Troodos Ophiolite Research Articles

Post-magmatic fracturing, fluid flow, and vein mineralization in supra-subduction zones: a comparative study on vein calcites from the Troodos ophiolite and the Izu\u2013Bonin forearc and rear arc

Based on the published data of pillow lava-hosted mineralized veins, this study compares post-magmatic fracturing, fluid flow, and secondary mineralization processes in the Troodos and Izu–Bonin supra-subduction zone (SSZ) and discusses the crucial factors for the development of distinct vein types. Thin section and cathodoluminescence petrography, Raman spectroscopy, fluid inclusion microthermometry, and trace element and isotope (87Sr/86Sr, δ18O, δ13C, Δ47) geochemistry indicate that most veins consist of calcite that precipitated from pristine to slightly modified seawater at temperatures < 50 °C. In response to the mode of fracturing, fluid supply, and mineral growth dynamics, calcites developed distinct blocky (precipitation into fluid-filled fractures), syntaxial (crack and sealing), and antitaxial (diffusion-fed displacive growth) vein microtextures with vein type-specific geochemical signatures. Blocky veins predominate in all study areas, whereas syntaxial veins represent subordinate structures. Antitaxial veins occur in all study areas but are particularly abundant in the Izu–Bonin rear arc where the local geological setting was conducive of antitaxial veining. The temporal framework of major calcite veining coincides with the onset of extensional faulting in the respective areas and points to a tectonic control on veining. Thus, major calcite veining in the Troodos SSZ began contemporaneously with volcanic activity and extensional faulting and completed within ~ 10–20 Myr. This enabled deep seawater downflow and hydrothermal fluid upflow. In the Izu–Bonin forearc, reliable ages of vein calcites point to vein formation > 15 Myr after subduction initiation. Therefore, high-T mineralization (calcite, quartz, analcime) up to 230 °C is restricted to the Troodos SSZ.

Petrogenetic implications of chromite-seeded boninite crystallization experiments: Providing a basis for chromite-melt diffusion chronometry in an oxybarometric context

Boninites are rare high magnesium andesites that often contain trace chromites. These chromites precipitate from primitive boninitic melts and are thought to be carried to the surface in melts that continue to crystallize a significant volume of silicate minerals. Such magmatic differentiation drives primitive chromite out of equilibrium with the residual melt with respect to both divalent and trivalent cation proportions. Diffusion then operates to alter primitive chromite toward a composition in equilibrium with residual melt. To simulate this process, we have performed internally heated pressure vessel experiments, providing insights into the processes of chromite-melt re-equilibration through time. While Fe-Mg exchange at magmatic temperatures equilibrates the tetrahedrally coordinated divalent cations in chromite in less than 6 hours, equilibration of trivalent cations in octahedral coordination with residual melts is slower. Our experimental results show that chromite Al concentrations are ubiquitously lower, and Fe2+/Fe3+ values are ubiquitously higher than modelled equilibrium values, indicating that there was insufficient time for significant Al and Fe3+ replacement of Cr. Similar observations can be made for natural chromite compositions in the extrusive sequence of the Troodos ophiolite (Cyprus). Based on a simple model of diffusive equilibration, we estimate that microphenocrystic chromites up to 60 μm in diameter take between 60 days and c. 170 years to equilibrate under conditions analogous to the physiochemical state of melt immediately prior to eruption. For the Troodos ophiolite extrusive sequence, this implies that mafic magmas are erupted less than c. 170 years after extraction from the mantle for disequilibrium textures and compositions to be preserved.

High-Ca boninitic melt inclusions in lavas of the Troodos ophiolite and a reappraisal of genetic relationships between different lava types

Abstract Boninites in many supra-subduction zone ophiolites are thought to mark the early stage of subduction. The Troodos ophiolite in Cyprus contains a variety of volcanic rocks including the lower pillow lavas and the boninite-bearing upper pillow lavas (UPL). Here we present major, trace element and Pb isotope data for melt inclusions in olivine from the UPL on the northern flank of the Troodos ophiolite. Melt inclusions hosted by high-Mg (Fo = 89.4–93.5) olivine grains have typical high-Ca boninitic compositions with high MgO (11.2–18.4 wt%) and SiO2 (51.2–55.0 wt%) and low TiO2 (0.2–0.6 wt%) contents, in line with the existence of boninites in the northern part of the ophiolite. The melt inclusions have trace element compositions similar to the Troodos boninitic lavas and can be regarded as representing the parental magma of the boninites. The boninitic magmas were derived from high-degrees of partial melting of a refractory source at 1400 °C and 1.5 GPa based on thermobarometers and phase equilibria. The melt inclusions have significantly variable Pb isotopic compositions (208Pb/206Pb = 2.059–2.122; 207Pb/206Pb = 0.828–0.862) and are enriched in fluid-mobile trace elements (e.g., U, Sr, and Pb), indicating that the refractory source was metasomatized by slab fluids. In comparison, recalculation of literature data suggests that the lower pillow lavas were derived from a less refractory and shallower source. We propose a subduction initiation model in which melting of two unique sources was needed to form the chemostratigraphy of lavas in the Troodos ophiolite.

Magmatic volatiles episodically flush oceanic hydrothermal systems as recorded by zoned epidote

Circulation of seawater at oceanic spreading centers extracts heat, drives rock alteration, and transports leached metals to shallower levels of the crust, where they may precipitate and form ore deposits. Crystallization of the lower crust, may exsolve and introduce magmatic volatiles into the seawater-dominant system. However, the role of magmatic volatiles added to the hydrothermal system, including pathways of these fluids are lesser known. Here we present coupled in-situ strontium isotope and rare earth element data of distinct domains in epidote, a common hydrothermal mineral throughout the Troodos ophiolite, to track magmatic fluid input and flow. Epidote crystal domains characterize three distinct strontium isotope-rare earth element signatures—suggesting sequential growth from magma-derived fluids (0.704, negative europium anomalies), rock-buffered fluids (0.7055, positive europium anomalies) and seawater-derived fluids (0.7065, negative cerium anomalies). Epidote records episodic fluxing of magmatic fluids from plagiogranites, through epidosites in the upflow zone and into metal ore deposits.

Regionally variable timing and duration of celadonite formation in the Troodos lavas (Cyprus) from Rb-Sr age distributions

Celadonite (K[Mg2+,Fe2+][Fe3+,Al3+](Si4O10)(OH2)) is a common void-filling mineral in ocean floor and ophiolite lavas that forms during low-temperature, off-axis, hydrothermal alteration. Its occurrence provides important information about the sink of K from seawater into the upper oceanic crust. New age determinations for celadonitic samples from a ~20 km section of the lavas in the Troodos ophiolite, along with published ages, show that most celadonite formed within the first ~20 Myrs after crust accretion. However, significant regional differences in the duration of celadonite formation are observed. In comparison to a region of flat paleo-seafloor, in a paleo-seafloor topographic low celadonite started forming later and continued forming for ~20 Myrs longer. The paleo-seafloor low is associated with abundant hydrothermal sediments which are interpreted to have played a key role in controlling the chemical conditions required for celadonite precipitation. It is hypothesised that celadonite formed for longer largely because these hydrothermal sediments acted as a source for labile Fe for a longer time than in the region where the Fe required for celadonite formation was mainly leached from silicate phases. In the region of flat paleo-seafloor, sediment did not begin accumulating for ~20 Myrs after crustal accretion allowing prolonged oxidative alteration of the upper crust. This led to Fe released by silicate phase breakdown being incorporated in stable Fe3+ containing phases, such as Fe-oxyhydroxides, rather than being transported as aqueous Fe2+ to sites where celadonite could form. If this model is correct, the uptake of K from seawater into the oceanic crust may be highly spatially heterogeneous and intimately linked to the subsurface redox conditions and the distribution of hydrothermal sediments.

Biogeochemical response of Pinus brutia and Olea europaea to lithological variations and Cu mineralisation in Cyprus

Regional geochemical mapping programs are predominantly based on regolith materials as the sampling media. The use of plant organs has been more limited, despite potential advantages demonstrated in more localised studies that have examined biogeochemical responses to various types of mineralisation or contamination, or sought to identify species with phytoremediation potential. This study investigated the effects of variations in lithology and mining wastes on the multielement chemistry of Pinus brutia (pine) needles and O. europaea (olive) leaves, obtained from low-density traverses across Cyprus and detailed sampling around two Cyprus-style VHMS Cu-deposits. A total of 476 samples were analysed by a combination of inductively-coupled plasma mass spectrometry (ICP-MS) and portable X-ray fluorescence spectrometry (pXRF). At the regional scale there are substantial differences in the composition of the pine needles, especially for Ni, Ba, Sr, Mn and Zn, between the major lithological groups that range from the ultramafic and mafic units of the Troodos Ophiolite to the mineralised pillow basalts and various marine carbonate units. Both pine needles and olive leaves display intense Re anomalies in areas with mine wastes and consistently low concentrations in background areas. The response for Cu and other macro- or micronutrient elements to contamination is less pronounced and may reflect barriers to uptake by the plants. The relative magnitude of variations between the different lithologies in median plant metal concentrations is equivalent to that displayed in soils of the Geochemical Atlas of Cyprus (e.g. REE, Zn and Sr), but for some elements the patterns differ substantially. There is close correlation between the ICP-MS and pXRF data for elements where most values are above pXRF detection limits. The study further demonstrates the potential of plants in regional geochemical mapping for a range of applications, in combination with or as an alternative to conventional regolith media.

Controls on soil geochemistry fractal characteristics in Lemesos (Limassol), Cyprus

This study extends the program of urban soil geochemical mapping in Cyprus to Lemesos, the country's second largest and fastest developing urban centre. Data from 411 soil samples are combined with samples previously analysed as part of the detailed soil geochemical atlas of Cyprus (GAC). Parent lithology is the dominant control on the distribution of most elements with soils derived from carbonate units contrasting with those developed on alluvium-colluvium that contains input from both the carbonates and mafic-ultramafic units of the nearby Troodos Ophiolite. Anthropogenic contamination is best indicated by the Pb-Sn-Ag-Bi suite with elevated values in a belt crossing the southern and older section of the city that is unrelated to parent lithology or current land use classification. As in Lefkosia, children's playgrounds display much higher As contents than at other sites due to application of a natural secondary carbonate (‘pouri’) as the surface layer, and elevated concentrations of Cu, Fe, Ti, Al, S and Mn associated with basaltic soils used in most school grounds. Multivariate patterns in the Lemesos data largely reflect those of similar parent lithologies within the national soil atlas, but there is closer clustering of the contamination-related elements and an ultramafic suite involving Ni-Cr-Pt-Pd-V-Co in Lemesos. Whereas most elements display multifractal populations using number-size fractal modelling, in urban and contaminated areas some higher concentration element populations do not display fractal behaviour, thus providing a means of identifying anthropogenic influences.

The Boninite‐like Dolerites in the Xigaze Ophiolites, Tibet: Similar to the MORB‐like Dolerites

AbstractThe Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (&gt; 52 wt%), high MgO (&gt;8 wt%) and low‐TiO2 (&lt;0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation.

Origin of magmatic harzburgite as a result of boninite magma evolution – An illustration using layered harzburgite-dunite cumulate from the Troodos ophiolite complex

Olivine (Ol) and orthopyroxene (Opx) are the primary liquidus phases of boninite in modern subduction settings and in many ophiolite complexes. It is thus straightforward to expect the formation of harzburgite cumulate resulting from boninite magma evolution. However, such magmatic harzburgite has been rarely studied. Here, we report the results of our study on such harzburgite from the Troodos ophiolite complex.The Troodos cumulate harzburgite (locally lherzolite) is characteristically interlayered with dunite, showing varying thickness on millimeter to decimeter scales, as the result of volumetrically varying multiple pulses of melt injection into the evolving magma chamber. We illustrate the development of the interlayered cumulate by phase equilibrium analysis. The parental melt of each pulse begins to crystallize olivine to form a dunite layer before reaching the Ol-Opx cotectic, along which Ol and Opx coprecipitate to form a harzburgite layer. Periodical replenishment will result in dunite-harzburgite interlayered cumulate. In cases when replenishment may be delayed, the melt along the Ol-Opx cotectic can evolve to the Ol-Opx-clinopyroxene (Cpx) eutectic to form harzburgite with some Cpx or lherzolite. The calculated melts in equilibrium with spinels in the cumulate are characteristic of boninite in major element compositional spaces. The calculated melts in equilibrium with Cpx and Opx in the cumulate share the same as, or identical to, trace element patterns of the Troodos boninite (both glasses and bulk-rock compositions). Petrological modeling of the boninite magma evolution shows a crystallization order of Ol, Opx, Cpx, plagioclase. Our study also emphasizes the importance in considering dunite-harzburgite/lherzolite cumulate when interpreting seismic structure of the crust in subduction settings, especially in rock sequences associated with subduction initiation thought to be indicated by boninite magmatism.

Low-temperature silica-rich gold mineralization in mafic VMS systems: evidence from the Troodos ophiolite, Cyprus

The Troodos ophiolite Cyprus hosts the on-land analogue for actively forming mafic, Cu-rich, or Cyprus-type VMS deposits. In addition to high-temperature (> 350 °C) sulfide mound-related mineralization, other fossil seafloor mineralizing systems are known to operate, including those characterized by an enrichment in Au and abundant silicification. In this study the mineralogy and geochemistry of four Au and silica-rich localities in Troodos are considered, and these include Kokkinovounaros, Mathiatis South, Touronjia, and Alpen Rose. We present whole rock geochemical and mineralogical data characterizing the distribution of Au in the hydrothermal systems of Troodos. Samples from silica-rich localities have two distinct sample mineralogies: supergene samples that contain predominantly goethite, hematite, and jarosite and hypogene samples that contain quartz, amorphous silica and minor hematite. Hypogene samples from Mathiatis South and Kokkinovounaros are enriched in Au with a median concentration of 1.5 ppm relative to supergene samples that contain 0.1 ppm (n = 107). This indicates that Au enrichment occurred on the seafloor and is not solely related to supergene weathering of silica-rich mineralization. We suggest that silica-rich Au mineralization in Troodos formed during the migration of newly formed crust off-axis or as white smokers proximal to known VMS deposits. In silica-rich mineralization that is located distally from known massive sulfide deposits, Au was probably remobilized from shallow crustal reservoirs during the low-temperature fluid flow (100–300 °C) in the lower extrusive sequence as the crust cooled and migrated off-axis. Based on modern seafloor analogues, we propose a revised model for the Troodos hydrothermal system that explains the distribution of Au in silica-rich mineralization that formed off-axis and postdates VMS formation and in zones proximal to VMS deposits that formed simultaneously as white smoker-type vents. We suggest that silica-rich mineralized zones formed an intermediary between high-temperature (> 350 °C) on-axis VMS deposits and the low-temperature silicification of umbers (< 100 °C).

The Origin of Plagiogranites: Coupled SIMS O Isotope Ratios, U–Pb Dating and Trace Element Composition of Zircon from the Troodos Ophiolite, Cyprus

AbstractU–Pb ages, trace element content and oxygen isotope ratios of single zircons from five plagiogranite intrusions of the Troodos ophiolite were measured to determine their crystallization age and assess the importance of fractional crystallization versus crustal anatexis in their petrogenesis. The results indicate that oceanic magmatism in Troodos took place at 94·3 ± 0·5 Ma, about 3 Myr earlier than previously recognized. Later hydrothermal alteration has affected most of the Troodos plagiogranitic rocks, resulting in growth of new zircon and/or partial alteration of zircon domains, causing slightly younger apparent crystallization ages. The new age inferred for seafloor spreading and ocean crust accretion in Troodos nearly overlaps that of the Semail ophiolite in Oman (95–96 Ma), strengthening previous indications for simultaneous evolution of both ophiolites in similar tectonic settings. Average δ18O(Zrn) values in the Troodos plagiogranites range between 4·2 and 4·8 ‰. The lower values in this range are lower than those expected in equilibrium with mantle-derived melt (5·3 ± 0·6 ‰), indicating variable contribution from hydrothermally altered, deep-seated oceanic crust in most of the Troodos plagiogranite intrusions. The inferred substantial involvement of crustal component is consistent with the existence of a shallow axial magma chamber, typical of fast-spreading mid-ocean ridge settings, within the Troodos slow-spreading ridge environment. This apparent contradiction may be reconciled by episodically intense magmatism within an otherwise slow, magmatically deprived spreading axis.

Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater

Low-temperature (<100°C) alteration of oceanic crust plays an important role in determining the chemical composition of the oceans. Although a major sink of seawater potassium, little is known about the effects of low-temperature basalt alteration on the potassium isotopic composition of seawater (δ41K∼0‰), which is ∼0.50‰ enriched relative to bulk silicate Earth (BSE, δ41K= -0.54‰). Here, we present a suite of isotopic systems (δ41K, δ26Mg, δ7Li, 87Sr/86Sr) and major/minor elements in bulk rock, veins and mineral separates from the upper volcanic section of Cretaceous (Troodos ophiolite) and Jurassic (Ocean Drilling Program Hole 801C) oceanic crust. We use these data to estimate the K isotopic fractionation associated with low-temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of Mg and K in seawater. We find that hydrothermally altered basalts from the Troodos ophiolite and ODP Hole 801C, most of which are enriched in K relative to the unaltered glass compositions, have δ41K values both higher and lower than BSE, ranging from +0.01‰ to -1.07‰ (n=83) and +0.04‰ to -0.88‰ (n=17), respectively. Average δ41K values of bulk-rock samples from Troodos and Hole 801C are indistinguishable from each other at ∼-0.50‰, indicating that low-temperature basalt alteration is a sink of 39K from seawater, and explaining, in part, why seawater has a higher 41K/39K than BSE. In contrast to K, average δ26Mg values for both Troodos (∼0.00‰) and Hole 801C (∼0.20‰) indicate that altered oceanic crust (AOC) is a sink of 26Mg from seawater, likely contributing to the light δ26Mg composition of seawater (∼-0.8‰) relative to BSE (∼-0.2‰). We observe isotopically heavy δ26Mg values in basalt samples characterized by small to no changes in bulk Mg content, consistent with extensive isotopic exchange of Mg between seawater and oceanic crust during low-temperature oceanic crust alteration. Finally, we find that variability in δ7Li and δ41K across three sites in the Troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust.

New U-Pb zircon age and petrogenesis of the plagiogranite, Troodos ophiolite, Cyprus

The Troodos ophiolite is a classic and is one of the most complete and best-studied ophiolites on Earth. However, there are few studies on the geochronology of the plagiogranite in the Troodos ophiolite. With the advancement of zircon U-Pb in situ dating method developed in recent decades, it is necessary to revisit the age of the plagiogranite. In this study, we carried out detailed petrography, zircon U-Pb dating and mineral compositional analysis for the plagiogranite. Our new zircon ages of ~90–91 Ma confirm the previous age data. Our findings also offer new perspectives on the petrogenesis of the Troodos plagiogranite. It is common that some of the Troodos plagiogranite outcrops show intense “epidotization”. The Troodos plagiogranite can thus be divided into epidote-free plagiogranite and epidote plagiogranite. Zircons in the epidote plagiogranite have mottled texture with high contents of most elements (especially for P, Y, REEs, Nb and Ta), which differs distinctly from those in the epidote-free plagiogranite with a clean appearance and oscillatory zoning. The two types of zircons all having high Th/U ratio (0.25–1.46 with an average of 0.69) are consistent with being of magmatic origin. The texture and composition of zircons from the epidote plagiogranite suggest that they are crystallized from a highly dissolving agent that transforms existing minerals such as plagioclase and amphiboles while concentrating rare earth elements (REEs) and high field strength elements (HFSEs, including Nb, Ta, Ti, Zr, Hf, Th and U4+) to precipitate accessory minerals such as zircon and titanite as well as the more abundant epidote. The intergrowth of quartz and skeletal plagioclase together with the precipitation of zircon, titanite and epidote indicates that the highly dissolving agent is best understood as late-stage magmatic fluid or more likely supercritical fluid/melt whose nature and origin deserve further investigation.

Volatile contents of volcanic glasses from the Troodos Ophiolite

Volatile contents of volcanic glasses from the Troodos Ophiolite

Experimental study on mafic rock dissolution rates within CO2-seawater-rock systems

Far-from-equilibrium batch experiments have been performed to study the low temperature dissolution potential of crystalline submarine basalts (from Juan de Fuca Plate and Mid-Atlantic Ridges) and of a highly altered gabbro from the Troodos ophiolite (Cyprus) in presence of seawater and carbon dioxide (CO2). The experiments have been carried out at 40 °C for up to 20 days with initial pH of ∼4.8 and under ∼1 bar pCO2 to identify the progressive water-rock interactions. Elemental steady-state release rates from the rock samples have been determined for silicon and calcium, the solution concentrations of which were found to be the most effective monitors of rock dissolution. Mass balance calculations based on dissolved Si and Ca concentrations suggest the operation of reaction mechanisms focussed on the grain surfaces that are characteristic of incongruent dissolution. Also, basic kinetic modelling highlights the role of mass-transport limitations during the experiments. Ca release rates at pH ∼ 5 indicate significant contributions of plagioclase dissolution in all the rocks, with an additional contribution of amphibole dissolution in the altered gabbro. Si release rates of all solids are found to be similar to previously studied reactions between seawater and basaltic glass and crystalline basalt from Iceland, but are higher than rates measured for groundwater-crystalline basalt interaction systems. This comparison with previous experimental results resumes the debate on the role of experimental variables, such initial rock mass and crystallinity, pCO2, and fluid chemistry on dissolution processes. Our new data suggest that CO2-rich saline solutions react with mafic rocks at higher rates than fresh water with low pCO2, at the same pH. Most significantly, both ophiolitic gabbro and Juan de Fuca basalts show Si and Ca release rates similar or higher than unaltered crystalline basalt from Iceland, highlighting the potential substantial role that ophiolitic rocks and offshore mafic reservoirs could play for the geological storage of CO2.

Geochemistry of Vein Calcites Hosted in the Troodos Pillow Lavas and Their Implications for the Timing and Physicochemical Environment of Fracturing, Fluid Circulation, and Vein Mineral Growth.

Calcite veins hosted in pillow lavas of the Late Cretaceous Troodos suprasubduction zone ophiolite provide insights into the timing and physicochemical environment of postmagmatic fracturing and fluid circulation through oceanic crust. This study presents rare earth element and yttrium (REE+Y) concentrations, δ13C, δ18O, 87Sr/86Sr, and clumped isotopic (Δ47) compositions of vein calcites in order to investigate their fluid sources, formation temperatures, and precipitation ages. These geochemical data are combined with microtextural analyses. Intersections of 87Sr/86Sr ratios of vein calcites with the Sr isotope seawater curve suggest two distinct calcite veining phases. Major calcite veining within an interval of ~10 Myr after crust formation is characterized by microtextures that point to extensional fracturing related to crack and sealing, host rock brecciation, and advective fluid flow. These vein calcites show REE+Y characteristics, 87Sr/86Sr ratios, and clumped isotopic compositions indicative of precipitation from seawater at <50 °C. Extended fluid residence times intensified fluid‐rock interactions and lowered Y/Ho ratios of some blocky vein calcites, whereas crack and sealing resulted in pristine seawater signatures. Low 87Sr/86Sr ratios of localized high‐temperature blocky vein calcites point to the involvement of hydrothermal fluids. These calcites show Mn‐controlled oscillatory growth zonations that probably developed in a closed system out of equilibrium. Later calcite veining (<75 Ma) may have coincided with rotation and/or uplift of the Troodos ophiolite. Microtextures of these vein calcites indicate fluid diffusion and fracture‐independent crystallization pressure‐driven veining. Their variably modified seawater signatures resulted from diffusion‐related fluid interaction with hydrothermal sediments.

Enrichment of H2O and fluid-soluble trace elements in the Troodos Ophiolite: Evidence for a near-trench origin

The Troodos Ophiolite on the island of Cyprus is a Cretaceous supra-subduction zone ophiolite. It formed by seafloor spreading in the vicinity of a subduction zone, but the exact tectonic setting in which it formed is debated. We determined H2O contents together with other volatiles (S, Cl and CO2) and Sr, Nd and Pb isotope compositions of fresh Troodos volcanic glasses, previously measured for major and trace element contents. Glass compositions range from boninite to tholeiitic andesite. The least degassed glasses have H2O contents of 1.7 to 2.7 wt%, and H2O/Ce ratios of 4400 to 33,600. The most depleted glasses with low Zr/Yb have the highest fractionation-corrected H2O contents and H2O/Ce, and the highest subduction zone input (highest Th/La, Ba/Zr and 87Sr/86Sr). The high H2O contents, enrichment in fluid-mobile elements, and depletion in Zr, Hf and heavy rare-earth elements are consistent with formation above a subduction zone, very close to a trench in a fore-arc position, likely at water depths of ~5000 m. The enrichment in both fluid-mobile (e.g. Rb, Cs) and melt soluble (e.g. Th) elements indicate that the Troodos Ophiolite formed by propagation of a back-arc rift into the fore-arc region of an active subduction zone.

Pliocene–Pleistocene sedimentary and geomorphologic development of the Vasilikos river catchment, S Cyprus, in relation to uplift of the Troodos ophiolite and climate-related changes

AbstractThe Pleistocene development of the Vasilikos River exemplifies the interaction of focused, tectonically induced surface uplift and climate-influenced changes. The resulting sediments are well exposed in Vasilikos Quarry and in the main river catchment further east. An important erosional surface incises the highest-level (oldest) fluvial conglomerates, down into Late Pliocene – Early Pleistocene open-marine mudrocks (Nicosia Formation), allowing integration with the circum-Cyprus sedimentary–geomorphic development (F1–F4 stages). To determine where the quarry deposits lie in relation to the Vasilikos river catchment, the fluvial deposits were mapped and valley profiles were constructed, revealing four main episodes, each associated with incision and distinctive fluvial deposition. Source lithology strongly influenced channel morphology, infill and adjacent slope-sediment (colluvium) composition. Palaeosols, particularly red-brown terra rossa, developed on abandoned fluvial terraces and adjacent hillslopes, especially overlying F3 surfaces. The combined evidence allows close correlation of the Vasilikos river and quarry deposits. Relatively coarse (chalky conglomerate/breccia) and fine-grained colluvium (calcareous silt – Cyprus harvara) developed especially on lower hillslopes following incision (mainly above F2 and F3 surfaces). Based on regional comparisons, overall sediment aggregation ended during the Early Pleistocene. The F1–F2 surfaces and deposits are inferred to be Middle Pleistocene, the F3 ones later Middle Pleistocene and the F4 ones near the Middle–Late Pleistocene boundary. Geomorphology and deposition were tectonically forced during strong, focused Early–Middle Pleistocene surface uplift. Coarse clastic ruff-off and palaeosol development (terra rossa) and related sediment aggradation are inferred to have increased during warm, humid periods. Late Pleistocene geomorphology and deposition were more influenced by climatic change, with semi-perennial streamflow, rapid sediment aggradation and palaeosol (terra rossa) development during warm, humid periods (interglacials). Cooler (glacial) periods enhanced fluvial-incision, sediment-bypassing and hillslope colluvial processes (e.g. frost shattering, downslope creep and mass flow) when sediment transport (bypassing) exceeded sediment supply. Neotectonic faulting affected the catchment but did not greatly affect geomorphology or sediment supply. Although climate / climate change (and eustatic sea-level change) had an important influence, tectonics is interpreted as the fundamental driver of geomorphological development and fluvial sedimentation, with implications for other areas, regionally to globally.

Effects of magmatic volatile influx in mafic VMS hydrothermal systems: Evidence from the Troodos ophiolite, Cyprus

The Troodos ophiolite, Cyprus is the principal on- land analogue for mafic-hosted volcanogenic massive sulfide (VMS) deposits. This study, for the first time, presents sulfur isotope (δ34S) data on a regional scale from VMS deposits and other mineralised zones across the Troodos ophiolite. In combination δ34S, Se/S ratios and trace element chemistry (e.g., Se, Cu and Au) of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems.Sulfur isotope analyses (n=180) across 19 mineral deposits indicate a variable source of sulfur in the Troodos VMS hydrothermal system, this in turn allows a variable source of metals to be inferred. Pyrite δ34S range from −5.5‰ to +13.2‰ with an average of +4.6‰ (n=160) for all deposits investigated. These δ34S variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction (δ34S +18 to +19‰) and leaching of primary magmatic sulfur from igneous rocks (δ34S 0-1‰). Consequently, two processes are proposed, explaining the trace metal and δ34S variation across the Troodos ore-forming systems including, i) a variable source of metals in the sheeted dyke complex and ii) the addition of a magmatic volatile-rich phase to the VMS hydrothermal systems.Two distinct lava units exist in the Troodos stratigraphy, namely the upper and lower pillow lavas (UPL and LPL). The more primitive UPL are enriched in Au, Se and Cu relative to As, Sb and Zn that are concentrated in the LPL. Some VMS deposits pre-date the formation of the UPL (e.g., Agrokipia A) suggesting Se, Cu and Au depleted source rocks. Hence, the stratigraphic position of VMS deposits and the ratio of UPL:LPL affinity elements (e.g., As +Zn+Sb vs. Cu+Se+Au) imply a systematic relationship between trace element distribution and stratigraphic depth; this relates to the relative proportion of UPL and LPL affinity lavas within the metal source region.δ34S values <0‰ recorded in some VMS deposits that are less than the Troodos magmatic mean of 0- 1‰ may be related to anhydrite buffering during fluid ascent, microbial sulfate reduction or the direct contribution of magmatically derived sulfur, to the hydrothermal system from an underlying magma chamber via volatile exsolution. We propose that negative δ34S values combined with Se/S × 106 ratios >500 in pyrite suggest the contribution of a magmatic volatile component (e.g., Apliki and Skouriotissa). We demonstrate that the source of metals and sulfur in the Troodos VMS hydrothermal system is affected by regional scale processes related to i) variable source lithologies and, ii) the contribution of a magmatic volatile phase to some Troodos VMS hydrothermal systems.

Fossilized Endolithic Microorganisms in Pillow Lavas from the Troodos Ophiolite, Cyprus

The last decade has revealed the igneous oceanic crust to host a more abundant and diverse biota than previously expected. These underexplored rock-hosted deep ecosystems dominated Earth’s biosphere prior to plants colonized land in the Ordovician, thus the fossil record of deep endoliths holds invaluable clues to early life and the work to decrypt them needs to be intensified. Here, we present fossilized microorganisms found in open and sealed pore spaces in pillow lavas from the Troodos Ophiolite (91 Ma) on Cyprus. A fungal interpretation is inferred upon the microorganisms based on characteristic morphological features. Geochemical conditions are reconstructed using data from mineralogy, fluid inclusions and the fossils themselves. Mineralogy indicates at least three hydrothermal events and a continuous increase of temperature and pH. Precipitation of 1) celadonite and saponite together with the microbial introduction was followed by 2) Na and Ca zeolites resulting in clay adherence on the microorganisms as protection, and finally 3) Ca carbonates resulted in final fossilization and preservation of the organisms in-situ. Deciphering the fossil record of the deep subseafloor biosphere is a challenging task, but when successful, can unlock doors to life’s cryptic past.