Present-day Volcanism Research Articles

The remarkable parallels between the North East Atlantic and Arctic regions

Geological understanding of the NE Atlantic and Arctic regions has increased greatly over the last two decades, revealing remarkable similarities. Continental extension in both regions onset during pausing or cessation of adjacent orogenies – the Verkhoyansk-Chukotka and Alpine orogenies – which relaxed compressional stresses and permitted extension. Severe extension accompanied by high-volume magmatism did not onset abruptly but was the culmination of long-term, regional tectonic and magmatic unrest. It was complex and piecemeal, with extension and volcanism occurring along multiple axes simultaneously, a process still ongoing in Iceland today. Large Igneous Province emplacement did not drive breakup but occurred as a consequence of continental extension. The High Arctic Large Igneous Province did not progress to breakup. As a result, much of the Arctic Ocean is floored by hyper-extended continental crust overlain by seaward-dipping reflectors. This crust resembles the passive margins of the NE Atlantic. The Alpha-Mendeleev Rise in the Arctic Ocean is a double-sided passive margin underlain by hyper-extended continental crust draped with seaward-dipping reflectors. A similar structure has been proposed for the Greenland-Iceland-Faroe Ridge. If correct, present-day volcanism in Iceland is building SDRs today. Up to ∼50% of the NE Atlantic Ocean oceanward of the continental shelves is likely underlain by at least some continental crust. For the Arctic Ocean this figure is ∼80%. Similar structures are found elsewhere, including the Madagascar Channel, the NW Indian Ocean, and the South Atlantic. These new ideas suggest promising directions of future study including assessing the true extent of continental crust in the oceans globally.

Minerals with Mixed Anion Radicals in Fumarole-Transformed Crustal Micro Xenoliths as a New Phenomenon of Present-Day Volcanism

Minerals with Mixed Anion Radicals in Fumarole-Transformed Crustal Micro Xenoliths as a New Phenomenon of Present-Day Volcanism

МОДЕЛЬ ГЛУБИННОГО СТРОЕНИЯ ЮЖНОЙ КАМЧАТКИ ПО РЕЗУЛЬТАТАМ ПЛОТНОСТНОГО 3D-МОДЕЛИРОВАНИЯ И КОМПЛЕКСУ ГЕОЛОГО-ГЕОФИЗИЧЕСКИХ ДАННЫХ

In the south of Kamchatka, a number of deep geophysical studies have been conducted along the profile lines. The aim of the research was to study the lithosphere in the zone of present-day volcanism and active seismicity. Geological and geophysical models of the Earth's crust and upper mantle were constructed along the profiles. The results were obtained as part of two-dimensional modeling of geophysical fields. But the analysis of materials shows that the territory is characterized by a complex geological structure, which is reflected in three-dimensional distribution of gravitating masses. For the first time, the article presents the results of volumetric density modeling covering the territory of southern Kamchatka, including areas covered by the Sea of Okhotsk and the Pacific Ocean. The model is based on the technology of three-dimensional imaging of 2D modeling results obtained along the grid of intersecting profiles. The 3D modeling generated isodensity surfaces that enclose regions with layers of high density (≥ 3.33 g/cm3). Thus, the surface identified beneath the ocean is interpreted as a fragment of the top of the subducting plate and the surface under the peninsula is identified as the top of the paleosubduction zone. A subhorizontal high-gradient zone (3.0–3.3 g/cm3) is recognized in the density structures that intersect the 3D model, which is identified with the Moho boundary. A model of subduction interaction between oceanic and continental lithospheric plates is proposed. The two-dimensional model shows the formation of a transitional layer between the Moho boundary of the overhanging lithospheric plate and the top of the paleosubduction zone. In the transition layer, a low-density zone is distinguished, where individual areas of maximum low-density are associated with melting chambers. Conditions are shown for the formation of the crust block with abundant basic-ultrabasic intrusions and the diorite-granodiorite intrusive massif. All ore occurrences and gold deposits of the Karymshinsky ore cluster are located within the contours of projection onto the ground surface of the deep high-gradient zone that encloses the low-density zone. Ore occurrences are genetically related to the zones of crustal weakness where epithermal deposits are formed in closed hydrothermal systems. Based on the analogy, it is possible to prognosticate gold occurrences in other areas of the projection of the high-gradient zone.

Earth-Like: an education & outreach tool for exploring the diversity of planets like our own

AbstractEarth-Like is an interactive website and twitter bot that allows users to explore changes in the average global surface temperature of an Earth-like planet due to variations in the surface oceans and emerged land coverage, rate of volcanism (degassing) and the level of the received solar radiation. The temperature is calculated using a simple carbon–silicate cycle model to change the level of CO2 in the atmosphere based on the chosen parameters. The model can achieve a temperature range exceeding −100°C to 100°C by varying all three parameters, including freeze-thaw cycles for a planet with our present-day volcanism rate and emerged land fraction situated at the outer edge of the habitable zone. To increase engagement, the planet is visualized by using a neural network to render an animated globe, based on the calculated average surface temperature and chosen values for land fraction and volcanism. The website and bot can be found at earthlike.world and on twitter as @earthlikeworld. Initial feedback via a user survey suggested that Earth-Like is effective at demonstrating that minor changes in planetary properties can strongly impact the surface environment. The goal of the project is to increase understanding of the challenges we face in finding another habitable planet due to the likely diversity of conditions on rocky worlds within our Galaxy.

Present-day volcanism on Venus as evidenced from weathering rates of olivine.

At least some of Venus' lava flows are thought to be <2.5 million years old based on visible to near-infrared (VNIR) emissivity measured by the Venus Express spacecraft. However, the exact ages of these flows are poorly constrained because the rate at which olivine alters at Venus surface conditions, and how that alteration affects VNIR spectra, remains unknown. We obtained VNIR reflectance spectra of natural olivine that was altered and oxidized in the laboratory. We show that olivine becomes coated, within days, with alteration products, primarily hematite (Fe2O3). With increasing alteration, the VNIR 1000-nm absorption, characteristic of olivine, also weakens within days. Our results indicate that lava flows lacking VNIR features due to hematite are no more than several years old. Therefore, Venus is volcanically active now.

Mid-ocean ridge eruption rates, long-term climate change, and the importance of marine seismoacoustics

Recent work on mid-ocean ridges suggests that eruption rates may not be as steady-state as once thought. There is evidence that seafloor eruptive activity waxes and wanes with long-term climate cycles potentially both being driven by climatic forcings and possibly serving itself as a climatic forcing. An important piece of evidence in understanding mid-ocean ridge volcanic cycles is to look at present-day eruption rates and compare those with expected eruption rates given well-constrained plate spreading rates. However, given that mid-ocean ridge eruptions appear to be relatively infrequent, seismoacoustic coverage of as much of the ocean as possible is key. Work at Axial Seamount and the East Pacific Rise has documented an impulsive signal that is associated with lava erupting onto the seafloor. Thus with adequate marine seismoacoustic coverage, seafloor eruptions can be identified with some confidence without in situ confirmation. Such monitoring would provide for better quantification of present-day eruption rates in the marine environment and help understand the contribution of mid-ocean ridges to present-day volcanism, as well as possible long-term variability.

Мировые тенденции в развитии геотермальной энергетики. Часть 1. Геотермальные ресурсы по регионам мира

The article provides an overview of materials on the use of geothermal resources over regions of the world. The dependence of efficiency of using geothermal resources on the geophysical and geographical parameters of each region is shown. We present data on the countries where geothermal energy is developing most successfully and for what purposes it is used. The leaders are here the United States, the Philippines, Mexico, Indonesia, and New Zealand. All these countries are located in the areas of present-day volcanism, within the “Fiery Ring”, where the heat carrier has high parameters and is available on the Earth's surface, where the cost of building a geothermal power plant is minimal, and the prime cost of energy is competitive in the energy market. It is emphasized that geothermal energy is used very little on the global scale: the share of electricity generated from geothermal resources is only 0.5%. It is produced only in 24 countries. The direct use of geothermal heat is much more wide-spread: for space heating, including centralized heat supply, for and pools, greenhouses, air conditioning, for heating open ground, and pavement, in industrial processes, for melting snow and other purposes. For example, in the capital of China, Beijing, about three hundred wells were drilled to a depth of about 3 kilometers. Water with a temperature up to 70 °C flows from them and is used for central heating of high-rise buildings and for heating greenhouses. In Iceland, owing to geothermal heat, 90% of the country's heating capacity needs are met. Almost half of the volumes of geothermal energy are spent for heating and hot water. In the Paris basin, there are 37 circulation systems, including 50 installations for centralized heat supply. Heat is also supplied to greenhouses and fish farms. In 2007, a new draft of the circulation system for heating the Orly airport in Paris was implemented. In the vicinity of Paris, 170 thousand houses are heated by 33 geothermal plants.

Deep Structure of the Zone of Tolbachik Fissure Eruptions (Kamchatka, Klyuchevskoy Volcano Group): Evidence from a Complex of Geological and Geophysical Data

With the use of the method of low-frequency microseismic sounding, the configuration of the magmatic feeding system of the Tolbachinsky Dol—a regional zone of areal basaltic volcanism in the southern part of the Klyuchevskoy volcano group in Kamchatka—is studied. The initial data are obtained by a stepby-step recording of the background microseismic noise in 2010–2015 within a thoroughly marked-out survey area covering the zones of fissure eruptions in 1975–1976 and 2012–2013 and, partly, the edifice of the Ploskii (flat) Tolbachik volcano. The depth sections reflecting the distributions of the relative velocities of seismic waves in the Earth’s crust are constructed. For a more reliable interpretation of the revealed deep anomalies, the results of independent geological and geophysical studies are used. The ascertained low-velocity structures are closely correlated to the manifestations of present-day volcanism. It is shown that the feeding structure of the Tolbachinsky Dol is spatially heterogeneous, incorporating subvertical and lateral pipeshaped magma conduits, closely spaced magma feeding channels, and shallow magma reservoirs. A longlived local transcrustal magma conducting zone is revealed, and regularities in the deep structure of the feeding systems of fissure eruptions are identified. The configuration of the established subvertical magma conduits permits basalts moving to rise to the surface by different paths, which, inter alia, explains the contrasting magma compositions observed during a single eruption. Thus, based on the instrumental data, it is shown that the magmatic feeding structure of the Tolbachinsky Dol has a number of specific peculiarities and is significantly more complicated than has been previously thought about the areal volcanic fields.

Crustal recycling, mantle dehydration, and the thermal evolution of Mars

We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650 K, preferably a primordial crustal thickness of 30 km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500 Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core–mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100 ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14 m thick global surface layer, suggesting that volcanic outgassing of H 2O could have significantly influenced the early martian climate and increased the planet’s habitability.

Post-eruptive process and products of volcanic rock alteration (transformation and synthesis of secondary products)

The conditions of transformation of the primary material of extrusive rocks and the synthesis of secondary products during post-eruptive processes have been considered using the example of three large areas of presentday volcanism and hydrothermal activity in Kamchatka, viz., the Great Tolbachik Fissure Eruption of 1975–1976 (GTFE), the Karymskii Volcanic Center (eruption of 1996 in Lake Karymskii), and the Uzon volcanic-hydrothermal system. Post-eruptive transformation of rocks causes volcanic-hydrothermal lithogenesis and low-middle-temperature mineralization in areas of present-day contrast (basalt-andesite) volcanism in zones of crustal extension. Based on the permanent presence of a wide range of acid rocks, such as dacites, rhyolites, and pumices in the areas of Holocene basalt volcanism of Kamchatka, we concluded that in conditions of pre-rift crustal extension, acid volcanism is preceded by deep-seated differentiation (similar to liquation) of primary mantle melts with separation of fluids enriched in ore-forming elements, first of all Cu, Zn, Pb, As, Sb, and Hg. During volcanogenic-hydrothermal lithogenesis in the discharge areas of those fluid systems, the destruction and replacement of primary minerals of igneous rocks and formation of complex-composition metasomatites take place. Clay minerals are major components of metasomatites in subaqueous conditions. The role of microbiota in processes of volcanogenic-hydrothermal lithogenesis and occurrences of local mineralization was studied.

Characterization of the seismogenic process in the Aleutian island arc: III. Earthquakes at the western and eastern margins of the arc

Parameters of the focal mechanisms of earthquakes, as well as their relations to the characteristics of seismicity and geological structure are analyzed in the regions of the Komandorskie Islands in the west of the Aleutian arc, the Fox Islands, and the Alaska Peninsula coast in the east of the arc. Different types of ruptures are revealed in the western and eastern parts of the Aleutian arc. The leading type of ruptures at the southern slope of the Komandorskie Islands is steep reverse faults crossing the arc at azimuths from submeridional to northeastern. A similar type of rupture occurs in abundance on the Rat Islands and is predominant on the Near Islands. Steep strike-slips with small components of the normal or reverse fault manifest themselves at the northern side of the block uplift of the Komandorskie Islands. Seismogenic ruptures in the region of the Komandorskie Islands do not contradict geological data on the rupture tectonics on Medny and Bering islands. At the southern slope of the Fox Islands, as well as in the Andreanof Islands, steep reverse faults striking longitudinally (along the arc) with the dip toward the deep-sea trench are the predominant type of seismogenic ruptures. This type of seismogenic ruptures is the leading type for the structures of island arcs with present-day volcanism; an example is the Kurile-Kamchatka island. Different types of predominant seismogenic ruptures in the western and eastern parts of the Aleutian island arc probably reflect different stages of the tectonic development of these regions of the arc. Possible positions and sizes of sources of the largest historical earthquakes in the eastern part of the Aleutian island arc are considered

Magnetotelluric sounding of Kamchatka

The MTS data acquired in Kamchatka during the last 30 years have been analyzed and summarized. Our interpretation is based on curves oriented along and across Kamchatka. Longitudinal and transverse curves can be affected by local geoelectric inhomogeneities. These were suppressed by conformal averaging. A bimodal interpretation of average longitudinal and transverse curves yielded a deep geoelectric model, which can be adopted as a starting point to be subsequently refined by 3D numerical modeling. The model involves a crustal conductive layer extending along central Kamchatka. In the east of the peninsula this layer is connected with crustal transverse conductive zones as wide as 50 km. Those zones have extensions toward the Pacific Ocean. Major centers of present-day volcanism occur in the transverse zones. The upper mantle contains an asthenospheric conductive layer forming an uplift beneath the present-day volcanic belt of Kamchatka.

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO3 and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO3 and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined.Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain &gt;0.1 wt% S as SO3. Such high SO3 contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (&gt;2 wt%).The SO3 contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally &lt;0.1 wt%. On the other hand, the SO3 contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO3 contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high.In terms of the Cl and SO3 contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO3 contents of apatite is a feature of early stage magmatic differentiation. The SO3 contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.

Beyond Subduction and Plumes: A Unified Tectonic-Petrogenetic Model for the Mexican Volcanic Belt

The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to present-day volcanism in southern Mexico. Its origin has been controversial, although the majority opinion views it as a volcanic arc related to the subduction of the Cocos plate under the North American plate. Both calc-alkaline and alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objections to these explanations, and conclude on the basis of geological, geochemical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active extension. Calc-alkaline or alkaline geochemistry of magmas is not useful for inferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contamination, and the OIB-like geochemistry suggests an enriched mantle source. Our proposal of a heterogeneous mantle beneath the MVB comprising “normal” mantle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt.

Structure and morphology of submarine volcanism in the hotspot region around Réunion Island, western Indian Ocean

The form of the Deccan-Maldives-Mascarene-Réunion hotspot trace suggests that it has, at least in part, been strongly controlled by crustal structures, especially fracture zones. This makes it difficult to assess the present-day or past location of the hotspot, and thus complicates the interpretation of African plate motion reconstruction. We present here results of a cruise to the Réunion area of which the aims were: (a) to determine the extent of present-day volcanism associated with the Réunion hotspot in the region; and (b) to examine the rôle of pre-existing oceanic crustal structures in controlling the location of present-day volcanism. Additionally, we examined the morphology and geology of the important extinct spreading centre southwest of Réunion abandoned when spreading jumped to separate Seychelles from India during the Deccan flood basalt episode some 60–65 Ma ago. The extensive bathymetrie, seismic and geological investigation shows that significant present-day hotspot volcanism is confined to the Piton de la Fournaise edifice on Réunion Island itself. Apparently, the location of recent Réunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits. For plate motion reconstruction and plume flux calculation purposes, Piton de la Fournaise must be taken as the present location of the Réunion hotspot. Accretion at the extinct spreading centre progressively ceased at the time of anomaly A27 (63 Ma), and was associated with marked propagation of the rift tips.

Crustal assimilation versus subducted sediment input in west Sunda arc volcanics: An evaluation

New geochemical and Sr, Nd, and Pb isotopic analyses of Quaternary to Cretaceous sediments from the northeastern Indian Ocean are used to estimate the composition of the sedimentary material subducted along the Sunda Trench, and to evaluate the effects of crustal contamination versus subducted sediment input in the Quaternary volcanics of the west Sunda arc. Two sediment endmember components are identified: siliceousclastic (SS) and calcareous-organogenic (CS); the latter can be regarded as SS strongly diluted by organogenic material, mainly CaCO3. Siliceous-clastic sediments are characterised by a component isotopically and geochemically similar to “typical” average upper crust. Consequently, LILE and LREE concentrations, LILE/LILE and LILE/LREE values, and Sr, Nd, and Pb isotopic ratios in West Sunda arc volcanics cannot easily distinguish between assimilation of crustal material by uprising magmas and contamination of their mantle source by bulk sediments. Post-Miocene siliceousclastic sediments sampled in the vicinity of the Sunda arc are largely derived from the arc itself, and therefore should not be used to evaluate the extent of sediment contamination of Indonesian arc volcanics. In addition, geophysical evidence suggests that post-Miocene sediments are largely accreted rather than subducted, and existing10Be isotopic data imply that post-Miocene sediments are not recycled by present-day volcanism. As the least contaminated arc volcanics occur in the eastern section of the west Sunda arc, where the highest sediment fluxes have been calculated, subduction of pre-Miocene northeastern Indian Ocean sediments or incorporation of fluids released from them into the sources of west Sunda are volcanics seem unable to reproduce the range and spatial distribution of Sr, Nd, and Pb isotopic values in the arc volcanics. By contrast, assimilation of crustal material by uprising melts derived from Indian Ocean-type mantle wedge, up to a maximum of approximately 10% for the most contaminated arc volcanics, seems better able to account for the Sr, Nd, and Pb isotope systematics of the arc volcanics, and is consistent with the variations in crustal thickness and composition along the arc, and with the spatial distribution of Sr, Nd, and Ph isotope values in mafic arc volcanics. These conclusions are also supported by the low10Be coupled with high B/Be values, and by the positive B/Be-SiO2 and B/Be-87Sr/86Sr correlations in calc-alkaline Sunda arc volcanics.

Sedimentary Fills of Izu-Bonin Fore-arc and Back-arc Rift Basins South of Japan, ODP Leg 126: ABSTRACT

From April to June 1989, Leg 126 of the Ocean Drilling Program successfully drilled the Izu-Bonin intraoceanic arc: Sites 787, 792, and 793 in the eastern, western, and central portions of a 40-70-km-wide fore-arc basin; Sites 790 and 791 on the 2-km-deep floor of the Sumizu back-arc rift; and Site 788 on the eastern rift footwall. Basaltic andesite and andesite basement of the fore-arc basin, initially 4-5 km deep, was produced by rifting or spreading that started 31-34 Ma (middle Oligocene) and has since been uplifted 1-2 km. Volcanism and erosion of surrounding highs provided debris flows and turbidites that began to fill the basin 250-300 m/m.y. Sharply declining volcanism and epiclastic supply are recorded in slowly accumulated Oligocene-Miocene (24-13 Ma) hemipelagic sediments. Regional explosive volcanism, renewed after 13 Ma, has left more than 200 thin ash layers in the uppermost (late Pliocene-Holocene) sediments. Total basin fill is 1.5-4 km thick. The Sumisu began to form 3.56-1.1 Ma. Prerift and present-day volcanism has been dominated by rhyolitic pumice eruptions. The eastern rift footwall, now 1.1 km below sea level, has been uplifted 0.2-1.7 km. Basaltic and arc-pyroclastic rift basement was 2 km deep prior to 1.1 Ma. From 1.1 tomore » 0.235 Ma, 100-400 m of predominantly hemipelagic sediment were deposited, although intrarift basaltic eruptions and rhyolitic eruptions were fairly common. Explosive arc volcanism increased dramatically 250 Ka, leaving 165-428 m of fine to coarse pumiceous sediments in layers that are each 30-50 m thick at Site 790.« less

Geochemistry and models of mantle circulation

Geochemical data help to constrain the sizes of identifiable reservoirs within the framework of models of layered or whole-mantle circulation, and they identify the sources of the circulating heterogeneities as mainly crustal and/or lithospheric, but they do not decisively distinguish between different types of circulation. The mass balance between crust, depleted mantle and undepleted mantle based on 143 Nd/ 144 Nd, Nb/U and Ce/Pb, and the concentrations of very highly incompatible elements Ba, Rb, Th, U, and K, shows that ca. 25- 70% (by mass) of depleted mantle balances the trace element and isotopic abundances of the continental crust. This mass balance reflects the actual proportions of mantle reservoirs only if there are no additional unidentified reservoirs. Evidence on the nature and ages of different source reservoirs comes from the geochemical fingerprints of basalts extruded at mid-ocean ridges and oceanic islands. Consideration of Nd and He isotopes alone indicates that ocean island basalts (oibs) may be derived from a relatively undepleted portion of the mantle. This has in the past provided a geochemical rationale for a two-layer model consisting of an upper depleted and a lower undepleted (‘primitive’) mantle layer. However, Pb-isotopic ratios, and Nb/U and Ce/Pb concentration ratios demonstrate that most or all oib source reservoirs are definitely not primitive. Models consistent with this evidence postulate recycling of oceanic crust and lithosphere or subcontinental lithosphere. Recycling is a natural consequence of mantle convection. This cannot be said for some other models such as those requiring large-scale vertical metasomatism beneath oib source regions. Unlike other trace elements, Nb, Ta, and Pb discriminate sharply between continental and oceanic crust-forming processes. Because of this, the primitive mantle value of Nb/U = 30 (Ce/Pb = 9) has been fractionated into a continental crustal Nb/U = 12 (Ce/Pb = 4) and a residual-mantle (morb (mid-ocean ridge basalt) plus oib source) Nb/U = 47 (Ce/Pb = 25). These residual mantle values are uniform within about 20% and are not fractionated during formation of oceanic crust. By using these concentrations ratios as tracers, it can be shown that the possible contribution of recycled continental crust to oib sources is limited to a few percent. Therefore, recycling must be dominated by oceanic crust and lithosphere, or by subcontinental lithosphere. Oceanic crust normally bears a thin layer of pelagic sediment at the time of subduction, and this is consistent with oib sources that are dominated by subducted oceanic crust with variable but always small additions of continental material. Primordial 3 He, 36 Ar, and excess 129 Xe, in oceanic basalts demonstrate that the mantle has been neither completely outgassed nor homogenized, but they do not constrain the degree of mixing or the size of reservoirs. Also, helium does not correlate well with other isotopic data and may have migrated into the basalt source from other regions. The high 3 He/ 4 He ratios found in some oibs suggest that, even though the basalts are not derived from primordial mantle, their sources may be located close to a reservoir rich in primordial gases. This leads to models in which the oib sources are in a boundary layer within the mantle. The primordial helium migrates into this layer from below. The interpretation of the rare-gas data is still quite controversial. It is often argued that the upper mantle is a well-homogenized reservoir, but the data indicate heterogeneities on scales ranging from 10° to 10 6 m. The 206 Pb/ 204 Pb ratios in the oceanic m antle range from 17 to 21, which is similar to the range in most continental rocks. The degree of mixing cannot be directly inferred from these data unless the size and composition of the heterogeneities and the time of their introduction into the system are known. The relative uniformity of Nb/U and Ce/Pb ratios in the otherwise heterogeneous morb and oib sources indicates that this reservoir was indeed homogenized after the separation of the continental crust, and that the observed isotopic and chem ical heterogeneities were introduced subsequently. Overall, the results are consistent with, but do not prove, a layered mantle where the upper layer contains both morb and oib sources, and the lower, primitive mantle is not sampled by present-day volcanism. Alternative models such as those involving a chemically graded mantle have not been sufficiently explored.

Telemetry interference incorrectly interpreted as evidence for lightning and present‐day volcanism at Venus

Spike‐like responses to interruptions in the data recorded by the Orbiter Electric Field Detector (OEFD) on the Pioneer Venus Orbiter (PVO) have been incorrectly identified as broadband whistler noise and then interpreted as evidence of lightning and volcanism by Singh and Russell [1986; 1987]. We show that these spikes are non‐physical artifacts of the processing of interruptions in telemetry data and are readily distinguished from other non‐artificial noise appearing in the measurements. We independently analyze the spikes and the non‐artificial noise to show that key portions of the results of Singh and Russell are compromised by the inclusion of the artificial data. This conclusion is certified by the fact that (a) the distribution of the spikes‐only data matches the distribution of noise presented by Singh and Russell, while (b) the number and distribution of all non‐spike noise is quite incompatible with their results. This finding is consistent with our prior research, and leads us to reiterate that the PVO electric field data set used by Singh and Russell provides no basis for inferring the presence of either lightning or present‐day volcanism at Venus.

Tectonic stresses and volcanism: in-situ stress measurements and neotectonic investigations in the Etna area (Italy)

Mt. Etna has been building up for about 500,000 years on the eastern margin of Sicily, at the external border of the Apenninic—Maghrebian mountain chain and at the crossing of several important regional fault systems. The tholeiitic and alkaline magmatism of Mt. Etna represents the northernmost manifestation of present-day volcanism which was continuous in space and time from the Upper Miocene to the Lower Pleistocene in central eastern Sicily. In-situ stress measurements and neotectonic investigations have been carried out on Mt. Etna, on its periphery and on the Hyblean Mountains to obtain a better understanding of the stress field, which is connected to the present-day activity of the volcano, and a clearer picture of the regional geodynamical evolution. The investigations allow a comparison of the present stress field with that acting during the Quaternary. The neotectonic analysis has given evidence of strong compressional stresses exerted on the Pleistocene sediments overlying the substratum of the volcano and its immediate surroundings in addition to evidencing the already known extensional deformation. The compressional forces were oriented approximately N-S at the beginning of the volcanic activity. The in-situ stress measurements show now that this N-S compression is today active only in the northernmost part of the Etna area, whereas in the remaining area extensional stresses oriented approximately WNW-ESE prevail. It is concluded that the compressional stresses near the surface, today limited to the northernmost part of the Etna area, had a much more widespread distribution at the beginning of the volcanic activity. This conclusion is important for understanding the deep-seated magmatic process. Focal mechanisms of earthquakes indicate a complex tectonic situation with normal faulting down to a depth of 5 km and reverse faulting at deeper levels.