varied MM reflectors that have been detected [Waszek et al., 2018], instead indicating widespread compositional heterogeneities. Furthermore, our recent thermal model [Waszek et al., 2021] reveals a link between high temperatures in the MTZ and surface activity, indicating that some plumes are able to traverse this region unimpeded. Illuminating the detailed seismic structures of the upper and mid-mantle is key to determine the link between reflectors, temperature, composition, and dynamics.</p><p>Here, we present a new large global dataset of ScS reverberations, compiled using an automatic waveform identification code based on Convolutional Neural Networks [Garcia et al., 2021]. Mantle discontinuities and reflectors generate precursors to ScSn phases, and postcursors to sScSn. Here, we present a new method to correct for 3D mantle structure in which we remove the symmetry problem suffered by most of these phases. The data are stacked to reveal the small amplitude reverberation signals, and our correction method allows us to stack for five ScSn and sScSn phases simultaneously to obtain the highest possible data coverage. For the global MTZ discontinuities, we use “adaptive stacking”. Based on Voronoi tessellation, the method automatically adjusts for topography, noise, and data coverage. Regional-scale fixed bin parameterisations of varying sizes are used to search for the intermittent MM reflectors.</p><p>We incorporate our seismic observations with mineral physics modelling, inverting for a realistic range of potential temperatures and basalt-harzburgite mixtures to obtain the best-matching thermochemical model for the MTZ. We first compare our new ScS MTZ model with its counterpart generated from SS and PP precursors [Waszek et al., 2021], to benchmark observational differences between data types. We next investigate the link or lack thereof between our MTZ model and detections of MM signals, to place improved constraints on variations in properties with depth. The final step is interpretation of our observations and modelling in the context of geodynamical simulations of mantle convection. Our outputs will contribute to greater understanding of the complex relationship between MTZ discontinuities and MM reflectors, with implications for global mantle circulation, compositional layering beneath the MTZ, and even surface activity. </p><p> </p><p> </p><p>

The subducting Pacific slab stagnates in the mantle transition zone and creates a big mantle wedge (BMW) system in East Asia. A similar BMW structure may have already existed since the Early Cretaceous (>120 Ma), but how such a structure evolved from Early Cretaceous to the present day remains unclear. We address this issue by comparing compositions and source heterogeneity of the 106–58 Ma basalts from Liaodong Peninsula and its adjacent areas (LPAA) in eastern China, with those formed in the modern BMW setting. The LPAA basalts display oceanic island basalts–like trace element patterns. Elemental and isotopic compositions of these basalts and their olivine phenocrysts point to peridotite and two recycled components in their source. One recycled component is altered lower oceanic crust given the low δ18Oolivine (2.8–5.2‰) of the ~99 Ma Liaoyuan alkali basalts. The second component consists of altered upper oceanic crust and pelagic sediments indicated by high δ18Oolivine (>6.0‰), represented by the ~58 Ma Luanshishanzi alkali basalts. The depleted mantle-like isotopes of these two components suggest derivation from a young HIMU source with characteristics of the Izanagi plate (e.g. Indian Ocean-type Sr-Nd-Pb-Hf isotopes), which may have resided in the mantle transition zone at that time. Our results reveal strong similarities between chemical and source characteristics of the mantle sampled by the 106–58 Ma LPAA basalts and those derived from the modern BMW. This implies that the BMW structure has been present since the Early Cretaceous, probably having lasted more than 120 Myr, and modulating the chemical properties of the upper mantle and influencing a variety of geological processes.

The fate of subducting carbon tracked by Mg and Zn isotopes: A review and new perspectives

Nature of 1800–1600 Ma mafic dyke swarms in the North China Craton: Implications for the rejuvenation of the sub-continental lithospheric mantle

Insight into the subducted Indian slab and origin of the Tengchong volcano in SE Tibet from receiver function analysis

The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

Helium isotopes and olivine geochemistry of basalts and mantle xenoliths in Jeju Island, South Korea: Evaluation of role of SCLM on the Cenozoic intraplate volcanism in East Asia

Rhenium–Os isotopic systematics of late Cenozoic intraplate basaltic rocks from Korea: Implications for recycled slab materials in their mantle source

We present a new model to explain one of the biggest tectonic problems of Earth Sciences today, namely, how and why did the Archean sub-continental lithospheric mantle under the Eastern Block of the North China Craton (NCC) delaminate or thin so drastically in the Cretaceous? The eastern NCC is surrounded by several sutures: to the north the south-dipping Solonker (Permo-Triassic formation age) and Mongol-Okhotsk suture (Jurassic formation age), to the south the north-dipping Dabie Shan and Song Ma sutures (Permo-Triassic formation age), and to the east the Pacific oceanic plate (200-100 Ma). Water was carried down under the eastern NCC by the hydrated Pacific plate for at least 100 Ma, and by oceanic plates subducted along the Solonker, Dabie Shan and Song Ma sutures for at least 250 Ma from the early Paleozoic to the Permo-Triassic, and by the Mongol-Okhotsk suture for at least 200 Ma until the Jurassic. Tomographic images show that the Pacific plate has ponded along the top of the mantle transition zone under the eastern NCC for over ca. 2000 km from the Japan trench. An addition of 0.2 weight percent H 2 O lowered the solidus temperature of hydrous mantle peridotite by 150 °C, which led to extensive melting in the hydrous mantle transition zone, and to the rise of hydrous plumes into the overlying crust-mantle. By the time of formation of the Permo- Triassic sutures, the hydration caused by subduction of four oceanic plates had caused major garnetization of the Archean crustal root of the eastern NCC, and post-collisional thrusting in the Jurassic led to major crust/mantle thickening and this triggered collapse of the hydro-weakened garnet-enriched crustal root. During the extensional Cretaceous period, abundant mafic, adakitic and granitic intrusions and extensive gold mineralization were emplaced and metamorphic core complexes and sedimentary and foreland basins formed in and around the eastern NCC. Part of the root was chemically transformed and replaced by upwelling fertile asthenospheric material, which fed the extrusion of extensive alkaliflood basalts in the Cenozoic.