Karen St. Seymour

Magma mixing at Nisyros volcano, as inferred from incompatible trace-element systematics

Abstract Rocks of the calc-alkaline volcanic complex of Nisyros, at the eastern end of the Quaternary South Aegean volcanic arc, range in composition from basaltic andesite to rhyodacite. Major- and compatible trace-element systematics are dominated by crystal fractionation, involving olivine, clinopyroxene, plagioclase, and magnetite in the mafic to intermediate compositional range and plagioclase, clinopyroxene, hypersthene, Fe-Ti oxides, and apatite in the intermediate to silicic range, although there is a marked paucity of lavas with 60–66 wt.% SiO 2 . The occurrence of andesitic inclusions in pre- and post-caldera dacites as well as reversely zoned and/or partly resorbed phenocrysts, exotic mineralogy (e.g., forsteritic olivine), compositionally bimodal plagioclase phenocryst populations, and the observation of two distinct glass phases in melt inclusions and pumices all indicate that magma mixing processes have played a role in the petrogenesis of Nisyros. Incompatible trace-element systematics are consistent with mixing between mafic and silicic end members. Major-element binary mixing calculations between end members successfully reproduce the dacite composition. The eruptive sequence at Nisyros suggests the existence of a compositionally zoned magma chamber, in which mixing between underlying more mafic magma and overlying rhyodacitic melt result in the formation of dacitic magmas. Inherent mineral assemblages in basaltic andesites indicate that mixing between mafic liquids and between mafic and more silicic liquids played also a role in their petrogenesis.

Geochemistry of the Grenville Dyke Swarm: role of plume-source mantle in magma genesis

Major and trace element data of samples from the approximately 590 Ma Grenville dyke swarm, in the southeastern parts of the Canadian Shield, were used to evaluate the extent of crustal contamination of the dyke magma and to model its probable mantle source. The dyke rocks are mostly saturated, quartz tholeiites. A few samples represent transitional basalts. Their overall chemistries are similar to those of continental flood basalts. Their compositions resemble those of liquids initially produced in the garnet peridotite mantle zone and then subjected to polybaric fractionation. However, fractionation models fail to mass-balance Ti, Fe, K, and P. Geochemical and isotopic criteria indicate that crustal contamination was minimal. The incompatible elements Zr, Y, K, Rb, Nb, V, Ta, Hf, and Th show hyperbolic elemental ratio-ratio distributions indicative of mixing between two end-members. One end-member has elemental ratios similar to those of a “depleted” N-MORB source and the second of a P-MORB i.e. ◂ertile” or “plume” MORB source. The more enriched dyke compositions can be successfully reproduced by mixing models in the ratio of 6:1 between the most depleted “MORB-like” dyke magma and a liquid composition similar to metasomatic glass veinlets coexisting with mica peridotite mantle xenoliths, and subsequent crystal fractionation. The Grenville dykes are probably a synrift swarm that was emplaced along a rift arm related to an Iapetan rrr triple junction that is thought to have formed over a mantle plume. The chemistry of the dyke swarm is consistent with its derivation from a plume head, in that the minor enriched source can be identified with plume-source mantle and the major depleted source with entrained mantle. The magma probably originated in the upper cooler parts of the plume head.

The potential for future explosive volcanism associated with dome growth at Nisyros, Aegean volcanic arc, Greece

Monitoring of the explosivity-controlling parameters in magmas associated with dome growth in the Quaternary volcano at Nisyros, Aegean Sea involves determining the chemical composition, including dissolved volatiles (H2O, CO2, F, Cl, and S), of melt inclusions in phenocrysts and interstitial matrices in pre- and post-caldera volcanic products. Our results indicate that the volatile content of the pre-caldera magma was a dominant factor responsible for the catastrophic explosivity of the caldera-forming episode. The relatively high estimated volatile contents and apparent interstitial viscosities of the post-caldera magmas, as well as the geophysical evidence for the existence of an active magma chamber beneath Nisyros, all suggest that this volcano is presently dormant but has a high potential for future explosive volcanism.

A study of scale deposition: An analogue of meso- to epithermal ore formation in the volcano of Milos, Aegean arc, Greece

Abstract The viability of boiling as a mechanism for meso- to epithermal ore formation has been tested in the Milos volcano, Aegean Island Arc, by investigating rates of deposition and composition of scale, and by drawing an analogy between the results and actual field occurrences on the island. Milos offers ideal conditions for such testing: it consists of predominantly felsic volcanic rocks with numerous late-stage hydrovolcanic (phreatic) breccias. A geothermal system is at present active in the area, with numerous terrestrial and submarine hot spring outlets and kaoline and bentonite alteration zones. AuAg bearing polymetallic, as well as manganese and barite ore deposits, occur on the island; scaling tests were made possible after the installation of a 2 MW geothermoelectric plant on Milos. The rates of scale deposition were measured during different test times and analyses made on the chemistry and mineralogy of scales from the Milos plant; the results indicate that the rates of scale deposition are higher near the flashing valve, and that the metals Pb, Cu, Zn are deposited as sulphides in the vicinity of the flashing point, particularly PbS, which has the lowest solubility and, to a lesser degree, Cu-sulphide. ZnS is the dominant sulphide downstream. Higher scaling rates are observed in tests which are suspect of a higher incidence of two-phase flow in the pipe. Pressure, temperature and salinity conditions of the hydrothermal fluid are conducive to phase separation in the upper parts of the Milos geothermal system and down to a depth of 1000–1100 m but, as scaling tests indicate, self-sealing of the system could rapidly halt hydrothermal flow and ore formation; however, hydrovolcanic (phreatic) explosions may initiate new cycles of ore-deposition. The metals have mainly precipitated as sulphides, except for Fe, which is found mostly in the siliceous matrix of the scale. The paragenesis of the metals in the Milos scales is similar to those of AuAg bearing polymetallic deposits on the island. Domains with signs of multiple episodes and craters of hydraulic fracturing, argillic alteration, siliceous sinters and gossaneous cappings, should be targeted on Milos for exploration for meso- to epithermal polymetallic and precious metal ores.

Monitoring oxygen fugacity conditions in pre-, syn- and postcaldera magma chamber of Nisyros volcano, Aegean island arc, Greece

Abstract The magmatic oxygen fugacities of the pre-, syn-, and postcaldera products of the Quaternary volcano of Nisyros in the Aegean Volcanic Arc, eastern Mediterranean Sea, have been measured from analyzed compositions of titaniferous magnetite-ilmenite equilibrium pairs. Quench temperatures in the precaldera rhyodacitic flows range from 877 to 836°C and oxygen fugacities from −11.4 to −11.8 log units. One sample of the climactic pumice displays average quench temperatures of 853°C with f O 2 of −11.7 log units. Crystallization temperatures of the dacitic domes that intercept the Nisyros caldera range from 843 to 804°C while oxygen fugacities range from −11.4 to −12.1 log units. The rocks of the Nisyros volcano record no significant change in oxygen fugacity relative to buffer curves immediately prior, during and after the caldera-forming event. This is surprising given the considerable potential for interaction between the magmas and atmospheric oxygen during caldera collapse and the concomitant massive exsolution of volatiles. However, the Nisyros fugacity data may not be atypical of stratovolcanoes with products of similar composition and histories of caldera-forming events. We speculate that only in the volcanoes where caldera collapse coincides with magma-meteoric water interaction do the climactic or early postcaldera units show anomalously high oxygen fugacities.

Mapping the spatial and temporal SST variations in the Red Sea, revealing a probable regional geothermal anomaly using Pathfinder V5 data

The idea is presented that satellite-derived night-time thermal maps can be used for the identification of regional sea surface temperature (SST) anomalies that may be related to geothermal processes. The analysis of night-time monthly Pathfinder V5 SST data indicated that November is the month for which the SST seasonal variability is minimized while the impact of the assumed SST anomaly is maximized. The methods used included SST domain slicing, frequency histogram interpretation and time series analysis (TSA using standardized principal component analysis, PCA) of the multi-temporal dataset while K-means cluster analysis identified a negative (cold) SST anomaly. The spatial pattern of SST variations includes a major SST anomaly south of 21° N and a stair step SST variation to the north. SST November data for an 8-year period were used to verify the spatial pattern of the thermal anomaly. Ocean dynamics and water circulation, as well as possible perennial dust storms from the Arabian Shield over the Red Sea and the degassing of underwater soft sediments, were considered as dominant factors controlling the SST anomalies observed. Because of their spatial and temporal structure, the SST variations are more likely to be related to geothermal-geotectonic activity in the Red Sea.The idea is presented that satellite-derived night-time thermal maps can be used for the identification of regional sea surface temperature (SST) anomalies that may be related to geothermal processes. The analysis of night-time monthly Pathfinder V5 SST data indicated that November is the month for which the SST seasonal variability is minimized while the impact of the assumed SST anomaly is maximized. The methods used included SST domain slicing, frequency histogram interpretation and time series analysis (TSA using standardized principal component analysis, PCA) of the multi-temporal dataset while K-means cluster analysis identified a negative (cold) SST anomaly. The spatial pattern of SST variations includes a major SST anomaly south of 21° N and a stair step SST variation to the north. SST November data for an 8-year period were used to verify the spatial pattern of the thermal anomaly. Ocean dynamics and water circulation, as well as possible perennial dust storms from the Arabian Shield over the Red Sea and the degassing of underwater soft sediments, were considered as dominant factors controlling the SST anomalies observed. Because of their spatial and temporal structure, the SST variations are more likely to be related to geothermal-geotectonic activity in the Red Sea.

Recurrent Igneous Activity and Movements on Deep Faults Inherited from the Sutton Mountains Triple Junction

The Sutton Mountains salient of the Appalachians in southeastern Quebec and northern Vermont has been interpreted as having been inherited from an Eocambrian-early Cambrian RRR triple junction. The approximately N-S trending normal faults of the Champlain Valley and eastern Adirondacks, and the NE trending normal faults along the St. Lawrence Valley, probably represent east-dipping listric faults which formed parallel to the rift arms that underwent expansion to oceanic dimensions, the Ottawa graben being the failed arm.

Kos Plateau Tuff (KPT) on Kalymnos island, Aegean volcanic arc: A geochemical approach

Thirteen pumice samples from the D and E ignimbrite units of Kalymnos Tuff have been analyzed for their biotite and feldspar phenocryst mineral chemistry and for bulk major and 20 trace, including 14 Rare Earth elements, to define and compare their petrochemistry with the Kos Plateau Tuff (KPT). For the same purpose major element analyses were obtained from Kalymnos Tuff and KPT glasses. Both KPT and Kalymnos pumice lapilli are rhyolites characterized by a well-developed ‘silky’ texture and roundish quartz. Phenocrysts of biotite and feldspars (sanidine, oligoclase) from both tuffs display compositional overlap. Crystals are charac-terized by undulatory extinction (quartz), fractures (sanidine, oligoclase) and bent cleavages (biotite) due to the explosive origin of their host. Both tuffs show well-defined petrogenetic trends and extensive compositional overlaps on major and trace element variation diagrams suggesting that they are consanguineous. However, D ignimbrite samples are more evolved than those obtained from E ignimbrite as indicated from major elements, alkali earths (Ba, Rb, Sr), immobile (Zr, Y), compatible (V) and hygromagmatophile trace element (Th) distributions. This evidence indicates a stratified magma chamber under a ∼16 Km caldera superstructure which is mostly submarine.