This study investigates Mg isotopes (δ26Mg) in vent fluids from Milos, Aegean Sea, to evaluate phase separation and secondary mineral formation. The δ26Mg vary significantly in Milos, exceeding 0.66‰, allowing for the classification of the fluids into three sub-groups based on chemical characteristics: seawater-like, cave fluids, and submarine-brines. The seawater-like fluids exhibit large δ26Mg variation, −0.64 to −1.18‰, and mostly follow a Rayleigh fractionation trend, with a fractionation factor α = 1.00020 ± 0.00011. The cave fluids are highly acidic, have low Cl, are vapor-rich, and display heavy δ26Mg compositions (−0.52 to −0.63‰). The submarine-brines are characterized by high Cl, high non-volatile metals, and light δ26Mg (−0.65 to −1.00‰). The latter two fluid types represent vapors and brines, respectively, which underwent phase separation at depth in Milos. These δ26Mg values were combined with major/trace elements, as well as Li and B isotopes, to explore possible controlling mechanisms. We report for the first time a shallow submarine hydrothermal system that has a vapor component enriched in heavy δ26Mg, but with no detectable isotopic changes in the brines. It is evident that δ26Mg in vent fluids is unique for separating effects of water/rock interaction and secondary mineral and phase separation at shallow-water systems.

The active venting fluids of Milos Island, located within the southern Aegean Sea, belong to a shallow-water hydrothermal system (< 200 m depth) that shows chemical compositions and evolution processes comparable to those of mid-ocean ridges (MOR). In this study, we analyze Li and delta Li-7 in 69 vent water samples, grouped into two types based on their salt content. The low-Cl end-member (EM) Cave fluids show a relatively high Li content (0.39-0.54 mM) with MORB-like delta Li-7 (similar to 4.5 parts per thousand, MORB = 3.7 parts per thousand) compared to that of seawater, and the high-Cl brine fluids contain remarkably high Li (6.14-10.6 mM) and variable delta Li-7 (1.4-8.7 parts per thousand). The latter fluids may have derived from metamorphic basement modified by seawater interactions at similar to 300 degrees C. A scenario using a steady-state dissolution/precipitation model can generate consistent Li and delta Li-7 patterns, where linear correlations of Cl and Li suggest phase separation occurred after water/rock interaction at depth. On the contrary, no significant delta Li-7 variation in most Milos fluids suggests limited isotopic fractionation occurred during phase separation. More importantly, the detected Li enrichment in the high-Cl fluids implies a large Li flux, similar to 3.4 x 10(7) mol/yr, to the ocean from the Milos system. Assuming that 10% of the world's shallow-water systems discovered to date have similar Li outputs to those of Milos, this Li flux would represent similar to 1.8% of MOR hydrothermal fluxes which is on the order of similar to 13 x 10(9) mol/yr. These results emphasize the importance of Li flux derived from shallow-water hydrothermal systems, which should not be excluded from the calculation of the marine Li budget and its impact on the global silicate weathering cycles.

Magmatic sources may contribute a significant amount of volatiles in geothermal springs; however, their role is poorly understood in submarine hydrothermal systems worldwide. In this study, new results of B and δ11B in 41 hydrothermal vent waters collected from the shallow hydrothermal system of Milos island in the Aegean Sea were combined with previously published data from other tectonic settings and laboratory experiments to quantify the effects of phase separation, fluid/sediment interaction and magmatic contribution. Two Cl-extreme solutions were identified, high-Cl waters (Cl as high as 2000mM) and low-Cl waters (Cl <80mM). Both sets of waters were characterized by high B/Cl (~1.2–5.3×10−3mol/mol) and extremely low δ11B (1.4–6.3‰), except for the waters with Mg content of near the seawater value and δ11B=10.3–17.4‰. These high-Cl waters with high B/Cl and low δ11B plot close to the vent waters in sediment-hosted hydrothermal system (i.e., Okinawa Trough) or fumarole condensates from on-land volcanoes, implying B addition from sediment or magmatic fluids plays an important role. This is in agreement with fluid/sediment interactions resulting in the observed B and δ11B, as well as previously reported Br/I/Cl ratios, supporting a scenario of slab-derived fluid addition with elevated B, 11B-rich, and low Br/Cl and I/Cl, which is derived from the dehydration of subducted-sediments. The slab fluid becomes subsequently mixed with the parent magma of Milos. The deep brine reservoir is partially affected by injections of magmatic fluid/gases during degassing. The results presented here are crucial for deciphering the evolution of the brine reservoirs involved in phase separation, fluid/sediment interaction and magmatic contribution in the deep reaction zone of the Milos hydrothermal system; they also have implications in the understanding of the formation of metallic vein mineralization.

The mechanisms controlling Br/Cl and I/Cl distribution in hydrothermal fluids on seafloor remain ambiguous. In this study, we examined systematically the spatial and temporal distribution of Cl, Br, and I in shallow-water hydrothermal fluids at Milos in the Aegean Sea, collected during two field excursions in 2002 and 2003. These fluids were analyzed for halogens, as well as other major/trace elements by high resolution inductive coupled plasma mass spectrometry. Large temporal variations (56% depletion compared with ambient seawater) in Br/Cl ratios are detected in the vapor enriched cave fluids, discharged through rock fissures near sea level and showed low pH, Cl, Br and Ι. On the other hand, small temporal Br/Cl variability (31% depletion) is characterized in the submarine-brine fluids that show high halogen concentrations. The I/Cl ratios in both cave and submarine-brine fluids are more than 10 times higher than in seawater, but fall into a range between hydrothermal fluids from mid-ocean-ridge (MOR) and sediment-hosted ridges. These results highlight the importance of organic matter degradation. A conceptual model is developed to explain the observed temporal and spatial variation of Br/Cl and I/Cl in the vent fluids at Milos. This model involves geochemical processes of sediment diagenesis, phase separation, and halite formation/dissolution at various stages during vent fluid circulation. The high I/Cl data suggest a scenario in which fluids were influenced by sediment diagenesis prior to phase separation. The latter process boils off vapor with high Br/Cl continuously at deep seated reservoir to create low Br/Cl brines, which was then mixed with seawater to form high Br/Cl variability in vent fluids. In addition tidal regulation at shallow depths may have an effect if halite-coating or dissolution occurs. There was no discernible Br/Cl change reported in MOR fluids. In strong contrast, our results show substantial Br/Cl variations and shed light on the role of phase separation, halite, and sediment diagenesis in shallow hydrothermal systems.

[1] Three types of hydrothermal vent fluids, herein referred to as cave, submarine-brine and seawater-like, were recovered from a shallow submerged system at Milos in the Aegean Sea, Greece, for detailed chemical and isotopic analyses. The cave fluids discharge through rock fissures near sea-level and have low pH, chlorinity, and B concentrations relative to seawater. The submarine-brine fluids are characterized by high Cl and contain > 10 times seawater B concentrations. A scenario involving a two-cells circulation is proposed; one occurs at 1-2 km and another at shallower depth. The deeper saline reservoir has experienced subcritical phase separation, partitioning 0.42 mM B in vapor and 6.8 mM in brine with no detectable isotopic fractionation. The reaction temperature in the saline reservoir is 313 degrees C calculated from the Na-K-Ca geothermometry. The vapors rise directly to form the cave vents, whereas the saline fluids transport in different pathways and are influenced by seawater mixing to form the variable submarine-brine fluids. The seawater-like fluids circulate at shallower depths, where calculated temperature is 248 degrees C and show slightly diluted B (0.36-0.41 mM) and seawater delta B-11. These fluids probably resulted from heating of down-flow seawater and may have experienced groundwater discharge and partial Mg removal. This study represents the first two-cells circulation occurring at Milos and emphasizes the important role of phase separation in shallow submarine hydrothermal system. Citation: Wu, S.-F., C.-F. You, B.-S. Wang, E. Valsami-Jones, and E. Baltatzis (2011), Two-cells phase separation in shallow submarine hydrothermal system at Milos Island, Greece: Boron isotopic evidence, Geophys. Res. Lett., 38, L08613, doi:10.1029/2011GL047409.

Neogene igneous activity in the Aegean region comprised both extrusive volcanism and intrusive granitoid plutonism. Granitoid plutonism in the Cyclades was established in the mid-Miocene period and lasted until late Miocene (18 to 9 Ma). The timing of granitoid intrusion was associated with the initiation of extensional tectonics in the Aegean. These intrusions form a broad belt about 200 km long running from the west (Lavrium and Serifos) to the eastern Aegean. Both S-type and I-type granitoids are present, the former generally being emplaced earlier than the I-types (similar to 15 to 8.3 Ma). Major and trace element variations reveal that three end-member components are involved in the granitoids, but the proportions of these vary in the different plutons. Initial isotopic compositions of all the granitoids are typical of crust-derived magmas from heterogeneous metasedimentary sources (I-type: (87)Sr/(86)Sr = 0.7091-0.712, epsilon(Nd) = -6.4 to -10.4; S-type: 0.710-0715, epsilon(Nd) = - 7.5 to - 10.1). Three end-member sources have been identified: 1) One end-member appears to be a metasedimentary biotite-gneiss (greywacke-type) such as that forming the metamorphic core complexes (Naxos and Paros); this is a dominant (but not the only) component in the S-types. 2a) Major, trace element and Sr-Nd isotopic composition correlations of the S-type granitoids with basement gneiss require an extra source of Sr and Ca, having lower initial (87)Sr/(86)Sr indicating a more depleted metasedimentary source at depth, due to possible interaction, of metamorphic fluids with marbles and amphibolites and infiltration through the gneissic units, at mid-crustal levels. 2b) A possible second end-member could be the marble component, as indicated by the buffered values of the Initial Sr isotopic ratios. Major element variation of the mafic microgranular (quartz diorites and tonalites) enclaves are compatible with dehydration melting of a mafic source similar to the amphibolites (island arc tholeiites) at medium pressure (similar to 8 kb) conditions. 3) Amphibolite is another end-member which may contribute mostly to the source of the younger intrusions, along the western flank of the arc. (C) 2009 Elsevier B.V. All rights reserved.

Very rare rhyolite dykes cross-cutting a Miocene I-type biotite-granite were discovered on Ikaria Island in the Aegean back-arc region. Their intrusion postdates exhumation of the granite to brittle crust at about 6.0-3.6 Ma; hence a Pliocene age is inferred. Petrological, geochemical and isotopic arguments indicate an origin through melting of crustal lithologies (tourmaline greywackes/semipelites) with no detectable contribution from asthenospheric sources. Strontium isotope ratios are relatively low unlike values for sediments entering the Hellenic trench but similar to those for certain Miocene Cycladic I-type granites and low-Rb Permo-Carboniferous Cycladic basement acid orthogneisses. Crust-mantle Sr-Nd isotopic mixing modelling also requires a low-Sr crustal end-member. The presence of Pliocene rhyolitic volcanism in the Aegean back-arc region in places distant from any possible subduction zone influence is attributed to slab tear at the eastern end of the Hellenic trench and attendant entrance and lateral displacement of asthenosphere through the thus formed slab window that, in turn, brought about kinematically mismatched continental escape in the subduction hangingwall. Extension along the boundary between the differentially moving Aegean and Anatolian blocks caused the intrusion of basaltic sills at middle crustal levels and the formation of acid anatectic melts from low-Sr, B-bearing quartzofeldspathic metasedimentary protoliths.

The Titaros ophiolite in Greece is a coherent thrust sheet of oceanic rocL· that lies atop Permo-Carboniferous granitic orthogneisses and amphibolites of unknown age of the eastern Pelagonian margin in the area NW of Mt. Olympos. It comprises a harzburgite tectonite mantle sequence with chromite mineralisation and a welldeveloped magma chamber with cyclic units ofdunite-lherzolite-wehrlite-pyroxenite that pass upwards into massive gabbros cut by diabase dykes and locally containing plagiogranite ponds. Ophiolite mineral and whole-rock chemistry are strongly in favour of a supra-subduction zone origin in a purely oceanic setting with the occasional brawny signature of melts from subducted sediments. By contrast, the basement amphibolites display a clear within-plate tholeiitic affinity with a slight imprint of subduction-zone fluids. Mineral stretching lineations and kinematics indicators of the orthogneisses and amphibolites suggest a consistent transport direction to the WSW. It is proposed that the basaltic protoliths of the amphibolites were emplaced during Permo-Triassic rifting of the eastern Pelagonian margin that led to the subsequent formation of the Vardar Ocean. The Titaros ophiolite was formed during closure of the Vardar Ocean via northeast-directed intra-oceanic subduction and subsequent obduction towards the southwest onto the eastern Pelagonian margin, probably in the Lower Cretaceous.

Chatzitheodoridis, E. Kostopoulos, D. Lyon, I. Henkel, T. Cornelius, N. Baltatzis, E. Reischmann, T. 2 OXFORD 15 Suppl. S 200VC

Geothermal activity in the Aegean island of Milos (Greece), associated with island-arc volcanism, is abundant both on-and off-shore. Hydrothermal fluids venting from several sites, mainly shallow submarine (up to 10 m), but also just above seawater level in one locality, were sampled over four summer field seasons. Some of the discharging fluids are associated with the formation of hydrothermal edifices.Overall, the main characteristics of the hydrothermal fluids are low pH and variable chlorinity. The lowest recorded pH was 1.7, and chlorinity ranged from 0.1 to 2.5 times that of seawater. The highest fluid temperatures recorded on site were 115 degrees C. Two main types of fluids were identified: low-chlorinity fluids containing low concentrations of alkalis (potassium, lithium, sodium) and calcium, and high concentrations of silica and sulphate; and high-chlorinity fluids containing high concentrations of alkalis and calcium, and lower concentrations of silica and sulphate. The type locality of the high-chlorinity fluids is shallow submarine in Palaeochori, near the cast end of the south coast of the island, whereas the type locality of the low-chlorinity fluids is a cave to the west of Palaeochori. The two fluid types are therefore often referred to as "submarine" and "cave" fluids respectively. Both fluid types had low magnesium and high metal concentrations but were otherwise consistently different from each other. The low-chlorinity fluids had the highest cobalt, nickel, aluminium, iron and chromium (up to 1.6 mu M, 3.6 mu M, 1586 mu M, 936 mu M and 3.0 mu M, respectively) and the high-chlorinity fluids had the highest zinc, cadmium, manganese and lead (up to 4.1 mu M, 1.0 mu M, 230 mu M and 32 mu M, respectively). Geochemical modelling suggests that metals in the former are likely to have been transported as sulphate species or free ions and in the latter as chloride species or free ions. Isotopic values for both water types range between delta D -12 to 33 parts per thousand and delta O-18 1.2 to 4.6 parts per thousand.The range of fluid compositions and isotopic contents indicates a complex history of evolution for the system. Both types of fluids appear to be derived from seawater and thus are likely to represent end members of a single fluid phase that underwent phase separation at depth. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.

D. Liotta and G. Ranalli.................................................................. 1 Heat flow, heat transfer and lithosphere rheology in geothermal areas: Features and examples G. Ranalli and L. Rybach.................................................................. 3 Thermo-mechanical evolution and rheology of the northern sector of the Tyrrhenian–Apennines system M. Verdoya, V. Pasquale and P. Chiozzi........................................................ 20 Relationship between recent heat flow and seismic properties: Some notes from crustal research in Germany E. Lüschen........................................................................... 31 Geofluid evidence from analysis of deep crustal seismic data (Southern Tuscany, Italy) F. Accaino, U. Tinivella, G. Rossi and R. Nicolich.................................................. 46 Crustal structures in the geothermal areas of southern Tuscany (Italy): Insights from the CROP 18 deep seismic reflection lines A. Brogi, A. Lazzarotto, D. Liotta, G. Ranalli and CROP18 …

Samples of Fe-Mn metasediments (hematite quartzites, mica-bearing quartzites and garnet-bearing amphibole schists) from the lower sub-unit of Kythnos island, part of Attic-Cycladic complex of Alpine age, have been collected and analysed for 45 elements, including rare earth elements. The hematite quartzites and mica-bearing hematite quartzites occur as intercalations in the upper part of a metavolcanic sequence or in the lower part of an overlying metasedimentary sequence of the northern part of Kythnos island. They show trace elements concentration and rare earth element (REE) patterns similar to those of hydrothermal deposits. The garnet-bearing amphibole schists occur in the sedimentary sequence of the southeastern part of the island. They have lower Fe/Mn ratios and higher trace element and REE contents in comparison with the hematite-quartzites and mica-bearing hematite quartzites. According to these geochemical characteristics, the protolith of Fe-Mn metasediments may have been formed under oceanic hydrothermal activity. In this process, circulating heated seawater reacted with basic volcanic rocks to form metal bearing hydrothermal solutions enriched in Fe, Mn and trace elements. These hydrothermal solutions were discharged on the ocean floor, through shear and/or fracture zones in an environment of decreasing temperature. The decreasing negative Eu anomalies from the lower (hematite quartzites) to the upper part (garnet-bearing amphibole schists) of the sequence possibly indicate a change in the condition (T, fO(2)) of the deposition as well as an increase of the detrital-hydrogenous components in relation to the hydrothermal fluids.

Tamarugite [NaAl(SO4)(2). 6H(2)O], found on the south-east coast of Milos island, occurs in the form of globular encrustations, in areas associated with fumaroles and hydrothermal activity. Its mode of occurrence suggests a recent origin, perhaps as a result of the mixing of gaseous emanations, enriched in H2S, with seawater spray.

The Lavrion area is part of the Attic-Cycladic massif. Blue amphibole analyses revealed that they are glaucophane or ferroglaucophane. Ca-amphiboles are characterized as actinolite or actinolitic hornblende. The Ps component of epidotes from the glaucophane-bearing rocks varies from 25.42-30.89%, whereas the Ps component of epidotes from the greenschist assemblages ranges from 23.81-26.88%. Chlorites show narrow compositional variations of ferromagnesian ratios (X(mg)=0.48-0.53). Albites are almost pure Ab(100). The K-feldspar present has a low Ab content. The evolution of the prasinites studied is characterized by the progressive transformation of eclogite facies (?) rocks through epidote blueschists into greenschists. A P-T path for the prasinites is presented indicating epidote blueschist facies at P-T conditions of around 7.0-7.5 kbar and 300-340 degrees C. Pressures of 4.0-3.5 kbar and temperatures around 340-360 degrees C are estimated for the subsequent overprint in the greenschist facies. The path EBS to GS conditions followed under nearly isothermal uplift.

Geochemical investigation of samples from 20 granitic intrusions in six tectonic zones of the Hellenides shows that both I-type and S-type granites occur in the region. The I-type granites from four of the zones, namely the Rhodope Massif (RM), the Serbomacedonian Massif (SMM), the Perirhodope Zone (PRZ) and the Attico-cycladic Zone (ACZ), show some systematic differences in their geochemistry. In particular, the Rb, Y, Nb, K and Ti contents increase in the sequence PRZ, SMM, RM and ACZ. The PRZ granites are of Jurassic age, those of the SMM and RM are Eocene to Oligocene and the ACZ ones are Miocene. The differences between zones are attributed to a combination of differences in partial melting and high-pressure fractionation processes. Geochemical differences within zones are explained by variable degrees of amphibole and apatite fractionation and accumulation.

Low grade metasediments from the Zarouchla Group of the Phyllite-Quartzite series in northern Peloponnesus have been investigated. Mineralogically, there is a clear distinction between the lowermost and the overlying formations. Rocks of the former contain characteristic minerals such as chloritoid or garnet whereas the other formations contain the assemblage muscovite + chlorite + qz ± paragonite ± paragonite/muscovite. Illite crystallinity values are low to middle anchizone in the uppermost formation and increase progressively through upper anchizone values in the intermediate formations, reaching low epizone values in the lowermost formation. Pumpellyite-actinolite facies metabasic rocks are sandwiched between metaclastites with upper anchizone or anchizone-epizone illite crystallinity values; and chloritoid bearing quartzites with low epizone illite crystallinity values. Although geothermometric data obtained from metasediments of the lowermost formation do not support a simple burial-related pattern of metamorphism, illite crystallinity data point to a progressive increase in metamorphic grade with stratigraphic depth.

The distribution of Fe2+ and Mg between coexisting phengite and chlorite has been examined in a suite of low-grade rocks. The distribution coefficientKD(Mg) is influenced in part by the content of tetrahedral or octahedral Al of either chlorite or phengite and in part byXMg of phengite. The tie lines for the mineral pairs on a SAF triangular diagram show a subparallel trend.