In arc lavas, elements of the extended high field strength element group (HFSE; Nb, Ta, Zr, Hf, W, and Mo) are valuable tracers to unravel magma source processes. These elements can also help to ...identify residual mineral assemblages in subducting slabs and in the mantle. Most high-precision studies on HFSE behaviour to date only focused on intra-oceanic arc suites and data for mafic lavas of the K-rich series (medium-K, high-K and shoshonitic) are scarce. Arguably, K-rich series are the most incompatible element-rich end-members of subduction zone magmatism, and they often record sediment recycling into the mantle. Understanding HFSE fractionation in K-rich lavas can therefore provide important insight into the global HFSE budget. Here we present a comprehensive extended HFSE dataset obtained by isotope dilution on well-characterised K-rich lavas from the Eastern Mediterranean, also including subducting sediment samples drilled during DSDP Leg 13 and ODP Leg 160 South and West of Crete. The volcanic samples include mafic calc-alkaline lavas from the active Aegean Island arc (Santorini) and post-collisional Tertiary lavas from SE Bulgaria. The Santorini lavas record a hydrous sediment melt-mediated source overprint of a depleted mantle source by components from the subducting African plate. The Bulgarian lavas tap lithospheric mantle sources that were overprinted by fluid- and melt-like subduction components during Eocene subduction of the African Plate. The sediments in this study comprise silts/sands, marl oozes, limestones and clay-rich debris flows and approximate the bulk sediment subducted beneath the Hellenic arc.
The marked enrichment of all HFSE in the lavas is controlled by the composition of the subducted sediments as shown by low 176Lu/177Hf (0.008630–0.02433) and Zr/Nb (11.3–29.4), combined with variable εHf (−3 to +11) and elevated W contents (up to 2.45ppm) in the lavas. Nevertheless, the lavas display unfractionated ratios of Nb/Ta and Zr/Hf of 12.3–16.5 and 34.4–38.6, respectively, with respect to MORB. This feature may be explained by the Nb/Ta in the sediments that are on average higher (15.2±2) compared to typical upper crustal estimates (12.5±1.8), indicating a strong regional control on the HFSE ratios in Eastern Mediterranean sediments. The lavas exhibit low Ta/W (<1.33), but do not show elevated W/Th, as expected for island-arcs entirely dominated by subduction zone fluids. Ratios of Mo/W in all lavas are low (<1.79), which, according to experimental evidence, can be explained by low fluid salinities at moderately oxidised conditions. Collectively, our data therefore confirm the mobility of W in subduction zones, but the selective mobility of W relative to the similar incompatible Th and Mo depends on the proportion of melt-like components, the composition of the material being subducted and ambient redox conditions in the subducted material.
The seafloor northeast of Santorini volcano in Greece consists of a small, elongated rifted basin that has been the site of recent submarine volcanism. This area lies within the Cyclades back-arc ...region of the present Hellenic subduction zone where the seafloor of the eastern Mediterranean Sea is descending beneath the Aegean microplate. The Cycladic region and the Aegean Sea as a whole are known to be regions of north–south back-arc extension and thinning of continental crust. Nineteen submarine volcanic cones occur within this small rift zone, the largest of these being Kolumbo which last erupted explosively in 1650AD, causing significant damage and fatalities on the nearby island of Santorini. Previous SEABEAM mapping and seismic studies from HCMR indicate that many of the smaller v'olcanic cones have been built above the present seafloor, while others are partly buried, indicating a range of ages for the activity along this volcanic line. None of the cones to the northeast of Kolumbo had been explored in detail prior to a cruise of the E/V Nautilus (NA007) in August 2010. The ROV Hercules was used to explore the slopes, summits and craters of 17 of the 19 centers identified on multibeam maps of the area. Water depths of the submarine volcano's summits ranged from 18 to 450m. In general, the domes/craters northeast of Kolumbo were sediment covered and showed little evidence of recent volcanic activity. Outcrops of volcanic rock were found in the crater walls and slopes of some of the cones but they typically consisted of volcanic fragments of pumice and lava that have been cemented together by biological activity, indicative of the lack of recent eruptions. Geochemical analysis of samples collected on the northeast cones showed evidence of low temperature hydrothermal circulation on the summit and upper flanks in the form of stream-like manganese precipitates emanating from pits and fractures.
Mounting evidence exists that variations in sulphur content in stalagmites are closely linked to changes in volcanic or anthropogenic atmospheric sulphur. The strong dependency of sulphur on soil pH ...and ecosystem storage, however, can result in a delay of several years to decades in the registration of volcanic eruptions and anthropogenic emissions by stalagmites. Here we present synchrotron-radiation based trace element analysis performed on a precisely-dated section of a stalagmite from Sofular Cave in Northern Turkey. As this section covers the time interval of the intensively studied Minoan volcanic eruption between 1600 and 1650 BC, we can test whether this vigorous eruption can be traced in a stalagmite. Of all measured trace elements, only bromine shows a clear short-lived peak at 1621±25 BC, whereas sulphur and molybdenum show peaks later at 1617±25 and 1589±25 respectively. We suggest that all trace element peaks are related to the Minoan eruption, whereas the observed phasing of bromine, molybdenum and sulphur is related to differences in their retention rates in the soil above Sofular Cave. For the first time, we can show that bromine appears to be an ideal volcanic tracer in stalagmites, as it is a prominent volatile component in volcanic eruptions, can be easily leached in soils and rapidly transferred from the atmosphere through the soil and bedrock into the cave and stalagmite respectively. Highly resolved oxygen and carbon isotope profiles indicate that the Minoan eruption had no detectable climatic and environmental impact in Northern Turkey.
•Geochemical imprint of the Minoan eruption in a stalagmite from Northern Turkey.•Peaks in bromine, molybdenum and sulfur are indicative for the Minoan eruption.•No climatic and environmental impact of the eruption in Northern Turkey.
In this study a relation is established between differential stress (σd), driving magma pressure ratio (R′) and tensile strength (T) of a rock mass in a volcanic system. It is shown that in σd vs. R′ ...space, the T curve follows equation of rectangular hyperbola and for dyking or volcanic eruption to occur, this T curve must be exceeded. It is proposed that this theoretical relation can be exploited to understand magma reservoir failure and for volcanic eruption forecasting. Applicability of this concept is demonstrated on Santorini volcanic system (Greece) for which considerable background information is available from previous studies. Published dyke orientation and aspect ratio data from Santorini yield σd and R′ values of 8.04 MPa and 0.22 respectively. Considering a weak quality host rock mass in Santorini, tensile strength curve for T = 1.5 MPa is plotted in σd vs R′ space. Considering magma reservoir volume and rock physical property estimates to be valid, and assuming σd and absolute stress to be constant, it is argued that change in reservoir dynamics would be controlled by variations in volume of magma released from the deeper magma reservoir to shallow reservoir (Vr) in Santorini. Values of R′ and Vr for the events of non-dyking in Santorini are also calculated. Minimum R′ and Vr required to induce dyking and eruption are estimated and it is concluded that the proposed theory can be generally applied for eruption forecasting.
•Differential stress (σd) is related to magma pressure ratio (R′) and tensile strength (T) of rock mass in a volcanic system.•Theoretical relation between σd, R′ and T is tested on Santorini volcano using available dyke data.•For constant σd and T, R′ is controlled by magma volume released into the magma reservoir.•Eruption forecasting can be done using σd, R′ and T relation for any volcanic system.
This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within ...the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely‐operated vehicle that were analyzed for major element, trace element and Sr‐Nd‐Hf‐Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave‐bearing pumices that are nearly identical in composition (73 wt.% SiO2, 4.2 wt.% K2O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52–60 wt.% SiO2). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206Pb/204Pb (<18.82) that has not been recognized in the Santorini volcanic products. The strong dissimilarity in both petrogenesis and inferred mantle sources between Kolumbo and Santorini suggests that pronounced source variations can be manifested in arc magmas that are closely associated in space and time within a single volcanic field.
Key Points:
A trace element and Sr‐Nd‐Hf‐Pb isotope study of Kolumbo submarine volcano within the Santorini volcanic field
The magmatic systems of Kolumbo and Santorini are unrelated and Kolumbo taps a distinct enriched mantle source
Mantle source variations can be manifested in temporally associated arc magmas within the same volcanic field
The late-seventeenth century BC Minoan eruption of Santorini discharged 30–60 km
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of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and ...accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE–SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption—a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite–diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it.
Measuring the low bromine abundances in Earth's materials remains an important challenge in order to constrain the geodynamical cycle of this element. Suitable standard materials are therefore ...required to establish reliable analytical methods to quantify Br abundances. In this study we characterise 21 Br-doped glasses synthesized from natural volcanic rocks of mafic to silicic compositions, in order to produce a new set of standards for Br analyses using various techniques. The nominal Br contents (amounts of Br loaded in the experimental samples) of 15 of 21 glasses were confirmed within 20% by instrumental neutron activation analysis (INAA). Using this new set of standards, we compare three micro-analytical approaches to measure Br contents in silicate glasses: synchrotron X-ray fluorescence (SR-XRF), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS). With SR-XRF, the Br contents of the standard glasses were determined with the highest accuracy (<10% for Br≥10ppm; >25% for Br≤5ppm), and high precision (<10% for Br contents >10ppm; 20–30% for Br≤10ppm). The detection limit was estimated to be <1ppm Br. All those factors combined with a high spatial resolution (5×5μm for the presented measurements), means that SR-XRF is well suited to determine the low Br abundance in natural volcanic glasses (crystal-hosted melt inclusions or matrix glasses of crystallized samples). At its current stage of development, the LA-ICP-MS method allows the measurement of hundreds to thousands ppm Br in silicate glasses with a precision and accuracy generally within 20%. The Br detection limit of this method has not been estimated but its low spatial resolution (90μm) currently prevents its use to characterise natural volcanic glasses, however it is fully appropriate to analyse super liquidus or sparsely phyric, Br-rich experimental charges. Our study shows that SIMS appears to be a promising technique to measure the low Br contents of natural volcanic glasses. Its spatial resolution is relatively good (~15μm) and, similarly to SR-XRF, the detection limit is estimated to be ≤1ppm. Using our new set of standards, the Br contents of two MPI-DING reference glasses containing ≤1.2ppm of Br were reproduced with precision <5% and accuracy <20%. Moreover, SIMS presents the advantage of being a more accessible instrument than SR-XRF and data processing is more straightforward.
•Three sets of bromine-doped silicate glasses have been synthesized from basaltic, andesitic and rhyodacitic volcanic rocks.•The Br contents of the synthetic glasses have been determined by INAA.•These glasses are homogeneous and can be used as standards to quantify Br contents in silicate glasses by micro-analytical techniques.•We present the first Br measurements by SIMS in silicate glasses (the Br-doped glasses and two MPI-DING reference glasses).•Both SIMS and SR-XRF are well suited methods to determine accurately low Br contents (≤10ppm) in natural volcanic glasses.
The structural evolution of the South Aegean Sea is little explored due to the lack of marine seismic data. Our present day understanding is mainly based on some island outcrops and GPS measurements. ...In this study we discuss the rather incremental opening of the Anydros Basin in the Pliocene during six major tectonic pulses and the subsequent basin fill processes by interpreting seismic data and derived time isochore maps. Between the active pulses basin floor tilting persisted on a much lower rate. Seismic data illustrate the depositional processes in the emerging Anydros Basin. The observation of onlap fill strata, divergent reflection pattern, moat channels and contourite drifts imply that deposition was controlled by turbidity and contour currents as well as the tilting basin floor. The metamorphic Attico–Cycladic basement shows a rise that aligns along an NW–SE directed axis crossing Anydros island. This axis marks a structural change of the Santorini–Amorgos Ridge and thus represents a major structural boundary. Dip angles of NE–SW trending major faults, like the Santorini–Amorgos Fault, indicate normal faulting to be the superior mechanism forming the present horst and graben environment. Hence, the area is likely to be in a state of NW–SE directed extensional stresses forming the asymmetric graben structure of Anydros. Secondary fault clusters strike the same direction but show much steeper dip angles, possibly indicating strike-slip movement or resulting from deformational stresses along the hinge zones of the normal faults. The majority of the faults we discovered are located in the area of earthquake clusters, which is another indication of recent faulting. Ring faults around Kolumbo submarine volcano, result from caldera collapse and mark the diameter of the magma chamber approximately to 20km.
•NE-SW rifting occurred during six tectonic pulses, parallel to the eastern part of the Hellenic trenches with NW-SE extension•Ring faults around Kolumbo submarine volcano emerged during its last eruptive phase•Multi-channel seismic data which resolve the entire basin fill deposits down to the metamorphic basement
Decades of research have focused on establishing the exact year and climatic impact of the Minoan eruption of Thera, Greece (c.1680 to 1500 BCE). Ice cores offer key evidence to resolve this ...controversy, but attempts have been hampered by a lack of multivolcanic event synchronization between records. In this study, Antarctic and Greenland ice-core records are synchronized using a double bipolar sulfate marker, and calendar dates are assigned to each eruption revealed within the 'Thera period'. From this global-scale sequence of volcanic sulfate loading, we derive indications toward each eruption's latitude and potential to disrupt the climate system. Ultrafine sampling for sulfur isotopes and tephra conclusively demonstrate a colossal eruption of Alaska's Aniakchak II as the source of stratospheric sulfate in the now precisely dated 1628 BCE ice layer. These findings end decades of speculation that Thera was responsible for the 1628 BCE event, and place Aniakchak II (52 ± 17 Tg S) and an unknown volcano at 1654 BCE (50 ± 13 Tg S) as two of the largest Northern Hemisphere sulfur injections in the last 4,000 years. This opens possibilities to explore widespread climatic impacts for contemporary societies and, in pinpointing Aniakchak II, confirms that stratospheric sulfate can be globally distributed from eruptions outside the tropics. Dating options for Thera are reduced to a series of precisely dated, constrained stratospheric sulfur injection events at 1611 BCE, 1561/1558/1555BCE, and c.1538 BCE, which are all below 14 ± 5 Tg S, indicating a climatic forcing potential for Thera well below that of Tambora (1815 CE).
Simplified model integrating magma emplacement, fluid characteristics and porphyry-epithermal mineralizing processes during Miocene to recent in the Hellenic Volcanic Arc. Modified from Voudouris et ...al. (2019).
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•Describes the submarine hydrothermal mineralization processes in an holistic approach based on modern models.•Useful in underwater mineral exploration for precious metals at convergent plate boundaries.•It provides the clues of the hydrothermal history of the Santorini volcano, the most famous volcano of the world.•The synthesized geochemical mineralogical and geophysical data can predict.•sites of new volcanic eruptions.•Since the Hellenic Volcanic Arc moves southwards it will affect the modern civilization.
Hydrothermal mineralization processes are investigated along the Hellenic Volcanic Arc (HVA) and associated environments in order to describe the factors that control the spatial and time variability in the hydrothermal intensity and the composition of the mineral deposits formed. Santorini is the most active hydrothermal center of the HVA. Two Fe-rich submarine hydrothermal deposits were formed. One in the Palaea Kameni (PK) embayment and the other in the Nea Kameni (NK) embayment. Hydrothermally introduced elements such as Fe, Mn, As and Si are more enriched in these metalliferous sediments than in any other similar deposit of the arc. Over a hundred volcanic events occurred in Santorini and this volcano is considered as one of the most violent and most famous in the world. The detailed study of these two deposits in combination with the tectonic regime provided an excellent opportunity to understand the hydrothermal mineralization processes in a convergent plate environment. The Santorini magma below the thinner continental crust and nearest to the greatest lithospheric extension prevents the early crystallization of Fe-Ti oxides, leading to extreme Fe oxide enrichments in the final hydrothermal fluids. The physicochemical conditions of formation of the two deposits are deduced on the basis of their mineralogical and geochemical features. The greater average values of Zn compared to those of Cu in PK deposit, suggest higher temperature of formation than that of NK deposit, where Cu concentrations are higher than those of Zn. This is in consistence with the abundance of pyrite in PK. The greater Mo and V values found in NK compared to those of PK Fe-rich sediments are indicative of the less oxygenated environment, favoring the biological activity of bacteria, known to be involved in the diagenetic transformation of Fe-oxyhydroxides to nontronite. This is in accordance with the plotting of the NK deposits in the nontronite field on the Fe2O3- Al2O3- MgO diagram. The high Si and low Al levels and the stagnant conditions are also in favor of the nontronite formation. Similar deposits were described from the Aeolean Arc and from EPR 18 °N. Applying the Dill et al. (1994) model on the PK and NK mineralogical, lithological and geochemical data, all processes involved during the formation of the final deposits and their fate are deduced. An examination of sediment geochemical data from the HVA fore-arc areas in relation to their tectonic setting, the seismic activity and geophysical data led to the production of a map showing the areas of potential presence of metal sulfides on the rock basement. Maleas basin is one of these areas which is characterized by high seismic activity and high heat flow measurements, associated with major fault planes along which magmatic material was injected. The sediment thickness there is very small. Other sites are the crossing points of the faulting lines with volcanic intrusive bodies and/or with a volcanic ridge of a NW-SE direction, near sites of high rates of lithospheric extension. A southward movement of the arc front is deduced.
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