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Gerhard Bohrmann - One of the best experts on this subject based on the ideXlab platform.
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The silicon isotope record of early silica diagenesis
Earth and Planetary Science Letters, 2015Co-Authors: Michael Tatzel, Friedhelm Von Blanckenburg, Marcus Oelze, Jan A. Schuessler, Gerhard BohrmannAbstract:The heavy isotopes of silicon are strongly enriched in some of the youngest, early diagenetically formed Porcellanite layers from the Southwest Indian Ridge (Pleistocene) and the Maud Rise (Pliocene). These Porcellanite layers are composed of opal-CT and were formed by the conversion of amorphous silica (opal-A) from siliceous sediment via dissolution–reprecipitation. Their bulk δSi30 values range between 1.7 and 2.3‰. Detritus-poor siliceous sediment surrounding these layers is significantly lower at −0.3 to 1.5‰. Sequential chemical extractions of bulk siliceous sediment show (i) preferential dissolution of diatoms featuring higher δSi30 than radiolaria and Al–Si components. The detailed investigation of Porcellanite layers by micro-scale Si isotope and Al/Si analyses using UV femtosecond laser ablation ICP mass spectrometry show that (ii) precipitation of authigenic aluminum silicates enriched in light Si isotopes drives pore waters to even higher δSi30. We suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert. We infer that past environmental conditions can be reconstructed with high fidelity from the stable Si isotope composition of chert when initial seawater Si concentrations were high (such as in the Precambrian). Exchange of Si between layers during phase transformation (from opal-A to opal-CT and from opal-CT to quartz) is impeded when variable amounts of detrital minerals are present, because they control rates of silica phase transformation and hence the timing of dissolution–reprecipitation during burial.
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physical properties of a Porcellanite layer southwest indian ridge constrained by geophysical logging
Marine Geology, 1997Co-Authors: Sebastian Gerland, Gerhard Kuhn, Gerhard BohrmannAbstract:Abstract A distinct Porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick Porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the Porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of Porcellanite. The Porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the Porcellanite bed. Images from the X-ray CT survey indicate that the Porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed.
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Pure siliceous ooze, a diagenetic environment for early chert formation
Geology, 1994Co-Authors: Gerhard Bohrmann, Andrea Abelmann, Rainer Gersonde, Hans-wolfgang Hubberten, Gerhard KuhnAbstract:The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, Porcellanites and cherts in land sections and Deep Sea Drilling Project and Ocean Drilling Program drill sites are usually found in sediment sections of Miocene age and older. During R.V. Polarstem cruises ANT-IX/3 and 4, young Porcellanites were recovered for the first time in contact with their host sediment in two cores from the Atlantic sector of the southern ocean. Chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation. In both cores the Porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid transformation of biogenic silica into Porcellanites.
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Reflector Pc a prominent feature in the Maud Rise sediment sequence (eastern Weddell Sea) Occurrence, regional distribution and implications to silica diagenesis
Marine Geology, 1992Co-Authors: Gerhard Bohrmann, Volkhard Spieß, Heinrich Hinze, Gerhard KuhnAbstract:Condensed pelagic deposits characterize the uppermost sediment sequence of the Maud Rise in the eastern Weddell Sea. The comparison of subbottom profiling data with the sediments drilled at two ODP sites shows that Lithologic Unit Ib deposited during late Miocene to late Pliocene times is generally characterized by acoustic transparency. A prominent reflector with strongly increased amplitude (called Reflector “Pc”) occurs within this mainly homogeneous diatom ooze unit and is correlated with the Porcellanite horizon found at ODP Site 689. As evidenced by sediment findings, Reflector “Pc” marks a distinct stratigraphic layer of early Pliocene age. The development of the Reflector “Pc” and its amplitude of reflection in the echosounding records is most likely caused by the silica cementation of Porcellanite within this layer. The analyses of the sediment echosounder profiles from five R.V. Polarstern cruises make it possible to map the regional distribution of Reflector “Pc”. In large areas of the central Maud Rise, a patchy but also continuous distribution of Reflector “Pc” was found. The reflector does not occur in sediments below 3000 meters of water. On average Reflector “Pc” is detected around 11 and 12 meters below seafloor, but depth occurrence is highly variable within the range of 8 m to more than 18 m. The apparent amplitude of Reflector “Pc” is higher in those areas where the diatom ooze Unit Ib reaches greater thicknesses, which may be caused by local enhanced tectonic subsidence. In general a good correlation of Reflector “Pc” amplitude and thickness of lithologic Unit Ib is observed. This may imply that the diatom ooze Unit Ib serves as a silica source, since the extent of silica cementation of the Porcellanite layer is controlled by the amount of dissolved silica supply. Other possible interpretations for silica sources include silica-rich interstitial water ascending from deeper sediment levels using tectonic structures for migration.
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Low-temperature opal-CT precipitation in Antarctic deep-sea sediments evidence from oxygen isotopes
Earth and Planetary Science Letters, 1991Co-Authors: Reiner Botz, Gerhard BohrmannAbstract:Porcellanite has been found in Antarctic deep-sea sediments of shallow burial depth at four different sites in host sediments younger than 4 Ma. Oxygen isotope analysis shows that the opal-CT samples are extremely rich in18O (δ18O= 41.2to44.7‰ rel. SMOW). According to the quartz/water fractionation the calculated isotopic opal-CT formation temperature is in the range of 0 to 4°C. This agrees well with sediment temperature measurements. The low opal-CT precipitation temperature contrasts with current ideas about later diagenetic formation of opal-CT at higher temperatures of 18 to 56°C.
A Basu - One of the best experts on this subject based on the ideXlab platform.
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u pb age and chemical composition of an ash bed in the chopan Porcellanite formation vindhyan supergroup india
The Journal of Geology, 2018Co-Authors: Meenal Mishra, M E Bickford, A BasuAbstract:AbstractWe report a newly determined age (1647±18 Ma) for a rare magmatic zircon extracted from a very fine-grained felsic ash bed in the type area of the Porcellanite Formation, Vindhyan Supergroup, India, and chemical compositions of two samples from this ash bed. The new results, in the context of published literature, (1) firmly establish the age of the Porcellanite Formation between ∼1640 and 1630 Ma and (2) suggest that Plinian eruptions took place from isolated vents along a 300-km crustal fracture.
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u pb age and hf isotopic compositions of magmatic zircons from a rhyolite flow in the Porcellanite formation in the vindhyan supergroup son valley india implications for its tectonic significance
The Journal of Geology, 2017Co-Authors: M E Bickford, Juergen Schieber, Meenal Mishra, Paul A Mueller, George D Kamenov, A BasuAbstract:AbstractThe Porcellanite Formation in the basal Semri Group of the Proterozoic Vindhyan Supergroup, exposed along the Son River Valley, India, primarily consists of intercalated silicified felsic tuff beds (Porcellanite), rhyolitic breccia, and shale. Lensoid patches of very coarse-grained rhyolite occur in a stretch of about 10 km near the eastern end of the valley. These are newly interpreted as rhyolite flows, with flow structures including aligned volcanic fragments and phenocrysts, a few of which are ∼10-cm-long feldspars. Microphenocrysts consist of fragments of patchy perthite and quartz, with subordinate Fe-oxide minerals set in a very fine-grained groundmass of quartz and feldspar. Volcanic rock fragments and fiamme are common. Bulk chemical compositions of nine samples also identify them as rhyolite (SiO2 = 75.24%; Na2O + K2O = 7.06%, CaO = 1.66%). Rare earth element distribution is typical of granitic rocks, with a negative Eu anomaly of 0.57. The samples are enriched in U, Th, La, and Rb and d...
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New U-Pb ages of zircons in the Owk Shale (Kurnool Group) with reflections on proterozoic Porcellanites in India
Journal of the Geological Society of India, 2013Co-Authors: M E Bickford, D. Saha, J. Schieber, G. Kamenov, A. Russell, A BasuAbstract:Felsic tuff beds with some presumed sedimentary components were reported from the Owk Shale (Kurnool Group; bearing Neoproterozoic fossils) in the upper part of the sedimentary succession in the Cuddapah basin in India by Saha and Tripathy (2012a). Our optical and SEM petrographic study of three thin sections, however, indicates that the parent samples are sandy mudstones with variable amounts of a felsic volcaniclastic component. New highquality U-Pb (SHRIMP and LA-MC-ICPMS) ages of 133 detrital zircon grains from a sample show that one grain is ca. 1880 Ma, one grain is ca. 3300 Ma, and the ages of the remaining 131 grains fall between 2690 Ma and 2429 Ma, the population averaging 2522 ± 36 Ma. The data indicate that the zircons are detrital grains derived from the ca. 2.5 Ga granitic/gneissic/greenstone basement of the Dharwar cratons that also host minor older Archean enclaves. The single 1880 Ma grain could have come from a ca. 1.9 Ga LIP. In the absence of any younger magmatic zircon, the absolute age of the Owk Shale remains elusive.
Meenal Mishra - One of the best experts on this subject based on the ideXlab platform.
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evidences of felsic volcanism and hydrothermal activities from clays associated with the palaeoproterozoic Porcellanite formation of the vindhyan supergroup central india
Geochemistry, 2019Co-Authors: G Mageswarii, Meenal Mishra, J P ShrivastavaAbstract:Abstract Chemico-mineralogical attributes of authigenic clays associated with the altered volcanic tuffs that occur in the Palaeoproterozoic Porcellanite Formation contain evidences of hydrothermal alteration and diagenetic processes in a marine environment. Previous sedimentological and geochemical studies on Porcellanite Formation were restricted to the Chopan area, but, the details related to provenance, nature and source of volcanism archived in these clays have not been ascertained. In order to understand these aspects, present study on these authigenic clays were carried out. Clay minerals represent dominance of illite with subordinate amount of montmorillonite. Moreover, low abundance of kaolinite is also noticed. The illite fibers and plates associated with the kaolinite indicate illitization. The kaolinite to illite transformation is favoured by incorporation of K + ions, derived from the K-feldspar dissolution and its overgrowth. Major oxide contents of these clays and their ratios when plotted over diagrams marked with standard illite, kaolinite, smectite and chlorite compositional fields show clustering within or close to the illite field. Thermodynamic components calculated for these clays when plotted over AR 2 3+ AlSi 3 O 10 (OH) 2 − R 2 3+ Si 4 O 10 (OH) 2 − AR 2+ R 3+ Si 4 O 10 (OH) 2 ternary diagram, data plots lie within the illite, mixed layer I/S and smectite fields. Binary major oxide data plots between bulk rock and authigenic clay compositions showed felsic affinity. Montmorillonite and illite predominated in the eastern and western marginal areas of the Vindhyan Basin, respectively. However, former resulted from the hydrothermal alteration of volcanic glass associated with the ferruginous breccia and altered tuffs and remnants of the volcanic vents, whereas, later is associated with the tuffaceous beds. Owing to the adsorption, Ba, Rb and Sr is enriched in clays comparing to the bulk rock composition. Low ( insitu weathering. The dominance of K-feldspar alteration and insitu weathering is also evident from clustering of clay data plots in the A-CN-K ternary diagram. Pronounced negative Eu anomaly together with higher LREE/HREE values associated with these clay minerals implied proximity to source and their possible derivation from the silicified felsic tuffs available in the provenance.
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u pb age and chemical composition of an ash bed in the chopan Porcellanite formation vindhyan supergroup india
The Journal of Geology, 2018Co-Authors: Meenal Mishra, M E Bickford, A BasuAbstract:AbstractWe report a newly determined age (1647±18 Ma) for a rare magmatic zircon extracted from a very fine-grained felsic ash bed in the type area of the Porcellanite Formation, Vindhyan Supergroup, India, and chemical compositions of two samples from this ash bed. The new results, in the context of published literature, (1) firmly establish the age of the Porcellanite Formation between ∼1640 and 1630 Ma and (2) suggest that Plinian eruptions took place from isolated vents along a 300-km crustal fracture.
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Petrological study of the early Mesoproterozoic Glauconitic Sandstone and Olive Shale members from the Semri Group, Vindhyan Supergroup in Central India: Implications to input from intrabasinal felsic volcanic source and glauconitization
Geological Journal, 2017Co-Authors: Meenal MishraAbstract:The Semri Group of the Vindhyan Supergroup contains widespread intrabasinal felsic volcanic rocks in the Porcellanite Formation formed during the early Mesoproterozoic. This widespread felsic volcanism was followed by the deposition of siliciclastic sedimentary rocks of the Kheinjua Formation containing the Olive Shale, Fawn Limestone, Glauconitic Sandstone members, the last one dominantly arkose and litharenite. The volcanic products resulting from volcanic activity during deposition of Porcellanite Formation were predominantly fine volcanic ash or dust that settled very slowly in the water column of the basin. The ranges of 28 elemental ratios for the Olive Shale and Glauconitic Sandstone members overlapped with those of the Porcellanite Shale Formation. The contemporaneous intermingling of the detrital input from volcanic products with the terrigenous detritus received by the basin from the Bundelkhand Craton in the north is reflected in the geochemical signatures. Various plots of elemental ratios and ternary plots for the siliciclastic rocks overlap those of the Porcellanite Shale Formation and Bundelkhand Tonalite-trondhjemite-granodiorite (TTG) suggesting that they might be the source rocks. The mixing of detritus from volcanic products was perhaps responsible for the widespread appearance of glauconite-bearing sediments in the subsequent deposits. Glauconitization of detrital K-feldspars, clay peloids, and volcanic lithic fragments is evident from the petrography. The shallow marine environment, slow sedimentation rates, elevated surface temperatures and mildly reducing conditions (CO2 got released from explosive volcanism) are considered as the favourable conditions for glauconitization, which prevailed within the Vindhyan Basin. Under these conditions the detrital input received as the result of previous volcanism during deposition of the Porcellanite shale (glass, pumice lithic fragments, and feldspars) were transformed into clays such as illite and glauconite during the deposition of siliciclastic rocks of the Kheinjua Formation within the basin.
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Explosive felsic volcanism at Palaeo-Mesoproterozoic boundary from Vindhyan Supergroup, Son Valley, Central India—Evidences of crustal contamination
Geochemistry International, 2017Co-Authors: Meenal Mishra, Kiran KumariAbstract:Vindhyan basin witnessed a widespread explosive type of felsic volcanism at Palaeo-Mesoproterozoic boundary which is manifested as Chopan Porcellanite shale. This is exposed as a linear belt along the Son valley in Central India. Porcellanite shale is pyroclastics deposit comprising strongly welded to unwelded ignimbrites. CIA values coupled with A–CN–K systematics provide strong evidence regarding their igneous origin and proximity of the source. The pyroclastics are rhyodacitic to rhyolitic in nature. The enriched LREE, LILE, depleted HFSE and incompatible element ratios such as Nb/Th, La/Sm and Zr/Nb indicate contamination and mixing between mantle-derived rocks and the average continental crust. Five distinct phases of volcanic activity have been identified based on field observations and petrological evidences. Pyroclastics at various stratigraphic levels indicate repeated occurrences of intrabasinal felsic volcanism, pointing to episodic extension, rifting and eruption over a period of time. The present studies have suggested that volcanic activity in Son valley and pyroclastic detritus resulted from a common chamber due to the rejuvenation and activation of deep seated faults like Son-Narmada lineament.
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u pb age and hf isotopic compositions of magmatic zircons from a rhyolite flow in the Porcellanite formation in the vindhyan supergroup son valley india implications for its tectonic significance
The Journal of Geology, 2017Co-Authors: M E Bickford, Juergen Schieber, Meenal Mishra, Paul A Mueller, George D Kamenov, A BasuAbstract:AbstractThe Porcellanite Formation in the basal Semri Group of the Proterozoic Vindhyan Supergroup, exposed along the Son River Valley, India, primarily consists of intercalated silicified felsic tuff beds (Porcellanite), rhyolitic breccia, and shale. Lensoid patches of very coarse-grained rhyolite occur in a stretch of about 10 km near the eastern end of the valley. These are newly interpreted as rhyolite flows, with flow structures including aligned volcanic fragments and phenocrysts, a few of which are ∼10-cm-long feldspars. Microphenocrysts consist of fragments of patchy perthite and quartz, with subordinate Fe-oxide minerals set in a very fine-grained groundmass of quartz and feldspar. Volcanic rock fragments and fiamme are common. Bulk chemical compositions of nine samples also identify them as rhyolite (SiO2 = 75.24%; Na2O + K2O = 7.06%, CaO = 1.66%). Rare earth element distribution is typical of granitic rocks, with a negative Eu anomaly of 0.57. The samples are enriched in U, Th, La, and Rb and d...
Gerhard Kuhn - One of the best experts on this subject based on the ideXlab platform.
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physical properties of a Porcellanite layer southwest indian ridge constrained by geophysical logging
Marine Geology, 1997Co-Authors: Sebastian Gerland, Gerhard Kuhn, Gerhard BohrmannAbstract:Abstract A distinct Porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick Porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the Porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of Porcellanite. The Porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the Porcellanite bed. Images from the X-ray CT survey indicate that the Porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed.
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Pure siliceous ooze, a diagenetic environment for early chert formation
Geology, 1994Co-Authors: Gerhard Bohrmann, Andrea Abelmann, Rainer Gersonde, Hans-wolfgang Hubberten, Gerhard KuhnAbstract:The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, Porcellanites and cherts in land sections and Deep Sea Drilling Project and Ocean Drilling Program drill sites are usually found in sediment sections of Miocene age and older. During R.V. Polarstem cruises ANT-IX/3 and 4, young Porcellanites were recovered for the first time in contact with their host sediment in two cores from the Atlantic sector of the southern ocean. Chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation. In both cores the Porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid transformation of biogenic silica into Porcellanites.
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Reflector Pc a prominent feature in the Maud Rise sediment sequence (eastern Weddell Sea) Occurrence, regional distribution and implications to silica diagenesis
Marine Geology, 1992Co-Authors: Gerhard Bohrmann, Volkhard Spieß, Heinrich Hinze, Gerhard KuhnAbstract:Condensed pelagic deposits characterize the uppermost sediment sequence of the Maud Rise in the eastern Weddell Sea. The comparison of subbottom profiling data with the sediments drilled at two ODP sites shows that Lithologic Unit Ib deposited during late Miocene to late Pliocene times is generally characterized by acoustic transparency. A prominent reflector with strongly increased amplitude (called Reflector “Pc”) occurs within this mainly homogeneous diatom ooze unit and is correlated with the Porcellanite horizon found at ODP Site 689. As evidenced by sediment findings, Reflector “Pc” marks a distinct stratigraphic layer of early Pliocene age. The development of the Reflector “Pc” and its amplitude of reflection in the echosounding records is most likely caused by the silica cementation of Porcellanite within this layer. The analyses of the sediment echosounder profiles from five R.V. Polarstern cruises make it possible to map the regional distribution of Reflector “Pc”. In large areas of the central Maud Rise, a patchy but also continuous distribution of Reflector “Pc” was found. The reflector does not occur in sediments below 3000 meters of water. On average Reflector “Pc” is detected around 11 and 12 meters below seafloor, but depth occurrence is highly variable within the range of 8 m to more than 18 m. The apparent amplitude of Reflector “Pc” is higher in those areas where the diatom ooze Unit Ib reaches greater thicknesses, which may be caused by local enhanced tectonic subsidence. In general a good correlation of Reflector “Pc” amplitude and thickness of lithologic Unit Ib is observed. This may imply that the diatom ooze Unit Ib serves as a silica source, since the extent of silica cementation of the Porcellanite layer is controlled by the amount of dissolved silica supply. Other possible interpretations for silica sources include silica-rich interstitial water ascending from deeper sediment levels using tectonic structures for migration.
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a young Porcellanite occurrence from the southwest indian ridge
Marine Geology, 1990Co-Authors: Gerhard Bohrmann, Gerhard Kuhn, Andrea Abelmann, Rainer Gersonde, Dieter K FuttererAbstract:A Porcellanite layer, probably younger than 0.6-0.4 Ma, of a nearly monomineralic composition of opal-CT was sampled on the Southwest Indian Ridge during Polarstern cruise ANT-VI/3. The intense cementation of the rock, together with recent findings by the Ocean Drilling Program (Legs 113 and 120) and the occurrence of a unique older Porcellanite from Eltanin Core 47-15, provides evidence of very early silica precipitation in pure diatom oozes of the Southern Ocean. Such Porcellanites occur in shallowly buried young sediments and provide a contrast to the established concepts of Porcellanite formation.
Dieter K Futterer - One of the best experts on this subject based on the ideXlab platform.
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a young Porcellanite occurrence from the southwest indian ridge
Marine Geology, 1990Co-Authors: Gerhard Bohrmann, Gerhard Kuhn, Andrea Abelmann, Rainer Gersonde, Dieter K FuttererAbstract:A Porcellanite layer, probably younger than 0.6-0.4 Ma, of a nearly monomineralic composition of opal-CT was sampled on the Southwest Indian Ridge during Polarstern cruise ANT-VI/3. The intense cementation of the rock, together with recent findings by the Ocean Drilling Program (Legs 113 and 120) and the occurrence of a unique older Porcellanite from Eltanin Core 47-15, provides evidence of very early silica precipitation in pure diatom oozes of the Southern Ocean. Such Porcellanites occur in shallowly buried young sediments and provide a contrast to the established concepts of Porcellanite formation.