Sr isotopic composition of Paleoproterozoic C-rich carbonate rocks: The Tulomozero Formation,SE Fennoscandian Shield

Rb–Sr systematics has been studied in ¹³C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ¹⁸O/¹⁶O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ⁸⁷Sr/⁸⁶Sr values in the least altered (Mn/Sr < 2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ⁸⁷Sr/⁸⁶Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.     

The comparative strontium isotopic composition of alkaline rocks: New data from Southern Portugal and East Africa

Rb-Sr whole rock isochrons for the Monchique and Mt. Kenya complexes, together with a series of ⁸⁷Sr/⁸⁶Sr ratios for 7 other alkaline suites, reveal limited initial ratio variation within the oceanic basalt range. Such variation is ubiquitous in alkaline suites and renders interpretation of their Sr behaviour highly subjective; this has led to major interpretive inconsistencies between previous studies. A consideration of wall-rock reaction, responsible for a few anomalous ratios in the present data, is believed to reconcile many of these inconsistencies; the subjective element can also be minimised by using the total of available data as a basis for comparative interpretation instead of interpreting individual sets of data absolutely. This approach shows available data to be remarkably unified, implying less diversified petrogenesis than is apparent from the sum of interpretations for individual alkaline complexes. In particular, Sr data lend little support to various models invoking crustal participation in the origin of nephelinites, nepheline syenites, ijolites or peralkaline oversaturated rocks, but suggest that these generally are straight-forward derivatives of mantle liquids. The consanguinity of most alkaline complexes is also confirmed.     

Rubidium,strontium content and strontium isotopic composition of strongly alkalic basaltic rocks from the Cape Verde islands

The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr⁸⁷/Sr⁸⁶ ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K₂O/K₂O + Na₂O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part.     

Carbon and oxygen isotopic composition of the Vendian-Cambrian carbonate rocks and paleoecological reconstructions

The carbon isotopic composition of carbonate rocks is widely used for the reconstruction of sedimentation paleoenvironment. Of special interest is the study of the Upper Proterozoic-Cambrian interval—the turning point in the Earth’s geological evolution. Rocks of this age show the widest variations in the carbon isotopic composition of carbonates typically correlated with epochs of global glaciations and change in the CO₂ regime. In this paper, we attempted to show that carbon isotopic variations often indicate postsedimentary alterations of carbonates and reflect the specific geochemical transformations of the rocks. Study of variations of carbon and oxygen isotopic compositions in the Vendian-Cambrian rocks provide insight into lithogenetic processes.     

The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr⁸⁷/Sr⁸⁶ ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr⁸⁷/Sr⁸⁶ ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr⁸⁷/Sr⁸⁶ ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr⁸⁷/Sr⁸⁶ ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr⁸⁷/Sr⁸⁶ ratio or (ii) selective diffusion of material of high Sr⁸⁷/Sr⁸⁶ ratio into kimberlitic fluids.     

辽西凌源雾迷山组碳酸盐岩碳氧同位素特征研究

辽西凌源雾迷山组碳酸盐岩碳氧同位素和Mn/Sr比值数据反映了碳酸盐岩的原始沉积特征。δ¹³C_PDB数值范围-2‰~2‰,δ¹⁸O_PDB数值范围为-10‰~-4‰,其平均值分别为0.15‰,-6.2‰,碳同位素组成具有旋回性变化特征。在雾迷山组一段中上部,δ¹³C表现出明显的正漂移,可能为藻类的大量繁殖、有机碳增加所引起。δ¹⁸O的增加为气候变冷和冰川作用的结果。在δ¹³C为负值的层段,δ¹⁸O数值较高。碳氧同位素组成的这种特征可能与海平面变化有关。     

Boron isotopic composition of subduction-zone metamorphic rocks

Many arc lavas contain material derived from subducted oceanic crust and sediments, but it remains unresolved whether this distinctive geochemical signature is transferred from the subducting slab by aqueous fluids, silicate melts, or both. Boron isotopic measurements have the potential to distinguish between slab transfer mechanisms because ¹¹B fractionates preferentially into aqueous fluids whereas little fractionation may occur during partial melting. Previous studies have shown that δ¹¹B values of island arc lavas (−6 to +7) overlap the range of δ¹¹B values for altered oceanic crust (−5 to +25) and pelagic sediments and turbidites (−7 to +11). Secondary ion mass spectrometry (SIMS) analyses of minerals in subduction-zone metamorphic rocks yield δ¹¹B=−11 to −3 suggesting that slab dehydration reactions significantly lower the δ¹¹B values of subducted oceanic crust and sediments. In order to explain the higher δ¹¹B values reported for arc lavas as compared to subduction-zone metamorphic rocks, the B-bearing component derived from the metamorphosed slab must be enriched in ¹¹B relative to the slab, favoring an aqueous fluid as the slab transfer mechanism.     

Rubidium,strontium and the isotopic composition of strontium in ultramafic nodule minerals and host basalts

The concentrations of rubidium and strontium and the isotopic composition of strontium have been determined in minerals separated from ultramaflc nodules occurring in late Tertiary and Quaternary basalts of wide geographic distribution. Clinopyroxene, orthopyroxene and olivine from each of three Iherzolite nodules show a relatively wide range of ⁸⁷Sr/⁸⁶Sr disequilibrium and none of the minerals is in isotopic equilibrium with its host basalt. In two cases there is a correlation between ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios of the nodule minerals, indicating apparent isochron relationships which may represent relict mantle events. Clinopyroxene and olivine from each of two wehrlite nodules are not in isotopic equilibrium, although the magnitude of the disequilibrium is smaller than that observed in the Iherzolite nodules. None of these ultramafic nodules can be a crystal cumulate from its host basalt, and it is doubtful that any type of genetic relationship exists. The strontium isotopic disequilibrium between nodule minerals seems to be a primary feature inherited from past mantle histories.     

湖北宜昌埃迪卡拉系陡山沱组下部无机碳同位素再研究：探寻碳酸盐岩碳同位素组成的原始海水信号

湖北宜昌地区埃迪卡拉系陡山沱组记录了多次显著的无机碳同位素(δ13Ccarb)波动事件,它们可以在华南乃至全球范围内进行广泛对比。然而,在一些特定剖面中,陡山沱组底部盖帽碳酸盐岩记录了δ13Ccarb极负值(＜30‰,VPDB),以及上覆陡山沱组二段底部地层中δ13Ccarb在正负值之间频繁波动的现象。一般认为,成岩作用中形成的碳酸盐矿物导致了这些分布不规律的δ13Ccarb波动,并对利用碳酸盐岩全岩无机碳同位素(δ13Cbulk)数据进行古海洋碳同位素组成恢复的可靠性造成了干扰。为了评估成岩作用对δ13Cbulk的影响,本文利用宜昌雾河地区埃迪卡拉系的一口钻井岩芯,对陡山沱组下部地层进行了详细的岩石学观察和碳同位素分析。研究结果表明,陡山沱组下部的碳酸盐岩中存在形成于成岩过程中的方解石矿物,而碳酸盐岩中的白云石组分并未遭受严重的后期成岩作用改造,白云石组分的碳同位素(δ13Cdolo)组成能够更加准确地反映沉积时的海水信息。为获得样品的δ13Cdolo信息,本研究首先将全岩粉末样品与30%磷酸反应2 h,去除其中的方解石组分,然后测试残余样品的碳同位素。实验结果显示,该方法可以有效去除岩石粉末中的方解石组分,最终获得的数据能够准确地反映δ13Cdolo特征。陡山沱组二段下部样品δ13Cdolo为连续稳定的正值,且明显高于δ13C bulk,δ13Cdolo 曲线更加真实地反映了新元古代冰期结束之后海洋碳同位素组成的变化特征。     

Pb-Pb age of Jatulian carbonate rocks: The Tulomozero Formation of southeast Karelia

The U-Pb systematics of 25 carbonate samples collected from the Upper Jatulian Tulomozero Formation in southeastern area of the Baltic shield has been studied. The U-Pb systems of Jatulian carbonates metamorphosed here under conditions of the greenschist facies likely have not been reset completely. Samples have been collected from core sections recovered by drilling 60 to 70 km apart from each other in western and eastern areas of the Onega Lake region. In majority, the rocks sampled characterize a thick upper member of the Tulomozero Formation, and a few samples have been collected in addition from its middle and lower members. The analyzed dolomitic rocks (Mg/Ca=0.60–0.68) have variable Mn (54–1450 ppm) and Sr (31–438 ppm) concentrations. Powdered dolomite samples have been treated preliminary in 1 N solution of ammonium acetate in order to get material for analysis enriched in pre-metamorphic carbonate phases in terms of U-Pb systematics. Five samples have been subjected to stepwise dissolution in 0.5 N HBr to analyze their carbonate phases L-1, L-2, L-3 and siliciclastic fraction for getting a deeper insight into the U-Pb systematics. The U-Pb characteristics of carbonate and siliciclastic fractions suggest deposition of studied carbonate sediments in two different paleobasins. In addition, they show for same samples the partial Pb redistribution between carbonate and siliciclastic components in the course of metamorphism and Pb gain from allogenic metamorphic fluids. The Pb-Pb date of 2090 ± 70 Ma (MSWD = 2.0) is estimated for the least altered dolomite samples from upper member of the Tulomozero Formation, which represent marine sediments of a paleobasin and contain a minimum of siliciclastic material, being the least-contaminated by gained Pb. The date obtained is well consistent with U-Pb and Sm-Nd ages established for the Jatulian volcanogenic rocks in northern and western areas of the Baltic shield.     

Neodymium and strontium isotopic characteristics of New Zealand granitoids and related rocks

Initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of Phanerozoic granitoids and related intrusions of the New Zealand block display a mixing-type array indicative of the involvement in their sources of old continental crustal material, most likely of Proterozoic age. _Sr(T) values range from –4 to +273 (⁸⁷Sr/⁸⁶Sr=0.7041–0.7233), while _Nd(T) ranges from +2.7 to –11.0. Preexisting metasedimentary rocks have generally higher _Sr and lower _Nd (ranging to present-day values of +646 and –15.0, respectively), and, particularly for the Mesozoic intrusives, are isotopically appropriate mixing end-members. The widespread, early Paleozoic Greenland Group graywackes, which are derived from Proterozoic sources, are modeled as the source of the crustal end-member mixing with mantle-derived mafic magmas to produce the intrusive rocks. Four different types of models are applied to the isotopic and trace-element (Rb, Sr, Ba, REE) data: simple mixing; mixing with a partial melt of the metasedimentary rock, with or without isotopic equilibrium; and assimilation-fractional crystallization. Based on these models, some constraints may be applied on petrogenesis (e.g., the lack of high Rb concentrations points to the presence of biotite, and HREE depletion points to the presence of garnet); however, the models fail to adequately explain all the data. The New Zealand granitoids show similarities in isotopic character not only to rocks from offshore islands on the New Zealand block, but also to similar-aged granitoids in adjacent regions of Antarctica and Australia. This points to similarities in crustal character between continental blocks formerly proximal in Gondwanaland. We note an overall increase in _Nd and decrease in _Sr in felsic magmas from the Paleozoic to the Mesozoic to the Cenozoic in New Zealand, indicative of a decrease over time in the level of influence of recycled continental crust in subduction-related magmatism.Division Contribution No. 4538 (582)     

豫西宜阳地区三叠纪早期孙家沟组上段湖相碳酸盐岩碳氧同位素和古环境分析

孙家沟组是华北板块二叠纪和三叠纪转折时期的沉积记录,选择记载气候变化的材料,揭示气候的连续性变化,对于分析华北板块在这一转折时期古环境的重大变革具有重要意义。本项研究选择地层出露良好的华北板块南部豫西宜阳地区,以孙家沟组上段呈韵律型发育的薄层碳酸盐岩为研究对象,分析这些碳酸盐岩层中碳酸盐矿物的碳氧同位素组成及其变化,结合宏观沉积环境的分析,揭示这些碳酸盐岩层的成因,以及它们所蕴含的古环境信息。研究表明,孙家沟组上段碳酸盐岩的碳酸盐矿物主要为自生碳酸盐,且没有发生显著的后期变化,碳氧同位素组成及其变化可以用于古环境的重建。根据孙家沟组上段碳酸盐岩碳氧同位素的组成和受控因素,结合宏观沉积环境分析,认为孙家沟组上段的盐酸盐岩形成于湖盆环境。δ~(18)O-7.97‰时,δ~(13)C和δ~(18)O呈明显的正相关关系,湖泊处于封闭环境,碳酸盐矿物富含白云石,且结晶程度较差;δ~(18)O-7.97‰时,δ~(13)C和δ~(18)O呈明显的负相关关系,湖泊处于开放环境,碳酸盐矿物富含镁方解石,且结晶程度较好。根据孙家沟组上段由下部到上部碳酸盐岩碳氧同位素的变化,揭示出当时的湖泊环境经历了由相对开放到相对封闭,再到相对开放的演化,并显示出温度变低,湿度变小,植被变差的演化趋势。孙家沟组上段湖泊环境的形成可能与南部北秦岭造山带的隆升有关,薄层碳酸盐岩层与上覆紫色泥岩层组成的韵律层主要与气候变化引起的降雨量有关。孙家沟组上段所记录的古环境及其变化与全球二叠纪末期生物大灭绝以后陆地环境的变化规律相吻合。     

Sr isotopic chemostratigraphy of Precambrian carbonate rocks in the Amderma Rise,Pai-Khoi Ridge

The Sr and C isotopic compositions of Precambrian carbonate rocks are determined for Amderma Rise, in the northeastern margin of Pai-Khoi Ridge. Based on the Sr isotopic chemostratigraphy, it is established for the first time that the Amderma Formation is referred to the Early Vendian, while the Morozovsk Formation is Late Riphean in age. This conclusion along with detailed mapping proves that the Precambrian “section” of the Amderma Rise is a series of tectonic plates combined in a nonchronostratigraphic order. Volcanic and sedimentary rocks of the Morozovsk and Sokolninsk formations make up the allochthon proper, while carbonate rocks of the Amderma Formation make up the para-autochthon. The high values of δ¹³С (up to +9.5‰) identified in limestones of both formations suggest a considerable distance of the Pai-Khoi paleobasin from the passive margin of the Baltic Region upon facies similarity to the Laurentia active margins.     

Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks

In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in ²H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.     

A strontium isotopic study of the volcanic rocks of the Myoko volcano group,central Japan

Thirty-one selected volcanic rocks from the Myoko volcano group which comprises a volcanic chain of four independent volcanoes of Quaternary to Recent age are analyzed for ⁸⁷Sr/⁸⁶Sr ratios. The rocks of the lizuna volcano, the oldest among the Myoko volcano group, have higher ⁸⁷Sr/⁸⁶Sr ratios and show a larger scatter ranging from 0.7043₇ to 0.7055₆ than those of other volcanoes. The Kurohime volcanic rocks have a restricted range of ⁸⁷Sr/⁸⁶Sr ratios (0.7040₃∼0.7043₅). ⁸⁷Sr/⁸⁶Sr ratios of the Myoko volcanic rocks are almost the same in average to those of the Kurohime volcanic rocks, although somewhat varied ranging from 0.7037₈ to 0.7046₁. A single analysis of the Yakeyama volcanic rock yielded a ⁸⁷Sr/⁸⁶Sr ratio of 0.7042₇. A characteristic pattern in ⁸⁷Sr/⁸⁶Sr ratios is observed through the volcanic activity of the Myoko volcano group; ⁸⁷Sr/⁸⁶Sr ratios are high in the early stage of the volcanic activity and then decrease to low values, the late eruptives being characterized by constant ⁸⁷Sr/⁸⁶Sr ratios. The negative correlation between ⁸⁷Sr/⁸⁶Sr and Rb/Sr, and positive correlation between ⁸⁷Sr/⁸⁶Sr and Sr found in the rocks of the Iizuna volcano are interpreted to show the occurrence of contamination by materials with high ⁸⁷Sr/⁸⁶Sr ratios (>0.7056), low Rb/Sr ratios (<0.01) and high Sr contents (>300 ppm). Sialic crustal contamination may have played only a minor role.     

The isotopic composition of strontium and sulfur in seawater of Late Permian (Zechstein) age

The Paleozoic minima of ⁸⁷Sr/⁸⁶Sr ratios and δ³⁴S values in seawater are observed in anhydrite layers of the Stassfurt (A-2) and Aller (A-4) cycles of evaporation of Late Permian age in Germany. These minima were probably caused by extremely low rates of continental runoff and bacterial sulfate reduction. Thereafter conditions changed suddenly, with a steeply increasing supply of radiogenic Sr and of nutrients into seawater. The latter induced a large growth in the production of organic material, in bacterial activity in sediments, and in the flow of isotopically heavy sulfur back into the oceans. This large change in the cycles of S and Sr (as well as of C and O) within the relatively short Zechstein period (2–5 Ma) reflects a considerable modification of the plate tectonic and environmental conditions shortly before the beginning of the Mesozoic era. The Ochoan deposition in the USA is probably of the same age as the Stassfurt-Leine evaporation in W Europe.     

The influence of carbonate textures and rock composition on durability cycles and geomechanical aspects of carbonate rocks

Acta Geotechnica - One of the most important geotechnical parameters in studying the engineering behavior of a rock mass is slake durability. The major goal of this research is to test how a series...