Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-¹⁸O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.     

Hydrologic precursors to earthquakes: A review

This review summarizes reports of anomalous flow rates or pressures of groundwater, oil, or gas that have been interpreted as earthquake precursors. Both increases and decreases of pressure and flow rate have been observed, at distances up to several hundred kilometers from the earthquake epicenter, with precursor times ranging from less than one day to more than one year. Although information that might rule out nontectonic causes does not appear in many published accounts of hydrologic anomalies, several recent studies have critically evaluated the possible influences of barometric pressure, rainfall, and groundwater or oil exploitation. Anomalies preceding the 1976 Tangshan, China, and the 1978 Izu-Oshima-Kinkai, Japan, earthquakes are especially well-documented and worthy of further examination.Among hydrologic precursors, pressure head changes in confined subsurface reservoirs are those most amenable to quantitative interpretation in terms of crustal strain. The response of pressure head to earth tides determines coefficients of proportionality between pressure head and crustal strain. The same coefficients of proportionality should govern the fluid pressure response to any crustal strain field in which fluid flow in the reservoir is unimportant. Water level changes in response to independently recorded tectonic events, such as earthquakes and aseismic fault creep, provide evidence that a calibration based on response to earth tides may be applied to crustal strains of tectonic origin.Several models of earthquake generation predict accelerating stable slip on part of the future rupture plane. If precursory slip has moment less than or equal to that of the impending earthquake, then the coseismic volume strain is an upper bound for precursory volume strain. Although crustal strain can be only crudely estimated from most reported pressure head anomalies, the sizes of many anomalies within 150 kilometers of earthquake epicenters appear consistent with this upper bound. In contrast, water level anomalies at greater epicentral distances appear to be larger than this bound by several orders of magnitude.It is clear that water level monitoring can yield information about the earthquake generation process, but progress higes on better documentation of the data.     

Cellular automata to describe seismicity: A review

Cellular Automata have been used in the literature to describe seismicity. We first historically introduce Cellular Automata and provide some important definitions. Then we proceed to review the most important models, most of them being variations of the spring-block model proposed by Burridge and Knopoff, and describe the most important results obtained from them. We discuss the relation with criticality and also describe some models that try to reproduce real data.     

A monogenetic,Surtla-type,Surtseyan volcano from the Eocene-Oligocene Waiareka-Deborah volcanics,Otago, New Zealand: A model

The relics of a small, monogenetic, continental-shelf, Surtseyan volcano are preserved on the North Otago coast, South Island, New Zealand, in the late Eocene-early Oligocene Waiareka-Deborah volcanics. The succession consists of two parts, i. e. a lower interval of bedded lapilli tuffs and lapillistones, representing the eruptive, aggradational-cone-building phase, and an upper epiclastic sequence, representing the post-eruptive degradational phase. All of the preserved succession appears to have been deposited below storm wave base. The lapilli tuffs and lappillistones are subaqueous fall deposits, modified contemporaneously by downslope grain flow and turbidity current redeposition, and perhaps by local reworking caused by turbulent thermal eddies. The absence of major discordances in the lapilli tuffs suggests that the active eruptive period was very short-lived, perhaps lasting only a few days. The epiclastic succession consists of redeposited volcanic, skeletal, lime mud and glauconitic detritus, transported by debris flows and other mass flows. The initial epiclastic unit, a debris flow, appears to represent the sector collapse of a significant part of the cone. The appearance of fossils and rounded clasts low in the epiclastic succession coincides with stabilisation of the top of the submarine volcanic edifice, development of a wave-planed top, and its colonisation by a diverse fauna. Periodic storm activity swept material off the platform, redepositing it as marginal talus ramps. Surtla, a wholly submarine satellite volcanic centre of the 1963–1967 eruptive activity of Surtsey, is an excellent modern analogue for both the eruptive and post-eruptive phases of the Bridge Point-Aorere Point volcanic centre. By analogy with Surtla, the 120 metres of lapilli tuffs and lapillistones exposed on Bridge Point and Aorere Point accumulated in only several days. The 25 metres of reworked, glauconitic and fossiliferous volcaniclastics, represent thousands of years based on the time required for glauconite to form.     

Linking hydrologic signatures to hydrologic processes: A review

Hydrologic signatures are metrics that quantify aspects of streamflow response. Linking signatures to underlying processes enables multiple applications, such as selecting hydrologic model structure, analysing hydrologic change, making predictions in ungauged basins, and classifying watershed function. However, many lists of hydrologic signatures are not process-based, and knowledge about signature-process links has been scattered among studies from experimental watersheds and model selection experiments. This review brings together those studies to catalogue more than 50 signatures representing evapotranspiration, snow storage and melt, permafrost, infiltration excess, saturation excess, groundwater, baseflow, connectivity, channel processes, partitioning, and human alteration. The review shows substantial variability in the number, type, and timescale of signatures available to represent each process. Many signatures provide information about groundwater storage, partitioning, and connectivity, whereas snow processes and human alteration are underrepresented. More signatures are related to the seasonal scale than the event timescale, and land surface processes (ET, snow, and overland flow) have no signatures at the event scale. There are limitations in some signatures that test for occurrence but cannot quantify processes, or are related to multiple processes, making automated analysis more difficult. This review will be valuable as a reference for hydrologists seeking to use streamflow records to investigate a particular hydrologic process or to conduct large-sample analyses of patterns in hydrologic processes.     

Mantle process beneath Philippine Sea back-arc spreading ridges: A synthesis of peridotite petrology and tectonics

Abstract In order to obtain a general view of the mantle process beneath a back‐arc basin spreading ridge, the diversity of peridotite petrology and tectonic occurrences in two back‐arc basin spreading ridges from the Philippine Sea were examined: the Parece Vela Rift and the Mariana Trough. The Parece Vela Basin spreading ridge (Parece Vela Rift) was a physically fast/intermediate‐spreading ridge, although many tectono‐magmatic features resemble those of slow‐ to ultraslow‐spreading ridges. Two unusual features of the Parece Vela Rift further demonstrate the uniqueness of the ridge: full‐axial development of oceanic core complexes and exposure of mantle peridotite at segment midpoints. The Parece Vela Rift yields a lithological assemblage of residual but still fertile lherzolite/harzburgite, plagioclase‐bearing harzburgite and dunite; similar assemblages are reported from the equatorial Mid‐Atlantic Ridge at the Romanche Fracture Zone and the ultraslow‐spreading ridges from the Indian and Arctic Oceans. The tectono‐magmatic characteristics of the Parece Vela Rift suggest that diffuse porous melt flow and pervasive melt–mantle interaction were the important mantle processes there. Globally, this ‘porous melt flow‐type’ mantle process is likely to occur beneath a segment midpoint of the ridge having a thick lithosphere, typically an ultraslow‐spreading ridge. In contrast, the Mariana Trough is a typical slow‐spreading ridge, exposing mantle peridotite at segment ends. The Mariana Trough yields a lithological assemblage of residual harzburgite and veined harzburgite, a common assemblage among the global abyssal peridotite suite. The tectono‐magmatic characteristics of the Mariana Trough suggest that channeled melt/fluid flow and limited melt–mantle interaction are the important mantle processes there, because of the colder wall‐rock peridotite in the segment end. This ‘channeled melt flow‐type’ mantle process is likely to occur in the shallow lithospheric mantle at the segment ends of any spreading ridges.     

Erratum to: Dissolved organic carbon in permafrost regions: A review

The article Dissolved organic carbon in pennafrost regions: A review, written by Qiang MA, Huijun JIN, Congrong YU, and Victor F. BENSE, was erroneously originally published online without open access. After publication in Vol 62 Issue 2 this was corrected and the article is now an open access publication.     

Coda waves: A review

The analysis and interpretation of coda waves have received increasing attention since the early seventies. In the past few years interest in this subject has spread worldwide, and the study of high-frequency seismic coda waves has become a very important seismological topic. As a conclusion of the studies accomplished in this time, coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves.P andS waves play a particularly important role in this interaction. The process introduces an attenuation which, added to the intrinsic absorption, gives the observed apparent attenuation. Therefore, coda waves constitute a thumbprint left by the heterogeneities on the seismograms. Coda waves offer decisive information about the mechanism of how scattering and attenuation take place. This review describes coda waves in detail, and summarizes the work done in this subject to 1986. The relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed. Particular attention has been given to the application of coda waves to estimate source and medium parameters. The state-of-the-art of the temporal variations of coda wave shape, and the possible use of these variations as an earthquake precursor also are presented. Care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering.     

Silicate perovskites: A review

Since the first discovery of silicate perovskites at high pressures and high temperatures in the laboratory in 1974, silicate perovskites have probably become the most studied materials in the geophysical community during the past decade or so and it is nearly established that these silicates are the most abundant materials making up the bulk of the Earth. There are basically two groups of silicate perovskites. Ferromagnesian silicates with or without Al₂O₃ crystallizing in a common orthorhombic perovskite structure at high pressures and temperatures (HPT) are preservable at ambient conditions. Silicates of large cations such as Ca and Na crystallizing in an ideal cubic perovskite structure at HPT cannot be preserved at ambient conditions. Thus, the lattice parameters, crystal structure, thermal expansion and compressional data have been studied, both experimentally and theoretically, mainly for orthorhombic silicate perovskites, and for MgSiO₃ in particular. For MgSiO₃ perovskite, the recommended lattice parameters area=4.777±0.003,b=4.931±0.003 andc=6.899±0.004 Å; bulk modulusB ₀=2.4±0.2 Mbar; and volume thermal expansivity =(3±1)×10^–5 deg^–1 at ambient conditions. Cubic CaSiO₃ perovskite is probably less compressible than orthorhombic MgSiO₃ perovskite. The lattice parameters of MgSiO₃ perovskite increase linearly with increasing contents of both FeSiO₃ and Al₂O₃, forming limited solid solutions. The degree of distortion of orthorhombic silicate perovskites does not appear to change at HPT.     

Aquatic primary production in relation to microalgal responses to changing light: A review

Dynamic aspects of algal photosynthesis are set against the background of physical water motions which change the light experienced by the phytoplankton. These time-dependent photosynthetic responses are reviewed in relation to the proposition that phytoplankton primary production may be incorrectly estimated by the commonly used static incubation of light and dark bottles for periods significantly longer than the response-time of phytoplankton to changing light. This proposition is supported by the clear overlap between the timescales which characterize water motions and the timescales reported for the complex responses of algae to changing light. Empirical studies comparing static and dynamic incubations have been inconclusive, as have models incorporating some representation of the dynamic photosynthetic response to changing light. These results reflect weaknesses in the simple formulations used to describe photosynthesis in relation to irradiance, the simplicity of physical schemes used to generate changes in irradiance with time, and a lack of data (field and laboratory) on dynamic responses of microalgae to changing light. The quantitative significance of many physiological mechanisms is not known in relation to their effect on photosynthesis.     

Radiofluorescence of quartz: A review

Radiofluorescence (RF) is the luminescence emitted during exposure to ionizing radiation. Charged particles or high-energy photons can be used as stimulation sources, and different designs for measurement equipment have been published. Only few studies have successfully used the quartz RF signal for dosimetry and dating. However, RF is a valuable tool in deciphering charge trafficking in quartz crystals, and also provides information for identifying types of defects causing specific luminescence emissions. Based on models for charge transfer in quartz, RF is seen as resulting from direct recombination of electrons with holes captured in recombination centers (or vice versa) during ionizing irradiation. Competition between reservoir and luminescent centers explains the initial decay of the modeled RF curve followed by a steady rise and also the observed ‘pre-dose’ effect. Emission spectra have been found to be similar to thermoluminescence (TL) spectra, with prevalent emissions in the UV and further emissions for some samples in the blue-green and red range. The high intensity levels and the possibility of choosing longer accumulation times compared to TL and OSL are advantages of RF for spectral measurements. Relative peak intensities in the emission spectra change with dose and absolute intensities with dose rate. Investigating the RF signal with changing measurement temperature allows calculating physical parameters of individual emissions that control thermal quenching. The degree of thermal quenching varies between the emissions, with most intense quenching in the UV. Sensitization of RF by several orders of magnitude has been observed after annealing at 500 °C.     

Ensemble flood forecasting: A review

Operational medium range flood forecasting systems are increasingly moving towards the adoption of ensembles of numerical weather predictions (NWP), known as ensemble prediction systems (EPS), to drive their predictions. We review the scientific drivers of this shift towards such ‘ensemble flood forecasting’ and discuss several of the questions surrounding best practice in using EPS in flood forecasting systems. We also review the literature evidence of the ‘added value’ of flood forecasts based on EPS and point to remaining key challenges in using EPS successfully.     

Initiation of CMEs: A review

Solar coronal mass ejections (CMEs) are a striking manifestation of solar activity seen in the solar corona, which bring out coronal plasma as well as magnetic flux into the interplanetary space and may cause strong interplanetary disturbances and geomagnetic storms. Understanding the initiation of CMEs and forecasting them are an important topic in both solar physics and geophysics. In this paper, we review recent progresses in research on the initiation of CMEs. Several initiation mechanisms and models are discussed. No single model/simulation is able to explain all the observations available to date, even for a single event.     

Oceanic electromagnetic studies: A review

The review covers the main results of the oceanic electromagnetic studies presented during last 4–5 years. Seafloor electromagnetic observations are dedicated to studies in solid Earth geophysics and oceanography. Thus, a large range of objectives and targets are under investigation. Technological and theoretical advances provide an increase of quantity and quality of the collected seafloor data. The host conductivity models for oceanic crust and mantle are being developed; the possibility of obtaining useful structural constraints in heterogeneous environment are discussed. Significant oceanographic results were derived from electromagnetic field observations; these appeared to be a new powerful method for study of long-term large-scale ocean variability.     

Radio Tomography: A New Experimental Technique

The technique of radio tomography has developed during the past fifteen years from a theoretical concept to an established experimental method, used for geophysical investigations ofsolar-terrestrial processes. It also has potential in the mapping and modelling of the ionised atmosphere for application to practical radio systems. The method involves measurement of the electron content of the ionosphere along a large number of intersecting satellite-to-receiver ray paths, with tomographic inversion of the data to give a two-dimensional image of the spatial distribution of plasma density in the region of ray-path intersection. The emphasis in this review is on experimental tomographic observations, which have highlighted the capabilities and potential of the technique. Examples are presented from the equatorial sector where the equatorial anomaly is a significant feature, the mid-latitude sector where radio propagation is often influenced by the presence of the main ionisation trough, and the auroral and polar regions where footprints of solar-terrestrial coupling processes are frequently to be seen.     

Archaean metallogeny: A synthesis and review

Granitoid rocks interspersed with greenstone belts together comprise Archaean cratons throughout the world. The greenstone belts contain a wide variety of volcanic rocks which, despite cyclical variations in composition, generally change from ultramafic komatiites toward the stratigraphic base of the successions, upward through tholeiitic basalts and calc-alkaline andesites, to silici-alkalic rhyodacites toward the top. These extrusive rocks are intruded by rocks of a similar wide compositional range, which are probably comagmatic and subvolcanic to the former. The volcanic rocks are also intercalated with, and flanked by, volcaniclastic and distinctive immature sedimentary strata, including turbiditic greywacke and polymictic conglomerate. All are products of the prolonged volcanism that dominated Archaean supracrustal evolution and metallogenesis.Rare element pegmatites are associated with the Archaean granitic intrusions. Four important types of metalliferous ores, iron-manganese, nickel-chrome, gold-silver and copper-zinc occur in the greenstone belts, often co-regionally with one another in the same mining districts. Algoma type iron-formations of oxide, carbonate, silicate and sulfide facies occur throughout the volcano-sedimentary successions from base suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimerare chromite deposits are restricted to the stratigraphically lower, ultramafic komatiites. Important gold ores are hosted primarily in the tholeiitic basalts, particularly where these are intercalated with ankeritic-pyritic chemical sedimentary strata, but smaller gold deposits are also known in stratigraphically lower ultramafic and higher felsic volcanic rocks. The largest massive base metal sulfide deposits occur in the stratigraphically higher felsic rhyodacitic members.The close spatial associations between deposits of these metals in Archaean rocks, particularly those of certain nickel, gold and base metal ores with iron-formation, together with their many similar geological characteristics, suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimentary precipitates from ferruginous hydrothermal fluids that were periodically discharged on the sea floor during the prolonged Archaean subaqueous volcanism. The massive base metal deposits are of similar origin, essentially Cu-Zn-rich varieties of sulfide-facies iron-formation. The auriferous cherty, ankeritic or pyritic chemical sedimentary strata were also formed by similar sea floor exhalative hydrothermal activity. Although seldom of mineable gold content themselves, these constituted important, pre-enriched source rocks for later metamorphic generation of gold veins. Although many of the nickel sulfide and chromite bodies are of magmatic generation, others closely associated with iron-formation, and themselves delicately interbedded with cherty or talc-carbonate laminae, may be due to similar sea floor hydrothermal discharge that accompanied ultramafic extrusive activity. Considering their close spatial and genetic links, the occurrence of any one of these four types of deposit suggests the possibility of the others wherever the favourable Archaean host rocks are present.Different Archaean cratons however, have differing proportions of these four types of deposit, and of their distinctive host rocks. Greenstone belts in all cratons throughout the world contain the iron-formations and gold deposits. Greenstone belts of southern Africa and Western Austrialia, however, have more abundant ultramafic rocks and more important nickel-chrome deposits. Some of them may be older than comparable belts in Canada which contain more rhyodacitic rocks and more important copper-zinc ores. Some belts of Brazil and West Africa may be still younger, contain more pyroclastic-volcaniclastic rocks, lack both the nickel-chrome and copper-zinc ores, but contain important manganese in their iron-formations. These relations suggest worldwide diachroneity of Archaean greenstone belt generation, late-Archaean granitic orogeny and ensuing Proterozoic sedimentation.