Rb-Sr dating of Paleozoic glauconite from the Llano region,central Texas

Glauconite from eight stratigraphic horizons (Cambrian to Pennsylvanian) in the Llano Uplift, central Texas and two Cretaceous glauconites were analyzed by the Rb-Sr method. Only two untreated samples provide ages in agreement with those anticipated from current best estimates of the geologic time scale. With one exception all the other apparent ages fall short of the estimated age of deposition by as much as 22%. Low ages, the pattern customarily observed, are attributed to postdepositional loss of radiogenic ⁸⁷Sr from expandable layers by weathering or during diagenesis.Detailed leaching experiments using a variety of reagents were performed to characterize the behavior of glauconite. The most promising treatment, which we recommend as standard procedure in all future studies, is with ammonium acetate which is able to purge the mineral of loosely-bound Rb and Sr while leaving tightly-bound components intact. After appropriate leach, three other Rb-Sr ages were brought into coincidence with their estimated ages of deposition.In contrast an Upper Cambrian glauconite was found to be extremely resistant to further alteration by chemical attack′, providing an age of 429 ± 17 M yr. Although 17% short of the age of deposition, this age is interpreted as the time of a real event: diagenetic recrystallization induced by burial. Comparison of data from four samples indicates that for Paleozoic glauconite, conditions exist in which the Rb-Sr system is less susceptible to mild disturbance than is the K-Ar system.     

Composition of the Precambrian Llano Uplift,central Texas,U.S.A.

The Llano uplift exposes rocks of approximately 1000 m.y. age. The weighted average composition of the exposed crust is: 70.7% SiO₂; 0.35% TiO₂; 13.6% A1₂O₃; 3.4% total Fe as Fe₂O₃; 1.1% MgO; 2.6% CaO; 3.3% Na₂O; and 4.4% K₂O. This composition is similar to, but more potassic, than equivalent estimates for the Canadian shield.     

Rubidium-Strontium Age of some Metamorphic Rocks from the Llano Uplift, Texas1

Four total-rock samples of the Valley Spring Gneiss give a Rb-Srisochron age of 1120?25 million years and an initial Sr⁸⁷/Sr⁸⁶ratio of 0.706?0.002. This age is significantly older than the1,000–1,050 and 920?20 million year dates previously determinedfor the igneous rocks of the Llano Uplift. Mineral isochrons for the individual gneiss samples yield anage of metamorphism of 1,000?15 million years, in good agreementwith the time of major plutonism. The strontium in each of theolder metamorphic rocks was isotopically homogenized at thistime, resulting in Sr⁸⁷/Sr⁸⁶ ratios ranging from 0?716 to 0?775.From these data it is concluded that the plutons, which appearto have formed contemporaneously with a Sr⁸⁷/Sr⁸⁶ ratio of 0?706,could not have obtained their strontium from the surroundingmetamorphic rocks unless some special mechanism operated toremove excess radiogenic strontium. Only if the granite remainedan open system, unable to retain radiogenic strontium duringits early history, could it be as old as the total-rock ageof the gneiss.     

A Geochronologie Study of the Lone Grove Pluton from the Llano Uplift, Texas

A field and petrologic study of the Lone Grove granitic plutonand surrounding rocks from the Llano Uplift, Texas, suggeststhat this area has been involved in a single orogenic cyclewith no later general metamorphism. Samples of granite, aplite,pegmatite, rhyolite, and metamorphic rocks were investigatedin order to determine the precision in Rb-Sr and K-Ar ages betweenvarious minerals and different localities. Refined chemicaland mass spectro-metric methods are capable of yielding ageson most highly radiogenic minerals to an analytical precisionof ? 1 ? per cent or better. Most of the ages from these rocksshow a spread commensurate with the experimental error. The average Rb-Sr age on microclines, muscovites, and biotitesis 1020 million years and the average K-Ar age on muscovites,biotites, and hornblendes is 1045 million years (Rb⁸⁷, _ß= 1?47? 10^–11 yr^–1; K⁴⁰, _ß = 4.72 ? 10^–10yr^–1, and = 0–585 ? 10^–10 yr^–1). Atotal rock Rb-Sr age on one of the granites gives no indicationof being older than those of the constituent minerals. The only rock to show a real age difference is a rhyolite porphyry,which gives an average Rb-Sr microcline age of 920 million years.A metasedimentary gneiss having a total rock Rb-Sr age of 1110million years may contain some radiogenic strontium from anearlier history. K-Ar determinations on several microcline andplagioclases give ages which are 5–20 per cent low relativeto the other minerals, presumably due to argon diffusion fromthe feldspar. Anomalously low Rb-Sr ages occur on several freshbiotites from pegmatites and granite. Evidence is presentedfor strontium or rubidium migration in these rocks althoughthe exact nature of the process is not known. Also somewhatlow K-Ar ages are obtained on the pegmatitic biotites. A study of the effects of weathering on the geochronologic systemsis made on two obviously altered granites. The only mineralto suffer any decrease in apparent age from such surface alterationis biotite, by the Rb-Sr method. The Sr⁸⁷/Sr⁸⁸ ratio of the original strontium incorporated intothe minerals of the granite is determined on several mineralshaving low Rb/Sr ratios and is found to be 0.0843?0–002(normalized to Sr⁸⁶/Sr⁸⁸ = 0.1194). A discussion of the distributionof rubidium, potassium, and normal strontium throughout thepluton is given and partitioning factors for the rubidium topotassium concentrations between different mineral species arecalculated. ²Present address: U.S. Geological Survey, Washington 25, D.C., U.S.A.     

Fluid evolution and transport during metamorphism: evidence from the Llano Uplift,Texas

A field, petrologic and stable isotopic investigation of the marbles and calc-silicates of the 1.15 b.y. Valley Spring Gneiss documents the dilution of internally evolved CO₂-rich fluids by externally derived aqueous fluids introduced along channelways. Reaction textures within calcsilicates record the evolution through time of initially CO₂-rich fluids toward increasingly more aqueous compositions. Assemblage zonations within calc-silicates require equilibration within local gradients of the mole fraction of CO₂ in the fluid, and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies. Localized depletions in ¹³C and ¹⁸O corroborate petrologic evidence for channelized infiltration. Isotopic compositions reflect both devolatilization and the introduction of low- ¹⁸O fluids; estimated minimum oxygen-equivalent fluid-to-rock ratios are near unity. Both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc-silicate formation. The calc-silicate assemblages which equilibrated with fluids of the lowest mole fraction of CO₂ record isotopic exchange equilibrium with fluids of ¹⁸O typical of those derived from normal granites, as do the granitic aplites and pegmatites which transect most calcsilicate occurrences. Thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism.     

P-T-X conditions of calc-silicate formation: evidence from fluid inclusions and phase equilibria; Llano Uplift, central Texas, USA

Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular-shaped bodies of coarse-grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer-grained granite. In the central part of the uplift, wollastonite-bearing calc-silicate rocks occur within the Valley Spring Gneiss. The development of these calc-silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc-silicate rocks equilibrated under H₂O-rich conditions with χCO₂ <0.10. Fluid inclusions present within the calc-silicate minerals are H₂O-rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO₂ in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H₂O-rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm³. Isochores for these fluid inclusions, when combined with the stability of the solid-solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc-silicates were emplaced at a pressure of at least 1–2 kbar.     

Rb-Sr isotopic studies of a polymetamorphic granulite terrane,Strangways Range,central Australia

Rb-Sr isotopic investigation of forty rocks from three different granulite localities in the Strangways Ranges, central Australia indicated two distinct granulite metamorphisms, namely M1: 1860±80 m.y., and M2: 1470±60 m.y. and 1430±60 m.y., λ=1.39x10^?11yr^?1. M2 has been recorded in the felsic granulites of two of the three localities. Although the Sr isotopic systems in the felsic granulites seem to show a normal growth in a closed system during M2, the Rb-Sr systems for some calcareous and mafic granulites have not completely re-equilibrated during M2, as shown by the scatter with respect to the reference isochrons for M1 and M2. Ca-carbonate rocks display unusually low level of Sr (20–120 ppm. Sr, and high Ca/Sr ratios). Schistose zones of retrogression in the granulites, dated by K-Ar and ⁴⁰Ar-³⁹Ar methods at ≈350 m.y., yield anomalous Rb-Sr data.     

Rb-Sr and K-Ar isotopic systems of the Upper Devonian sediments in the Pripyat Trough,Belarus

The results of Rb-Sr and K-Ar isotope studies of the Famennian upper salt and suprasalt complexes sampled from the core material of exploratory boreholes drilled at the northwestern margin of the Pripyat Trough (Belarus) are presented. The suprasalt and saliferous portions of the sequence define an age of approximately 470 Ma. Despite multiple reworking of the material, we interpret this value as preservation of the provenance “memory.” Regardless of the Sr content in sample, the strontium isotope ratios obtained for low-Rb minerals (celestine, carbonate, and halite) varies throughout the section within 0.7083–0.7095, indicating the preservation of some primary features of the matter. The lowest isotope values obtained are slightly higher than those inferred for the upper Famennian. This fact confirms the concept that the studied complex was formed in a shallow-water setting. Data on sylvites show that the present-day isotopic characteristics of potassium salts were formed significantly later.     

Metabasalts and metagabbros from the Llano Uplift,Texas: Petrologic and geochemical characterization with emphasis on tectonic setting

The 1.0-1.2 b.y. old rocks of the southeastern Llano Uplift, Texas include a 7 km thick sequence of amphibolite-grade, stratified, mafic metasedimentary rock (Packsaddle Schist) which is intruded by a varied suite of syntectonic and late-kinematic intrusions. The metasediments contain large blocks of serpentinized peridotite (Coal Creek serpentinite) and coarse hornblendite and metagabbro. Prior to the end of maximum deformation, the sequence was intruded by low to medium-K₂O tholeiitic basalts (0.40-0.72% K₂O). Late-kinematic low-K₂O tholeiitic basalts (0.38-0.40% K₂O) were intruded as dikes into the folded rocks. The Coal Creek serpentinite contains both syntectonic and late-kinematic low-K₂O tholeiitic metabasalts (0.13–0.36% K₂O). The Llano metabasalts and metagabbros are characterized by low Cr (67–378 ppm) and Ni (36–170 ppm), variable Rb (1.5–14.7 ppm), Sr (140–1229 ppm), TiO₂ (0.40–2.20%), P (568–2707 ppm), and Zr (18–230 ppm), and Y (16–45 ppm), Co (40–57 ppm), and Sc (36–49 ppm) similar to modern MORB. The metabasalts have La abundances from 7 to 39 times chondritic and exhibit light REE enrichment with ¦La/Sm¦_N from 1.13 to 1.45 and ¦La/Yb¦_N from 1.12 to 2.99. The metabasalts show a strong correlation of increasing Zr, Ti, and Y and decreasing Eu/Eu^* (1.56–1.00), CaO/TiO₂, Al₂O₃/TiO₂, and MgO/ MgO+FeO^* with increasing REE enrichment (La_N). The Llano metabasalts and metagabbros have initial ⁸⁷Sr/⁸⁶Sr=0.7029±0.0001. A likely petrogenetic model for these metabasalts is an island arc in which events from early arc development to final late-kinematic intrusion were dominated by tholeiitic volcanism and intrusion. The chemical systematics of the Llano mafic metaigneous rocks suggest they are products of fractionation of olivine, clinopyroxene, and plagioclase from more primitive basaltic magmas generated beneath the island arc complex.     

Rb-Sr, K-Ar, H-and O-Isotope systematics of the Middle Riphean shales from the Debengda Formation, the Olenek Uplift, North Sibera

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ¹⁸O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.     

北大别主簿源花岗岩和片麻岩矿物的

对大别造山带北部主簿源中生代花岗岩侵入体及其围岩片麻岩进行了矿物氧同位素分析，同时对同一样品进行了矿物 Rb- Sr内部等时线定年。结果表明，花岗岩和片麻岩矿物的氧同位素温度大小顺序为：角闪石 >磁铁矿 >榍石 >石英 >黑云母 >长石，遵循缓慢冷却条件下扩散控制的氧同位素交换封闭顺序，指示这些岩石没有受到后期热液蚀变的扰动。根据黑云母-长石-磷灰石-全岩内部 Rb- Sr等时线测定，花岗岩的年龄为 (118± 3) Ma，代表了岩浆侵位冷却年龄；片麻岩的年龄为 (122± 1) Ma，代表了片麻岩受大面积燕山期岩浆侵位热烘烤达到高温同位素平衡后的冷却年龄。因此，矿物之间的氧同位素平衡与否 ,能够对矿物 Rb- Sr体系封闭后平衡状态的保存性以及矿物内部等时线定年的有效性予直接制约。     

A Rb-Sr geochronologic study of the pegmatites of the Middletown area,Connecticut

The pegmatites of Eastern Connecticut have a mineralogy consistent with a magmatic origin yet occur in a non-igneous environment. Various theories of genesis have been investigated by the Rb-Sr geochronologic method.Rb-Sr measurements on early stage pegmatite minerals indicate an age of 258±1 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.734±0.0096. Previously reported K-Ar and U, Th-Pb ages for pegmatite minerals are 249±8 m.y. and 260±3 m.y. respectively. Rb-Sr whole rock data for the host rocks vary between 285±10 m.y. and 472±15 m.y. in age and between 0.705±002 and 0.7167±0.0016 in initial Sr⁸⁷/Sr⁸⁶. A direct genetic relationship between the pegmatites and their host rocks is thus precluded. In addition, whole rock samples of the Brimfield schist taken at variable distances from the Strickland Quarry pegmatite have remained essentially closed systems with respect to Rb and Sr and thus an in situ origin for this pegmatite is unlikely. Mixing of pegmatite and country rock systems has occurred only locally, and isotopic studies of these mixed rocks yield a date of 231±4 m.y. with initial Sr⁸⁷/Sr⁸⁶=0.7188±0.004, an age not inconsistent with previously reported K-Ar and Rb-Sr mineral dates on host rock minerals (approximately 220 to 240 m.y.).Late stage cleavelandites are anomalously enriched in radiogenic Sr-87, the source of which was most probably other zones within the crystallizing pegmatite. This is indicated by analyses of pegmatite whole rocks which show both enrichment and depletion of radiogenic Sr-87 in local systems. The conclusion is drawn that there was widespread movement of radiogenic Sr-87 within each pegmatite system, but that pegmatite-host rock reactions were minimal.     

Rb-Sr and K-Ar age of globular phyllosilicates and biostratigraphy of the Riphean deposits of the Olenek Uplift (North Siberia)

This work presents results of the complex mineralogical, geochemical, and isotope-geochronological investigation of globular dioctahedral 2: 1 phyllosilicates (GPS) of the illite–glauconite series from the Riphean sequences of the Olenek Uplift. It is established that GPS (glauconite, Al-glauconite, Fe-illite) in deposits of the Arymass, Debengda, and Khaipakh formations are represented by mixed-layer varieties of two types: (1) with relatively low (<10%) and (2) higher (10–20%) contents of expandable layers. Among the mixed-layer varieties are those with disordered alternation of micaceous and smectite layers (R = 0), as well as with tendency to ordering (R ≥ 1). The parameter b of an elementary cell of minerals varies from 9.18 to 9.72 Å. The Rb–Sr age dating of GPS was first carried out in combination with the calculation of theoretical pattern of the cation distribution in the mineral structure and comparison of the calculation results obtained with the Mössbauer and IR spectroscopy data. This approach is based on the assumption that development and evolution of isotope systems in GPS are synchronous with the evolution of the crystalline structure of the mineral at various stages of the geological and geochemical history of the development of sedimentary units. Analysis of the obtained data allows us to state that the structural features of the Riphean GPS from the Olenek section reflect the early diagenetic stages of the formation of the minerals studied. The 87Sr/86Sr initial ratios in the studied sediments are consistent with the range of variations in this ratio in the Middle Riphean Ocean (0.7049–0.7061). The Rb–Sr and K–Ar ages of the GPS of the Arymass (1305 ± 8 and 1302 Ma, respectively), Debengda (1265 ± 12 and 1284 ± 22 Ma), and Khaipakh (1172 ± 18 and 1112 ± 24 Ma) formations in the Olenek Uplift section are close to the accumulation time of corresponding deposits and, correspondingly, have significance for stratigraphic correlations.     

Leaching of quartz from precambrian hypabyssal rhyolite porphyry,Llano County,Texas

A Precambrian hypabyssal rhyolite porphyry in central Texas has retained its original texture, although alkali feldspar phenocrysts have inverted and unmixed from zoned, single-phase high-temperature feldspar to zoned microperthite with an intermediate microcline host.Blue quartz phenocrysts owe their color to dispersion by zircon inclusions. Neutron activation analysis reveals substitution of tetravalent cations for silicon in the quartz.In two outcrops, quartz and albite have been completely leached from the rock and K-feldspar added, leaving the texture intact. The leaching was most likely accomplished by residual brine from an overlying Lower Cretaceous evaporite deposit subsequently removed by erosion.     

Late Cambrian microbial build-ups,Llano Area,Central Texas: A three-phase morphological evolution

Although Late Cambrian microbial build-ups were recognized in the Point Peak Member of the Wilberns Formation in Central Texas (USA) nearly 70 years ago, only a few studies focused specifically on the build-ups themselves. This study focuses on the interpretation of the regional (15 measured sections described in literature representing an area of 8000 km²) and local (field and drone photogrammetry studies in a 25 km² area from within south Mason County) microbial build-up occurrence, describes their growth phases and details their interactions with the surrounding inter-build-up sediments. The study establishes the occurrence of microbial build-ups in the lower and upper Point Peak members (the Point Peak Member is informally broken up into the lower Point Peak and the upper Point Peak members separated by Plectotrophia zone). The lower Point Peak Member consists of three <1 m thick microbial bioherms and biostrome units, in addition to heterolithic and skeletal/ooid grainstone and packstone beds. One, up to 14 m thick, microbial unit associated with inter-build-up skeletal and ooid grainstone and packstone beds, intercalated with mixed siliciclastic–carbonate silt beds, characterizes the upper Point Peak member. The microbial unit in the upper Point Peak member displays a three-phase growth evolution, from an initial colonization phase on flat based, rip-up clast lenses, to a second aggradation and lateral expansion phase, into a third well-defined capping phase. The ultimate demise of the microbial build-ups is interpreted to have been triggered by an increase of water turbidity caused by a sudden influx of fine siliciclastics. The lower Point Peak member represents inner ramp shallow subtidal and intertidal facies and the upper Point Peak member corresponds to mid-outer ramp subtidal facies. Understanding the morphological architecture and depositional context of these features is of importance for identifying signatures of early life on Earth.     

Preliminary isotopic studies in the Bida basin,central Nigeria

The area of the Bida basin is lacking in essential data for effective groundwater management. Hydrological studies using environmental isotopes (²H,³H,¹³C,¹⁸O) were carried out in the basin during the 1987–1988 hydrological year on samples of rainfall, streams, springs, and groundwater. The results obtained results do not indicate the presence of paleowaters; rather all groundwaters in the basin are found to be of meteoric origin as bothD and ¹⁸O values of the samples, plotted in the conventional way, align with the meteoric water line (MWL). Moreover, these data show the deeper, and apparently older, groundwater to be more depleted in the heavy stable isotopes than the shallower, relatively younger groundwaters, indicating recharge under cooler climatic conditions for the former. Measured tritium values range from 0.7 to 23.8 TU and show a variation with both depth and lithology, while at the same time indicating that active recent recharge is taking place basin-wide. The few¹³C data available were used to illustrate the effect of pumping on the carbonate geochemistry of the groundwaters. Also, the stable isotope data from the basin were compared with similar data from other parts of the country to show the continental effect.     

K-Ar and Rb-Sr Dating of blue amphiboles,micas, and associated minerals from the Western Alps

The results of 63 new radiometric K-Ar and Rb-Sr measurements on metamorphic minerals from the internal units of the Western Alps show Hercynian, Permian, as well as three Alpine age groups. The first of the Alpine ages cover the period between 78 and 100 m.y. and refer to high pressure parageneses. The second group comprises K-Ar 39 to 50 m.y. ages; these values are affected by some inherited argon, as indicated by Rb-Sr measurements which point to 35–36±4–5 m.y., i.e. similar to the culmination of the Lepontine crystallization. The final group includes 15 to 30 m.y. ages. It is not yet clear which geologic processes have led to this isotope re-equilibration. Large amounts of inherited argon have been found in Alpine metamorphic minerals of the basement rocks.     

K-Ar isotopic geochronology of Lianhuashan tungsten mining district,Guangdong Province

Reported in this paper are K-Ar ages of volcanic rocks, biotite-granite and greisen from the Lianhuashan tungsten mine in Guangdong Province. The isotopic dilution method was adopted in this work. Whole-rock isochron ages of the volcanic rocks are in the range 120.4±12.1 m.y. The petrogenetic age of biotite-granite and the minero genetic age of tungsten ore are close to those of the volcanic rocks. The apparent ages of various types of rock in the mining district generally bias towards younger values, which may be attributed to the effects of late thermal events which took place in the circum-Pacific metallogenetic belt.     

Rb-Sr and K-Ar systems of biotite in surface environments regulated by weathering processes with implications for isotopic dating and hydrological cycles of Sr isotopes

Biotite is widely used for Rb-Sr and K-Ar isotopic dating and influences Sr isotope geochemistry of hydrological regimes. The isotopic system of biotite behaves diversely in response to surface weathering; i.e. the complete preservation of original Rb-Sr and K-Ar isotopic ages or dramatic reduction. In this study, we have explored the relation between the behavior of isotopic systems and complex weathering processes of biotites in the weathering profiles distributed on the Mesozoic granitoids in South Korea. In the lower parts of the profiles, biotite in the early stages of weathering was transformed into either oxidized biotite or hydrobiotite, with a mass release of ⁸⁷Sr and ⁴⁰Ar forced by the rapid oxidation of ferrous iron. During the transformation to oxidized biotite, ⁸⁷Sr and ⁴⁰Ar were preferentially released relative to Rb and K, respectively, via solid-state diffusion through the biotite lattice, resulting in a drastic reduction of original isotopic age. The reduction of Rb-Sr age was greater than that of K-Ar age because K was preferentially released over Rb whereas ⁸⁷Sr and ⁴⁰Ar were released proportionally to each other. However, during the transformation of biotite to hydrobiotite (i.e., to regularly interstratified biotite-vermiculite), ⁸⁷Sr, Rb, ⁴⁰Ar, and K were completely retained in the alternating biotite interlayer, and thus the original isotopic age can be preserved. In the upper parts of the profiles, where iron oxidation was almost completed, ⁸⁷Sr, Rb, ⁴⁰Ar, and K were gradually and proportionally released, with no further significant change in isotopic age during the gradual transformation of the early-formed oxidized biotite into hydrobiotite and vermiculite or during their final decomposition to kaolinite. The ratios and amounts of isotopes released from weathered biotites are dependent upon the degree of iron oxidation and the pathways of mineralogical transformation. Regional and local variations in isotopic systems affected by particular weathering processes should be considered when dating biotite or biotite-bearing rocks in weathering environments, modeling the transfer of Sr isotopes to hydrologic regimes, and tracking the provenance of sediments.     

Rb-Sr and K-Ar Characteristics of Upper Vendian Clayey Rocks in the Russian Platform

The paper considers geochemical behavior of Rb-Sr and K-Ar systems in Upper Vendian clayey rocks of the Russian Platform. The use of additional material on size fractions of sediments recovered from boreholes drilled in the Gavrilov Yam area made it possible to confirm the previous inference on two stages of the epigenetic matter transformation (approximately 600 and 400 Ma ago). Distortions are related to transformation of sediments due to interaction in the water-rock system. The degree of interaction was more intense in the upper part of the sedimentary section relative to its lower strata. These inferences are substantiated by materials from boreholes that characterize different types of Vendian sections and different tectonic zones.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 381–402.Original Russian Text Copyright © 2005 by Vinogradov, Burzin, Bujakaite, Golovin, Gorozhanin, Veis, Murav’ev.