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1.
Concerning the question of how and when free oxygen started to accumulate in the Earth’s atmosphere and hydrosphere, we report Mo concentrations and isotopic compositions as well as Platin group element and Rhenium (PGE-Re) data from black shales of the Transvaal Supergroup (Ghaap Group, 2.64-2.5 Ga, and Pretoria Group, 2.45-2.15 Ga). For comparison we also include new data from the 3.23 Ga Fig Tree Group, Barberton Greenstone Belt. This time range covers the period between the first robust evidence for cyanobacteria at 2.7 Ga and the disappearance of mass independent sulfur isotope fractionation (MIF) at 2.32 Ga. Due to the redox dependent solubility of Mo, Re and Os, such data are important proxies for changes in oxygen levels of the early atmosphere and oceans. In particular, Mo isotope fractionation can only occur when Mo is in solution as oxyanions, requiring free oxygen. In the Fig Tree Group samples, Mo is not fractionated relative to the continental crust, and PGE-Re abundance patterns reflect those of komatiites, indicating a purely detrital input of these element, a sign of no free oxygen. A general increase in Mo concentration and isotope fractionation, as well as an enrichment in Re, compared to the continental crust and Fig Tree Group, can be seen within the Ghaap Group indicating a gradual rise of oxygen between 2.64 and 2.5 Ga. However, Mo concentrations and δ98/95Mo values vary strongly in this period. Local conditions of sedimentation (changes in redox and input conditions) and/or global oxic/anoxic fluctuations could have caused these Mo variations.Samples of the overlying Pretoria Group (2.45-2.15 Ga) show Mo concentrations and isotopic compositions somewhat below the continental input, while PGE-Re abundance patterns are quite similar to those for the Ghaap. The apparent contradiction between the decoupled Mo and PGE-Re values can be resolved assuming a stratified ocean with almost total scavenging of Mo. Such an increased scavenging can result from enhanced biological sedimentation between and especially after glacial events. This period coincides with the Lomagundi-Jatulian carbon isotope excursion, which indeed indicates very enhanced organic carbon burial.  相似文献   

2.
Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO42−) to thiomolybdate (MoS42−) in overlying bottom waters. The average δ97/95Mo (0.72 ± 0.10‰, 2σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H2S]aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H2S]aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30-70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively.The Re-Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re-Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2σ, n = 21, MSWD = 85, Model 3). This age is younger than U-Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re-Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb-Zn-Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re-Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41-58 ppm), but are characterized by variable δ97/95Mo (0.3-0.8‰) and DOP (0.57-0.92). The lightest δ97/95Mo values occur near the base of the black shale unit. Based on the Re-Os systematics, hydrothermal fluids have probably overprinted the authigenic δ97/95Mo signature in those shales. However, the heaviest δ97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ97/95Mo at 1.73 Ga.  相似文献   

3.
Combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains from the auriferous Eldorado Reef conglomerate, upper Central Rand Group, reveal new insights into the provenance of the sediments and thus, by implication, possibly also into that of the gold. Most of the detrital zircon grains, which are of magmatic origin, yielded Mesoarchaean ages clustering around 2.94 and 3.06 Ga. A subordinate zircon population gave ages with maxima at 3.28 and 3.44 Ga. The Mesoarchaean zircon grains mostly show super-chondritic ?Hft of up to +5.2, whereas the Palaeoarchaean zircon grains have nearly chondritic composition with ?Hft between −1.3 and +2.0. The new dataset of the Mesoarchaean zircon populations provides the first unambiguous evidence of the formation of juvenile crust not only at 3.06 but also at 2.94 Ga. As the analysed zircon grains are from the ruditic fraction, they must be derived from a comparatively proximal source in close vicinity to the Central Rand Basin. Based on currently available data, this source was most likely a magmatic arc that existed at the northern edge of the Witwatersrand Block at 3.06 Ga. An additional source might be the 2.94 Ga magmatic rocks of the Kraaipan Greenstone Belt that occurs to the west of the Witwatersrand Block. The minor fraction of Palaeoarchaean zircon grains in the Eldorado Reef perhaps stem from sources that are isotopically similar to the Barberton Greenstone Belt and the Limpopo Belt but were more proximal to the Central Rand Basin.  相似文献   

4.
The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13 ± 0.14‰, 1σ, n = 11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n = 14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE + Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.  相似文献   

5.
The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45 Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend.  相似文献   

6.
Pillow basalt and chert form integral lithologies comprising many Archean greenstone belt packages. To investigate details of these lithologies in the >3.7 Ga Isua Greenstone Belt, SW Greenland, we measured silicon isotope compositions of quartz crystals, by secondary ion mass spectrometry, from a quartz‐cemented, quartz‐amygdaloidal basaltic pillow breccia, recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions. The recrystallized chert, chert clasts and quartz cement have overlapping δ30Si values, while the δ30Si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age, despite amphibolite facies metamorphism. We suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting, consistent with the pillow breccia origin. On the basis of the present data, we conclude that the geological context of each sample must be carefully evaluated when interpreting δ30Si values of quartz.  相似文献   

7.
UPb zircon systematics for granitic and gneissic clasts from the Moodies conglomerate of the Barberton Greenstone Belt, South Africa, exhibit discordancy patterns that suggest variable and multi-episodic lead loss and indicate crystallisation ages for the parent granitoids between 3.3 and 3.47 Ga. Metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2.8–2.9 Ga ago that is ascribed to a regional thermal event as also shown by published RbSr data for a nearby granitoid pluton. Since the Moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments, a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early Archaean granite-greenstone terrains. The maximum age of 3.3 Ga for the Moodies Group also shows that greenstone belt evolution in the Barberton region took place over a period of at least 150 Ma from about 3450 to 3300 Ma ago.The Kaap Valley and Stentor plutons intrude the lower part of the greenstone succession, and their diapiric emplacement caused large-scale deformation of earlier structures in the entire greenstone belt sequence at about 3.23–3.25 Ga ago.Detrital zircons in the sediments of the Fig Tree Group date the mean age of their source terrain at c. 3520 Ma which is also the maximum age of deposition for these strata. This age is indistinguishable from the oldest age reported from the Barberton greenstones and suggests the possibility that pre-greenstone granitoid crust may exist in the eastern Kaapvaal craton.  相似文献   

8.
Shales of the ca. 3.0 Ga Buhwa Greenstone Belt, Zimbabwe, were derived from a compositionally diverse provenance whose ages, determined by ion probe analyses of detrital zircons in interbedded sandstones, range from 3.8 to 3.1 Ga. Geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source. REE concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed, with components of felsic (tonalite and alkalic granitoid) and mafic rocks. Sm/Nd and Nd isotopic compositions of these rocks can be used to model initial Nd isotopic ratios at the time of sedimentation (εNdsed), as well as model crustal formation ages (TDM). The former, at the age of sedimentation, range from +0.6 to −10.8, consistent with a range of provenance ages. The latter range from 4.46 Ga to 2.99 Ga. The oldest crustal formation ages, up to 0.7 Ga older than known detrital components, are interpreted here to indicate that the Sm-Nd system of the sediments experienced open system behavior. The implied alteration would have included an increase in Sm/Nd by about 20-25 percent, probably in the form of preferential loss of Nd with respect to Sm. The Pb isotopic compositions of whole rock samples are quite radiogenic, with a range of 206Pb/204Pb from 25.5 to 154. An array of ten samples lies scattered about a line with a 207Pb/204Pb -206Pb/204Pb slope age of about 2.73 Ga. Five individual samples were sequentially leached to further test the timing and characteristics of this U-Th-Pb alteration event. These arrays of a whole rock, three leach steps, and a residue also form linear Pb-Pb arrays (one is more scattered) with ages ranging from 2260 ± 360 Ma to 2824 ± 170 Ma, suggesting that all samples experienced a latest Archean to earliest Proterozoic enrichment in U/Pb. This age range also may be the approximate age of Sm/Nd enrichment for the shales. All samples, both whole rocks and leached samples, lie grouped on a 208Pb/204Pb - 206Pb/204Pb diagram around a line with 232Th/238U = 3.5 (2.9 to 3.9). Because of the lack of large differences in the Th/U of the samples through large ranges of U/Pb, we interpret this consistency in Th/U to mean that the shales of the Buhwa belt experienced Pb loss, rather than U and Th gain. Circumstances that may be responsible for Pb loss in a sedimentary basin include loss of saline fluids during basin dewatering. Such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent Limpopo Belt, suggesting that uplift, dewatering, and geochemical and isotopic alteration can be genetically related.  相似文献   

9.
Regional recumbent folds, inverted stratigraphy, nappes and olistostromes are described from the southern part of the 3.3–3.5 Ga Barberton Greenstone Belt. Overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness, occurred over minimum distances of 86 km. More ductile and coherent units were overfolded up to at least 2 km during their emplacement. The glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns, driven by heat sources from igneous intrusions. The upwelling areas of the geothermal convection cells were sites of mud-pools and hydrothermal vents which may mark the trailing edges (pull aparts) of the overthrust units. Progressive silica and carbonate precipitation due to decreasing temperatures, within the zones of fluid migration distant from the areas of high heat flow, probably acted as built-in braking systems below the travelling slabs. Active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt. The recognition of nappe and overthrust tectonics in the Barberton Belt, processes which may have been commonplace in Archaean terrains, necessitates a re-evaluation of the stratigraphy of this belt.  相似文献   

10.
In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.  相似文献   

11.
The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C.

If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.  相似文献   


12.
Measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open-system behavior of Re–Os in molybdenites. Molybdenite samples from six porphyry copper deposits, one epithermal polymetallic vein deposit, four skarns, and three Fe-oxide Cu–Au deposits were analyzed. The δ97Mo‰ (where ) for all samples varied from 1.34 ± 0.09‰ to −0.26 ± 0.04‰. This is the largest molybdenum isotopic variation in molybdenite from high-temperature ore deposits recorded to date. δ97Mo‰ of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples. Isotope values for Mo also vary within the individual deposits. In general, molybdenites from porphyry copper deposits have the lightest values averaging 0.07 ± 0.23‰ (1σ). Molybdenites from the other deposit types average 0.49 ± 0.26‰ (1σ). The variations could be related to the fractionation of Mo into different mineral phases during the ore-forming processes. A comparison of the Mo isotope ratios and the Re–Os ages obtained from the same aliquot may possess a geochronological evaluation tool. Samples that yielded robust ages have different Mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages. Another observed relationship between the Re–Os and Mo isotope data reveals a weak correspondence between Re concentration and Mo isotope composition. Molybdenites with higher concentrations of Re correspond to lighter Mo isotope values.  相似文献   

13.
Lowe DR 《Geology》1994,22(12):1099-1102
The 3.55-3.22 Ga Barberton Greenstone Belt, South Africa and Swaziland, and surrounding coeval plutons can be divided into four tectono-stratigraphic blocks that become younger toward the northwest. Each block formed through early mafic to ultramafic volcanism (Onverwacht Group), probably in oceanic extensional, island, or plateau settings. Volcanism was followed by magmatic quiescence and deposition of fine-grained sediments, possibly in an intraplate setting. Late evolution involved underplating of the mafic crust by tonalitic intrusions along a subduction-related magmatic arc, yielding a thickened, buoyant protocontinental block. The growth of larger continental domains occurred both through magmatic accretion, as new protocontinental blocks developed along the margins of older blocks, and when previously separate blocks were amalgamated through tectonic accretion. Evolution of the Barberton Belt may reflect an Early Archean plate tectonic cycle that characterized a world with few or no large, stabilized blocks of sialic crust.  相似文献   

14.
High spatial resolution multiple sulfur isotope studies undertaken by multi‐collector secondary ion mass spectrometry (SIMS) commonly use well‐characterised sulfide reference materials that do not (or are assumed not to) exhibit mass‐independent fractionation in 33S and 36S, taking advantage of the three‐isotope plot to evaluate the extent of such fractionation in unknown targets. As a result, few studies to date have used a mass independently fractionated reference sulfide to demonstrate accuracy of measurement and/or data reduction procedures. This article evaluates two mass independently fractionated sulfides, a pyrite from the 3.7 Ga Isua greenstone belt and a pyrrhotite from a 2.7 Ga gold deposit in Minas Gerais, Brazil, which may be used to provide additional confidence in the obtained multiple sulfur isotope data. Additionally, the article presents a method for measuring quadruple sulfur isotopes by SIMS at a comparable spatial and volume resolution to that typically employed for triple sulfur isotopes. This method has been applied to the Isua pyrite as well as to a sample of 2.5 Ga pyrite from the Campbellrand, Transvaal, South Africa, previously investigated using SIMS for triple sulfur isotopes, illustrating its potential for quadruple sulfur investigations.  相似文献   

15.
Using improved analytical techniques, which reduce the Re blanks by factors of 8 to 10, we report new Re-Os data on low Re and low PGE pallasites (PAL-anom) and IIIAB irons. The new pallasite samples nearly double the observed range in Re/Os for pallasites and allow the determination of an isochron of slope 0.0775 ± 0.0008 (T = 4.50 ± 0.04 Ga, using the adjusted λ187Re = 1.66 × 10−11 a−1) and initial (187Os/188Os)0 = 0.09599 ± 0.00046. If the data on different groups of pallasites (including the “anomalous” pallasites) are considered to define a whole-rock isochron, their formation would appear to be distinctly younger than for the iron meteorites by ∼60 Ma. Five IIIAB irons (Acuna, Bella Roca, Chupaderos, Grant, and Bear Creek), with Re contents ranging from 0.9 to 2.8 ppb, show limited Re/Os fractionation and plot within errors on the IIAB iron meteorite isochron of slope 0.07848 ± 0.00018 (T = 4.56 ± 0.01 Ga) and initial (187Os/188Os)0 = 0.09563 ± 0.00011. Many of the meteorites were analyzed also for Pd-Ag and show 107Ag enrichments correlated with Pd/Ag, requiring early formation and fractionation of the FeNi metal, in a narrow time interval, after injection of live 107Pd (t1/2 = 6.5 Ma) into the solar nebula. Based on Pd-Ag, the typical range in relative ages of these meteorites is ≤10 Ma. The Pd-Ag results suggest early formation and preservation of the 107Pd-107Ag systematics, both for IIIAB irons and for pallasites, while the younger Re-Os apparent age for pallasites suggests that the Re-Os system in pallasites was subject to re-equilibration. The low Re and low PGE pallasites show significant Re/Os fractionation (higher Re/Os) as the Re and PGE contents decrease. By contrast, the IIIAB irons show a restricted range in Re/Os, even for samples with extremely low Re and PGE contents. There is a good correlation of Re and Ir contents. The correlation of Re and Os contents for IIIAB irons shows a similar complex pattern as observed for IIAB irons (Morgan et al., 1995), and neither can be ascribed to a continuous fractional crystallization process with uniform solid-metal/liquid-metal distribution coefficients.  相似文献   

16.
Fifty-eight rock chips from fifteen samples of sedimentary rocks from the Ramah Group (approximately 1.9 Ga) in northeastern Labrador, Canada, were analyzed for major and minor elements, including C and S, to elucidate weathering processes on the Earth's surface about 1.9 Ga ago. The samples come from the Rowsell Harbour, Reddick Bight, and Nullataktok Formations. Two rock series, graywackes-gray shales of the Rowsell Harbour, Reddick Bight and Nullataktok Formations, and black shales of the Nullataktok Formation, are distinguishable on the basis of lithology, mineralogy, and major and trace element chemistry. The black shales show lower concentrations than the graywackes-gray shales in TiO2 (0.3-0.7 wt% vs. 0.7-1.8 wt%), Al2O3 (9.5-20.1 wt% vs. 13.0-25.0 wt%), and sigma Fe (<1 wt% vs. 3.8-13.9 wt% as FeO). Contents of Zr, Th, U, Nb, Ce, Y, Rb, Y, Co, and Ni are also lower in the black shales. The source rocks for the Ramah Group sediments were probably Archean gneisses with compositions similar to those in Labrador and western Greenland. The major element chemistry of source rocks for the Ramah Group sedimentary rocks was estimated from the Al2O3/TiO2 ratios of the sedimentary rocks and the relationship between the major element contents (e.g., SiO2 wt%) and Al2O3/TiO2 ratios of the Archean gneisses. This approach is justified, because the Al/Ti ratios of shales generally retain their source rock values; however, the Zr/Al, Zr/Ti, and Cr/Ni ratios fractionate during the transport of sediments. The measured SiO2 contents of shales in the Ramah Group are generally higher than the estimated SiO2 contents of source rocks by approximately 5 wt%. This correction may also have to be applied when estimating average crustal compositions from shales. Two provenances were recognized for the Ramah Group sediments. Provenance I was comprised mostly of rocks of bimodal compositions, one with SiO2 contents approximately 45 wt% and the other approximately 65 wt%, and was the source for most sedimentary rocks of the Ramah Group, except for black shales of the Nullataktok Formation. The black shales were apparently derived from Provenance II that was comprised mostly of felsic rocks with SiO2 contents approximately 65 wt%. Comparing the compositions of the Ramah Group sedimentary rocks and their source rocks, we have recognized that several major elements, especially Ca and Mg, were lost almost entirely from the source rocks during weathering and sedimentation. Sodium and potassium were also leached almost entirely during the weathering of the source rocks. However, significant amounts of Na were added to the black shales and K to all the rock types during diagenesis and/or regional metamorphism. The intensity of weathering of source rocks for the Ramah Group sediments was much higher than that of typical Phanerozoic sediments, possibly because of a higher PCO2 in the Proterozoic atmosphere. Compared to the source rock values, the Fe3+/Ti ratios of many of the graywackes and gray shales of the Ramah Group are higher, the Fe2+/Ti ratios are lower, and the sigma Fe/Ti ratios are the same. Such characteristics of the Fe geochemistry indicate that these sedimentary rocks are comprised of soils formed by weathering of source rocks under an oxygen-rich atmosphere. The atmosphere about 1.9 Ga was, therefore, oxygen rich. Typical black shales of Phanerozoic age exhibit positive correlations between the organic C contents and the concentrations of S, U, and Mo, because these elements are enriched in oxygenated seawater and are removed from seawater by organic matter in sediments. However, such correlations are not found in the Ramah Group sediments. Black shales of the Ramah Group contain 1.7-2.8 wt% organic C, but are extremely depleted in sigma Fe (<1 wt% as FeO), S (<0.3 wt%), U (approximately l ppm), Mo (<5 ppm), Ni (<2 ppm), and Co (approximately 0 ppm). This lack of correlation, however, does not imply that the approximately 1.9 Ga atmosphere-ocean system was anoxic. Depletion of these elements from the Ramah Group sediments may have occurred during diagenesis.  相似文献   

17.
The radiation of macroscopic animals in the Early Cambrian, commonly be called as ‘Cambrian explosion’, had a close relationship with Earth oxygenation. These macrofossils are widely preserved within the Early Cambrian in the northern and western Yangtze Craton. In order to show the casual relationship between animal evolution and Earth redox environment, Mo content and isotopic composition analyses have been conducted on dolomites of the Tianzhushan Formation and the black shales of the Shuijingtuo Formation from the lowermost Cambrian profile in the eastern Huangling dome, northern Yangtze Craton. The calcitic dolostones from the Tianzhushan Formation display lower concentrations of Mo contents (0.055–0.666 ppm) than that of the black silty shales from the Shuijingtuo Formation (6.0–172 ppm). This magnitude difference is most likely related to the different lithologies due to Mo enrichment in shales. The Mo isotope values (δ98/95Mo) of the calcitic dolostones for the Tianzhushan Formation vary from −0.73‰ to 0.41‰. In comparison, the black sandy shales from the Shuijingtuo Formation exhibit δ98/95Mo of −1.04–1.84‰, including 5 samples from the earliest Shuijingtuo Formation displaying systematically higher δ98/95Mo values (0.09–1.84‰) than those of the other 5 samples from upper layers (δ98/95Mo = −1.04 to −0.19‰). These geochemical data suggest that the redox environment of the early Cambrian is fluctuating, and the peak oxygenation of the ocean had already reached a level similar to modern environments. According to the Mo concentrations and δ98/95Mo data, the sedimentary succession can be divided into five intervals, which are closely correlated to the episodic evolution of early life in the Cambrian. Together with the available published data from other profiles in the Yangtze Craton, our new data reveal that the higher oxidation environment had a positive effect on the evolution of early life.  相似文献   

18.
We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O2 in the environment ∼50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ∼2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (δ98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ98/95Mo compared to average upper continental crust. In addition, the upper (S1) and lower (S2) black shale units within the Mt. McRae Shale exhibit systematic differences in average isotopic compositions and distinct patterns of δ98/95Mo variation. Heavier δ98/95Mo values occur in the S1 unit, where δ98/95Mo correlates with Mo enrichments. In the S2 unit, δ98/95Mo is not as heavy and is relatively invariant.Based on sedimentary Fe proxies we infer that S1 sediments were deposited under euxinic conditions, so that Mo removal from bottom waters was likely quantitative. Thus, δ98/95Mo in this interval likely records coeval seawater. The lighter δ98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ98/95Mo values in the S2 unit represent a lower limit on the value in contemporaneous seawater.After evaluating a range of hypotheses, we conclude that the isotopically heavy δ98/95Mo values seen throughout the Mt. McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces on land or in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.  相似文献   

19.
The Yangchang granite‐hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA‐ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc‐alkaline with K2O/Na2O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large‐ion lithophile elements (LILEs) and depleted in high‐field‐strength elements (HFSEs) with negative Nb and Ta anomalies in primitive‐mantle‐normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar 206Pb/204Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the εNd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo‐Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

20.
Iron(Fe) is abundant in nature while molybdenum(Mo) is the most abundant transition metal in seawater. Due to their high sensitivity to the redox state of the environment, the isotopic compositions of Fe and Mo as well as variations have been widely used to probe the redox conditions and the evolution of ancient ocean chemistry in favor of improved analytical techniques. Here, we summarized isotopic fractionation mechanisms and natural distribution of both iron and molybdenum isotopes, and further we summarized and partially reinterpreted the redox evolution of ancient oceans through time based on available Fe-Mo data compiled in this study. The process that causes the largest iron isotope fractionation is redox reaction and the iron in oxidation state is generally enriched in 56Fe. Biotic and abiotic pyrite formations also produce a large Fe isotope fractionations. Isotopic fractionation of molybdenum in seawater is mainly caused by the adsorption process of dissolved Mo onto ferromanganese oxides or hydroxides in sediments. Fe-Mn (hydro)oxides tend to adsorb isotopically light molybdenum resulting in the isotopic composition of Mo in seawater heavier. However, the Mo sinks in euxinic settings cause almost no molybdenum isotope fractionation. The Fe Mo isotope isotopic records through geological timegenerally suggest similar ocean redox evolution: Oceans older than 2.3 Ga was mainly dominated by ferruginous condition, and there was a slight increase in oxygen content between 2.6 and 2.5 Ga. Earth’s surface was initially oxidized during 2.3 to 1.8 Ga, during which euxinic deposition of sulfide was elevated. Euxinic waters may have expanded greatly between 1.8 and 0.8 Ga, and after that, Earth’s surface had being gradually oxidized and the euxinic waters shrank substantially.Finally, suggestions are proposed for further work on the Fe-Mo isotope research in the context of ancient ocean chemistry.  相似文献   

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