Rb-Sr Isochron Diagram Research Articles

Rb–Sr systematics of Holocene pelitic sediments and their bearing on whole-rock dating

In many cases, when dealing with argillaceous fine-grained sedimentary rocks, the isotopic ages obtained from Rb–Sr whole-rock isochron calculations are of geological significance, despite the fact that the initial conditions of Sr isotopic homogenization are not fulfilled. To explain this, a mechanical mixing during deposition has been suggested, leading to an aggregate of mixed material with fairly uniform 87Sr/86Sr values, whatever Rb/Sr ratio is found in the analysed samples. This investigation of the behaviour of Rb and Sr during sedimentation and early diagenesis, involves study of more than 60 samples of fine-grained recent sediments from selected coastal localities of Brazil. The results indicate that pelitic samples from some recent to present-day transitional or shallow marine environments, such as the Jacarepaguá tidal flat and the Amazonas River mouth, where halmirolysis could have occurred, may produce nearly horizontal best-fit lines in a Rb–Sr isochron diagram. Moreover, the initial isotopic 87Sr/86Sr ratios appear to be well above 0.710. In open marine environments, such as the Campos Basin, where sampling was spread over more than 100 metres, exchanges between the argillaceous sediments and seawater potentially happen at low temperature, inducing a reduction of the 87Sr/86Sr isotopic composition of the sediments to nearly 0.710. An almost horizontal best-fit line is produced for the entire stratigraphic section, independent of the sedimentation age. We believe that in similar environments such ‘zero age isochrons’ are maintained for some time, testifying to exchanges between the sediment and the interstitial fluids, before the onset of burial diagenesis. The above described studies on fine-grained and recent argillaceous sediments seem to confirm the production of horizontal best-fit lines in isochron diagrams, justifying the application of the Rb–Sr whole-rock method to pelitic sedimentary rocks. The nature of the material is critical. The pelitic sediments to be analysed should contain only very limited amounts of coarse clastic material (especially feldspars and mica fragments) and consist mainly of fine clay minerals in which smectites, illite and mixed layers illite–smectite largely predominate.

Inferences from SR-isotope systematics for magma evolution of NW Bavarian granitic intrusion sequences

Many granites occur as sequences of intrusions which show certain relations between its members. In this work the systematics of isochron slopes (i.e. isochron ages) and initial 87Sr/86Sr ratios are derived mathematically for two different types of granite intrusion sequences: fractional crystallization and magma generation by partial melting. The equations derived in this paper are applicable to any radioactive mother–radiogenic daughter system as Rb/Sr, Sm/Nd, U/Pb, etc. If granites are produced by magma fractionation, it can be shown that in an Rb–Sr isochron diagram the average isotope ratios yi=(87Sr/86Sr) and xi=(87Rb/86Sr) of a member Gi of a granitic intrusion sequence plot at the intersection of the individual isochrons of the two subsequent granites Gi and Gi+1 which have intrusion ages of ti and ti+1. In a Compston-Jeffery diagram, where the initial ratios bi are plotted vs the ages ti (or isochron slopes ai), bi increases monotonically with decreasing age ti. Two subsequent granites Gi and Gi+1 with the coordinates xi, yi and xi+1, yi+1 are connected by a line with a minimum slope of xi and a maximum slope of xi+1. If granites Gi are produced by progressive partial melting of a source rock, the present-day average values (xi, yi) of all granites Gi plot on an isochron which yields the age and the initial ratio of the source rock. The Compston-Jeffery diagram, contrary to the fractional crystallization model, does not necessarily yield a monotonic curve. The isotopic criteria derived in this paper, together with geochemical criteria, are applied to a granitic intrusion sequence in the Fichtelgebirge where the different granites are probably produced by fractional crystallization. A granitic intrusion sequence which covers a large area in the Oberpfalz only a few kilometres east of the KTB and approximately 30 km south of the Fichtelgebirge, however, probably is derived from magma generated by partial melting.

Rb-Sr Ages of Diagenesis of Mg-rich Clay in Permian Sediments, Palo Duro Basin, Texas Panhandle, U.S.A.

ABSTRACT Mid- to upper Permian sedimentary rocks in the shallow (< 3 km), intracratonic Palo Duro Basin, Texas Panhandle, occur in multicyclic carbonate-evaporite-redbed sequences, interpreted to be hypersaline shelf depositional assemblages. Certain marker horizons can be traced continuously over the entire basin. Clay minerals include distinctive varieties of Mg-rich smectite, chlorite, or mixed-layer species resulting from diagenesis in an evaporitic environment. Discrete particle-size fractions of clay were separated from core recovered from the Tubb (Leonardian), San Andres Unit #4 (lower Guadalupian), upper Seven Rivers (upper Guadalupian), and Salado-Tansill (Guadalupian-Ochoan) Formations to determine the times of mineral genesis. Rb-Sr isotopic data, for aliquots of a clay sample that has been leached by diverse ion exchange reagents, plot as a linear array in a Rb-Sr isochron diagram. For ultra-fine clay (< 0.05 µm), interpreted to be diagenetic, the slope of the data array corresponds to the time of diagenesis. Ages of clay diagenesis in the four sampled stratigraphic units agree generally with ages of deposition according to the geologic time scale, signifying that diagenesis was penecontemporaneous with deposition. Analyses of coarser clay fractions (0.2-0.5 µm and 2-4 µm) indicate the presence of a detrital component. In most instances, initial 87Sr/86Sr in diagenetic clay is higher than the Sr isotope ratio measured directly in associated anhydrite. Introduction of radiogenic Sr from occasional stream influx or from concurrent diagenetic dissolution of silicate detritus could have elevated 87Sr/86Sr in the ambient brine that mediated clay diagenesis.

New evidence for cenozoic resetting of KAr ages in volcanic rocks of the Northern Portion of the Armiralty Bay, King George Island, Antarctica

This paper presents six KAr whole-rock ages of the Znosko Glacier Formation (Cardozo Cove Group) that crops out between Mackellar Inlet (type-area) and Ezcurra Inlet, inner Admiralty Bay, King George Island, Antarctica. Originally attributed to the Upper Mesozoic, a RbSr isochron diagram place the lithostratigraphic unit in the Paleocene (60±5 Ma). The same samples were isotopically analyzed by the KAr method and the calculated ages, which range between 40 and 50 Ma, confirm the effect of an important tectonic-thermal event, responsible for structural deformations and metasomatic alterations in that unit. We discuss the origin of the tectonic event and the related KAr reset on rocks cropping out around Admiralty Bay.

Influences of the nature of the initial Rb [sbnd]Sr system on isochron validity

The theoretical basis for the Rb Sr isochron is questioned and the influence of both 86Sr and 87Sr/ 86Sr as variables in an initial Rb Sr system on the validity of isochron is theoretically discussed. It is mathematically shown that linear correlations on a Rb Sr isochron diagram can be enhanced by using 86Sr as a common denominator and that the linear arrays can be produced only when initial 87Sr /86Sr ratios define a horizontal line, or a linear correlation exists on the isochron diagram, at the initial instance of time. In the case of the initial horizontal line the linear arrays can still yield valid isochrons giving the true ages and initial ratios of the system, while in the case of the initial linear correlation anomalous isochrons are yielded which have no distinct geological meaning. An apparent isochron is then used to generalize the anomalous isochrons published in the literature so far and an apparent isochron equation is derived accordingly. Effects of fractional crystallization, partial melting, mixing and metamorphism on Rb-Sr isochrons are discussed respectively. It is concluded that an observed Rb Sr isochron can be obtained as long as the initial system fulfills the linear correlation between 87Sr/ 86Sr and 87Rb/ 86Sr ratios. As it is impossible to distinguish a valid isochron from an apparent isochron in the light of Rb Sr isotopic data alone, caution must be taken in explaining the Rb Sr isochron age of any geological system.

Anomalous isotopic compositions of Sr, Ar and O in the Mesozoic diabase dikes of Liberia, West Africa

The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using λ(87Rb)=1.42 × 10−11y−1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.

Effect of chemical weathering on the RbSr date of feldspar in neogene till, Mount Fleming, South Victoria Land, Antarctica

Cenozoic till deposits occur in presently ice-free areas at high elevations in the Transantarctic Mountains. These tills could have been deposited by local ice caps that no longer exist, or by ancestral outlet glaciers of the Antarctic ice sheet. Because the possible sources of the sediment differ significantly in age, RbSr dates of detrital feldspar in till can be used to evaluate these hypotheses. However, chemical weathering products adversely affect RbSr dates and must be removed to obtain dates that reflect the time of formation of the feldspar and, hence, the provenance of the sediment. A feldspar-quartz concentrate of till from Mr. Fleming was cleaned ultrasonically in 36 incremental steps for a total of 85 hr. in order to remove weathering products from the feldspar. Aliquots of the feldspar grains and the weathering products removed from them were collected after each cleaning step for SEM, XRD and RbSr analysis. Illite is the primary weathering product of K-feldspar; kaolinite forms from plagioclase, and gibbsite seems to occupy the outermost layer on both feldspars. The average total thickness of the altered layer is 4.5μm. Progressive cleaning causes the position of the feldspar on a RbSr isochron diagram to move in a way that affects the date derived from it. Illite has the greatest effect and causes RbSr dates of weathered feldspar to be lowered. The presence of an X-ray amorphous precursor of illite apparently has no effect on the RbSr date. Cleaned feldspar from Mt. Fleming till yields a date of 506 Ma, which is similar to the age of the granitic basement rock of the Transantarctic Mountains. There is no evidence that the till contains feldspar from other sources. We conclude that deeply weathered feldspar should be ultrasonically cleaned for at least 30 hr. to remove the alteration products that affect the RbSr date.

Effect of chemical weathering on the Rb?Sr date of feldspar in neogene till, mount fleming, south victoria land, antarctica*1

Cenozoic till deposits occur in presently ice-free areas at high elevations in the Transantarctic Mountains. These tills could have been deposited by local ice caps that no longer exist, or by ancestral outlet glaciers of the Antarctic ice sheet. Because the possible sources of the sediment differ significantly in age, RbSr dates of detrital feldspar in till can be used to evaluate these hypotheses. However, chemical weathering products adversely affect RbSr dates and must be removed to obtain dates that reflect the time of formation of the feldspar and, hence, the provenance of the sediment. A feldspar-quartz concentrate of till from Mr. Fleming was cleaned ultrasonically in 36 incremental steps for a total of 85 hr. in order to remove weathering products from the feldspar. Aliquots of the feldspar grains and the weathering products removed from them were collected after each cleaning step for SEM, XRD and RbSr analysis. Illite is the primary weathering product of K-feldspar; kaolinite forms from plagioclase, and gibbsite seems to occupy the outermost layer on both feldspars. The average total thickness of the altered layer is 4.5μm. Progressive cleaning causes the position of the feldspar on a RbSr isochron diagram to move in a way that affects the date derived from it. Illite has the greatest effect and causes RbSr dates of weathered feldspar to be lowered. The presence of an X-ray amorphous precursor of illite apparently has no effect on the RbSr date. Cleaned feldspar from Mt. Fleming till yields a date of 506 Ma, which is similar to the age of the granitic basement rock of the Transantarctic Mountains. There is no evidence that the till contains feldspar from other sources. We conclude that deeply weathered feldspar should be ultrasonically cleaned for at least 30 hr. to remove the alteration products that affect the RbSr date.

Identification and age of neoformed Paleozoic feldspar (adularia) in a Precambrian basement core from Scioto County, Ohio, USA

Fourteen core samples of Precambrian granitic gneisses from a well drilled in the Green Township, Scioto County, Ohio were studied to determine the origin of alkali feldspar in these rocks. The well intersected the basement at a depth of 1,700 m and penetrated 11.3m of Precambrian crystalline rocks. Petrographically the samples in the upper 6.4 m of the basement core show evidence of severe alteration by the presence of hematite, limonite and chlorite and by the absence of plagioclase. Alkali feldspars from this part of the core are turbid, have a low 2 V of about 10°, are highly enriched in K, have low Na and Rb concentrations, lack cathode luminescence, and form a straight line on a Rb-Sr isochron diagram yielding a date of 599±69 Ma. Core samples from below 6.4 m appear relatively fresh and unaltered. Alkali feldspar from this portion of the core is orthoclase, shows uniform blue luminescence and gives a Rb-Sr date of 1,162±11 Ma. These results indicate that feldspars in the lowest part of the core are primary minerals that crystallized during the Grenville Orogeny, whereas the K-feldspar in the top of the core is of low-temperature secondary origin. The formation of this feldspar is explained as a consequence of chemical weathering of primary feldspar during late Precambrian time to clay minerals that were later reconstituted under low-temperature hydrothermal conditions as K-feldspar (adularia) by reactions with brines derived from the overlying Mt. Simon Formation of Cambrian age.

Rb-Sr provenance dates of feldspar in glacial deposits of the Wisconsin Range, Transantarctic Mountains

Glacial deposits in the Wisconsin Range (lat. 85° to 86°30′S, long. 120° to 130°W) of the Transantarctic Mountains include a deposit of till on the summit plateau at an elevation of 2,500 m above sea level and glaciolacustrine sediments along the Reedy Glacier. The plateau till and underlying sediments consist of six units that appear to record the replacement of ice-free, periglacial conditions by ice cap glaciation of pre-Pleistocene age. Alternatively, the plateau till may have been deposited by the East Antarctic ice sheet either when it was thicker than at present or when the Wisconsin Range was lower in elevation. Feldspar size fractions from the plateau till have Rb/Sr ratios that increase with grain size from 1.4 (67 to 125 µm) to 4.24 (500 to 1,000 µm). These size fractions define a straight line on a Rb-Sr isochron diagram and yield a date of 480 ± 21 Ma that is indistinguishable from the age of the granitic basement rocks of the Wisconsin Range dated previously. This result therefore supports the hypothesis that the plateau till was deposited by a local ice cap and suggests that the Wisconsin Range was sufficiently elevated to permit an icecap to form prior to the growth of the East Antarctic ice sheet. The glaciolacustrine sediments along Reedy Glacier probably were deposited in an ice-marginal melt-water pond along the margin of a temperate Reedy Glacier soon after the East Antarctic ice sheet first reached full size. The Rb/Sr ratios of feldspar in this sediment are lower than those of feldspar in the plateau till and range only from 0.46 to 0.66. Nevertheless, three feldspar fractions form a straight line on the Rb-Sr isochron diagram, the slope of which indicates a date of 576 ± 21 Ma. The difference in the date derived from the feldspar of the glaciolacustrine sediment may be caused by the presence of a component of Precambrian feldspar derived from the East Antarctic Shield.

Geologic evolution of the Serra dos Carajás, Pará, Brazil

The Serra dos Carajas is located in southern Para, in the region between the Araguaia and Xingu rivers, and has been regarded very recently as the most promising and potentially interesting metallogenic province in South America. ln order to characterize its geologic evolution, a geochronological program was carried out by a cooperative effort of Docegeo and the CPGeo-USP, with the help of structural, petrologic and geological information. In the region, a polymetamorphic basement occurs in which gneisses arid related rocks are associated to the supracrustal rocks of the Salobo Formation. The iron formations and low grade metamorphics of the Grao-Para Group overlie the basement, and are covered by the sedimentary rocks of the Rio Fresco Formation. Post-orogenic and cratogenic granites occur, cutting through all the above mentioned units. The granitic rocks indicated Rb-Sr whole rock ages up to 1,850 m.y., K-Ar determinations on micas from the metamorphic indicated that the regional cooling occurred at about 1.950 m.y., after the main events of the Transamazonian orogeny. Some transamazonian results were also obtained on micaceous schists of the Salobo Formation. However, several analytical points, in a Rb-Sr isochron diagram, indicated a Late Archean age for their main rnetamorphic episode.

Age of emplacement of the schistes lustres nappe, Alpine Corsica

Structural studies in the Schistes lustrés nappe west of Bastia, Corsica, demonstrate the existence of a tectonic mélange in which km-scale blocks and smaller lozenges of basement granite gneiss, thick-layered marble and dismembered Mesozoic ophiolite are enveloped in a matrix of calc-schist and blueschist. The main ( S 1) foliation is developed in both block and matrix and is concordant with lithologie contacts. Blueschist facies metamorphism was syn-kinematic with the main foliation. The S 1 in the Schistes lustrés was refolded about ENE-WSW trending, tight similar and monoclinal fold axes ( F 2). These second folds verge to the southeast and show km-scale axial culminations and depressions that are reflected by topography and residual Bouguer gravity anomalies. Parautochthonous Hercynian basement (Tenda-Corte complex) beneath the western edge of the Schistes lustrés nappe contains a mylonitic foliation which is concordant with the main foliation in the Schistes lustrés. The intensity of deformation in the basement decreases away from this contact and undeformed granites are found 3 km to the west. Whole rock samples of the deformed basement immediately beneath the Schistes lustrés yield an Rb-Sr isochron diagram ( n = 4) which has an age of 105 ± 8 Ma (1σ) and initial 87Sr 86Sr ratio of 0.7228 ± 0.0005 (1σ). This result is more precise than our preliminary age and initial ratio estimate of 98 ± 14 and 0.7296 ± 0.0068, respectively (Cohen et al., 1979). It is similar to a recently published mid-Cretaceous (90 Ma) 40Ar- 39Ar age from glaucophane mineral separates. We interpret this date as the age of a metamorphic overprint related to the emplacement of the Schistes lustrés nappe and associated ophiolites, the formation of the main foliation and blueschist facies metamorphism. These results indicate that the mid-Cretaceous blueschist facies metamorphism documented in the Western Alps formerly extended farther south of its present terminus. The data are consistent with mid-Cretaceous obduction of Tethyan oceanic crust onto the present-day eastern continental margin of Corsica. We postulate that during Eocene—early Oligocene time a polarity flip occurred outboard of the obducted crust and a new, southfacing subduction zone developed. This change in polarity was responsible for the development of southeast-vergent second folds and for the resetting of 40Ar− 39Ar and K-Ar geochronologic clocks described in the literature.

Rb-Sr Dating of Detrital Feldspar: A New Method to Study Till

Late Wisconsin till in the midwestern U.S.A. contains feldspar concentrated in the sand-size fractions (). The feldspar originated almost exclusively from the igneous and metamorphic rocks of the Precambrian Shield of North America. Ice-flow directions support the hypothesis that feldspar in Late Wisconsin till of Ohio and Indiana is a mixture of grains derived from the Superior structural province (2.70 b.y.) by the Huron ice lobe and from the Grenville structural province (1.07 b.y.) by the Erie and Ontario lobes. Feldspar is well suited for dating by the Rb-Sr method, and such dates can therefore be used to study regional variations in its provenance. The K-feldspar/plagioclase ratios of feldspar and their Rb/Sr ratios increase with grainsize. Feldspar size fractions from two till samples form a linear array of data points on the Rb-Sr isochron diagram. The slope of this line is related to the proportion of mixing and is expressed as a provenance date of b.y. Feldspar in the 2.0 to 3.0ø fractions of 24 samples of till from the Powell-Union City Moraine between Mansfield, Ohio, and the Wabach River, Indiana, have Rb-Sr provenance dates that increase from 1.2 b.y. (east) to 1.8 b.y. (west). The westerly increase in the provenance dates is probably due to an increase in the abundance of feldspar derived from the Superior Province. The close grouping of the provenance dates and the absence of a bimodal distribution of dates along the strike of the Powell-Union City Moraine indicate that thorough mixing of sediment occurred when the Erie lobe advanced over glacial deposits laid down earlier. Such extensive reworking implies that the older deposits were not frozen and may have been water-logged. The final advance of the Erie lobe was probably not caused by a deterioration of climate in Ohio and Indiana, but may have been a result of changes in the dynamics of the Laurentide ice sheet.

Rb-Sr mantle isochrons from oceanic regions

Existing data for 87Sr/ 86Sr and Rb/Sr ratios of basalts from oceanic islands and mid-ocean spreading ridges show significant positive correlations on a Rb-Sr isochron diagram (when data are averaged by island group). Furthermore, tholeiites and alkali basalts occupy distinct non-overlapping fields on this plot. The tholeiite correlation is interpreted as a mantle isochron, and the agreement of this age (1.6 ± 0.2 b.y.) with that reported for Pb-Pb isochrons from oceanic basalts lends strong support to the use of such isochrons for tracing mantle evolution. Oceanic basalts are apparently sampling a mantle in which chemical heterogeneities have persisted for at least 1.5–2.0 b.y. The data support a kinematic model for the mantle in which a relatively uniform and non-radiogenic asthenosphere is penetrated by, and mixed with, blobs or plumes derived from an isolated (1.5–2 b.y.) and chemically heterogeneous mesosphere.

Systematic variations of sialic and volcanic detritus in piston cores from the Red Sea

The 87Sr 86Sr ratios and Sr concentrations of the non-carbonate fractions of sediment from two cores taken in the median valley of the Red Sea indicate that the detrital fractions are mixtures of two components originating from old sialic and young volcanic rocks of the surrounding land areas. The mixing equations were derived from the data and were used to estimate the Sr concentrations of the two components. The volcanogenic detritus has a Sr content between 770 and 800 ppm while the component derived from old sialic rocks contains from 50 to 70 ppm. The volcanic component consists primarily of unweathered particles of alkali basalt and volcanic glass while the sialic component is represented by clay minerals and X-ray amorphous material. Systematic variations of the concentration of volcanogenic detritus were used to define layers of sediment which correlate over a distance of about 200 km separating the two cores. The concentrations of quartz, feldspar, amphibole, illite, kaolinite and chlorite were found to vary systematically with the content of volcanogenic detritus as calculated from the Sr data. The carbonate concentrations of the sediment range from 33 to 87% and appear to increase in a southerly direction. The rate of carbonate deposition increased at times of more efficient input of sediment derived from old sialic rocks. This relationship is explained by the hypothesis that both were influenced indirectly by brief warming trends during the Würm glaciation. The occurrence of sediment layers enriched in sediment derived from old sialic rocks correlates approximately with interstadials of the Würm glaciation in northern Europe. Increased input of volcanogenic detritus occurred around 37,000 and 26,000yr ago and during the past 5000 yr. A plot of 87Sr 86Sr and 87Rb 86Sr ratios defines a line on the Rb-Sr isochron diagram yielding a fictitious date of about 230 million yr.

Petrologic and Rb-Sr Isotopic Study of the Chiquimula Pluton, Southeastern Guatemala

The Chiquimula pluton is one of a chain of plutons that extends across southern Guatemala and northern Honduras. Its exposed area of about 300 sq km in southeastern Guatemala is highly irregular. Exposed parts of the epizonal pluton are dominantly granodiorite; gabbro may be abundant, but it is exposed only in the deepest canyons. Diorite is predominant along the southern and western margins, and the rock is an intergrading mixture of granodiorite with subordinate adamellite and granite near the northern and eastern margins. A whole-rock Rb-Sr isochron diagram shows data points falling in two distinct groups. Granite and adamellite form a scattering of points indicating relatively high Rb/Sr (3.3 to 4.6) and high Sr 87 /Sr 86 (0.72 to 0.75). Points from the volumetrically more important gabbro and grand diorite, indicative of low Rb/Sr (1.2 or less) and Sr 87 /Sr 86 (0.706 to 0.708), form a well-defined isochron corresponding to an age of 50 ± 5 m.y. and initial Sr 87 /Sr 86 = 0.7060 ± 0.0002. K-Ar and Rb-Sr biotite ages from granite of 84 m.y. and 95 m.y., respectively, are younger than the whole-rock Rb-Sr age (215 m.y.) of the same specimen, but older than the age of emplacement of the pluton as suggested by the whole-rock isochron from granodiorite and gabbro. Field relations, petrographic data, and isotopic data are consistent with the interpretation that the Chiquimula pluton is just beginning to be unroofed. The compositional variation probably resulted from a combination of assimilation and differentiation of a granodioritic magma. The more granitic parts may be due to partially assimilated more siliceous exotic blocks of basement rock.