Tectonic evolution of the Western Limassol Forest Complex, Cyprus

The Western Limassol Forest Complex (WLFC), Cyprus, forms an anomalous ophioliteterrain in the south of the Troodos Massif. Detailed studies have revealed magmatic and structural histories that differ markedly from the Penrose-type ophiolite of the Troodos Massif to the north. In the WLFC, a tectonised harzburgite of upper mantle origin has been intruded by multiple ultramafic and gabbroic plutons and swarms of mainly NE-SW trending dykes. The entire complex has been sheared along E-W trending serpentinite shear zones, the orientation of which indicate sinistral displacement. The various styles of deformation from ductile to brittle, and the progressive cooling history of the Intrusive plutons and dykes indicate a history of about 4km of uplift for the host upper mantle lithologies, while in a sea floor setting. The geochemistry of the intrusive plutons and dykes is similar to the lavas that crop out around the periphery of the WLFC, and Indicate derivation from a depleted upper mantle source. Geochemical comparison with the Troodos massif basalts suggests a tectonic history involving rapid extension across the WLFC and adiabatic melting of the upper mantle producing boninitic magmas. The regional setting for the WLFC suggests a model of formation involving the development of a transtensional transform fault zone and an extensional relay zone that off-set to the south, sinistral transform movement along the Arakapas fault belt. Comparison of the WLFC with a transpressional palaeo-transform fault preserved in the Antalya complex of Turkey suggests that the Neo-Tethyan spreading system (within which the Troodos massifformed) was experiencing an anticlockwise rotational torque during the final stages of oceanic crustal formation

Near-seafloor magnetic signatures unveil serpentinization dynamics at ultramafic-hosted hydrothermal sites

A near-seafloor magnetic and bathymetric survey conducted by the autonomous underwater vehicle AutoSub 6000 over intermediate-temperature, ultramafic-hosted Von Damm Vent Field (Mid-Cayman spreading center, Caribbean Sea) revealed a moderate positive magnetic anomaly, in accordance with the magnetic response of other known ultramafic-hosted hydrothermal vent fields. However, compared with low-temperature ultramafic-hosted hydrothermal activity, the magnetic signature of this intermediate-temperature site indicates a slightly stronger magnetization contrast between the hydrothermal system and its host, but it remains considerably weaker than at high-temperature ultramafic-hosted hydrothermal vent fields. This observation highlights the nonlinear increase of magnetization production with temperature, as iron partitions into weakly magnetic brucite under 200 °C, but magnetite dominates above this temperature, leading to a sudden increase in the magnetic signature of a site. Our study is consistent with recent laboratory experiments and unveils the dynamics of the serpentinization reaction, enabling fine tuning of the magnetic technique for remotely locating hydrothermal systems. In addition to refining our understanding of the magnetic behavior of hydrothermal vent fields, these new results also reveal the orientation of the fluid pathway feeding the hydrothermal site and indicate the nonvertical structure of the complex network of fissures within the host rock and its associated tectonic feature—an oceanic core complex

Carlsberg Ridge and Mid-Atlantic Ridge: Comparison of slow spreading centre analogues

Eighty per cent of all mid-ocean spreading centres are slow. Using a mixture of global bathymetry data and ship-board multibeam echosounder data, we explore the morphology of global mid-ocean ridges and compare two slow spreading analogues: the Carlsberg Ridge in the north-west Indian Ocean between 57°E and 60°E, and the Kane to Atlantis super-segment of the Mid-Atlantic Ridge between 21°N and 31°N. At a global scale, mid-ocean spreading centres show an inverse correlation between segment length and spreading rate with segmentation frequency. Within this context, both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge are similar: spreading at 22 and 26 mm/yr full rates respectively, being devoid of major transform faults, and being segmented by dextral, non-transform, second-order discontinuities. For these and other slow spreading ridges, we show that segmentation frequency varies inversely with flank height and ridge axis depth. Segments on both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge range in aspect ratio (ridge flank height/axis width), depth and symmetry. Segments with high aspect ratios and deeper axial floors often have asymmetric rift flanks and are associated with indicators of lower degrees of melt flux. Segments with low aspect ratios have shallower axial floors, symmetric rift flanks, and evidence of robust melt supply. The relationship between segmentation, spreading rate, ridge depth and morphology, at both a global and local scale, is evidence that rates of melting of the underlying mantle and melt delivery to the crust play a significant role in determining the structure and morphology of slow spreading mid-ocean ridges

Deep-ocean mineral deposits: metal resources and windows into earth processes

Deep-ocean mineral deposits could make a significant contribution to future raw material supply. Growing metal demand and geopolitics are focussing increasing attention on their resource potential and economic importance. However, accurate assessment of the total amounts of metal and its recoverability are very difficult. Deep-ocean mineral deposits also provide valuable windows through which to study the Earth, including the evolution of seawater and insights into the exchange of heat and chemicals between the crust and the oceans. Exploration for, and potential extraction of, deep-ocean mineral deposits poses many geological, technical, environmental and economic challenges, as well as regulatory and philosophical questions. Great uncertainty exists, and the development and stewardship of these deposits requires an incremental approach, encouraging transparency and scientific and civil societal input to balance the interests of all

Modern seafloor hydrothermal systems: new perspectives on ancient ore-forming processes

Seafloor massive sulfides are deposits of metal-bearing minerals that form on and below the seabed as a result of heated seawater interacting with oceanic crust. These occurrences are more variable than previously thought, and this variability is not necessarily reflected in the analogous volcanogenic massive sulfide deposits that are preserved in the ancient rock record. The geological differences affect both the geochemistry and the size of seafloor massive sulfide deposits. Current knowledge of the distribution, tonnage, and grade of seafloor massive sulfides is inadequate to rigorously assess their global resource potential due to the limitations in exploration and assessment technologies and to our current understanding of their 3-D characteristics

Geochemical evidence of Milankovitch cycles in Atlantic Ocean ferromanganese crusts

Hydrogenetic ferromanganese crusts are considered a faithful record of the isotopic composition of seawater influenced by weathering processes of continental masses. Given their ubiquitous presence in all oceans of the planet at depths of 400–7000 meters, they form one of the most well-distributed and accessible records of water-mass mixing and climate. However, their slow accumulation rate and poor age constraints have to date limited their use to explore 100 ka paleoclimatic phenomena. Here it is shown how the Pb isotope signature and major element content of a Fe-Mn crust from the north-east Atlantic responded to changes in the intensity and geographic extent of monsoonal rainfall over West Africa, as controlled by climatic precession during the Paleocene. The studied high-spatial resolution (4 μm) laser-ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) Pb isotope data is a nearly 2 order of magnitude improvement in spatial and temporal resolution compared to micro-drill subsamples. The record demonstrates cyclicity of the 206Pb/204Pb and 208, 207Pb/206Pb ratios at the scale of single Fe-Mn oxide laminae, in conjunction with variations in the Fe/Mn ratio, Al, Si and Ti content. Time-frequency analysis and astronomical tuning of the Pb isotope data demonstrates the imprint of climatic precession (∼20 ka) modulated by eccentricity (∼100 and 405 ka), yielding growth rates of 1.5–3.5 mm/Ma consistent with previous chemostratigraphic age models. In this context, boreal summer at the perihelion causes stronger insolation over West Africa, resulting in more intense and geographically extended monsoonal rainfalls compared to aphelion boreal summer conditions. This, in turn, influences the balance between the weathering endmembers feeding the north-east Atlantic basin. These results provide a new approach for calibrating Fe-Mn crust records to astronomical solutions, and allow their isotopic and chemical archive to be exploited with an improved temporal resolution of 1000–5000 years

A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge

Mantle oxygen fugacity (fO2) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO2 proxies: Fe3+/Fetot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO2 proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe3+/Fetot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium isotope analyses. Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the VAA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range (VAA = −1.09 to −0.86‰; 1SD = 0.02–0.09) and does not co-vary with fractionation-corrected Fe3+/Fetot (0.134–0.151; 1SD = 0.005) or V/Sc (6.6–8.5; 1SD = 0.1-1.3) ratios. However, on a global scale, basaltic VAA may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO > 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe3+/Fetot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe3+/Fetot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe3+/Fetot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland. Comparison of observed and modeled Fe3+/Fetot indicate that source variation in fO2 is present under the Reykjanes Ridge, with higher Fe3+/Fetot closer to Iceland. This source variability in fO2 cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes

Controls on metal enrichment in ferromanganese crusts: temporal changes in oceanic metal flux or phosphatisation?

Oceanic hydrogenetic ferromanganese (Fe-Mn) crusts are a major repository for many metals, such as Co, Ni, Cu, Pt, Te and REE, which are essential for decarbonisation of transport and energy systems. Secondary mineralisation processes, occurring during phosphatisation episodes, commonly impregnate the shallower deposits with carbonate fluorapatite (CFA). The suboxic oceanic conditions during such events are frequently invoked to explain the lower Co content and unusually high Ni, Cu, Zn and Pt content of older phosphatised crusts. Here, the hypothesis of suboxic diagenetic recrystallization induced by phosphatisation episodes as a driving mechanism for Ni, Cu, Zn and Pt enrichment and Co depletion is evaluated. Accurately dated geochemical profiles, spanning 75 Ma of depositional history, for a shallow (1100 mbsl) phosphatised sample and a deeper (3100 mbsl) unphosphatised sample from Tropic Seamount in the north-east Atlantic, are compared. An isocon analysis, which allows to quantitively evaluate chemical gains and losses in mass transfer and therefore permits compensation for the dilution effect induced by the addition of CFA in the Fe-Mn crusts, demonstrates that no loss of Co has occurred in the phosphatised crust, whilst Pt, Te, Cu, Ni and Zn are enriched relative to younger, unphosphatised Fe-Mn crust. Both geochemical profiles show sympathetic trends and similar amplitudes of variation in concentration. This excludes phosphatisation as the driving mechanism for the metal enrichment and depletion. Systematic differences in metal content between the two samples, such as higher Cu and lower Co content in the deeper sample, are consistent with the depth profile of dissolved metal concentrations in the water column. The variability observed in the geochemical profiles is consistent with temporal changes in metal fluxes to the ocean, as a result of the evolving climate and oceanographic configuration of the north-east Atlantic Ocean through the Cenozoic. It is concluded that changing metal fluxes, rather than secondary mineralisation process associated with phosphatisation, is the dominant control on the primary metal content in Fe-Mn crust deposits at Tropic Seamount

Boninite and Harzburgite from Leg 125 (Bonin-Mariana Forearc): A Case Study of Magma Genesis during the Initial Stages of Subduction

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component. The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250°C and 1.5 kb for an average boninite) of boninite segregation. We thus envisage that boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 boninites and boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting