Search

Highlights • Nisyros (rhyo)dacites are unhybridized melts extracted from a cumulate mush. • The peritectic crystallisation of amphibole buffers the composition of the melts. • High compatible element contents emphasise the role of cumulate resorption. • Crustal melting is not required to generate rhyolitic melts. • Peritectic melts are a common global endmember in the monotonous intermediates. 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.% SiO2) 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.

The morphology and the activity of a submarine caldera, Avyssos, at the northern part of Nisyros volcano in the South Aegean Sea (Greece), has been studied by means of remotely operated underwater vehicle dives. The recorded time series of temperature and conductivity over the submarine volcano have been analyzed in terms of the Generalized Moments Method. The findings of the mathematical analysis shed light on the volcanic activity, but also on the morphology (shape) of the submarine volcano. The conductivity time series indicates the volcano is at rest in agreement with other types of observations. On the other hand, temperature fluctuations, which in general describe a multifractal process, show that the submarine caldera operates as an open system that interacts with its surroundings. This type of analysis can be used as an indicator for the state of activity and the morphological structure (closed or open system) of a submarine volcano.

Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.

Στο νησί της Νισύρου, σαν αποτέλεσμα της ηφαιστειακής δραστηριότητας και των ενεργών τεκτονικών διεργασιών, αναπτύσσεται ένα υδροθερμικό σύστημα, το οποίο παρουσιάζει πέντε τύπων επιφανειακές εκδηλώσεις: α) θερμές πηγές, β) ατμίδες γ) υδροθερμικούς κρατήρες, δ) θερμά εδάφη και ε) αναβράζουσες ηφαιστειακές λίμνες λάσπης. Οι αναβράζουσες ηφαιστειακές λίμνες λάσπης μπορούν να περιγραφούν και σαν όξινες θερμές πηγές ή ατμίδες, με περιορισμένη ποσότητα νερού, σε υψηλής θερμοκρασίας ηφαιστειακά περιβάλλοντα. Κατά την παρούσα μελέτη, ελήφθησαν δείγματα νερού και αποθέσεων από τον υδροθερμικό κρατήρα του Στέφανου. Η περιοχή του κρατήρα είναι το μόνο σημείο στο οποίο εμφανίζονται αναβράζουσες ηφαιστειακές λίμνες λάσπης στη Νίσυρο. Το νερό είναι ιδιαιτέρα όξινο (pH=2.4), με υψηλές συγκεντρώσεις θεϊκών (1375mg/L), λόγω του αερίου H2S) και θερμοκρασία κοντά στο σημείο βρασμού. Αποτέλεσμα του γεγονότος αυτού είναι η δημιουργία του στοιχειακού θείου σε κρυστάλλους που εντοπίζεται στις αποθέσεις. μαζί με προϊόντα της υδροθερμικής εξαλλοίωσης των περιβαλλόντων πετρωμάτων. Κατά τη χημική ανάλυση του νερού και των αποθέσεων εντοπίστηκαν υψηλές συγκεντρώσεις σε μια σειρά 1839 στοιχείων (π.χ. στο νερό: 55mg/L Fe; 19.5mg/L Zn, στις αποθέσεις: 430mg/Kg Pb; 72mg/Kg Cu; 60mg/Kg Cr) σαν αποτέλεσμα των διεργασιών της υδροθερμικής εξαλλοίωσης που λαμβάνει χώρα στην περιοχή., On Nisyros island as a result of the volcanic activity and active tectonic, a hydrothermal system develops and it is expressed by 5 types of surface manifestations: i) thermal springs, ii) fumaroles iii) hydrothermal craters, iv) hot grounds and v) mudpots. In general, a mudpot could be described as an acidic hot spring and fumarole with limited water which it is formed in high temperature geothermal areas. Water sample and depositions of mudpots collected, analyzed and studied from Stefanos hydrothermal crater, which is the only site on Nisyros Island, where mudpots occur. Mudpots water is very acidic (pH=2.4), with high sulfate concentration (1375mg/L), due to the H2S(gas) and temperature near the boiling point. As a result, elemental sulfur is found inside the depositions alongside with products of the hydrothermal alteration of the surrounding rocks. In the water and in the depositions were found high concentrations in several elements (e.g. in water: 55mg/L Fe; 19.5mg/L Zn, in depositions: 430mg/Kg Pb; 72mg/Kg Cu; 60mg/Kg Cr) reflecting the alterations processes which are taking place.

Almost three-quarters of known volcanic activity on Earth occurs in underwater locations. The presence of active hydrothermal vent fields in such environments is a potential natural hazard for the environment, society, and economy. Despite its importance for risk assessment and risk mitigation, the monitoring of volcanic activity is impeded by the remoteness and the extreme conditions of many underwater volcanoes. The morphology and the activity of the submarine caldera, Avyssos, at the northern part of Nisyros volcano in the South Aegean Sea (Greece), were studied using a remotely operated underwater vehicle. The recorded time series of temperature and conductivity over the submarine volcano have been analyzed in terms of the Generalized Moments Method. This type of analysis can be used as an indicator for the state of activity of a submarine volcano. Here, we expand the work conducted for the first time in 2018. We present the findings of the geological exploration and the mathematical analysis, obtained from the data collected in October 2010. The temperature and conductivity time series show minor fluctuations in a rather stable environment. Based on these results, the impact of developing appropriate mechanisms and policies to avoid the associated natural hazard is expected to be important.