Magmatic significance and hydrothermal alteration of layered chromitites from the Bracco Gabbro Complex Ophiolite, Ligurian Ophiolites, Italy

Re-Os geochronology, O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt, China

Depletion and refertilization of the Tethyan oceanic upper mantle as revealed by the early Jurassic Refahiye ophiolite, NE Anatolia—Turkey

Seawater-basalt interaction taking place at mid-ocean ridges was studied using numerical modeling to determine the compositional evolution of hydrothermal fluids and associated alteration mineralogy forming within newly emplaced crustal material. Geochemical modeling was carried out in a closed seawater-basalt system at discrete temperature intervals between 2 and 400°C at 500 bars, varying fluid/rock ratios, and secondary mineral assemblages representative of basalt alteration in natural systems. In addition to temperature, the fluid/rock ratio has a fundamental control on the resulting system chemistry. At rock-buffered conditions (low fluid/rock ratios), the mineral-solution equilibrium was characterized by high cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and very low dissolved Mg concentrations due to the precipitation of smectites and chlorite. A complex secondary mineral alteration assemblage dominated by Ca- and Na-bearing minerals including zeolites, calcite, epidote, prehnite, clinozoisite, and albite was predicted to form. The resulting fluid composition was alkaline and reduced relative to ambient seawater, with Eh values ranging between −0.2 and −0.6 V. In contrast, seawater-buffered conditions (high fluid/rock ratios) resulted in lower cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and higher dissolved Mg concentrations comparable to the value of this element in ambient seawater. A more simple mineral assemblage was predicted to form at these conditions with the predominance of Al-Si- and Mg-bearing minerals including kaolinite, quartz, and talc in addition to large amounts of anhydrite. The resulting fluid composition was mildly acidic and oxidized relative to seawater with Eh values ranging between −0.2 and 0 V. These modeling results were compared to a compilation of submarine hydrothermal vent fluid compositions from mid-ocean ridge settings and analogous basalt-dominated environments. The agreement obtained between the simulations and the compiled fluid data indicates that mid-ocean ridge hydrothermal processes can be closely reproduced by mineral-solution equilibria for a broad range of temperatures and fluid/rock ratios.

[1] Field observations and petrological and geochemical data are used to constrain a conceptual model for the formation of a gabbro-peridotite section from Ligurian ophiolites (Italy). The studied section is attributed to an intraoceanic domain of the Jurassic Ligurian-Piedmontese basin and is characterized by the lack of a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed model shows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase facies conditions occurred mostly in the form of grain-scale porous flow. We recognize a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the moderately depleted precursor mantle. In particular, localized infiltrations of MORB-type melts gave rise to formation of spinel websterite layers close to the lithosphere-asthenosphere boundary. The peridotite-websterite association was involved in a spinel facies deformation attributed to emplacement of asthenospheric material at the base of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes. Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal. The growth of a gabbroic pluton (up to ∼400 m thick) that is intruded into the mantle sequence is attributed to accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solidification is characterized by ductile shearing developed from near-solidus to amphibolite facies conditions.

CRUSTAL METASOMATISM IN SUBDUCTED MANTLE: RECORDS FROM THE ULTEN PERIDOTITES (UPPER AUSTROALPINE, EASTERN ALPS)

After nearly 50 years of research on hydrothermal circulation, the global hydrothermal on-axis element turnover is still not well constrained. Existing estimates of hydrothermal element fluxes typically invoke a basalt-hosted black smoker archetype hydrothermal vent fluid that is imposed to be responsible for the global hydrothermal cooling of oceanic lithosphere. The diversity of hydrothermal vent fluid compositions, especially to be found at slow to ultra-slow spreading mid-ocean ridges (due to varying degrees of fluid rock interaction with peridotites), has not been properly addressed yet.Here we present a study that for the first time considers the diversity of hydrothermal vent fluids by analyzing a global database of hydrothermal vent fluid compositions (MARHYS Database Version 3.0). We derive a proper weighting of these fluid types by analyzing strike lengths and substrate types of the mid-ocean ridge system and estimate the partitioning of these hydrothermal fluid types to improve quantification of hydrothermal element fluxes at mid-ocean ridges. We show that the element‑to‑energy flux ratio in peridotite-hosted (or peridotite-influenced) hydrothermal vent fluids is significantly different to the one in purely basalt-hosted fast spreading ridges. Consequently, for many compounds significantly higher (e.g. H2, CH4, Fe) or lower (e.g. H2S, CO2) element fluxes are found to be associated with hydrothermal cooling at slow- and ultra-slow spreading ridges. Our results show that, despite their lower power output (compared to fast spreading ridges), slow and ultra-slow spreading centers, with their serpentinization‑derived hydrothermal fluids, play a major role for the element transfer between the ocean crust and the ocean.

Geochemical characteristics of granitoid rocks from within the Archean Michipicoten Greenstone Belt, Wawa Subprovince, Superior Province, Canada: implications for source regions and tectonic evolution

We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 °C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of ‘epithermal suite’ elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2–0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A δ13C value of −5.4‰ in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (<0.026 mmol/kg and <0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as δ34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 ± 60 yr.

U–Pb zircon geochronology of the Ligurian ophiolites (Northern Apennine, Italy): Implications for continental breakup to slow seafloor spreading

This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts). Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes. These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences. Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism. The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading. Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults. The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts. Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere.

Ophiolite complexes represent fragments of ocean crust and mantle formed at spreading centers and emplaced on land. The setting of their origin, whether at mid-ocean ridges, back-arc basins, or forearc basins has been debated. Geochemical classification of many ophiolite extrusive rocks reflect an approach interpreting their tectonic environment as the same as rocks with similar compositions formed in various modern oceanic settings. This approach has pointed to the formation of many ophiolitic extrusive rocks in a supra-subduction zone (SSZ) environment. Paradoxically, structural and stratigraphic evidence suggests that many apparent SSZ-produced ophiolite complexes are more consistent with mid-ocean ridge settings. Compositions of lavas in the southeastern Indian Ocean resemble those of modern SSZ environments and SSZ ophiolites, although Indian Ocean lavas clearly formed in a mid-ocean ridge setting. These facts suggest that an interpretation of the tectonic environment of ophiolite formation based solely on their geochemistry may be unwarranted. New seismic images revealing extensive Mesozoic subduction zones beneath the southern Indian Ocean provide one mechanism to explain this apparent paradox. Cenozoic mid-ocean-ridge–derived ocean floor throughout the southern Indian Ocean apparently formed above former sites of subduction. Compositional remnants of previously subducted mantle in the upper mantle were involved in generation of mid-ocean ridge lavas. The concept of historical contingency may help resolve the ambiguity on understanding the environment of origin of ophiolites. Many ophiolites with “SSZ” compositions may have formed in a mid-ocean ridge setting such as the southeastern Indian Ocean.

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

Oceanic serpentinisation is the principal process of water incorporation into the oceanic lithosphere, thus playing a significant role in the elements cycle. The conditions of serpentinisation vary in temperature, water-rock ratio, and fluid composition, but the investigation of the interplay of these factors, rather than their individual variation, is rarely attempted. This study examines mid-ocean ridge (MOR) serpentinite samples from Mid-Atlantic Ridge 15°20′N Fracture Zone (Leg 209, Sites 1272 and 1274) and from Hess Deep (Leg 147, Site 895D and 895E) to gain insight into the combined effects of these variables. Various in situ geochemical tools are used to evaluate the mutual influence of the different factors in the successive stages of serpentinisation. The Cl/B content of serpentine is employed as a proxy for fluid salinity, enabling a more precise temperature calculation when the constant Cl/B correlates with varying δ18O. The combined in situ temperature-dependent oxygen and boron isotope compositions reveal instances of localised fluctuations in fluid pH that impact the boron isotope composition of serpentine. The δ18O compositions of serpentine vary between 0.8 and 7.8 ‰, implying temperature variations within the range of ∼115 °C to 290 °C. Where the δ11B compositions of serpentine exceed the expected temperature-related variations of circa 9–13 ‰, we propose that pH variations during progressive serpentinisation are the cause (δ11B up to 22 ‰ variability observed between serpentine textures from a single sample). Moreover, this study emphasises the divergence in serpentinisation conditions between MOR and passive margin settings. Serpentinisation can occur at higher temperatures (up to 290 °C) with a more saline fluid (Cl/B > 25) under variable pH conditions in MOR settings, while the serpentinisation in PaMa settings takes place at lower temperatures (< 200 °C) with a less saline fluid (Cl/B < 25) and probably more alkaline conditions.

The Prince William Sound region is located approximately 100 km east-southeast of Anchorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Historically, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologically and are characteristically stratiform to strata bound. The deposits vary in size from less than 1 million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type environments.The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite, pyrrhotite, chalcopyrite, and sphalerite. Minor phases include cubanite, arsenopyrite, tetrahedrite, and secondary marcasite. Silicate gangue is comprised predominantly of quartz and chlorite, with minor sericite, talc, and calcite. Primary sedimentary features preserved within massive sulfide are ubiquitous within the deposits and include planar bedding, crossbedding, and graded bedding.Fluid inclusion microthermometry of quartz gangue from the massive sulfide deposits is consistent with hydrothermal fluid temperatures found in genetically related modern seafloor hydrothermal environments. Minimum trapping temperatures of fluid inclusions in quartz from the stockwork feeder zone of the volcanic-hosted deposits generally range from 210 degrees to 260 degrees C. Fine-grained anhedral quartz gangue in massive sulfide layers also contains sparse primary fluid inclusions; typically the homogenization temperature is lower in the sediment-hosted deposits. Hydrothermal fluids in the volcanic-hosted settings exited the volcanic pile directly into seawater and were rapidly quenched, thus fluid inclusions only record high-temperature fluid conditions. Similar hydrothermal fluids in the sediment-hosted settings exited the volcanic pile into warm, seawater-saturated sediments and were cooled more slowly, in an environment where lower temperature fluid inclusions were formed and preserved.Sulfur and oxygen isotope data support a submarine origin for the deposits. Sulfur isotope values range from delta 34 S GDT = 0.8 to 5.9 per mil for volcanic-hosted deposits, and from 2.7 to 9.2 per mil for sediment-hosted deposits. The higher values for sediment-hosted ores reflect increased amounts of reduced seawater sulfate in the hydrothermal fluid, with sulfate reduction occurring in the seawater-hydrothermal fluid mixing zone within the sediment pile below the seawater-sediment interface. Oxygen isotope values for quartz gangue associated with massive sulfide also reflect the lower temperature deposition that is characteristic of the sediment-hosted deposits: delta 18 O quartz = 7.0 to 7.8 per mil (SMOW) for volcanic-hosted deposits and delta 18 O quartz = 9.6 to 16.9 per mil for sediment-hosted ores.Whole-rock geochemical analyses of the basalts and related igneous rocks from both the Valdez and Orca Groups show that these rocks are consistent with formation in midocean ridge and/or seamount environments. The geochemistry of the Valdez Group rocks suggests that sediment contamination of basaltic magma has occurred, imparting a primitive arclike signature. Discrimination diagrams show that the rocks from both groups have normal midocean ridge basalt, enriched midocean ridge basalt, and low K arc tholeiite affinities, and plot along the tholeiitic-calc-alkaline boundary.The fluid inclusion, stable isotope, and whole-rock data collected in this study are all consistent with the hypothesis that the massive sulfide deposits in the Prince William Sound district are sea-floor volcanogenic massive sulfide deposits that formed in both sediment-starved and sedimented midocean ridge settings.

The Izmir‐Ankara‐Erzincan suture zone is the major Neotethyan suture in northern Anatolia. Two areas of ophiolitic rocks from this zone have been studied. The Beşdeǧirmen area, north of Kütahya, includes dismembered ophiolitic rocks (the Kinik Ophiolite) and consists of serpentinized pyroxenites and periodotites together with gabbros and gabbro dykes. The Kaynarca area, south‐east of Kütahya, contains medium to low grade metamorphic rocks (mainly amphibolites, amphibole schists and quartz schists) at the base of the Kinik Ophiolite. These two areas are probably joined under the Neogene cover.The Beşdeǧirmen pyroxenites contain spinel with a Cr number [Cr/(Cr + Al)] &lt; 0·60 and are similar to peridotites from mid‐ocean ridge (MOR) settings. Gabbro samples contain very calcic plagioclase (An87–100) and magnesio‐hornblende as the principal phases; prehnite and zeolite are the most common late stage fracture infillings. The chemistry and mineralogy of the gabbros have features similar to rocks from both MOR and arc settings.For the Kaynarca amphibolites there are at least two stages of amphibole growth. The first stage amphiboles are brownish green calcic varieties. In contrast, the second stage amphiboles record a medium to high pressure overprint with the growth of fibrous, bluish green varieties. The low crossite content of the amphiboles, and the absence of lawsonite, suggest the transition between blueschist and greenschist facies metamorphism. Trace element and rare earth element data from the amphibolite samples are comparable with oceanic island basalts.Isotopic dating suggests a Coniacian‐Ypresian age for the formation of the Beşdeǧirmen gabbros, significantly younger than the Jurassic ages previously suggested for the formation of the main Izmir‐Ankara ocean. Unlike many other Tethyan ophiolites the Kinik Ophiolite appears to represent a fragment of a Neotethyan ocean, rather than a supra‐subduction zone environment. We suggest a back‐arc basin setting for the formation of the Beşdeǧirmen ophiolitic rocks.The Kaynarca rocks formed as an oceanic island or seamount and the Kaynarca amphibolites were formed by intra‐oceanic thrusting during the closure of the Izmir‐Ankara‐Erzincan ocean. Preliminary isotopic data suggest an Albian‐Campanian age for this sub‐ophiolite metamorphism. The sub‐ophiolite metamorphic rocks were overprinted by low temperature‐high pressure metamorphism related to Late Cretaceous subduction. Final emplacement is related to the terminal collisional event which occurred at the end of the Cretaceous.