Galena Crystallization and the Origin of Sulfur in the Oklo and Bangombé Natural Reactors: The Effects of a ca. 900 Ma Thermal Event

2000 ◽  
Vol 663 ◽  
Author(s):  
Lena Zetterström ◽  
Torbjörn Sunde

ABSTRACTGalena in the ca. 1950 Ma old natural fossil fission reactors in Gabon crystallized sometime between 980 Ma and 750 Ma during a period of regional extension and the intrusion of a dolerite dyke swarm. The S isotopic composition of galena, containing radiogenic Pb from uraninite, gives information about the origin of the S. Results from ion microprobe analyses of galena from the reactor zones indicate that S mainly originates from the surrounding sediment. Galena in a thin, altered dolerite dyke also contains non-magmatic S. The presented data gives no positive evidence for the involvement of magmatic S during the ca. 900 Ma galena crystallisation, however, the possibility cannot be ruled out.

GFF ◽  
2016 ◽  
Vol 138 (1) ◽  
pp. 6-16 ◽  
Author(s):  
Victor Puchkov ◽  
Richard E. Ernst ◽  
Michael A. Hamilton ◽  
Ulf Söderlund ◽  
Nina Sergeeva

2003 ◽  
Vol 88 (10) ◽  
pp. 1583-1590 ◽  
Author(s):  
Mostafa Fayek ◽  
Satoshi Utsunomiya ◽  
Rodney C. Ewing ◽  
Lee R. Riciputi ◽  
Keld A. Jensen

2021 ◽  
pp. SP518-2021-46
Author(s):  
Arnab Dey ◽  
Sisir K. Mondal

AbstractDolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE - SW to NNE - SSW, NW - SE to WNW - ESE, E - W, and N - S directions. The WNW - ESE trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of the Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe-Ti-oxide minerals and texture across the trend. At places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. Primary silicates are clinopyroxene (augite: Mg# = 65.7 - 82.6; En37-48Fs11-17Wo36-41), orthopyroxene (clinoenstatite: Mg# = 68.5 − 78; En63-70Fs20-29Wo4-5), plagioclase (An11-39Ab44-82Or1-7) and Fe-Ti oxides are titanomagnetite (FeO = 34.38 − 39.50 wt%, Fe2O3 = 48.26 − 56.21 wt%, TiO2 = 5.05 − 9.60 wt%) and ilmenite (FeO = 40.75 − 43.79 wt%, Fe2O3 = 3.54 − 10.03 wt%, TiO2 = 47.82 − 50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065oC to 978oC, and coexisting titanomagnetite-ilmenite pairs reveal 731.39oC to 573.37oC at the oxygen fugacity (fO2) condition NNO+0.3 to FMQ-1.03. The dyke contains disseminated sulfides at the interstices of Fe-Ti-oxides, and silicates. Major sulfide minerals are pyrite, chalcopyrite, and vaesite; Pyrite-vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentration of Co = 0.01 − 5.70 wt% and Ni = 0.02 − 1.95 wt%. Coexisting vaesite contains Co = 2.42 − 10.44 wt%, Ni = 26.40 − 47.88 wt%, and Fe = 7.32 − 26.55 wt%. Texture, sulfide-silicate assemblage, and presence of low metal/S sulfides such as the pyrite-vaesite assemblage indicate primary Fe-Ni- sulfides (pyrrhotite-pentlandite) that segregated from immiscible sulfide liquid at high temperature is modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic-mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe-Ni-sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5643989


2020 ◽  
Author(s):  
Lidiia Shpakovich ◽  
Sergey Malyshev ◽  
Valeriy Savatenkov

<p>Geodynamic reconstructions are largely based on information contained in mafic igneous rocks, including dykes and sills. The age and isotope-geochemical characteristics of such rocks are inevitable for understanding of geodynamic history of the Proterozoic cratons. The regions in Siberian Craton, where Precambrian mafic dyke swarms are known are following: Anabar Shield and Olenek Uplifts, Aldan-Stanovoi Shield, SE area of Siberian Craton, and smaller Uplifts on the SW margin of Siberian Craton.</p><p>The Udzha paleo-rift is located in the northern part of Siberian Craton between Anabar and Olenek Uplifts is also associated with mafic dyke swarm. These dykes cross-cut the pre-Neoproterozoic sedimentary successions. The age of the largest dyke in Udzha paleo-rift (Great Udzha Dyke) presented by medium-grained dolerite was determined to be 1386 ± 30 Ma (Malyshev et al., 2018).</p><p>We present new data of Sr, Nd and Pb isotopic composition on the Udzha paleo-rift dykes, determined by TIMS. The initial isotopic composition of Pb in the dykes was obtained using the leaching method by Savatenkov et al., 2019. The Sr isotopic composition of the dykes demonstrates substantial variation (εSr varies from 8.4 to 110.4). We do not consider this fact as a result of crust contamination, because Nd isotopic composition does not vary significantly (εNd varies from -1.4 to 0.7). Obtained results indicate that initial for the Udzha paleo-rift dykes melts were generated from two mantle reservoirs of DM and EMII-type. The initial Pb isotopic composition of the dykes reveals EMII source participation in the melts generation too (<sup>206</sup>Pb/<sup>204</sup>Pb varies from 16.133 to 16.266, <sup>207</sup>Pb/<sup>204</sup>Pb varies from 15.343 to 15.458). The presence of enriched component is likely associated with lithospheric mantle, metasomatized by fluids, derived from subducted terrigenous material.</p><p>The studies were supported by the Russian Science Foundation project No. 19-77-10048.</p><p>References</p><p>Malyshev, S. V., Pasenko A. M., Ivanov A. V., Gladkochub D. P., Savatenkov V. M., Meffre S., Abersteiner A., Kamenetsky V. S. & Shcherbakov V. D. (2018): Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data. – Minerals, 8(12), 555</p><p>Savatenkov V. M., Malyshev, S. V., Ivanov A. V., Meffre S., Abersteiner A., Kamenetsky V. S., Pasenko A. M. (2019): An advanced stepwise leaching technique for derivation of initial lead isotope ratios in ancient mafic rocks: A case study of Mesoproterozoic intrusions from the Udzha paleo-rift, Siberian Craton. – Chemical Geology, 528, 119253</p>


Clay Minerals ◽  
2014 ◽  
Vol 49 (4) ◽  
pp. 569-594 ◽  
Author(s):  
J. Środoń ◽  
J. Szulc ◽  
A. Anczkiewicz ◽  
K. Jewuła ◽  
M. Banaś ◽  
...  

AbstractMudstones and claystones from the southern marginal area of the European Upper Triassic, midcontinental Keuper basin (Silesia, southern Poland) were investigated using XRD, organic and inorganic geochemistry, SEM, K-Ar of illite-smectite, AFT, and stable isotopes of O and C in carbonates in order to unravel the consequent phases of the geological history of these rocks, known for abundant fossils of land vertebrates, and in particular to evaluate the diagenetic overprint on the mineral composition. The detected and quantified mineral assemblage consists of quartz, calcite, dolomite, Ca-dolomite, illite, mixed-layer illite-smectite, and kaolinite as major components, plus feldspars, hematite, pyrite, chlorite, anatase, siderite, goethite as minor components. Palygorskite, gypsum, jarosite and apatite were identified in places.The K-Ar dates document a post-sedimentary thermal event, 164 Ma or younger, which resulted in partial illitization of smectite and kaolinite. The maximum palaeotemperatures were estimated from illite-smectite as ∼125°C. Apatite fission track data support this conclusion, indicating a 200–160 Ma age range of the maximum temperatures close to 120°C, followed by a prolonged period of elevated temperatures. These conclusions agree well with the available data on the Mesozoic thermal event, which yielded Pb-Zn deposits in the area. Organic maturity indicators suggest the maximum palaeotemperatures <110°C.Palygorskite was identified as authigenic by crystal morphology (TEM), and calcite by its accumulation in soil layers and by its isotopic composition evolving with time, in accordance with the sedimentary and/or climatic changes. Dolomite isotopic composition indicates more saline (concentrated) waters. Palygorskite signals a rapid local change of sedimentary conditions, correlated with algal blooms. This assemblage of authigenic minerals indicates an arid climate and the location at the transition from a distal alluvial fan to mudflat.Fe-rich smectite, kaolinite, and hematite were products of chemical weathering on the surrounding lands and are therefore mostly detrital components of the investigated rocks. Kaolinite crystal morphology and ordering indicates a short transport distance. Hematite also crystallizedin situ, in the soil horizons. A large variation in kaolinite/2:1 minerals ratio reflects hydraulic sorting, except of the Rhaetian, where it probably signals a climatic change, i.e. a shift in the weathering pattern towards kaolinite, correlated with the disappearance of hematite. Quartz, 2M1illite, and minor feldspars and Mg-chlorite were interpreted as detrital minerals. The documented sedimentation pattern indicates that in more central parts of the Keuper playa system, where an intense authigenesis of the trioctahedral clays (chlorite, swelling chlorite, corrensite, sepiolite) took place, illite and smectite were the dominant detrital clay minerals.Cr/Nb and Cr/Ti ratios were found as the best chemostratigraphic tools, allowing for the correlation of all investigated profiles. A stable decrease of these ratios up the investigated sedimentary sequence is interpreted as reflecting changes in the provenance pattern from more basic to more acidic rocks.


2012 ◽  
Vol 18 ◽  
pp. 69-84 ◽  
Author(s):  
Kenneth G. MacLeod

Oxygen isotopic analysis of the phosphate in bioapatite has become a standard paleoclimatological tool with results documented in a rapidly expanding literature. Phosphate-based measurements are particularly important for samples where carbonates preservation is suspect (as is the case for many Paleozoic sites). Important analytical and observational advances that have fueled the expansion of phosphate-based studies include: 1) Oxygen isotopic ratios of biogenic apatite can be measured on small enough samples (≥ ~300 μg), quickly enough, cheaply enough, and accurately enough to permit meaningful high resolution paleoclimatic studies of trends through time, along spatial transects, and/or among taxa, 2) biogenic apatite is precipitated in approximate equilibrium with ambient waters and thus records the interplay of temperature and the isotopic composition of the water in which a sample grew, 3) tooth enamel and conodont crown material are quite resistant to diagenetic alteration and are preferred targets for both paleotemperature and paleoecological studies, 4) Paleozoic conodont δ18O records seem to provide robust paleotemperature information on time scales ranging from thousands of years to 100's of millions of years, and generation of increasingly refined paleotemperature records from this diagenetically resistant phase is likely to continue to be a useful field of study, 5) paleoenvironmental variations in δ18O values of seawater have been documented (e.g., differences between glacial and interglacial oceans), but whether and by how much the δ18O value of the hydrosphere may have increased since the Cambrian remains unresolved, and 6) differences in δ18O values among conodont taxa are increasingly well documented and, coupled with the potential to study growth series using ion microprobe techniques, are providing novel perspectives on and important tests of conodont paleoecology.


1998 ◽  
Vol 135 (4) ◽  
pp. 509-517 ◽  
Author(s):  
G. J. H. OLIVER ◽  
R. R. McALPINE

A sheeted dolerite dyke swarm has been discovered at Duniewick Fort, Ballantrae. It forms part of the Early Ordovician Ballantrae Complex of Ayrshire, southwest Scotland. Asymmetric chilled margins, parallel to sub-parallel dykes and multiple dykes (dykes within dykes) are found. Although the mineralogy has been altered by metamorphism, the geochemistry is comparable with modern day back-arc marginal basin basalt. Cross-cutting sills have the chemistry of within-plate basalt. The regional geological setting suggests that an ophiolite sequence has been dismembered and incorporated into a serpentinite mélange. This is the first description of a 100% sheeted dyke complex (senso stricto) from the Ballantrae ophiolite.


1970 ◽  
Vol 85 ◽  
pp. 1-115
Author(s):  
C.H Emeleus ◽  
W.T Harry

The Igaliko Nepheline Syenite Complex is in the Julianehåb District of Southern Greenland, at about 61°N and 45°W. The syenites and related rocks cover an area of about 450 square kilometres in the country between Tunugdliarfik and Igaliko Fjord on the west, and the Inland lce to the east. The Complex belongs to the group of Precambrian igneous intrusions, comprising the Gardar 19neous Province, exposed in the country around Ivigtut and Julianehåb. The Complex comprises four distinct intrusive centres, termed, in order of decreasing age, the Motzfeldt Centre, the North Qoroq Centre, the South Qoroq Centre and the Igdlerfigssalik Centre. With these are associated a number of small satellitic syenites which generally pre-date the rocks of the main centres of activity and numerous alkali trachyte and Big Feldspar Dykes, which, for the most part, belong to the regional Mid-Gardar swarms. The satellitic syenites and the three earlier syenite centres are earlier than the Mid-Gardar dykes swarms. Three members of the Igdlerfigssalik Centre are also earlier than the dykes but the four late intrusions in this centre cut the majority of the dykes. Each centre is made up of several intrusive members; including satellitic intrusions, the Complex consists of 28 separate units as well as several small dykelike bodies of syenogabbro and alkali gabbro. Within each centre the individual intrusions have arcuate, steep-sided outcrops with discordant, intrusive relations towards earlier members although, occasionally, there is evidence that petrographically and texturally distinct syenites were intruded at only short intervals thus possibly representing rapid pulses of magma from the same source. The mineralogy of the syenites is usually simple: perthitic alkali feldspar, nepheline and alkali pyroxene are the dominant minerals; these are accompanied by fayalitic olivine, alkali amphibole, biotite, magnetite-rich opaques, analcite, natrolite and apatite. Pegmatites are not numerous, but the Complex does contain the celebrated Narssârssuk pegmatite which lies within the outer member of the later group of syenites in the Igdlerfigssalik Centre. Except where locally contaminated by assimilation of country rocks, the Igaliko syenites are all nepheline bearing; the commonest type is foyaite. The quartz syenite – alkali gabbro intrusive of Klokken, about 5 km south of the Complex, is not regarded as a satellite. Igneous layering and mineral lamination are common internal structures in the syenites. These generally define concentric, inward-dipping structures within individual intrusions; the frequency with which the well-developed internal structures in one syenite may be sharply truncated by a later syenite indicates that, in some instances, an appreciable amount of time must have elapsed between successive intrusions. The structures found in the Igaliko syenites are identical with those described from other layered alkaline, basic and ultrabasic intrusions. 19neous activity within the Complex began at least as early as the Mid-Gardar. Syenites of the earliest centre intrude sediments, agglomerates and lavas belonging to the Eriksfjord Formation and also cut a dolerite dyke which may belong to the relatively early Gardar dyke swarm. The early Østfjordsdal Syenite (pre-South Qôroq Centre) cuts a small swarm of nepheline porphyry dykes. The dykes of the Mid-Gardar swarms maintain their regional WSW-ENE strike throughout most of the Complex except near Qôroq where there is a marked swing into a more northerly direction. Two major sinistral faults, striking ESE and east-west, displace the earlier centres of the Complex and the dykes. The faults are members of a regional set developed throughout southern Greenland. Since no alkaline dyke cuts the younger of the two faults, it is considered likely that the 2 km sinistral transcurrent movement of this fault post-dates the late syenites in the Igdlerfigssalik Centre; the late syenites are cut by a number of alkaline dykes petrographically identical with those displaced by the sinistral faults. Thus, the latest syenites at Igaliko may be of slightly earlier date than the later members of the Ilímaussaq Intrusion and the Central Complex of Tugtutôq, although still belonging to the Late-Gardar group of intrusions.


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