Kameni Islands Research Articles

Chemical and textural diversity of Kameni (Greece) dacites: role of vesiculation in juvenile and mature basal crystal masses

Dacite lavas erupted from Kameni Islands volcanic centre (Greece) during the last 2000 years have a limited range in chemical composition (SiO2 = 64.0–68.5%) which contrasts with their wide range in plagioclase abundance (3–22%) and crystal size distributions. Most plagioclase crystals have simple zoning and occur independently or in loose clusters with finer-grained cores. We propose that magmatic diversity was produced by the interaction between crystals that formed at the base of a magma reservoir and bubbles produced by injection and vesiculation of more mafic magma. Two end-member situations can be identified: in juvenile systems, the basal crystal mass is loosely connected and readily disrupted by bubble formation. The crystal–bubble couples accumulate at the top of the reservoir, from where they can enter the sub-volcanic plumbing system to produce high-crystal content, chemically unevolved magmas. In the mature system, the crystal mass is well connected so bubbles displace the evolved, interstitial magma and liberate only a smaller number of crystals from the crystal mass. This process produces chemically evolved magmas, with lower crystal contents. The oldest lavas seem to have been produced from mature systems, whereas the youngest eruptions were of lavas produced from juvenile systems. This progression may reflect an overall reduction in repose times during the last 2000 years.

Magma chambers versus mush zones: constraining the architecture of sub-volcanic plumbing systems from microstructural analysis of crystalline enclaves.

There are clear microstructural differences between mafic plutonic rocks that formed in a dynamic liquid-rich environment, in which crystals can be moved and re-arranged by magmatic currents, and those in which crystal nucleation and growth are essentially in situ and static. Crystalline enclaves, derived from deep crustal mushy zones and erupted in many volcanic settings, afford a unique opportunity to use the understanding of microstructural development, established from the study of intrusive plutons, to place constraints on the architecture of sub-volcanic systems. Here, we review the relevant microstructural literature, before applying these techniques to interrogate the crystallization environments of enclaves from the Kameni Islands of Santorini and Rábida Volcano in the Galápagos. Crystals in samples of deep-sourced material from both case studies preserve evidence of at least some time spent in a liquid-rich environment. The Kameni enclaves appear to record an early stage of crystallization during which crystals were free to move, with the bulk of crystallization occurring in a static, mushy environment. By contrast, the Rábida enclaves were sourced from an environment in which hydrodynamic sorting and re-arrangement by magmatic currents were common, consistent with a liquid-rich magma chamber. While presently active volcanoes are thought to be underlain by extensive regions rich in crystal mush, these examples preserve robust evidence for the presence of liquid-rich magma chambers in the geological record. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Towards reconstruction of the lost Late Bronze Age intra-caldera island of Santorini, Greece

During the Late Bronze Age, the island of Santorini had a semi-closed caldera harbour inherited from the 22 ka Cape Riva Plinian eruption, and a central island referred to as ‘Pre-Kameni’ after the present-day Kameni Islands. Here, the size and age of the intracaldera island prior to the Late Bronze Age (Minoan) eruption are constrained using a photo-statistical method, complemented by granulometry and high-precision K-Ar dating. Furthermore, the topography of Late Bronze Age Santorini is reconstructed by creating a new digital elevation model (DEM). Pre-Kameni and other parts of Santorini were destroyed during the 3.6 ka Minoan eruption, and their fragments were incorporated as lithic clasts in the Minoan pyroclastic deposits. Photo-statistical analysis and granulometry of these lithics, differentiated by lithology, constrain the volume of Pre-Kameni to 2.2–2.5 km3. Applying the Cassignol-Gillot K-Ar dating technique to the most characteristic black glassy andesite lithics, we propose that the island started to grow at 20.2 ± 1.0 ka soon after the Cape Riva eruption. This implies a minimum long-term lava extrusion rate of ~0.13–0.14 km3/ky during the growth of Pre-Kameni.

Historical bathymetric charts and the evolution of Santorini submarine volcano, Greece

Historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes, such as submarine volcanism. The British Admiralty, for example, have been involved in lead line measurements of seafloor depth since the early 1790s. Here, we report on an analysis of historical charts in the region of Santorini volcano, Greece. Repeat lead line surveys in 1848, late 1866, and 1925–1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth. These data reveal that the flanks of the Kameni Islands, a dacitic dome complex in the caldera center, have shallowed by up to ∼175 m and deepened by up to ∼80 m since 1848. The largest shallowing occurred between the late 1866 and 1925–1928 surveys and the largest deepening occurred during the 1925–1928 and 2001 and 2006 surveys. The shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866–1870 and 1925–1928 at rates of up to 0.18 and 0.05 km3 a−1, respectively. The deepening is attributed to a load-induced viscoelastic stress relaxation following the 1866–1870 and 1925–1928 lava eruptions. The elastic thickness and viscosity that best fits the observed deepening are 1.0 km and ∼1016 Pa s, respectively. This parameter pair, which is consistent with the predictions of a shallow magma chamber thermal model, explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from InSAR analysis.

Stratigraphic relations of Santorini’s intracaldera fill and implications for the rate of post-caldera volcanism

We propose a new volcanological interpretation of Santorini’s intracaldera fill deposits as revealed by seismic reflection profiles. This interpretation, along with supplementary gravity and geophysical studies, reveals three distinct volcaniclastic units (1–3). From top to bottom these units are attributed to (1) modern infilling sediment, (2) shallow marine phreatomagmatic volcanism associated with the relatively recent formation of the Kameni Islands, and (3) downfaulted ‘Minoan’ pyroclastic deposits, which formed during caldera collapse towards the end of the Bronze Age eruption. Estimates of the volumes of the seismic units and Kameni Islands yield a dense rock equivalent magma volume of 4.85 ± 0.7 km 3 . The average rate of volcanism over the past c . 3641 years is estimated at 1.3 ×10 −3 km 3 a −1 , and is similar to the rate since the AD 1707 eruption of 1.2 ×10 −3 km 3 a −1 based on historical lava volume estimates.

The emergence and growth of a submarine volcano: The Kameni islands, Santorini (Greece)

The morphology of a volcanic edifice reflects the integrated eruptive and evolutionary history of that system, and can be used to reconstruct the time-series of prior eruptions. We present a new high-resolution merged LiDAR-bathymetry grid, which has enabled detailed mapping of both onshore and offshore historic lava flows of the Kameni islands, emplaced in the centre of the Santorini caldera since at least AD 46. We identify three new submarine lava flows: two flows, of unknown age, lie to the east of Nea Kameni and a third submarine flow, located north of Nea Kameni appears to predate the 1925–1928 lava flows but was emplaced subsequent to the 1707–1711 lava flows. Yield strength estimates derived from the morphology of the 1570/1573 lobe suggest that submarine lava strengths are approximately two times greater than those derived from the onshore flows. To our knowledge this is the first documented yield strength estimate for submarine flows. This increase in strength is likely related to cooling and thickening of the dacite lava flows as they displace sea water. Improved lava volume estimates derived from the merged LiDAR-Bathymetry grid suggest typical lava extrusion rates of ∼2–3m3s−1 during four of the historic eruptions on Nea Kameni (1707–1711, 1866–1870, 1925–1928 and 1939–1941). They also reveal a linear relationship between the pre-eruption interval and the volume of extruded lava. These observations may be used to estimate the size of future dome-building eruptions at Santorini volcano, based on the time interval since the last significant eruption.

Seismicity, Vp/Vs and shear wave anisotropy variations during the 2011 unrest at Santorini caldera, southern Aegean

The Santorini caldera has been the focus of several large explosive eruptions in the past, the last of which occurred in the early 1950s. The volcano was dormant until early 2011 when increasing number of earthquakes accompanied significant intra-caldera uplift. This seismic activity was recorded by 8 temporary as well as 19 permanent seismic stations that were installed on Santorini and nearby islands after the onset of the unrest. Using data from January 2011 until June 2012 we calculated accurate relative locations for 490 events utilizing both catalog and waveform cross-correlation differential travel times of P- and S-phases. The distribution of relocated events exhibits a large cluster between Thera and Nea Kameni islands along the caldera rim, suggesting the activation of a preexisting ring fault. All hypocenters are located between 5 and 11km resulting in a sharp cutoff of seismicity above and below these depths. We also used P and S travel times in order to calculate average Vp/Vs ratios and estimated shear wave splitting parameters (fast direction φ, delay time δt) for events within the shear wave window. The Vp/Vs ratios at several stations exhibit a majority of values consistently below the regional one (~1.77). Their temporal variations can be explained as periods of gas influx and depletion in the upper crust beneath the caldera. A comparison of δt for a number of earthquake doublets shows a progressive decrease of delay times towards the end of the unrest probably as a result of cracks closing owing to stress relaxation. The seismological observations presented here are compatible with petrological models that suggest the existence of a deep (11–14km) dacitic magma reservoir and a shallower (<5km) rhyolitic magma chamber.

Mapping inflation at Santorini volcano, Greece, using GPS and InSAR

Recent studies have indicated that for the first time since 1950, intense geophysical activity is occurring at the Santorini volcano. Here, we present and discuss the surface deformation associated with this activity, spanning from January 2011 to February 2012. Analysis of satellite interferometry data was performed using two well‐established techniques, namely, Persistent Scatterer Interferometry (PSI) and Small Baseline Subset (SBAS), producing dense line‐of‐sight (LOS) ground deformation maps. The displacement field was compared with GPS observations from 10 continuous sites installed on Santorini. Results show a clear and large inflation signal, up to 150 mm/yr in the LOS direction, with a radial pattern outward from the center of the caldera. We model the deformation inferred from GPS and InSAR using a Mogi source located north of the Nea Kameni island, at a depth between 3.3 km and 6.3 km and with a volume change rate in the range of 12 million m3 to 24 million m3 per year. The latest InSAR and GPS data suggest that the intense geophysical activity has started to diminish since the end of February 2012.

Submarine volcanoes along the Aegean volcanic arc

Abstract The Aegean volcanic arc has been investigated along its offshore areas and several submarine volcanic outcrops have been discovered in the last 25 years of research. The basic data including swath bathymetric maps, air-gun profiles, underwater photos and samples analysis have been presented along the four main volcanic groups of the arc. The description concerns: (i) Paphsanias submarine volcano in the Methana group, (ii) three volcanic domes to the east of Antimilos Volcano and hydrothermal activity in southeast Milos in the Milos group, (iii) three volcanic domes east of Christiana and a chain of about twenty volcanic domes and craters in the Kolumbo zone northeast of Santorini in the Santorini group and (iv) several volcanic domes and a volcanic caldera together with very deep slopes of several volcanic islands in the Nisyros group. The tectonic structure of the volcanic centers is described and related to the geometry of the arc and the neotectonic graben structures that usually host them. The NE–SW direction is dominant in the Santorini and Nisyros volcanic groups, located at the eastern part of the arc, where strike–slip is also present, whereas NW–SE direction dominates in Milos and Methana at the western part, where co-existence of E–W disrupting normal faults is observed. The volcanic relief reaches 1100–1200 m in most cases. This is produced from the outcrops of the volcanic centers emerging usually at 400–600 m depth and ending either below sea level or at high altitudes of 600–700 m on the islands. Hydrothermal activity at relatively high temperatures observed in Kolumbo is remarkable whereas low temperature phenomena have been detected in the Santorini caldera around Kameni islands and in the area southeast of Milos. In Methana and Nisyros, hydrothermal activity seems to be limited in the coastal areas without other offshore manifestations.

Vulcanian explosion cycles: Patterns and predictability

Research Article| September 01, 2007 Vulcanian explosion cycles: Patterns and predictability S.F.L. Watt; S.F.L. Watt 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar T.A. Mather; T.A. Mather 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar D.M. Pyle D.M. Pyle 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (9): 839–842. https://doi.org/10.1130/G23562A.1 Article history received: 21 Nov 2006 rev-recd: 23 Apr 2007 accepted: 03 May 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S.F.L. Watt, T.A. Mather, D.M. Pyle; Vulcanian explosion cycles: Patterns and predictability. Geology 2007;; 35 (9): 839–842. doi: https://doi.org/10.1130/G23562A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Repetitive and violent Vulcanian explosion sequences are common hazards at many volcanoes. Statistical analyses of such sequences form the basis of forecasting models and reveal underlying explosive processes. However, no single statistical model describes interexplosion repose intervals in Vulcanian systems. Soufrière Hills, Montserrat, is best described by a log-logistic model, while Sakurajima, Japan, shows a transition from a log-logistic to a Weibull model as activity intensifies. Anak Krakatau, Indonesia, displays two failure modes on different time scales, both described by Weibull distributions. At the Kameni Islands, Santorini, Greece, model-fitting parameters vary between eruption cycles. Rates of magma rise and pressurization may be the most important controls in determining the statistical distribution of repose intervals in Vulcanian systems. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Textural analysis of magmatic enclaves from the Kameni Islands, Santorini, Greece

The Kameni Islands have been the focus of historic volcanic activity on Santorini, Greece. The islands comprise a series of dacite lavas, the erupted compositions of which have remained approximately constant for the last ∼2200 years. The dacite lava flows of Nea Kameni contain a variety of mafic enclaves, distinguished by differences in texture, composition and mechanical behaviour. The mafic enclaves represent chilled fragments of mafic magma that crystallised rapidly on contact with the cooler, more silicic host magma. The absence of chilled margins on the enclaves suggests that the replenishing magma initially formed a dense layer at the base of the chamber. Groundmass textures are interpreted as an indication of the degree of undercooling between the enclaves and the host. Groundmass textural information obtained from the enclaves has been used to determine the relative volumes of replenishing magmas injected into the host magma before eruption, which indicates that the volume of erupted magma is directly proportional to the volume of replenishing magma emplaced in the chamber prior to eruption, which thus may act as a potential eruption trigger. The variety of replenishing magmas implies the existence of a complex conduit and chamber system beneath Santorini volcano. Andesitic enclaves with linear crystal size distributions are thought to have originated as an aphyric andesitic melt expelled from a crystal mush by filter-pressing. Disequilibrium phenocryst assemblages in the lavas and phenocryst-bearing enclaves provide evidence for the entrainment and recycling of older phenocryst populations.

Quantitative morphology, recent evolution, and future activity of the Kameni Islands volcano, Santorini, Greece

Linking quantitative measurements of lava fl ow surface morphology with historical observations of eruptions is an important, but underexploited, route to understanding eruptions of silicic magma. We present here a new, high-resolution digital elevation model (DEM) for the intracaldera Kameni Islands, Santorini, Greece, which reveals the potential of high-resolution imaging (at ~1 m per pixel) of lava-fl ow fi elds by airborne light detection and ranging laser radar (LiDAR). The new DEM has an order-of-magnitude better resolution than earlier models, and reveals a wealth of surface morphological information on the dacite lava fl ows of the Kameni Islands. In turn, this provides quantitative constraints on the bulk rheology of the emplaced lava fl ows. When combined with a reanalysis of contemporary eruption accounts, these data yield important insights into the behavior of dacite magma during slow effusive eruptions on Santorini and elsewhere, and allow the development of forecasts for the style and duration of future eruptions. Kameni Island lava fl ows exhibit classic surface morphologies associated with viscous magma: levees and compression folds. Levee heights and fl ow widths are consistent with a Bingham rheology, and lava yield strengths of 3‐7 ◊ 104 Pa. Compression folds have long wavelengths (15‐25 m), and change only a little downstream; this is consistent with observations of other terrestrial silicic lava fl ows. The blocky a‘a dacite lava-fl ow margins show a scale-invariant morphology with a typical fractal dimension that is indistinguishable from basaltic Hawaiian a‘a, confi rming that the fractal dimension is insensitive to the composition of the fl ow. Dome-growth rates during eruptions of the Kameni Islands in 1866 and 1939 are consistent with a model of slow infl ation of a dome with a strong crust. Lava domes on the Kameni Islands have a crustal yield strength (4 ◊ 107 Pa) that is lower by a factor of 2‐4 than the domes at Pinatubo and Mount St. Helens. The dome-height model combined with the apparent time-predictable nature of volcanic eruptions of the Kameni Islands allow us to suggest that should an eruption occur during 2006, it will last for more than 2.7 yr and produce a dome ~115‐125 m high.

Information about open-system magma chambers derived from textures in magmatic enclaves: the Kameni Islands, Santorini, Greece

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939–1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry.

Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc

The timescales of element transfer at volcanic arcs provide important insights into magma generation, movement and storage beneath destructive margin volcanoes. Here we present major, trace and Sr-, Nd- and U-series isotope data on <200 ka old samples from Santorini in the Aegean volcanic arc as a case study of a hazardous destructive margin volcano. Samples range from low-K calc-alkaline basalts to high-K rhyolites, are enriched in light rare earth elements and fluid mobile trace elements, and have negative Ta, Nb and Ti anomalies characteristic of arc magmatism. 87Sr/ 86Sr ratios range from 0.7035 to 0.7054 and are negatively correlated with 143Nd/ 144Nd ratios, ranging from 0.51285 to 0.51263. Lavas from the second eruptive cycle (∼180 ka–∼3.6 ka) are in U–Th radioactive equilibrium, and a whole-rock, plagioclase, magnetite U–Th mineral isochron from a 67±9 ka dacite yields an age of 85 (+22/−19) ka (1σ). Samples from the Kameni islands (46 AD–1950) show small but significant uranium excesses, with ( 238U/ 232Th)=0.91–1.04 and ( 230Th/ 232Th)=0.91–0.97, suggesting that the time since U–Th differentiation is ≤147 (+27/−21) ky (1σ). A U–Th mineral isochron from the 1940 Kameni dacite yields an age indistinguishable from its eruption age. The Kameni samples show variable Ra depletion, with ( 226Ra/ 230Th)=0.92–0.99. The range in Sr and Nd isotope data reflects progressive crustal assimilation. The combined trace element and isotope data are consistent with a three-component source model involving mantle wedge, sediment melts and slab-derived fluids. While initial 238U-excesses in the second eruptive cycle have decayed, implying slab-derived fluid addition >180 ky prior to eruption, for the Kameni lavas fluid addition to the wedge is ≤150 ky. U–Th mineral isochrons indicate that crystal residence times in the subvolcanic plumbing system are short. Fractional crystallisation less than ∼1 ky prior to eruption of the Kameni dacites is suggested by ( 226Ra/ 230Th)<1. The combined data imply that timescales of magmatic differentiation in crustal reservoirs are short, that transfer time of the slab-fluid signature through the mantle wedge varies through time.

Plagioclase residence times at two island arc volcanoes (Kameni Islands, Santorini, and Soufriere, St. Vincent) determined by Sr diffusion systematics

The diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers. Initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase-melt partition coefficient. Trace element diffusion will subsequently act to modify this initial profile and – given enough time – produce a profile that is in equilibrium with the anorthite variations within the crystal. We argue that the trace element partition coefficient Da/b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal-liquid partition coefficients, and that the equilibrium profile of the crystal can be calculated. The time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase. Strontium plagioclase-melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions. A one-dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir. The model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the Kameni Island dacites, Santorini, and from the 1979 Soufriere andesite, St. Vincent. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase. Maximum crystal residence times range from 100 to 450 years. This is in good agreement with estimates from crystal size distribution and from Ra-Th disequilibrium studies for the Kameni Islands. For Soufriere, however, such short residence times are incompatible with U-Th mineral errorchron data that suggest residence times of >40 ka in a thermally buffered magma reservoir. To reconcile these apparently different ages, we invoke a more complicated magmatic history for Soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption.

U-Th-Ra Isotope Evidence for Rates of Fluid Transport and Magmatic Processes Beneath Santorini, Aegean Volcanic Arc, Greece

The island of Santorini is situated in the Aegean and is part of the Hellenic Volcanic Arc, formed by the northward subduction of the African Plate under the Eurasian Plate. Santorini has been volcanically active since the middle Pleistocene. The two youngest (<200 ka) volcanic sequences are the second explosive cycle that terminated with the caldera forming Minoan eruption of ~1600 BC, and the Kameni island dacites, which comprise twelve historic lava eruptions from 46 AD to 1950. Selected rocks from these two sequences have been analysed for major and trace elements, for Sr and Nd isotopes, and for the short lived isotopes of U-Th-Ra by TIMS. Second eruptive cycle The compositions of the second eruptive cycle range from basaltic andesite to rhyolite, and samples are enriched in LREE, LILE, U and Th. Sr, Nd and Th isotopic compositions are also enriched relative to MORB, and they preserve good isotope-isotope correlations with S7Sr/S6Sr = 0.7035-0.7052 and 143Nd/144Nd = 0.51285-0.51267 (Fig. 1), and (23~ = 0.97-0.90. The rocks tend to have similar (23~ and (238U/232Th) activity ratios, and they are therefore in 23~ equilibrium. The within-suite trace element and Sr-Nd isotope trends can be modelled by AFC processes, indicating that variable assimilation of up to ~3% upper crustal material was responsible for the range in Sr, Nd and Th isotopes in the second eruptive cycle. The composition of the inferred pre-contamination magma reflects contributions from both sedimentderived melts (-0.3%) and hydrous fluids from the subducted slab. Relative to N-MORB, the enrichment in LREE, the high Th/Ta ratios and the low initial 143Nd/144Nd [~ 0.51285] and Th activity ratios [(23~ ~ 0.6] reflect modification of the mantle wedge by small degree melts of the subducted sediment. In contrast, the concentrations of fluid mobile trace elements [Rb, Sr, Ba, K, U and Pb] are significantly increased by introduction of slabderived fluids. The data are consistent with a fluid Sr isotope ratio of ~0.7035, as inferred from studies of other island arcs (e, g. Turner et al., 1997), and mass balance considerations indicate that fluid addition accounts for ~40% of the U budget. The

Hydrothermal sedimentation in the caldera of Santorini, Hellenic Volcanic Arc

ABSTRACTSediments in the caldera of Santorini are receiving a hydrothermal input of iron and manganese from presently active hydrothermal vents off the Kameni Islands, and are enriched in these elements in their surface layer. However, greater Fe‐Mn enrichments occur in discrete layers at depth in the cores separated from the surface by Fe‐Mn poor sediments, suggesting that a past hydrothermal event may have been more intense than the present one. The buried Fe‐Mn enriched layers occur above a turbidite thought to have resulted from sediment slumping due to a major volcanic eruption and earthquake in 1650, and are thought to have formed consequent on the activation of faults related to the magma chamber by the eruption facilitating seawater‐rock interaction processes and the formation of metal‐rich hydrothermal solutions.

PIXE and EMP analyses of volcanites from Nea Kameni island (Aegean Sea, Greece)

The major and trace element compositions of several minerals and their surrounding groundmass in the volcanic rocks from the post-caldera Nea-Kameni island of the Santorini volcanic complex, Aegean Sea, have been determined, in-situ with particle-induced X-ray emission (PIXE) and wavelength dispersive X-ray analysis with an electron microprobe (EMP). The lavas are typically calc-alkaline dacites. All samples are porphyritic with a phenocryst mineralogy dominated by plagioclase, augite, hyperstene and Ti-magnetite. The phenocrysts range in size from 200 μm to 2.5 mm. The PIXE and EMP analyses were done on sections polished with diamond paste

The [formula omitted] ratios of siderite and organic matter of a modern metalliferous hydrothermal sediment and their implications for banded iron formations

A hydrothermal ferruginous sediment precipitated by the activity of the microaerophilic chemolithotrophic bacterium Galionella ferruginea has been sampled in a bay of Nea Kameni Island, Santorini, Greece. Due to high fugacity of carbon dioxide, precipitation of siderite also occurred. The δ 13C-value (PDB) was determined to be +6.5‰ in the siderite and −26.8‰ in the organic matter of mainly microbiological origin. The enrichment of isotopically light carbon in carbonates of banded iron formations ( δ 13 C = −10‰ ) has recently been discussed in literature. The dilution of these marine carbonates by isotopically light carbon is suggested to have resulted from bacterial oxidation of reduced organic carbon during early diagenesis. It is further suggested that Fe(III) in the newly precipitated sediment served as electron acceptor during oxidation. In a hydrothermal model for the origin of banded iron formations this is consistent with the data presented here. It is therefore postulated from the results presented that the reduced organic carbon in banded iron formations originates from microaerophilic chemolithotrophic bacteria.

Post-caldera dacites from the Santorini volcanic complex, Aegean Sea, Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period

The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An55 to An42). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO2). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10−9.6-10−9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H2O, a considerable amount (about 0.01–0.015 km3) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO2 provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than ∼30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.