Mineral Composition and Geochemical Zoning of Bottom Sediments in the Pobeda Hydrothermal Cluster (17°07.45′ N–17°08.7′ N Mid-Atlantic Ridge)

Lithology and Mineral Resources - The paper presents the mineral and chemical compositions of carbonate, metalliferous, and ore-bearing sediments developed within the Pobeda ore cluster based on...     

Sm-Nd and Rb-Sr isotopic systems and captured He and hydrocarbon gases as markers of melt sources and fluid regime under which the oceanic crust of the Mid-Atlantic Ridge was formed at 5°–6° N

The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals and data on He and hydrocarbons in fluid inclusions in the same samples. The materials presented in this publication were obtained by studying samples dredged from the MAR crest zone at 5°–6° N (U/Pb zircon dating, geochemical and petrological-mineralogical studies). It was demonstrated that the variations in the isotopic composition of He entrapped in the rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass of MAR (DM source) and atmospheric He. An increase in the fraction of atmospheric He in the plutonic rocks and, to a lesser degree, in their minerals reflects the involvement of seawater or the hydrated material of the oceanic crust in the magmatic and postmagmatic processes. This conclusion finds further support in a positive correlation between the fraction of mantle He (R) and the ⁸⁷Sr/⁸⁶Sr ratio. The high-temperature hydration of ultrabasic rocks (amphibolization) was associated with an increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by a drastic decrease in this fraction and a significant increase in the ⁸⁷Sr/⁸⁶Sr ratio. The insignificant variations in the ¹⁴³Nd/¹⁴⁴Nd (close to 0.5130) and ⁸⁷Sr/⁸⁶Sr (0.7035) in most of the gabbroids and plagiogranites, as well as the fraction of mantle He in these rocks, amphibolites, and their ore minerals, indicate that the melts were derived from the depleted mantle. Similar?_Nd values of the gabbroids, plagiogranites, and fresh harzburgites (6.77–8.39) suggest that these rocks were genetically related to a single mantle source. The value of ?_Nd = 2.62 of the serpentinized lherzolites likely reflects relations of these relatively weakly depleted mantle residues to another source. The aforementioned characteristics of the rocks generally reflect the various degrees of mixing of components of the depleted mantle with crustal components (seawater) during the metamorphic and hydrothermal processes that accompanied the formation of the oceanic crust.     

The Magmatic–Hydrothermal Transition and Origin of Brine in the Oceanic Core Complex of the Mid-Atlantic Ridge at 13° N

New data on local mineral associations and the microheterogeneity of minerals and fluid inclusions in gabbro were obtained for the gabbro–peridotite oceanic core complex with a long-lived detachment fault controlling the hydrothermal activity. It is assumed that the hydrothermal hydrogen-bearing fluid with a NaCl content of >30 wt % is formed in the seawater/harzburgite (~1/5) reaction of serpentinization. The brine residual after serpentinization interacted with gabbro at the final stages of crystallization of an intrusion and assimilated some components (K, REEs, and Ba) from the residual melt. The interaction was resulted in metamorphic transformations of gabbro at decreasing temperature below 500°C. The reaction of the decomposition of magmatic titanomagnetite with the transition of iron reduced to Fe²⁺ into newly formed chlorinebearing amphibole at 540–450°C and logf(O₂) from–20 to–24 is shown.

     

New hydrothermal sulfide fields of the Mid-Atlantic Ridge: Yubileinoe (20°09′ N) and Surprise (20°45.4′ N)

Two new sulfide fields (Yubileinoe, 20°09′ N, and Surprise, 20°45.4′ N) were discovered between 20°01′ and 20°54′ N within the Russian Application Area of the Mid-Atlantic Ridge (MAR). The Yubileinoe field is located at a depth of 2300–2550 m in the near-top area of the first rift ridge, which is a boundary of the western wall of the rift valley. This new field and the Zenith-Victory field, which was previously discovered in the eastern wall, occur symmetrically relative to the rift valley of this MAR segment. The Surprise field at a depth of 2800–2850 m is situated in the eastern wall of the rift valley, on the slope of the volcanic uplift. After the discovery of these inactive sulfide fields, the number of hydrothermal fields within the Russian Application Area reached ten.     

Hydrothermal metasomatic alteration of carbonate bottom sediments in the Ashadze-1 field (13° N Mid-Atlantic Ridge)

We have performed the first detailed study of hydrothermal alteration of the Holocene-upper Pleistocene sediments in the recent Ashadze-1 hydrothermal field sampled during the 26th cruise of R/V Professor Logachev in 2005. It has been established that alterations in mineralogy and geochemistry are caused by the dissolution of calcite shells in bottom sediments and their metasomatic replacement with sulfides and other hydrothermal minerals. A zone of sediments enriched in magnesian hydrothermal minerals has been revealed in the sediments of the MAR for the first time. It is suggested that metasomatism is related to diffuse percolation of hydrothermal fluids through sediments.     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Morphology and impurity elements of zircon in the oceanic lithosphere at the Mid-Atlantic ridge axial zone (6°–13° N): Evidence of specifics of magmatic crystallization and postmagmatic transformations

The paper presents newly obtained original data on the morphology, internal structure (as seen in cathodoluminescence images, CL), and composition of more than 400 zircon grains separated from gabbroids and plagiogranites (OPG) sampled at the axial zone of the Mid-Atlantic Ridge (MAR). The zircons were analyzed for REE by LA-ICP-MS and for Hf, U, Th, Y, and P by EPMA. Magmatic zircon in the gabbroids crystallized from differentiating magmatic melt in a number of episodes, as follows from systematic rimward increase in the Hf concentration, and also often from the simultaneous increase in the (U + Th) and (Y + P) concentrations. These tendencies are also discernible (although much less clearly) in zircons from the OPG. Zircon in the OPG is depleted in REE compared to the least modified zircons in the gabbro, which suggests that the OPG were derived via partial melting of gabbro in the presence of seawater-derived concentrated aqueous salt fluid. Another reason for the REE depletion might be simultaneous crystallization of zircon and apatite. The CL-dark sectors, which are found in practically all of the magmatic zircon grains, have Y/P (a.p.f.u.) ? 1 which most likely resulted from OH accommodation in the zircon structure, a fact suggesting that the OPG parental melt contained water. High-temperature hydrothermal processes induced partial to complete recrystallization of zircon (via dissolution-reprecepitation), a process that was associated with ductile and brittle deformations of the zircon-hosting rocks. The morphology of the hydrothermal zircons varies depending on pH and silica activity in the fluid from weakly corroded subhedral crystals with typical vermicular microtopography of the crystal faces to completely modified grains of colloform structure. Geochemically, the earlier hydrothermal transformations of the zircons resulted in their enrichment in La and other LREE, except only Ce, whose concentration, conversely, decreases compared to that of the unmodified magmatic zircons. The hydrothermal zircon displays a reduced Ce anomaly and its most altered domains typically host minute inclusions of xenotime, U and Th oxides and silicates, and occasionally also baddeleyite, which suggests that the hydrothermal fluid was reduced and highly alkaline. These features were acquired by the seawater-derived fluid when it circulated within the axial MAR zone area due to phase separation in the H₂O–NaCl system and particularly as a result of fluid interaction with the abyssal peridotites of oceanic core complexes. Our data demonstrate that zircon is a sensitive indicator of tectonic and physicochemical processes in the oceanic crust.     

Evidence for Granitoid Magmatism and the Composition of the Silicic Melt in the Gabbroid Assemblage of the Mid-Atlantic Ridge at 13° N: New Data on Melt Inclusions

Doklady Earth Sciences - Despite the local occurrence of silicic magmatism during the formation of the oceanic crust, the nature of felsic granitoid veins (“oceanic plagiogranite”)...     

Variolitic lavas in the axial rift of the Mid-Atlantic Ridge and their origin (Sierra Leone area, 6°18′N)

Altered variolites described for the first time in the axial zone of the Mid-Atlantic Ridge are represented by rounded globules of andesite (icelandite) composition with light trachyandesite rim embedded in a picrobasaltic matrix. The globules were transferred with picrobasaltic melt and then floated to the surface of lava flow, while formation of leucocratic rims was presumably related to thermodiffusion (Soret effect) in a cooling heterogeneous melt. This heterogeneous melt was formed by penetration of ascending column of picrobasaltic magma in already existing small intracructsal magmatic chamber filled with residual icelanditetype andesite melt and involvement of the latter into a general upward movement. The rapid ascent of the melts in the oceanic spreading zones by means of turbulent flowing caused dispersion of the extragenous melt into small drops in a jet of picrobasaltic magma, without their interaction. Variolites were formed during cooling of such heterogeneous lava flow. No signs of liquid immiscibility were found in the studied variolites.     

Chronology of Hydrothermal Activity Within the Yubileynoye Ore Field (Mid-Atlantic Ridge, 20°08′ N)

This paper reports on the integrated geochronological and geochemical methods used in studying the ore deposits and metalliferous sediments of the Yubileinoye field. This study gives the opportunity to carry out cross dating of hydrothermal deposits, including the ²³⁰Th/U dating of sulfides, the ²³⁰Th, ¹⁴C dating, and foraminiferal analysis of the sediments, and, on this basis, to reconstruct hydrothermal activity over time. It was established that the ores started forming about 100 000–123 000 years ago and were renewed 4–5 times with a frequency of 10–20 ka. As a result, the complex of pyrite-marcasite, chalcopyrite and sphalerite ores and the associated metal-bearing and ore-bearing sediments with consistent geochemical specialization were formed. The integrated geochronological and geochemical studies of the ores and sediments allow us to obtain detailed data on evolution of the hydrothermal ore-formation not only in the certain areas but also for the entire Mid-Atlantic Ridge.     

Conditions of the formation of plagiogranite from the Markov trough,Mid-Atlantic Ridge, 5°52′-6°02′N

Doklady Earth Sciences -     

Noble metals in sulfide assemblages from deep sectors of the active TAG mound (Mid-Atlantic Ridge, 26°08′ N)

Primary sulfides from cores of holes 957M, 957C, and 957H drilled during the ODP Leg 158 on the active hydrothermal TAG mound (Mid-Atlantic Ridge, 26°08′ N) were examined for the concentration of several elements. Based on 262 microprobe analyses, it has been established that the sulfides are characterized by an extremely heterogeneous distribution of noble metals (Au, Ag, Pt, and Pd) and several associated elements (Hg, Co, and Se). Noble metals are arranged in the following order in terms of decreasing abundance, i.e., concentration level above the detection limit (the number of analyses containing the specific element is given in parentheses): Au (65), Ag (46), Pt (21), and Pd (traces). The associated trace elements make up the following series: Co (202), Hg (132), and Se (49). Main carriers of the “invisible” portion of noble metals are represented by pyrite (Au, Hg), marcasite and pyrite (Ag, Co), sphalerite and chalcopyrite (Pt, Pd), and chalcopyrite (Se). The noble metal distribution in sulfides reveals a lateral zonality: the maximal concentration and abundance of Au in chalcopyrite (or Pt and Ag in chalcopyrite and pyrite) increase from the periphery (Hole 957H) to the center (holes 957C and 957M) of the hydrothermal mound, while the Au distribution in pyrite displays a reversed pattern. The Co distribution increases with depth. This work discusses the vertical zonality in the distribution of elements mentioned above and their response to the evolution of ore genesis.     

Geochemical nature and age of the plagiogranite-gabbronorite association of the oceanic core complex of the Mid-Atlantic ridge at 5°10′S

Our newly obtained data on the geochemistry and age of plagiogranite-gabbronorite association in the oceanic core complex of the Mid-Atlantic Ridge (MAR) at 5°10′S suggest close genetic relations between these rocks in this segment of the ridge. The U/Pb zircon age of an oceanic plagiogranite (OPG) sample is 1.059 ± 0.055 Ma and is in good agreement with the zircon age of plutonic rocks in the oceanic core complex of northern MAR. A distinctive geochemical feature of the rocks is their unusually depleted ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, which suggest that the plutonic rocks of the gabbronorite-plagiogranite association in MAR at 5°10′S could be derived from the most strongly depleted mantle reservoir of all known to occur beneath the axial MAR zone. The COMAGMAT-5.2 numerical thermodynamic simulation of the possible crystallization links between the plagiogranite and gabbronorite from the MAR segment at 5°10′S led us to conclude that the leading role in the origin of the plagiogranite was played by a two-stage process: the partial melting of the gabbronorite and the subsequent fractionation of the newly generated melt. The regional differences between the isotopic-geochemical parameters of MAR plagiogranites can, perhaps, reflect local specifics of so-called hydrothermal anatexis, such as the geochemical features of the rocks involved in this process and the parameters of the hydrothermal process, for example, variations in the W/R ratio.     

Influence of hydrothermal-metasomatic processes on the formation of present-day sulfide ores in carbonate bottom sediments of the mid-Atlantic Ridge (19°–20° N)

Based on materials obtained in Cruises 33 and 34 of the R/V Professor Logachev, the paper addresses formation conditions, morphology, structures, mineral composition of the present-day oceanic sulfide ores, and their relationships with the host (biogenic carbonate) bottom sediments in the 19°–20° N MAR (Zenith-Victoria and Petersburg hydrothermal fields) region. The grain size distribution, mineral composition of the carbonate (background) and ore-hosting sediments, as well as physicochemical parameters of their interstitial waters, are examined. The results suggest a significant role of hydrothermal-metasomatic processes in the formation of ores and ore-bearing sediments. A model is proposed for the formation of sulfide mineralization in oceanic sediments at the geochemical barrier in the zone of their interaction with the acid hydrothermal (diffuse-type) ore-bearing solutions delivered from rocks of the ocean floor.     

Recent massive sulfide deposits of the Semenov ore district, Mid-Atlantic Ridge, 13°31′ N: Associated rocks of the oceanic core complex and their hydrothermal alteration

The oceanic core complexes and large-offset detachment faults characteristic of the slow-spreading Mid-Atlantic Ridge are crucial for the structural control of large hydrothermal systems, including those forming sub-seafloor polymetallic sulfide mineralization. The structural-geological, petrographic, and mineralogical data are considered for the oceanic core complex enclosing the Semenov-1, -2, -3, -4, and -5 inactive hydrothermal sulfide fields recently discovered on the Mid-Oceanic Ridge at 13°31?? N. The oceanic core complex is composed of serpentinized and talc-replaced peridotites and sporadic gabbroic rocks, however, all hydrothermal fields reveal compositional indications of basaltic substrate. The volcanic structures superposed on the oceanic core complex are marked by outcrops of pillow lavas with fresh quenched glass. Dolerites regarded as volcanic conduits seem to represent separate dike swarms. The superposed volcanic structures develop largely along the near-latitudinal high-angle tectonic zone controlling the Semenov-1, -2, -5, and -3 hydrothermal sulfide fields. The manifestations of hydrothermal metasomatic alteration are diverse. The widespread talcose rocks with pyrrhotite-pyrite mineralization after serpentinite, as well as finding of talc-chlorite metabasalt are interpreted as products of hydrothermal activity in the permeable zone of detachment fault. Chloritization and brecciation of basalts with superposed quartz or opal, barite, and pyrite or chalcopyrite mineralization directly related to the sub-seafloor sulfide deposition. The native copper mineralization in almost unaltered basalts at the Semenov-4 field is suggested to precipitate from ore-forming fluids before they reach the level of sub-seafloor sulfide deposition. Amphibolites with plagiogranite veinlets are interpreted as tectonic fragments of the highest-temperature portions of hydrothermal systems, where partial melting of basic rocks in the presence of aqueous fluid with formation of plagiogranitic melt is possible. Silicic rocks (plagiogranite, tonalite and diorite) revealed in the tectonic zone controlling the Semenov-1, -2, -5, and -3 hydrothermal sulfide fields are related to both plutonic and subvolcanic bodies and considered to be products of partial melting of basic rocks at deep levels of the hydrothermal systems. The hydrothermal fields differ in their structural position. The giant Semenov-4 field is located at the area where the hanging-wall basalt wedges out and the detachment fault zone reaches the oceanic floor. The range of relatively small Semenov-1, -2, -3, and 5 fields develops on the oceanic core complex massif, being localized in the superposed volcanic structures within the near-latitudinal steeply dipping tectonic zone. The structural control of the hydrothermal fields at 13°31?? N is also interpreted in different ways. For the Semenov-4 field, the ascending fluid flow can be related to the permeable detachment fault zone. The root zone of the hydrothermal system with a magmatic heater could have been localized at a significant distance beneath the axial spreading zone. For the other four relatively small fields, it is suggested that the ascending fluid flows and roots of the hydrothermal systems are controlled by the volcanic structures superposed on the oceanic ore complex within the steeply dipping tectonic zone.     

Peridotite-gabbro-trondhjemite association of the Mid-Atlantic Ridge between 12°58′ and 14°45′N: Ashadze and Logachev hydrothermal vent fields

This study aimed at reconstructing the sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites of the oceanic core complexes of the Ashadze and Logachev hydrothermal vent fields. The study object was the collections of plutonic rocks made during cruises 22 and 26 of the R/V Professor Logachev, Cruise 41 of the R/V Akademik Mstislav Keldysh, and the Russian-French expedition Serpentine aboard the R/V Pourquoi pas? The data reported here suggest that the oceanic core complexes of the Ashadze and Logachev fields were formed via the same scenario in the two MAR regions. On the other hand, the analysis of petrological and geochemical characteristics of the rocks indicated that the oceanic core complexes of the MAR axial zone between 12°58′ and 14°45′N show a pronounced petrological and geochemical heterogeneity manifested in variations in the degree of depletion of mantle residues and the Nd isotopic compositions of the rocks of the gabbro-peridotite association. The trondhjemites of the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under the influence of hydrothermal fluid. It was supposed that the presence of trondhjemites in the MAR oceanic core complexes can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone in which petrologically and geochemically contrasting oceanic core complexes are spatially superimposed served as sites for the development of large hydrothermal clusters with a considerable ore-forming potential.     

Lithospheric structure and crust–mantle decoupling in the southeast edge of the Tibetan Plateau

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden change of the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.