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The effects of the composition and angle of the subducting slab and mantle wedge flow on tectonic and magmatic processes in island arcs and associated back-arcs are poorly understood. Here we analyse the ages and compositions of submarine lavas from the flanks and the floor of the back-arc Futuna Trough some 50 km east of Tanna Island in the New Hebrides arc front. Whereas >2.5 Ma-old back-arc lavas formed from an enriched mantle source strongly metasomatized by a slab component, the younger lavas show less slab input into a depleted mantle wedge. The input of the slab component decreased over the past 2.5 million years while the enriched mantle was replaced by depleted peridotite. The change of Futuna Trough lava compositions indicates rapid (10 s of km/million years) replacement of the mantle wedge by corner flow and slab steepening due to rollback, causing extensional stress and back-arc rifting in the past 2.5 million years. A compositional change in lavas from the Futuna trough at 2.5 Ma suggest a change in mantle source, attributed to slab rollback and the opening of the back-arc basin. [ABSTRACT FROM AUTHOR]

Porphyry deposits host various trace elements in economic amounts, but the hydrothermal processes causing their fractionation and enrichment are still not fully understood, but vital to target the most prospective mineralisation. We present the first micro-analytical study on the trace element composition of pyrite from the Koloula Cu-Au porphyry in the young and thin (<25 km) Solomon Islands arc. A statistical evaluation of the trace element data of pyrite indicates that mineral inclusions obscure the chemical signature of the mineralisation processes. The filtered pyrite data, from which the inclusions were excluded, correlate with variations in temperature and salinity, as defined by fluid inclusions from several alteration zones. Trace element ratios in pyrite show systematic variations with fluid temperature and salinity (Co/Ni, Se/Te), phase separation (Co/Ni, Co/As) and mixing of magmatic fluids and meteoric waters (Se/Ge). The mineralisation in the potassic alteration zone was controlled by the formation of a hypersaline liquid (Co/As > 1, Co/Ni > 50) from a magma-derived fluid (Se/Ge ~ 100) at lithostatic pressure conditions and temperatures of up to 700 °C (Se/Te > 50). This was followed by a transition to hydrostatic pressure conditions due to open-fracture continuity towards the paleo-surface, marking the onset of boiling in the chlorite-sericite to sericitic alteration zone (Co/As < 1), where minor proportions of meteoric water were involved (Se/Ge < 100). The shallowest part of the mineralisation is controlled by lower temperature (<300 °C, Se/Te < 50) vapour-rich fluids (Co/As < 1) that condensed into meteoric waters in an epithermal transition zone (Co/Ni < 50, Se/Ge < 100). Trace element systematics in pyrite from progressive alteration zones therefore preserve the time-space evolution of porphyry (-epithermal) systems in young and thin oceanic island arcs. [ABSTRACT FROM AUTHOR]

Immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings. Here we report on compositional in situ data of magmatic sulphides from mid-ocean ridge, back-arc and island arc magmatic systems. Iron-Ni-rich monosulphide solid solutions (mss) that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules. In subduction-related magmas mss are most abundant in relatively evolved melts and characterised by low Ni and high Fe contents, as well as low Ni/Cu ratios (< 1). In contrast, mss from mid-ocean ridges and back-arcs without subduction input occur in mafic rocks and have high Ni/Cu ratios (> 1). Thus, mss chemistry varies systematically in lavas from the different tectonic settings in terms of their Fe-Ni distribution and Ni/Cu ratio. The observed mineralogical and chemical variations between mss from the different settings reflect early S saturation accompanied by olivine fractionation along mid-ocean ridges compared to later stage S saturation in arc systems associated with Fe-Ti oxide fractionation. These systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid-ocean ridge melts. Large Ni/Cu variations in mss from the same locality and in individual samples suggest that S saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma. [ABSTRACT FROM AUTHOR]

Abstract: Dacitic to rhyolitic glasses from Brothers Seamount in the southern Kermadec island arc lie on very tight major element trends and have formed by fractional crystallization from a basaltic magma rather than by partial melting of amphibolitic lower crustal rocks. The radiogenic isotope and highly incompatible element ratios of the dacites resemble the composition of basalts from other volcanoes in the same segment of the island arc which probably represent analogues to the basaltic parental magma of the Brothers Seamount lavas. Glass compositions show that the magmas did not degas extensively but retained much of their volatile inventory with very high Cl (4000 to 7000 ppm) but low water contents (∼2 wt.%). The Cl/K of 0.25 is constant in the Brothers Seamount lavas and probably reflects the composition of a sedimentary slab component in the subarc mantle below the volcano. The Brothers Seamount Cl/K reflects a sedimentary fluid rather than a melt and is lower than the Cl/K observed in Valu Fa Ridge lavas showing an influence of subducted altered basaltic crust, probably due to the higher K contents of sediments than in basalts. [Copyright &y& Elsevier]

Boiling is a crucial process triggering ore formation in magmatic-hydrothermal systems and controlling the enrichment of precious and rare metal(loid)s in epithermal-porphyry mineralizations. Steep physicochemical gradients during boiling of hydrothermal fluids at shallow water depths caused metal(loid) precipitation along a 3 km long Pb-Zn-Ag vein system on Milos Island in the South Aegean Volcanic Arc. We present new trace element and Pb, Sr, and S isotope data from sulfides providing insights into the diversity of mineralization processes in shallow-marine hydrothermal systems. Lead and Sr isotope compositions of sulfides and sulfates reflect the mixing of fluids that reacted with metamorphic basement and the volcanic host rocks, whereas some of the S were derived from seawater. Investigation of mineralized samples along the Kondaros-Vani fault zone revealed distinct chemical variations that represent a vertical profile through the boiling zone of a hydrothermal system. Boiling during fluid ascent at decreasing temperatures (230–150 °C) and sulfur fugacities triggered the precipitation of sulfides rich in Zn, Pb, Fe, Cu, Ag, Sb, and As, resulting in increasing Pb, Ag, and Sb contents with decreasing depth. A pyrite group with high Tl/Cu and low As/Sb ratios, as well as δ34S VCDT values reaching as low as −6‰, is interpreted as precipitating from high-Cl liquids that underwent vigorous boiling at deeper crustal levels subsequent to tectonic faulting. Condensation of vapor (high Hg, Bi, and As contents) and mixing with seawater (high Mo contents) in the shallow-marine subseafloor sequences at Vani caused Hg-Bi-Mo-As-rich mineralization similar to the active hydrothermal system at Milos. [Display omitted] • Boiling controls the distribution of metal(loid)s in subseafloor veins. • Silver concentrations increase towards the seafloor during fluid ascent and boiling. • Negative δ34S values in pyrite are related to vigorous boiling at deeper levels. • Vapor condensation and seawater mixing cause high Hg, Mo, Tl, Bi, and As contents. [ABSTRACT FROM AUTHOR]

The calc-alkaline lavas of the submarine Pausanias Volcanic Field and the neighboring Methana peninsula represent the largest volcanic edifices of the western South Aegean Volcanic Arc (SAVA) in the Mediterranean Sea and erupted on 30–35 km thick continental crust. We report the first whole rock major and trace elements together with Sr-Nd-Pb isotope ratios for basaltic to andesitic lavas from the submarine Pausanias Volcanic Field consisting of six smaller, volcanic structures. The mafic lavas have the most primitive compositions of the SAVA with MgO of up to 9 wt% and Ni concentrations >100 ppm. The incompatible trace element abundance patterns of the Pausanias and Methana lavas overlap and have typical island arc patterns, but Pausanias lavas display a larger geochemical heterogeneity on a smaller spatial scale compared to those from Methana. The Pausanias lavas resemble those from Methana in Sr, Nd, and Pb isotopes reflecting higher sediment subduction in the western SAVA than in the central arc at Santorini. The variability of incompatible element concentrations at relatively constant isotope compositions suggests variable degrees of partial melting of a peridotite-sediment mélange diapir in the mantle wedge. The Pausanias magmas formed from a less depleted mantle source compared to the central SAVA and their ascent in an extensional basin supported relatively rapid ascent without the extensive stagnation in the crust observed in the Methana magmas. The compositional differences between Pausanias and Methana imply differences in their composition and/or transport in the mantle wedge and the crust, allowing highly variable melts to reach the surface within a spatial distance of few kilometers. • Small scale mantle heterogeneity recorded at the Pausanias Volcanic Field. • Melting of sediment-rich mélange diapirs contribute to Pausanias lavas. • At Pausanias, extension allows melts to ascend more direct through the crust compared to Methana. [ABSTRACT FROM AUTHOR]

We present here new bathymetric, petrological and geochemical whole rock, glass and mineral data from the submarine Epi volcano in the New Hebrides (Vanuatu) island arc. The structure has previously been interpreted to be part of a larger caldera structure but new bathymetric data reveal that the volcanic cones are aligned along shear zones controlled by the local tectonic stress field parallel to the recent direction of subduction. We aim to test if there is an interaction between local tectonics and magmatism and to what extent the compositions of island arc volcanoes may be influenced by their tectonic setting. Primitive submarine Epi lavas and those from the neighbouring Lopevi and Ambrym islands originate from a depleted mantle wedge modified by addition of subduction zone components. Incompatible element ratios sensitive to fluid input (e.g., Th/Nb, Ce/Yb) in the lavas are positively correlated with those more sensitive to mantle wedge depletion (e.g., Nb/Yb, Zr/Nb) amongst the arc volcanoes suggesting that fluids or melts from the subducting sediments have a stronger impact on the more depleted compositions of the mantle wedge. The whole rock, glass and mineral major and trace element compositions and the occurrence of exclusively normally zoned clinopyroxene and plagioclase crystals combined with the absence of inversely zoned crystals and water-bearing phases in both mafic and evolved lavas suggest that the erupted melt was relatively dry compared to other subduction zone melts and has experienced little disequilibrium modification by melt mixing or assimilation. Our data also imply that differentiation of amphibole is not required to explain the incompatible element patterns but may rather result from extensive clinopyroxene fractionation in agreement with petrographic observations. Thermobarometric calculations indicate that the melts fractionated continuously during ascent, contrasting with fractionation during stagnation in an established crustal magma reservoir. We interpret the occurrence of this fractional crystallisation end-member in a relatively thick island arc crust (~30 km thickness) to result from isolated and relatively rapid ascent of melts, most likely through a complex system of dykes and sills that developed due to the tectonic positioning of Epi in a complex tectonic zone between a compressional environment in the north and an extensional setting in the south. We can show that the alignment of the cones largely depends on the local tectonic stress field at Epi that is especially influenced by a large dextral strike-slip zone, indicating that structural features have a significant impact on the location and composition of volcanic edifices. [ABSTRACT FROM AUTHOR]

We provide new geochemical and isotope data for lavas from six young submarine volcanoes from the northernmost oceanic Tonga island arc. These lavas have low concentrations of moderately incompatible trace elements, but are enriched in Nb and Ta compared to all other Tonga island arc lavas, and their Nd, Hf and Pb isotope compositions resemble those of rejuvenated lavas from the nearby Samoa hotspot. These unusual compositions indicate that the mantle source of North Tonga lavas was depleted by prior melting events, before re-enrichment by a partial melt enriched in highly incompatible elements with Nd, Hf and Pb isotope compositions similar to Samoan intraplate lavas. Samoa plume mantle is flowing southwards beneath the northern Tonga island arc and Lau Basin, in part guided by rifting of the Tonga plate lithosphere. A fluid component was later added from a mixture of subducted pelagic sediment and subducting Louisville Seamount Chain volcanic material. Our new data show that the Louisville component is only found in lavas from volcanoes occurring along ∼100 km of the arc front, indicating that the oceanic crust loses Pb by dehydration over a relatively small depth interval during subduction. The compositions of lavas from the northern Tonga island arc are therefore influenced by material from two hotspots; southwards inflow of mantle from the active Samoa plume beneath the arc within the past 6 Ma, and a subduction component from >80 Ma old subducted Louisville Seamount Chain basalts. [ABSTRACT FROM AUTHOR]