Deep structure and volcanic activity of Mount Elbrus and a portion of the Elbrus–Tyrnyauz valley: Geological and geophysical data

Doklady Earth Sciences - A microseismic sounding profile was made along the Baksan River valley from the eastern summit of Elbrus volcano to the southern edge of town Tyrnyauz. The geological...     

Is the tilt of the Thames valley towards the east caused by a distal lobe of the Icelandic mantle plume?

Over the last 30 years, a growing body of research has shown that first-order control of the elevation of Earth's surface is exercised by thermal anomalies in the upper asthenosphere. One line of research is to test models and observations of mantle behaviour against the sedimentary record. A second line of research is to use the sedimentary record to further understanding of mantle behaviour. Here this second line of research is adopted: a particular hypothesis of mantle behaviour is tested against the Quaternary sedimentary record of the Thames valley, England. Schoonman et al. (2017) have proposed that a warm finger of mantle material extending from the Icelandic plume underlies southern England at the present day. That warm finger would represent the distal end of the influence of the Icelandic plume in this area, and would have advanced broadly from west to east, causing a progressive tilt of the surface of the Thames valley towards the east. The warm-finger hypothesis is supported by the evidence reviewed here. That evidence consists of two main sets of observations, both sets established beyond reasonable doubt by many researchers over many years. First, there is the progressive increase in elevation westward from the present-day coast of the North Sea of the 2.5–2 Ma shallow-marine Red and Norwich Crags. Second, there is the subsequent Quaternary record of progressive eastward tilting of the Thames valley shown in the river terraces.     

Zircon SHRIMP U–Pb age of Late Jurassic OIB-type volcanic rocks from the Tethyan Himalaya: constraints on the initial activity time of the Kerguelen mantle plume

This work presents zircon U–Pb age and whole-rock geochemical data for the volcanic rocks from the Lakang Formation in the southeastern Tethyan Himalaya and represents the initial activity of the Kerguelen mantle plume. SHRIMP U–Pb dating of zircons from the volcanic rocks yielded a ²⁰⁶Pb/²³⁸U age of 147 ± 2 Ma that reflects the time of Late Jurassic magmatism. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of TiO₂ (2.62 wt%–4.25 wt%) and P₂O₅ (0.38 wt%–0.68 wt%), highly fractionated in LREE/HREE [(La/Yb)_N = 5.35–8.31] with no obvious anomaly of Eu, and HFSE enrichment with no obvious anomaly of Nb and Ta, which are similar to those of ocean island basalts and tholeiitic basaltic andesites indicating a mantle plume origin. The Kerguelen mantle plume produced a massive amount of magmatic rocks from Early Cretaceous to the present, which widely dispersed from their original localities of emplacement due to the changing motions of the Antarctic, Australian, and Indian plates. However, our new geochronological and geochemical results indicate that the Kerguelen mantle plume started from the Late Jurassic. Furthermore, we suggest that the Kerguelen mantle plume may played a significant role in the breakup of eastern Gondwanaland according to the available geochronological, geochemical and paleomagnetic data.     

Origin of the LLSVPs at the base of the mantle is a consequence of plate tectonics——A petrological and geochemical perspective

In studying the petrogenesis of intra-plate ocean island basalts(OIB) associated with hotspots or mantle plumes, we hypothesized that the two large-low-shear-wave-velocity provinces(LLSVPs) at the base of the mantle beneath the Pacific(Jason) and Africa(Tuzo) are piles of subducted ocean crust(SOC)accumulated over Earth's history. This hypothesis was formulated using petrology, geochemistry and mineral physics in the context of plate tectonics and mantle circulation. Because the current debate on the origin of the LLSVPs is limited to the geophysical community and modelling discipline and because it is apparent that such debate cannot be resolved without considering relevant petrological and geochemical information, it is my motivation here to objectively discuss such information in a readily accessible manner with new perspectives in light of most recent discoveries. The hypothesis has the following elements:(1) subduction of the ocean crust of basaltic composition to the lower mantle is irreversible because(2) SOC is denser than the ambience of peridotitic composition under lower mantle conditions in both solid state and liquid form;(3) this understanding differs from the widespread view that OIB come from ancient SOC that returns from the lower mantle by mantle plumes, but is fully consistent with the understanding that OIB is not derived from SOC because SOC is chemically and isotopically too depleted to meet the requirement for any known OIB suite on Earth;(4) SOC is thus the best candidate for the LLSVPs, which are, in turn, the permanent graveyard of SOC;(5) the LLSVPs act as thermal insulators, making core-heating induced mantle diapirs or plumes initiated at their edges, which explains why the large igneous provinces(LIPs) are associated with the edges of the LLSVPs;(6) the antipodal positioning of Jason and Tuzo represents the optimal momentum of inertia, which explains why the LLSVPs are stable in the spinning Earth.     

El Ni?o as a consequence of the global oscillation in the dynamics of the earth’s climatic system

During study of the physical nature and potential precursor features of the El Ni?o phenomenon in the Pacific, it was found that a negative large-scale temperature anomaly on the Indian Ocean surface may be one of its significant precursors. This anomaly appears prior to the occurrence of El Ni?o and is accompanied by growth in atmospheric pressure. It gradually extends eastwards along the equator until the zone of planetary convection in the area of the Indonesian Region. The west wind that emerges on the eastern peripherals of the mentioned pressure anomaly leads to reversal of the Pacific segment of the Walker equatorial atmospheric circulation and to a subsequent change in the zonal thermal dipole polarity in the tropical zone of the Pacific (the latter means culmination of the El Ni?o phenomenon). In addition to the mentioned thermobaric anomaly in the Indian Ocean, other obvious signs of large-scale pressure anomalies have been found in the global atmospheric pressure field; these anomalies may be interpreted as manifestations of the intradecadal global oscillation in the dynamics of the modern climatic system. It is suggested that the whole known complex of events related to the El Ni?o phenomenon in the Pacific is a consequence and a regional link of the planetary structure of this global atmospheric phenomenon.     

Platinum group element (PGE) geochemistry to understand the chemical evolution of the Earth’s mantle

Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth.     

Geologic stages of the Paleogene and Neogene evolution of the Arctic shelf in the Ob’ region (West Siberia)

The paper is concerned with the structure of the Arctic shelf sediments in the Ob’ region in the Paleogene and Neogene, sampled from boreholes drilled on the Yamal Peninsula, in the lower reaches of the Pur and Taz Rivers. The specifics of Paleogene marine sedimentation in the central and northern West Siberian Plain are studied. The effect of abiotic (tectonic) factors on the completeness of the geologic record is considered as well as the effect of recent (Oligocene-Neogene) tectonic processes on topography and sedimentation. The borehole sections are compared with the main seismic sections of the Kara Sea and Lomonosov Ridge.     

New isotope–geochemical and mineralogical data on the ultramafic xenoliths in the volcanic rocks of the Kamchatka–Koryak region: Two types of mantle protolith in the modern island-arc system

The paper presents new isotope–geochemical and mineralogical data on mantle xenoliths of the “island-arc” (Avacha, Shiveluch, and Kharchinsky volcanoes) and “within-plate” (Valovayam River, Cape Navarin, and Bakening Volcano areas) types. In terms of paragenesis and mineral composition, the “islandarc” xenoliths correspond to the olivine–plagioclase depth facies, while the “within-plate” xenoliths came from spinel lherzolite to wehrlite facies, which is transitional to the olivine–plagioclase equilibrium. The majority of the “within-plate” xenoliths are enriched in high-field-strength elements (Ti, Nb, Hf, Zr, Yb). The “island-arc” xenoliths in general are depleted in REE, while the “within-plate” xenoliths are enriched in all REE. The former have low Pb isotope ratios, being in isotope equilibrium with lower crustal basites, while most of the latter group are enriched in radiogenic Pb. The island-arc xenoliths are of magmatic origin and were derived from the sublithospheric crust–mantle mixture, while the “within-plate” xenoliths reflect the composition of the asthenospheric mantle source. The primary appearance of the xenoliths is obliterated by secondary recrystallization and metasomatic reworking.     

The age and geochemistry of volcanic rocks on the eastern flank of the Umlekan–Ogodzha volcanoplutonic belt (Amur region)

⁴⁰Ar/³⁹Ar dating yielded the reliable ages of andesite from the Unerikan complex (102.1 ± 1.4 Ma) and basaltic andesite from the Burunda complex (107.3 ± 2.4 Ma). The established age of volcanism is close to one of the stages of formation of the Khingan–Okhotsk volcanoplutonic belt. The petrography and geochemistry of basic, normal-basic, and normal rocks point to their dual character: They combine features specific for tholeiitic and calc-alkalic rocks. Most likely, these rocks formed in the setting of transform continental margin.     

A study on Emei mantle plume activity and the origin of high-As coal in southwestern Guizhou Province

1IntroductionIn recent years deep-going investigations into thestructure of the deep interior of the Earth have provedthat the region of Southwest China where the Emeishanbasalts are widespread is an igneous rock provincecommonly accepted by the internati…     

Trace element modelling of the origin and evolution of the Oyaca‐Boyalik volcanic rocks (NE of Haymana,Ankara, Turkey)

Lower Miocene Boyalik volcanic rocks, situated approximately 80 km south of Ankara, exhibit both alkaline and calc‐alkaline characteristics. Alkaline products are trachybasaltic and trachyandesitic, whereas calc‐alkaline products are dacitic. The phenocrysts in the dacites consist primarily of plagioclase and hornblende, with lesser amounts of biotite. The groundmass contains plagioclase and quartz microcrysts. Trachyandesites are mainly composed of plagioclase and biotite phenocrysts with a groundmass of alkali feldspar microlites and minor clinopyroxene microcrysts. Trachybasalts are mainly composed of olivine and plagioclase phenocrysts, with minor clinopyroxene phenocrysts associated with alkali feldspar, plagioclase and clinopyroxene microlites and microcrysts in the groundmass. Oxides are common accessory phases in all products. Boyalik volcanic rocks have essentially homogeneous incompatible trace element patterns with variable Nb and Th anomalies, enrichment in Rb, Ba, K, La, Ce and Nd, and positive Sr anomalies. Some trace element ratios (e.g. Ba/Ta, Ba/Nb, Th/U and Ce/Pb) are variable among the series. For instance, dacites and trachyandesites have higher Ba/Ta (724–2509), Ba/Nb (45–173) and Th/U (3.5–8.7) and lower Ce/Pb (7.1–3.9) values than the trachybasalts. Trace element data indicate that the series are chemically distinct but probably were derived from a common lithospheric mantle source via variable degrees of partial melting. The magmas then underwent a process of evolution involving assimilation and fractional crystallization (AFC) during ascent to the surface. Although trachyandesites and dacites were generated from a lithospheric mantle source via ～1% and ～1.5% to ～5% degrees of partial melting, respectively, trachybasalts were derived from the same source via higher degrees of partial melting (～20%) with neglegible crustal contamination. Boyalik volcanism is linked to an intracontinental transpressional setting. However, the overall geochemical features are consistent with derivation from a mantle source that records earlier Eocene subduction between the Sakarya continental fragment and the K?r?ehir block during time.     

Formation of PGM-Cu-Ni deposits in the process of evolution of flood-basalt magmatism in the Noril’sk region

The timing of the emplacement of ore-bearing melts in the process of evolution of flood-basalt magmatism in the Noril’sk District is discussed. The current models of ore formation consider the emplacement of ore-bearing intrusions either under the conditions of a closed magmatic system as a product of a self-dependent magmatic event, or under the conditions of an open magmatic system, where intrusions are parts of the conduits feeding lava flows. In both cases, the composition of the initial magma, the content of volatile components therein, and the contribution of country rock assimilation are important for the development of a genetic model. The relationships between lavas and intrusions are exemplified in the South Maslov intrusion, which cuts through the rocks of the Nadezhdinsky Formation. No geological evidence for links of lavas to intrusions has been established. Substantial difference in geochemistry (Ti contents, Gd/Yb and La/Sm ratios, etc.) of the tuff and lava sequence on the northern shore of Lake Lama and the Maslov intrusions are demonstrated. It is concluded that the Noril’sk deposits were formed as products of emplacement of self-dependent portion of magma in the post-lower Nadezhdinsky time. The melt composition determined from melt inclusions in olivine corresponds to high-Mg tholeiitic basalt (up to 7–8 wt % MgO) containing up to 1 wt % H₂O and 0.3 wt % Cl and undersaturated with sulfur. The fluid regime of flood-basalt volcanism had no anomalous features—the fluid was aqueous-carbon dioxide. The melts of ore-bearing and barren intrusions had similar concentrations of volatile components. The distribution of major and trace elements in intrusive rocks of the contact zone with the lower part of the Nadezhdinsky Formation characterized by high (La/Sm)_N ratio in comparison with gabbroic rocks (2.8–2.3 and 1.3–1.6, respectively), indicates that contamination of the initial melt only took place in a narrow (1 m) contact zone or did not develop at all. New data on isotopic compositions of Sr (⁸⁷Sr/⁸⁶Sr)₂₅₁ = 0.7089 and Pb (²⁰⁶Pb/²⁰⁴Pb = 20.877–24.528 in anhydrite confirm that local assimilation did not play a substantial role in the formation of rock and ores. On the basis of chemical composition of ore-forming intrusions, their isotopic characteristics, and the composition of melt inclusions in olivine, it is suggested that the lower crustal rocks were a major source of ore-bearing magmas.     

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources,plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.     

Fe–S melt as a likely solvent of diamond under mantle conditions

The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P–T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.     

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO₂ contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO₂ contents and high Mg# (Mg# = 100 Mg/(Mg + Fe²⁺)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (⁸⁷Sr/⁸⁶Sr)_i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (⁸⁷Sr/⁸⁶Sr)_i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO₂ (77.63–82.52 wt.%) and low total Fe₂O₃, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.     

Melt impregnation phases in the mantle section of the Ślęża ophiolite (SW Poland)

The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al₂O₃ 3.00–4.00 wt.%, Cr₂O₃ 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al₂O₃ 2.20–3.20 wt.% and Cr₂O₃ 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al₂O₃ 0.00–1.00 wt.% and Cr₂O₃ 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre.     

Apatite fission track and (U-Th)/He thermochronology of the Rochovce granite (Slovakia) – implications for the thermal evolution of the Western Carpathian-Pannonian region

The thermal evolution of the only known Alpine (Cretaceous) granite in the Western Carpathians (Rochovce granite) is studied by low-temperature thermochronological methods. Our apatite fission track and apatite (U-Th)/He ages range from 17.5 ± 1.1 to 12.9 ± 0.9 Ma, and 12.9 ± 1.8 to 11.3 ± 0.8 Ma, respectively. The data thus show that the Rochovce granite records a thermal event in the Middle to early Late Miocene, which was likely related to mantle upwelling, volcanic activity, and increased heat flow. During the thermal maximum between ~17 and 8 Ma, the granite was heated to temperatures ? 60 °C. Increase of cooling rates at ~12 Ma recorded by the apatic fission track and (U-Th)/He data is primarily related to the cessation of the heating event and relaxation of the isotherms associated with the termination of the Neogene volcanic activity. This contradicts the accepted concept, which stipulates that the internal parts of the Western Carpathians were not thermally affected during the Cenozoic period. The Miocene thermal event was not restricted to the investigated part of the Western Carpathians, but had regional character and affected several basement areas in the Western Carpathians, the Pannonian basin and the margin of the Eastern Alps.     

Ca- and Mg-perovskite phases in the Earth’s mantle as a probable reservoir of Al: Computer-simulation evidence

Semi-empirical and quantum chemical studies of Al atom energy in CaSiO₃ and MgSiO₃ with the perovskite-type structure at pressures and temperatures of the Earth’s mantle are reported. The phase diagram for CaSiO₃ is reproduced and refined. Probable mechanisms of Al incorporation in the structures studied are considered. According to the results of the calculations, Al is preferably incorporated into MgSiO₃, rather than into CaSiO₃. Evaluation of the isomorphic capacity of perovskite phases in relation to Al shows that the Al content in MgSiO₃ may reach 2.4 mol % at 120 GPa and 2400 K. CaSiO₃ cannot be a source of Al atoms in the Earth’s mantle.