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The generation of continental crust of intermediate composition occurs predominantly in convergent margin settings, yet the mechanisms by which felsic, calc-alkaline arc magmas are generated remain poorly understood. Magma mixing appears to be a common process in voluminous intermediate arc rocks but the composition of the felsic mixing endmember is typically obscured by the mixing process. We investigate a suite of porphyritic (rhyo)dacitic magmas (65–72 wt.% SiO 2 ) from Nisyros, a young stratovolcano in the Aegean arc, Greece. These magmas are not affected by shallow process such as hybridisation or crystal-melt segregation and thus offer a valuable insight into the origin of felsic melts at convergent margins. We find that the Nisyros (rhyo)dacites form through a reaction in which earlier-formed wehrlite cumulates in the deep arc crust react with melts to form amphibole. This implies that melt major element compositions are effectively buffered by a low-variance mineral assemblage to follow this peritectic boundary such that the silica content of melts extracted from the deep crustal hot zone is controlled by the amount of amphibole crystallised. The resorption of cumulates is pivotal in imparting a distinct trace element signature that is decoupled from major element systematics. For example, high compatible element contents and a strong amphibole signature (low Y and Dy/Yb) cannot be captured by simple crystallisation models and require cumulate resorption. Variable radiogenic isotope systematics indicate minor crustal contamination although assimilation is not proportional to silica content and hence not a main driving force behind the generation of felsic melts. Instead, the Nisyros (rhyo)dacites formed through melt-cumulate reaction processes prior to emplacement as mush bodies at shallow depth and partial eruption. Magma mixing only becomes an important process in the youngest unit on Nisyros. On a global scale, peritectic boundary melts are rarely sampled in the whole rock or melt inclusion record. Conversely, peritectic boundary melts do form a suitable felsic mixing endmember for the generation of voluminous “ monotonous intermediate ” magmas. [ABSTRACT FROM AUTHOR]

We present new distal records of tephra deposits that overly the Kos ignimbrite in seven locations of Datça peninsula. Tephra in one of these locations were previously associated with Nisyros Kyra sub-unit based only on the field characteristics. We use different proxies such as field observations, petrography, mineral, glass, and whole-rock chemistry in order to characterize and correlate the previously and recently identified pumice fall deposits on Datça. The total thickness of the fall deposit reaches to 3.5 m. The size of the pumice clasts is generally within the range of lapilli, and they have vitrophyric texture consisting mainly of plagioclase (andesine to labradorite) with scarce clinopyroxene (diopside to augite), olivine (Fo 48–50 ), amphibole (magnesio-hastingsite), and biotite crystals. Amphibole is a ubiquitous phenocryst in all Datça tephra units and used as a criterion for the correlation. Glass major element analyses by EMPA reveal two different groups with andesitic and dacitic compositions. Difference in silica content (up to ca. 4 wt%) detected in the same specimen also designates the heterogeneity in pumice glass. This heterogeneity in glass composition is also supported by the frequent occurrence of banded pumice clasts in Datça tephra. Whole-rock composition of the pumice is mainly andesitic with calc-alkaline affinity. Multi-element patterns on primitive-mantle normalized diagram display typical arc-magmatism signature (i.e. depletion in Nb, Ta, Ti, and P). In order to check and eliminate the potential alternatives, we compared the distal deposits on Datça not only with Kyra, but also with other Nisyros tephra units. Yet, Kyra is the only unit that has comparable depositional characteristics, calcic amphibole crystals, andesitic-dacitic glass and whole-rock chemistry, and distal tephra deposits on neighboring islands (Tilos and Chalki). Therefore, we associate Datça tephra deposits with some proximal Kyra subunits of intermediate composition. However, if further geochemical (especially glass and mineral chemistry) data are provided for the Lakkí and Melisserí tephra units, a more thorough tephra correlation will be possible. Finally, our results may have implications on Nisyros tephra dispersal and geochemical characterization. [ABSTRACT FROM AUTHOR]

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920 mg/kg) and Nb (245 mg/kg), in the coastal sand. The low REE concentration (ΣREE + Y = 240 mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE. The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions. [ABSTRACT FROM AUTHOR]

Abstract: The Greek island of Nisyros is the source of tephra found in distal settings throughout the Aegean. In this contribution, we define geochemical fingerprints for the Lower and Upper Pumice eruptions of Nisyros based on micron-beam major (EMPA) and trace (LA-ICP-MS) element data. These two eruptions are compositionally distinct from each other. The high phenocryst content of proximal Lower and Upper Pumice tephra means that the differences between the two magmas are masked in whole-rock analyses. We demonstrate that all previous reports of distal marine and continental tephras correspond to the younger Upper Pumice not the older Lower Pumice. This has important implications for the marine tephrochronology of the Mediterranean and for the age of the Upper and Lower Pumice eruptions. We suggest an age of at least 47ka for the Upper Pumice and an older, unconstrained age for the Lower Pumice. [Copyright &y& Elsevier]

Abstract: A 27cm thick, loose tephra deposits has been observed within coarse-grained colluvium at Çardak area of Denizli on the northern apron of the Acıgöl graben in western Anatolia, Turkey. It is a biotite-rich tephra with mean grain-size of coarse silt. Chemically it is dacite and rhyolite with average of 66.35% SiO2 and 4.70% alcalia (Na2O+K2O) in bulk analyses. It was deposited a time between 5380±90 and 2395±65yrscal. BP, possibly between 4750 and 3385yrscal. BP according to 14C dating of two palaeosol layers within the colluvium. This is only and the thickest Holocene air-fall tuff layer found in terrestrial sediments in this region so far, and according to present knowledge there is no young volcanic source for such a formation in western Turkey. Analyses and comparisons of the analytic results with those of various young volcanic rocks suggest that the Çardak tephra originated from a volcanic source in the Aegean Sea. Besides, its geochemical composition, formation time and significant thickness represent probably a separate late Holocene explosive eruption in the eastern Mediterranean, apart from the Santorini event. [Copyright &y& Elsevier]

Abstract: The Kyra sequence is a volcanic eruption sequence originating from the eastern flank of Nisyros volcano, Greece. Its eruptions products can be found not only on Nisyros itself but also on the nearby non-volcanic island of Tilos. In an extensive sampling campaign, outcrops of the Kyra eruption products on Nisyros were sampled and corresponding samples on Tilos were taken. The clear stratigraphical relationship between the different units within in the individual outcrops, combined with the chemical information gained by the application of instrumental neutron activation analysis (INAA) to the samples, made a detailed chemo-stratigraphy of the complete eruption sequence possible. It can be shown that the sequence is separated into eight distinguishable eruptions. Furthermore, no eruption products of the caldera-forming eruptions from Nisyros (Lower- and Upper Caldera Pumice) or from Santorin were found on Tilos. [Copyright &y& Elsevier]

Abstract: In this work, we investigated the water–rock interaction processes taking place in the hydrothermal reservoir of Nisyros through both: (1) a review of the hydrothermal mineralogy encountered in the deep geothermal borehole Nisyros-2; and (2) a comparison of the analytically-derived redox potentials and acidities of fumarolic-related liquids, with those controlled by redox buffers and pH buffers, involving hydrothermal mineral phases. The propylitic zone met in the deep geothermal borehole Nisyros-2, from 950 to 1547m (total depth), is characterised by abundant, well crystallised epidote, adularia, albite, quartz, pyrite, chlorite, and sericite–muscovite, accompanied by less abundant anhydrite, stilpnomelane, wairakite, garnet, tremolite and pyroxene. These hydrothermal minerals were produced in a comparatively wide temperature range, from 230 to 300°C, approximately. Hydrothermal assemblages are well developed from 950 to 1360m, whereas they are less developed below this depth, probably due to low permeability. Based on the R H values calculated for fumarolic gases and for the deep geothermal fluids of Nisyros-1 and Nisyros-2 wells, redox equilibrium with the (FeO)/(FeO1.5) rock buffer appears to be closely attained throughout the hydrothermal reservoir of Nisyros. This conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite, since R H values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation. The pH of the liquids feeding the fumarolic vents of Stephanos and Polybote Micros craters was computed, by means of the EQ3 code, based on the Cl–δD relationship which is constrained by the seawater–magmatic water mixing occurring at depth in the hydrothermal–magmatic system of Nisyros. The temperature dependence of analytically-derived pH values for the reservoir liquids feeding the fumarolic vents of Stephanos and Polybote Micros craters suggests that some unspecified pH buffer fixes the acidity of these reservoir liquids at values of 4.72–4.85 and 4.88–5.23, respectively. Many of these pH values are lower than those expected for the full-equilibrium condition, although they are close to those of the reservoir liquids of Nisyros-1, 5.16, and Nisyros-2, 4.87. It is likely that this excess of acidity-producing species, chiefly CO2, promotes release of Fe(II) and Fe(III) to the reservoir liquids through rock dissolution, permitting the attainment of redox equilibrium with the (FeO)/(FeO1.5) rock buffer, as already suggested by the late Werner Giggenbach. [Copyright &y& Elsevier]

The investigation of heterogeneous magma systems enhances the understanding of magma differentiation and transfer processes in active volcanoes, thus constraining the dynamics driving the eruptions and the related hazard. Magma heterogeneity is generally preserved in the coeval juvenile products of explosive eruptions, as it occurs in the Upper Pumice sequence, emplaced by the last sub-Plinian explosive eruption at Nisyros volcano (Greece). The deposit comprises a basal fallout, overlaid by pyroclastic density current units, followed by a lag-breccia level. White-yellow, porphyritic, rhyolitic pumices constitute the main juvenile component. Grey, crystal-rich juvenile clasts (CRCs) are less abundant (up to 10–15%), and are characterised by three different texture types (Type-A, -B and -C), with specific recurrence in the different depositional units and well correlated to the magma evolution. In the basal unit CRCs occur as andesitic to dacitic lapilli with Type-A and -B vesicular textures associated with highly variable trace element and isotopic compositions. In the lag-breccia deposit, the juvenile clasts occur as bombs with crenulated or bread-crust surfaces, displaying diktytaxitic Type-C textures and less evolved andesitic compositions, covering a larger Nd-isotope range at lower Sr-isotopes compared to the others. The CRCs are interpreted as the result of the rapid cooling of more mafic magma blobs sequentially intruded in the cooler rhyolitic host magma, in which they attained variable textures by different undercooling conditions, due to their variable compositions. We suggest that a two-stage AFC (Assimilation plus Fractional Crystallisation) process occurred at different pressures, before intrusion in the host magma, accounting for their heterogeneous chemical and isotopic characteristics. Firstly, the most primitive melts variably assimilated gneissic wallrock at depth, acquiring a variable Nd-isotope signature. On the way to the surface, they later experienced shallow AFC processes within different small magma reservoirs, involving heterogeneous carbonate-rocks such as pure limestone, metasomatised marble and skarn. Sequential dynamics of ascent and intrusion into the rhyolitic magma chamber lead the more evolved and skarn-contaminated Type-A and -B melts to firstly move in the upper part of the reservoir to be erupted in the early fallout deposits. Type-C more mafic melts later intruded the rhyolitic reservoir and were erupted in the lag-breccia deposit. The lowest Nd-isotopes recorded by CRCs, with respect to all the volcanic products of the Kos-Nisyros volcanic field, reveal the peculiar transient history for these magmas at relatively shallow levels in the crust. The CO 2 release from the carbonate-rock assimilation has also possibly contributed to trigger the explosive eruption, discharging a large amount of CO 2 into the atmosphere. • Trace element and isotope decoupling reveals independent differentiation processes. • Selective assimilation of heterogeneous wallrock affects key trace element contents. • Small magma systems record the fingerprint of crustal assimilation. • The lowest Nd isotopes reveal the peculiar transient history of mafic magmas. • Textural versus chemical heterogeneity are induced by different processes. [ABSTRACT FROM AUTHOR]

Abstract: We use new on-land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at Nisyros Island (Greece), which has an active caldera that shows a complicated network of faults, fractures and volcano-tectonic structures. We measured 157 faults that show dominant dip-slip normal motions along planes mainly striking NE–SW, NNE–SSW, and NNW–SSE. Inside the caldera, dykes, necks and morphometric parameters of volcanic domes, explosion craters and fumarole pits indicate the control by NE-striking discontinuities on magma and gas paths. The NNW- and NE-striking faults bound a major block that underwent repeated downthrow and uplift movements during the late Pleistocene–Holocene. Experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass-shaped fault pattern, as seen in plan view, with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank. All these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres, whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses, can account for the overall deformation field. [Copyright &y& Elsevier]

Abstract: A composite 40.2 m sediment sequence from Megali Limni, Lesvos Island, Greece, spanning the interval from 22 to 62 ka BP, contains six Pleistocene tephra layers, which can be identified on the basis of their major and trace elemental compositions. The youngest tephra deposit correlates with the 22 ka Y-2 marker, erupted during the Cape Riva eruption of Santorini. The second tephra originates from the Campanian area in Italy and corresponds to the Campanian Ignimbrite eruption (the 39 ka Y-5 marker). The next two layers are compositionally identical, and originate from Eastern islands of the Hellenic Arc; they could potentially correspond to the Upper and Lower Pumice eruptions of Nisyros or some other previously unidentified eruption from Yali. The fifth stratum corresponds to the Y-6 marker, which erupted during the Green Tuff eruption of the Pantelleria Island. The source of the last tephra layer is either the Yali–Nisyros complex or Central Anatolia; no specific corresponding eruption has yet been found. Long distal tephra sequences within terrestrial settings are rare in the eastern Mediterranean and the Megali Limni record contains the most complete sequence to date. Moreover, its age model, based on independent chronologies such as AMS and conventional radiocarbon dates and high-resolution pollen stratigraphy, provides new ages for these eruptions and places them within the context of the millennial-scale palaeoenvironmental variability of the last glacial period. Finally, the recognition of the Pantelleria tephra in this locality considerably extends the confirmed Northerly and Easterly distribution of this widespread and distinctive unit, while the existence of the Nisyros–Yali tephras on Lesvos refutes the current belief of limited tephra dispersal and small magnitude of these eruptions. [Copyright &y& Elsevier]