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41.
C.R. German S.A. Bennett D.P. Connelly A.J. Evans B.J. Murton L.M. Parson R.D. Prien E. Ramirez-Llodra M. Jakuba T.M. Shank D.R. Yoerger E.T. Baker S.L. Walker K. Nakamura 《Earth and Planetary Science Letters》2008,273(3-4):332-344
We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an 18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a 24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at 3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest. 相似文献
42.
Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge 总被引:3,自引:0,他引:3
Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (∼365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different 143Nd/144Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1. 相似文献
43.
Seismic and geological information on the Lomonosov Ridge is considered with reference to drilling data obtained during the ACEX-302 expedition. A new zonation proposed for the composite section of the ridge sedimentary cover and based on marine microfossils (silicoflagellates and dinocysts) from several boreholes is correlated with biostratigraphic zones of Paleogene sections in northern West Siberia. Principal stages of the Arctic basin development in the Aptian-Cenozoic are defined in onshore and Lomonosov Ridge sections. Synchronous formation of sedimentary sequences in the ridge, an element of the Arctic basin, shelves, and epicontinental seas is established for the period under consideration. 相似文献
44.
Provenance of Precambrian Fe- and Al-rich Metapelites in the Yenisey Ridge and Kuznetsk Alatau, Siberia: Geochemical Signatures 总被引:2,自引:0,他引:2 下载免费PDF全文
Major, trace and rare earth element contents of Fe- and Al-rich metapelites from the Korda (Yenisey Ridge) and Amar (Kuznetsk Alatau) formations were determined to examine the nature, origin and evolution of their protoliths. Results indicate that these rocks are the redeposited and metamorphosed products of Precambrian kaolinitic weathering crusts, while the geochemical distinctions between the studied metapelites are determined by different weathering conditions in the source area and tectonic settings. The protolith of the Korda Formation metapelites was produced by erosion products of the post-Archean granitoid rocks, which accumulated under humid climate conditions in shallow-water basins along the continental margin. The geochemical characteristics of the deeper primary deposits of the Amar Formation suggest that volcanogenic material of mafic composition derived from an island-arc environment had a major role in supplying the erosion zone. These results agree with lithofacies data and with the geodynamic reconstruction of the evolution of the Yenisey Ridge and Kuznetsk Alatau during the Mesoproterozoic and Neoproterozoic, respectively. It was shown that REEs had limited mobility during contact metamorphism. The coherent mobility of REEs during collisional metamorphism may be attributed both to mineral reactions responsible for modal changes and to local chemical heterogeneity inherited from the initial protolith. 相似文献
45.
A.D. Nozhkin A.A. Postnikov K.E. Nagovitsin A.V. Travin A.M. Stanevich D.S. Yudin 《Russian Geology and Geophysics》2007,48(12):1015-1025
Trachybasalt-alkali trachyte volcanism in the Yenisei Ridge was found out to be synchronous with deposition of coarse tilloids and flysch of the Chivida Formation of the Neoproterozoic Chingasan Group. New 703±4 Ma 40Ar/39Ar biotite and titan-augite ages of subalkaline basalts in the Chivida Formation indicated that they erupted in the Late Neoproterozoic. According to microfossil evidence, the Chingasan sediments correlate with Late Neoproterozoic strata in the type sections of the southern Siberian craton. The Chingasan deposition apparently lasted no longer than 30 Myr judging by the isotope ages obtained for the underlying Upper Vorogovka Group and subalkaline basalts in the Chivida Formation. The fault-parallel position of grabens and coarse grain sizes and variable thicknesses of their lithological complexes, as well as syndepositional trachybasalt-alkali trachyte volcanism provided evidence that the volcanosedimentary rocks of the Chingasan Group formed in an environment of active rifting. 相似文献
46.
The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the
recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev
vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide
(VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb
and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb).
A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized
patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values
of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly
shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely
reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks. 相似文献
47.
Melt inclusion and host glass compositions from the easternend of the Southwest Indian Ridge show a progressive depletionin light rare earth elements (LREE), Na8 and (La/Sm)n, but anincrease in Fe8, from the NE (64°E) towards the SW (49°E).These changes indicate an increase in the degree of mantle meltingtowards the SW and correlate with a shallowing of the ridgeaxial depth and increase in crustal thickness. In addition,LREE enrichment in both melt inclusions and host glasses fromthe NE end of the ridge are compatible with re-fertilizationof a depleted mantle source. The large compositional variations(e.g. P2O5 and K2O) of the melt inclusions from the NE end ofthe ridge (64°E), coupled with low Fe8 values, suggest thatmelts from the NE correspond to a variety of different batchesof melts generated at shallow levels in the mantle melting column.In contrast, the progressively more depleted compositions andhigher Fe8 values of the olivine- and plagioclase-hosted meltinclusions at the SW end of the studied region (49°E), suggestthat these melt inclusions represent batches of melt generatedby higher degrees of melting at greater mean depths in the mantlemelting column. Systematic differences in Fe8 values betweenthe plagioclase- and the olivine-hosted melt inclusions in theSW end (49°E) of the studied ridge area, suggest that theplagioclase-hosted melt inclusions represent final batches ofmelt generated at the top of the mantle melting column, whereasthe olivine-hosted melt inclusions correspond to melts generatedfrom less depleted, more fertile mantle at greater depths. KEY WORDS: basalt; melt inclusions; olivine; plagioclase; Southwest Indian Ridge 相似文献
48.
Valery A. Vernikovsky A. E. Vernikovskaya A. B. Kotov E. B. Sal'nikova V. P. Kovach 《Tectonophysics》2003,375(1-4):147
The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton. 相似文献
49.
Seismic investigations along the western sector of Alpha Ridge, Central Arctic Ocean 总被引:2,自引:0,他引:2
Wilfried Jokat 《Geophysical Journal International》2003,152(1):185-201
50.