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1.
Beryllium isotopes (10Be and9Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of10Be and9Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 109 and 6 × 1016 atoms g−1 of10Be and9Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The10Be/9Be ratio in trap samples varies from 3 to 20 × 10−8. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of10Be into the traps at about 1500 m are estimated as 9 × 105 and 4 × 105 atoms cm−2 a−1 at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 105 atoms cm−2 a−1 at both locations. Good correlations exist between10Be,9Be and27Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.  相似文献   

2.
We have measured by accelerator mass spectrometry the26Al contents of 20 and the10Be contents of 14 iron meteorites. The26Al contents are typically 30% or more lower than values obtained by counting techniques; the10Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the4He/21Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP26(Fe) as 3–4 dpm/kg andP10(Fe) as 4–5 dpm/kg. The average P/10P26 activity ratio is close to 1.5. Exposure ages calculated from21Ne/26Al ratios cannot be calibrated so as to agree with both40KK/ ages and ages based on the shorter-lived nuclides39Ar and36Cl. If agreement with the latter is forced, then the disagreement with40KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 107 a.  相似文献   

3.
The10Be deposition rate in two Arctic Ocean cores, ～80°N, was measured to be ≤0.6 atoms cm?2 sec?1, much less than the10Be deposition rate reported for other oceanic regions. Possible causes for the low10Be deposition in the Arctic are: (1) latitudinal variation in the10Be delivery rate from the stratosphere; (2) the presence of an ice cover on the Arctic, which may inhibit the transfer of atmospherically supplied10Be from sea surface to sediments; and (3) locally low scavenging efficiency of sedimentary particles for10Be in the Arctic.  相似文献   

4.
We examine 10Be concentration in two pit profiles in the Parkajoki area at ∼67°N on the northern Fennoscandian shield in northern Sweden. Due to repeated cover by cold-based, non-erosive ice sheets, the area retains many relict non-glacial features, including tors and saprolites. In the examined pit profiles, gruss-type saprolite developed from weathering of intermediate igneous rocks is overlain unconformably by Weichselian till.Our results show that 10Be concentrations found in the till greatly exceed the levels of 10Be that can have accumulated since deglaciation at ∼11 ka and are comparable to those reported from Pliocene and Early Pleistocene tills in North America. Old tills with grussified boulders at depth were excavated in the Parkajoki area and correlations with neighbouring parts of Finland indicate a Middle Pleistocene or older age. Evidence from pit excavations and geochemistry shows that the underlying saprolites have been truncated by glacial erosion and that previously weathered material has been incorporated into the till sequence. Hence, 10Be inventories in the tills are dominated by material recycled from Middle Pleistocene or older soils, near-surface sediments and saprolite, and cannot be used to date the periods of till deposition. The retention of relict 10Be in the tills nonetheless confirms minimal glacial erosion.Concentrations of meteoric 10Be in the saprolites are lower than any reported saprolite concentrations measured in other settings. Uncertainty in the pre-glaciation 10Be concentrations in the saprolites makes age determinations difficult. One possibility is that that the saprolite had higher 10Be concentrations in the past but that saprolite formation ended after glaciation and burial by till and that the 10Be has substantially decayed. Modelling of the meteoric 10Be depth profiles in this case suggests that the saprolites in the Parkajoki area were formed at a minimum of 2 Ma. Erosion of the saprolite allows an older age of up to ∼5 Ma, with up to 250 cm of material removed and incorporated into later tills. A second possibility is that concentrations of meteoric 10Be in the saprolite were originally lower, with formation of the saprolite in a period or periods of ice- and permafrost-free conditions before 0.8 Ma.  相似文献   

5.
Understanding the extent to which local factors, including bedrock and structure, govern catchment denudation in mountainous environments as opposed to broader climate or tectonic patterns provides insight into how landscapes evolve as sediment is generated and transported through them, and whether they have approached steady-state equilibrium. We measured beryllium-10 (10Be) concentrations in 21 sediment samples from glaciated footwall and hanging wall catchments, including a set of nested catchments, and 12 bedrock samples in the Puga and Tso Morari half-grabens located in the high-elevation, arid Zanskar region of northern India. In the Puga half-graben where catchments are underlain by quartzo-feldspathic gneissic bedrock, bedrock along catchment divides is eroding very slowly, about 5 m/Ma, due to extreme aridity and 10Be concentrations in catchment sediments are the highest (~60–90 × 105 atoms/g SiO2) as colluvium accumulates on hillslopes, decoupled from their ephemeral streams. At Puga, 10Be concentrations and the average erosion rates of a set of six nested catchments demonstrate that catchment denudation is transport-limited as sediment stagnates on lower slopes before reaching the catchment outlet. In the Tso Morari half-graben, gneissic bedrock is also eroding very slowly but 10Be concentrations in sediments in catchments underlain by low grade meta-sedimentary rocks, are significantly lower (~10–35 × 105 atoms/g SiO2). In these arid, high-elevation environments, 10Be concentrations in catchment sediments have more to do with bedrock weathering and transport times than steady-state denudation rates. © 2020 John Wiley & Sons, Ltd.  相似文献   

6.
The vertical distributions of10Be and9Be at three locations in the Pacific (25°N, 170°E; 17°N, 118°W; 3°S, 117°W) are presented. The results show that both isotopes exhibit nutrient-like profiles. From the surface to the bottom, the increase for10Be is two- to threefold and that for9Be is about fivefold. While the inter-station variations in surface water concentrations may reach a factor of two, deep-water values tend to be much more uniform averaging about 2000 atoms/g for10Be and 30 pM for9Be. A similar situation applies to the10Be/9Be ratio; it varies approximately from 1 to 3 × 10−7 (atom/atom) at shallow depths but tends toward a value close to 1.1 × 10−7 in the deep ocean. The variation of10Be/9Be can be viewed as resulting from the fact that10Be in a given parcel of water consists of two components: recycled and primary. The recycled component is that part of10Be which has reached tracer equilibrium with9Be, as opposed to the primary component which, upon entering the sea from the atmosphere, has yet to equilibrate with9Be through particle cycling and mixing processes. It is estimated that 70% to nearly 100% of10Be at the three stations are being recycled, and the recycled beryllium bears an atomic ratio of10Be/9Be close to 1 × 10−7. The oceanic residence time of Be is of the order of 1000–4000 years, comparable to or slightly longer than the ocean mixing time.  相似文献   

7.
We present 9 bottom222Rn profiles measured from the western and southern Indian Ocean during the 1977–1978 GEOSECS expedition. These profiles can be grouped into three cypes: one-layer, two-layer, and irregular types. The one-layer profiles with quasi-exponential distributions allow one to estimate the apparent vertical eddy diffusivity,Kv, with a simple model. The two-layer profiles show that there is a benthic boundary layer of the order of 50–100 m in which the excess222Rn distribution shows a vertical gradient much smaller than that of the layer immediately above. Within the boundary layer, the STD potential temperature (θ) and density(σ4) profiles are practically constant, and theKv values are of the order of 1000 cm2/s. The STD profiles for the water column above the boundary layer show gradients of increasing stability, and theKv values are of the order of 100 cm2/s. Modeling of the Rn data in the water column above the boundary layer indicates that there is a transition layer which effectively reduces the penetration of excess Rn from the benthic boundary layer into the upper layer.Sarmiento et al. [10] have shown that the buoyancy gradient or stability is inversely correlated with the apparent vertical eddy diffusivity, and the resulting buoyancy flux is fairly uniform, ranging from 1 to 14 × 10?6 cm2/s3 in the Atlantic and Pacific Oceans. However, Sarmiento et al. [11] show that a much higher buoyancy flux is associated with an intensified flow of the bottom water through a passage. In the Indian Ocean basins, we have found that the buoyancy flux has a comparable range (3–14 × 10?6 cm2/s3), except for a couple of stations where both stability and apparent vertical diffusivity are higher, resulting in a much higher buoyancy flux, probably indicative of rapid bottom water flow.  相似文献   

8.
The content of K, Th and U in the continental crust is estimated based on the assumption that the concentration of these elements decreases with depth asAx = A0e?x/D [11], withAx andA0 the heat production rates at depthx and at the surface, respectively. Taking the weighted mean heat production rate of the intrusive rocks of the upper crust asA0 = 2.33 μWm?3, that of the granulites representing the lower crust asAx = 0.72 μWm?3, and the mean scale heightD= 9.5km [1] the average vertical distancex = b between these intrusives and granulites is 11.2 km. Withb known and the average concentrations of K, Th and U in granulites and intrusive rocks of the upper crust the scale heights of the vertical distribution of these elements areDK = 71km,DTh = 9.5km,DU = 5.8km. The knowledge of these parameters permits to calculate the average concentrations of these elements in a 33.3 km thick crust:K= 2.19%,Th= 4.43ppm,U= 0.66ppm; Th/U = 6.7 and K/U = 3.3 × 104. The resulting heat flow is 23.0 mW m?2 which is practically identical with the value deduced from heat flow measurements. Assuming that the Th/U ratio of the entire crust—including the sediments—is 3.9, the high ratio of 6.7 in the crystalline crust indicates that about 7.2 × 1012 t U were extracted from it. All rocks with Th/U ratios <3.9 are possible sinks of this U. About half that amount is deposited in sedimentary rocks, mainly in black shales. The second important sink are the volcanic rocks of the continental margins.  相似文献   

9.
Cosmogenic7Be(t1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess210Pb and/or239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale.In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)7Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10?7 cm2/s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess234Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact.Measurements of total (wet + dry) atmospheric deposition of7Be in New Haven give an average flux of 0.07 dpm/cm2 day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm2 in a perfect collector. Sediment cores from Long Island Sound contain about half this7Be inventory, consistent with either a mean residence time for7Be in the water column of about one half-life or with post-depositional loss of7Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm2, much nearer the atmospheric delivery. A higher inventory of7Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for7Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess210Pb and239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.  相似文献   

10.
The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.87Sr/86Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative εNdvalues ofεNd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (εNd = +2.5;87Sr/86Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average206Pb/204Pb≈ 19.4,207Pb/204Pb= 15.79–15.84,208Pb/204Pb≈ 41.2. The basic potassic rocks have similar206Pb/204Pb values but range in207Pb/204Pb and208Pb/204Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of87Sr/86Sr with Rb/Sr, 1/Sr and207Pb/206Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of87Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +εNd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?εNd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?εNd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.  相似文献   

11.
We have determined K, Rb and Sr concentrations and87Sr/86Sr ratios in fresh surface waters, a rain water sample and five geothermal waters from the Cantal volcanic area in the Massif Central, France. A comparison with appropriate rock types of the region showed no apparent chemical and isotopic fractionation occurring in the fresh water-surface rock system. The thermo-mineral water results suggest that all springs discharge dissolved Sr from the following contributors: Hercynian granito-metamorphic basement, lacustrian sediments underlying the volcano, Miocene-Pliocene volcanic rocks of basaltic to rhyolitic composition.  相似文献   

12.
A sequential chemical extraction procedure was developed and tested to investigate the utility of meteoric 10Be as a tracer for authigenic mineral formation beneath the East Antarctic Ice Sheet. Subglacial meltwater is widely available under the Antarctic Ice Sheet and dissolved gases within it have the potential to drive chemical weathering processes in the subglacial environment. Meteoric 10Be is a cosmogenic nuclide with a half-life of 1.39·106 years that is incorporated into glacier ice, therefore its abundance in the subglacial environment in Antarctica is meltwater dependent. It is known to adsorb to fine-grained particles in aqueous solution, precipitate with amorphous oxides/hydroxides, and/or be incorporated into authigenic clay minerals during chemical weathering. The presence of 10Be in chemical weathering products derived from beneath the ice therefore indicates chemical weathering processes in the subglacial environment. Freshly emerging subglacial sediments from the Mt. Achernar blue ice moraine were subject to chemical extractions where these weathering phases were isolated and 10Be concentrations therein quantified. Optimization of the phase isolation was developed by examining the effects of each extraction on the sample mineralogy and chemical composition. Experiments on 10Be desorption revealed that pH 3.2–3.5 was optimal for the extraction of adsorbed 10Be. Vigorous disaggregation of the samples before grain size separations and acid extractions is crucial due to the incorporation of the nuclide in clay minerals and its preferential absorption to clay-sized particles. 10Be concentrations of 2–22·107 atoms·g−1 measured in oxides and clay minerals in freshly emerging sediments strongly indicate subglacial chemical weathering in the catchment of the Mt. Achernar moraine. Based on total 10Be sample concentrations, local basal melt rates, and 10Be ice concentrations, sediment-meltwater contact in the subglacial environment is on the order of thousands of years per gram of underlying fine sediment. Strong correlation (R = 0.97) between 10Be and smectite abundance in the sediments supports authigenic clay formation in the subglacial environment. This suggests meteoric 10Be is a useful tool to characterize subglacial geochemical weathering processes under the Antarctic Ice Sheet.  相似文献   

13.
An initial survey has been made of the geochemistry of 9Be in natural waters with a view to utilizing the isotope with the cosmogenic species 10Be (t1/2 = 1.5 m.y.) in geochronological studies of sedimentary accumulation. The mobility of9Be in continental waters is a strong function of pH with acid streams (pH < 6) being strongly enriched as compared to alkaline carbonate rivers. Large-scale scavenging of beryllium occurs in the estuaries of rivers containing more than about 200 pM of the element. In alkaline systems of lower concentration the element appears to behave conservatively. The net flux of beryllium to the ocean is estimated as that equivalent to an average effective river concentration of about 150 pM. Beryllium is strongly enriched in ridge crest hydrothermal solutions over ambient seawater. The flux is approximately 70% of that from the continents. The deep water concentrations of 9Be vary by only 20% between the North Atlantic and North Pacific demonstrating that the “chemical saltation” produced by scavenging from the deep water column and release from the sediments predominates over simple advective concentration along flow lines. Isotopic homogenization of 10Be and 9Be is probably achieved in the ocean. The situation in continental waters appears much less promising.  相似文献   

14.
RB-Sr and Sm-Nd isotopic and trace-element-abundance values have been determined for 15 mafic and intermediate rocks from six Pleistocene volcanic centres of the Fly-Highlands province. 87Sr/86Sr and N d values range from 0.70362 to 0.70540, and +1.9 to +5.9, respectively. These new data can be accounted for by contamination of mantle-derived magmas by the continental crust through which the magmas have risen. They do not, however, preclude derivation of some of the Sr and Nd from subducted crust, nor are they inconsistent with Sr and Nd enrichments having taken place by means of mantle metasomatic events. Nevertheless, there is no Benioff zone beneath the Fly-Highlands province (although there is geological evidence for Cretaceous subduction). A preferred interpretation is that uncontaminated, mantle-derived magmas are related to the Pliocene crustal uplift that caused the development of the highlands and which formed in response to a mid-Tertiary continent/island-arc collision.  相似文献   

15.
The concentration profiles of 9Be, 10Be, 230Th, 232Th, 231Pa (via 227Th) and 238U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the 10Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ? cores raised from the manganese nodule field in the North Pacific have been measured. The 10Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator.The concentrations of 10Be, 230Thexc and 231Paexc and ratios of 10Be/9Be and 230Thexc/232Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (～ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on 230Thexc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on230Thexc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the 10Be based rate is less than the 230Thexc rate and in R/V Vitiaz the 10Be based rate is greater than the 230Thexc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from 230Thexc and 10Be profiles document changes in nodule growth rate with time rather than mixing effects on 230Thexc profiles.The 10Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface 10Be/9Be ratio of North Pacific nodules predicts a 9Be concentration within the limits of measured values. The inventory of 10Be and 230Thexc in the nodules is only ～ 10% of the total, the remaining being in sediments. The 10Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both 10Be and 230Th based on 230Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.  相似文献   

16.
Talat  Ahmad  Kabita C.  Longjam  Baishali  Fouzdar  Mike J.  Bickle  Hazel J.  Chapman 《Island Arc》2009,18(1):155-174
The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.  相似文献   

17.
Unsupported226Ra (t12 = 1620years) in marine sediments can provide a basis for measuring rates of accumulation of the order of centimeters per thousand years. The excess radium apparently enters the sediments incorporated in phytoplankton. The sensitivity of the method depends upon the initial value of the unsupported226Ra and of the value of230Th, a parent of226Ra, in the sedimentary components.226Ra dating was applied to a sediment taken from the slope of the San Clemente Basin in the Southern California coastal region. Rates of sedimentation over two half-lives of the nuclide were found to be either 5.2 or 5.3 cm/1000 years depending upon which of two models for the geochronology is used. One model assumes that the230Th brings to the deposit an amount of226Ra in equilibrium with it. The other is based upon the growth of the226Ra from the230Th in the sedimentary components.238+239Pu and210Pb levels in the upper strata indicated sedimentation rates of the order of 100–500 cm/1000 years, i.e. much faster accumulations. We suggest these derived rates are spurious and reflect bioturbative activities of surface-living organisms.  相似文献   

18.
18O/16O and 87Sr/86Sr ratios were determined for Quaternary calc-alkalic volcanic rocks from six volcanic rock suites in the central and western Japan arcs. The δ18O values relative to SMOW and 87Sr/86Sr ratios range from +6.3 to +9.90/00 and 0.70357 to 0.70684, respectively. Both the O- and Sr-isotopic compositions are higher than those for island-arc primitive magmas and their differentiates. The isotopic compositions of the calc-alkalic rocks cannot be derived by a simple fractional crystallization of the primitive magmas. On the other hand, the 18O- and 87Sr-enrichment is confined to the rock suites located in well-developed island arcs having thick continental-type crust with low or negative Bouguer anomalies. Involvement of 18O- and 87Sr-rich crustal material in the magma formation is suggested.The isotopic compositions vary remarkably within individual rock suites as well as from volcano to volcano. The data points in δ18O vs. 87Sr/86Sr plot accord with a mixing model between primitive magmas and crustal material of dioritic composition on an average, assuming their comparative Sr contents. The primitive magmas involved could not be low-Sr tholeiites, but magmas more or less enriched in incompatible elements including Sr, which correspond to high-alkali tholeiites or alkali basalts and their evolved magmas. The nature of the primitive magmas seems to change from tholeiitic to more alkalic with progressing island-arc evolution.Mixing of crust-derived melts is more plausible than assimilation of solid-rocks for involving 20 to 30% crustal material in the magmas along simple mixing curves. Isotopic variations between the rock suites are ascribed to variable Sr concentration radio of the end-members, variable isotopic compositions of crustal material or variable mixing ratio of the end-members. Extremely high-δ 18O rocks with moderate increase in 87Sr/86Sr ratio suggest another mixing process in shallower magma chambers between andesite magmas and metasedimentary rocks having high δ 18O and 87Sr/86Sr values but low Sr content. Subsequent fractional crystallization of once-derived magmas would be the prominent process for the rock suites showing gradual increase in 18O up to 10/00 with uniform 87Sr/86Sr ratios.  相似文献   

19.
The territory of Karelia (Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval (1650—1350 Ma). As regards the paleointensity H an, the huge interval 1–2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1–1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15° S and 40° S. The inferred value of H an is close to the lower boundary of the interval (1.36–11.56) × 1022 A m2, encompassing previously published intensity values of the paleofield 1–1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003].  相似文献   

20.
Existing methods of cosmogenic nuclide burial dating perform well provided that sediment sources undergo steady rates of erosion and the samples experience continuous exposure to cosmic rays. These premises exert important limitations on the applicability of the methods. And yet, high mountain sediment sources are rife with transient processes, such as non-steady erosion by glacial quarrying and/or landsliding, or temporary cosmic-ray shielding beneath glaciers and/or sediment. As well as breaching the premises of existing burial dating methods, such processes yield samples with low nuclide abundances and variable 26Al/10Be ratios that may foil both isochron and simple burial-age solutions. P–PINI (Particle-Pathway Inversion of Nuclide Inventories) is a new dating tool designed for dating the burial of sediments sourced from landscapes characterized by abrupt, non-steady erosion, discontinuous exposure, and catchments with elevation-dependent 26Al/10Be production ratios. P–PINI merges a Monte Carlo simulator with established cosmogenic nuclide production equations to simulate millions of samples (10Be–26Al inventories). The simulated samples are compared statistically with 10Be–26Al measured in field samples to define the most probable burial age. Here, we target three published 10Be–26Al datasets to demonstrate the versatility of the P–PINI model for dating fluvial and glacial sediments. (1) The first case serves as a robust validation of P–PINI. For the Pulu fluvial gravels (China), we obtain a burial age of 1.27 ± 0.10 Ma (1σ), which accords with the isochron burial age and two independent chronometers reported in Zhao et al. (2016) Quaternary Geochronology 34, 75–80. The second and third cases, however, reveal marked divergence between P–PINI and isochron-derived ages. (2) For the fluvial Nenana Gravel (USA), we obtain a minimum-limiting burial age of 4.5 ± 0.7 Ma (1σ), which is compatible with unroofing of the Alaska Range starting ∼ 6 Ma, while calling into question the Early Pleistocene isochron burial age presented in Sortor et al. (2021) Geology 49, 1473–1477. (3) For the Bünten Till (Switzerland), we obtain a limiting burial age of <204 ka (95th percentile range), which conforms with the classical notion of the most extensive glaciation in the northern Alpine Foreland assigned to the Riss glaciation (sensu marine isotope stage 6) contrary to the isochron burial age presented in Dieleman et al. (2022) Geosciences, 12, 39. Discrepancies between P–PINI and the isochron ages are rooted in the challenges posed by the diverse pre-burial 26Al/10Be ratios produced under conditions characteristic of high mountain landscapes; i.e., non-steady erosion, discontinuous cosmic-ray exposure, and elevation-dependent 26Al/10Be production ratios in the source region, which are incompatible with the isochron method, but easily accommodated by the stochastic design of P–PINI.  相似文献   

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