Isotopic Age Determinations Research Articles

Isotopic ages of Jurassic siliceous shale and Triassic bedded chert in Unuma, Central Japan.

Isotopic age determinations were carried out for Jurassic siliceous shale (assigned to Bajocian by radiolarian fossils) and for, Triassic bedded chert (assigned to Anisian by radiolarians and conodonts) in Unuma, Gifu Prefecture, Central Japan. Whole-rock samples of the Jurassic shale give a Rb-Sr isochron age of 179.8 ± 5.7 Ma with an initial 87Sr/86Sr ratio of 0.71011 ± 0.00036, whereas two of these Jurassic samples have K-Ar whole-rock ages of 151 and 152 Ma. For the Triassic chert, the Rb-Sr whole-rock isochron shows 211.9 ± 4.7 Ma with an initial 87Sr/86Sr ratio of 0.71325 ± 0.00020, and a K-Ar whole-rock age for one of these Triassic samples is 207 ± 7 Ma. Two Rb-Sr whole-rock isochron ages indicate the time when the respective sedimentary rocks became chemically closed with regard to the Rb-Sr system. The initial 87Sr/86Sr ratios in both sets of rocks are higher than the contemporaneous marine 87Sr/86Sr ratio, suggesting the existence of an older landmass. The K-Ar ages of the Jurassic siliceous shale presumably represent the time of a later geologic event.

A radiometric time scale of the Triassic

Abstract A review of the large number of stratigraphically controlled isotopic age determinations relevant to the Triassic shows that the ages of the Triassic‐Jurassic and Permian‐Triassic boundaries are approximately 200 Ma and 245 Ma, respectively. Estimates for the period boundaries within the Triassic are as follows: Norian‐Carnian—215 Ma; Carnian‐Ladinian—225 Ma; Ladinian‐Anisian—235 Ma; Anisian‐Scythian—240 Ma. Because of the many uncertainties inherent in these estimations, both stratigraphic and radiometric, the ages of the Triassic system and stage boundaries given above are assigned an error of ± 5 Ma.

Alteration of the oceanic crust: Processes and timing

87Sr/ 86Sr ratios, Sr, K, Rb and Cs contents and the petrology of basalts and secondary phases recovered from deep basement drilling at DSDP Sites 332B, 417A, 417D and 418A show that the oceanic crust alters in four distinct stages characterized by formation of palagonite, smectite and calcite (Stages I, II and III, respectively). Stage IV represents the final compaction of the crust, including a dehydration of the crust without major chemical changes. Isotopic age determinations by Hart and Staudigel [11] and Richardson et al. [13], show that, at least for Atlantic-type crustal regimes, Stage I and II last for a maximum of 3 m.y., and Stage III lasts beyond Stage I and II, but is probably completed in significantly less than 10 m.y. Stage IV is long-lived and may still be active at 100 m.y. Stages I and II, the phase of halmyrolysis, include geochemically significant interactions between seawater and basalt for the upper 500 m of layer II and involve volumes of seawater containing a large basaltic component. Stage III solutions show evidence of less seawater-basalt interaction, at least to depths of 500 m; calcites deposited from these solutions have Sr isotopic ratios close to seawater values; but also have very low Sr/Ca ratios indicating a large basalt Ca component in the solutions. Smectite formation is the result of the interaction of seawater and basalt. The initial 87Sr/ 86Sr ratios of smectites represent the Sr isotopic composition of the solution when the smectite is being formed. Thereafter, alkalies may be continuously added to interlayer positions in the smectite in order of decreasing hydration energy (Cs is more enriched than Rb, Rb more than K). The later-formed carbonates have very low alkali concentrations, and 87Sr/ 86Sr ratios identical to contemporaneous seawater. Therefore, since the alkali concentrations in a whole rock sample are affected by different alteration processes, the alkali concentrations alone are not reliable indicators of the degree of alteration.

Agents of low temperature ocean crust alteration

δ18O and87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are depositedafter this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.

The Ordovician, Silurian and Devonian time scales

A reassessment of published Ordovician, Silurian and Devonian isotopic age determinations available has necessitated the correction of several calculated dates and revision of several stratigraphic conclusions in the light of later work. The revised dates plot close to a single time scale line, except for Rb Sr dates on acid volcanic rocks. We conclude that the base of the Devonian is at 410 Ma, the base of the Silurian at 437 Ma, and the base of the Ordovician (Tremadoc Series) at 519 Ma. These dates are within the range of previously published dates for the base of the three periods. We place more reliance on our estimates for the Upper Ordovician, Silurian and Devonian than for the Lower Ordovician. A comparison with other time scales shows that our new scale is similar to many previously published, except for those which rely heavily on acid volcanics.

ISOTOPIC AGE DETERMINATIONS ON A BASALT INTRUSION AND ULTRABASIC XENOLITHS, SOUTH BUTE, SCOTLAND

SUMMARY K-Ar isotopic age determinations are reported from a basaltic sheet and some xenoliths from South Bute, Scotland. The age of the intrusion, 309 ± 5 Ma (pre-middle Carboniferous), is in agreement with the date inferred from its petrology. The results from the ultrabasic xenoliths do not permit a simple interpretation.

On the age of rocks and Precambrian time scales

Specific type regions are needed to adequately define Precambrian time intervals whose limits are established–relative to temporally and stratigraphically ordered sequences of intrusive and stratified rocks–through the use of field relationships, fossil content, paleomagnetic characteristics, and relevant isotopic age determinations. Other Precambrian rocks are then correlated with the standard by establishing the synchroneity of geologic events as recorded by isotopic, paleomagnetic, paleontologic, tectonic, stratigraphic, and lithologic attributes of the rocks. A time-stratigraphic system comparable to that for the Phanerozoic cannot be established for the Precambrian, as individual beds of stratified sequences cannot be correlated with the same precision. The [North American] “Code of Stratigraphic Nomenclature” reflects neither this fundamental difference between Precambrian and Phanerozoic rocks nor the distinction between a general temporal ordering of both intrusive and stratified rocks and a stratigraphic ordering alone. These distinctions are basic to understanding the different requirements for establishing geologic age categories for Precambrian and Phanerozoic rocks and are manifested in the need to redefine time-rock, time-stratigraphic, magnetostratigraphic, and magnetochronologic concepts in the Code.

Isotopic age determinations on the composite sill and associated olivine dolerite, South Bute

Synopsis K-Ar isotopic age determinations on the Tertiary composite sill indicate it is at least 56.3 ± 1.0 Ma old. The olivine dolerite sheet is likewise of a Tertiary age, with a similar age to the composite sill, and may be related to the crinanite sills of southern Arran. The whole almost certainly belong to the ‘Major Phase’ of igneous activity identified as occurring at 59 Ma in the British Tertiary igneous province.

Polyorogenic Nature of the Southern Caledonian Fold Belt in East Greenland: An Isotopic Age Study

The southern part of the Caledonian fold belt in East Greenland comprises a complex of pre-Caledonian rock units as demonstrated by numerous age determinations and interpretation of field work. U-Pb isotopic age determinations on zircons, Rb-Sr analyses of whole-rock and mineral samples and one K-Ar mineral age are presented. They date some of the key rock units fundamental to the unravelling of the development of this part of the fold belt, and support the polyorogenic nature of the Caledonian fold belt. Three main groups and two orogenic events with related migmatizations are observed: (1) Archean basement: early intrusive granites and basic dikes imply a minimum age of 2500 m.y. for the basement; (2) Proterozoic intrusions: bodies and sheets of augen granites were emplaced about 1000 m.y. before present. The intrusions coincide with the first (i.e. Proterozoic) orogenic event as well as the first migmatization recognized in the area; (3) Caledonian intrusives: a massive hypersthene monzonite sheet representing an early phase of the second (i.e. Caledonian) orogeny, was emplaced about 475 m.y. ago. Younger granitic stocks as well as sheets of granite associated with the second migmatization were formed in the period between 425 and 400 m.y. ago.

Isotopic age determinations on granitic and gneissic rocks from the Ubendian-Usagaran system in southern Tanzania

Isotopic age measurements on granites and gneisses are reported from the Ubendian-Usagaran System in southern Tanzania. For the Lukumburu-Wino Granites a Rb-Sr whole-rock isochron (9 points) of 1771 ± 145 Ma with initial 87 Sr 86 Sr of 0.704 ± 0.003 was obtained ( λ 87 Rb = 1.42 · 10 −11 a −1; errors 95% confidence level). Two samples from neighbouring granites and three gneisses from the contact zone of the Lukumburu-Wino Granites also fit to this isochron; inclusion into the isochron calculation yields an age of 1747 ± 85 Ma with the same initial Sr composition. Five parts of a banded Usagaran gneiss sample produce an isochron of 589 ± 70 Ma with initial 87 Sr 86Sr of 0.7194 ± 0.0005. This internal isochron is interpreted as reflecting Sr isotopic equilibration through the sample during the main phase (or one of the main phases) of the Pan-African thermotectonic episode. Rb-Sr and K-Ar ages of 17 micas show a general resetting by the Pan-African thermotectonic episode. The Rb-Sr ages of the muscovites and the biotites average 528 Ma and 463 Ma, respectively, and are interpreted as reflecting subsequent stages in the cooling history after the termination of the event. Biotites have K-Ar ages persistently higher than the corresponding Rb-Sr ages, possibly reflecting a high partial 40Ar pressure through the crust during the resetting of the biotite K-Ar systems.

The age of mineralisation at Parys Mountain, Anglesey

Summary Borehole exploration of the mineralisation in the Ordovician and Silurian rocks of Parys Mountain, Anglesey has produced two hypotheses to explain the emplacement of the ore deposits. Mineralisation was either syngenetic, associated with volcanicity in the Ordovician, or epigenetic, occurring during or after the Caledonian orogeny. An outline of the geological evidence is given together with the results of K-Ar isotope age determinations on thirty-two core samples. Two isotopic events have emerged from the K-Ar determinations, 394±9 Ma and 353±7 Ma; these ages are discussed in the light of the evidence available.

Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 2. Geochronology

The region is divided into two domains, which are defined by their contrasting petrology, structure, and aeromagnetic anomaly pattern. The northern domain is underlain by east-trending, steeply south-dipping, migmatized garnet–biotite paragneiss. The southern domain is underlain by gneissic tonalite and trondhjemite, intruded by granite plutons, and with abundant amphibolite inclusions.Twelve U–Pb isotopic age determinations of zircons from two gneisses and four granite bodies from the eastern Lac Seul region of the English River Gneiss Belt indicate the following sequence of events. The tonalite gneiss of the southern domain was emplaced definitely more than 3008 m.y. ago and probably more than 3040 ± 40 m.y. ago. Local anatexis or injection associated with regional metamorphism produced a local pegmatite leucosome in the paragneiss 2681 ± 20 m.y. ago. Late- or post-orogenic granites, which invade both the paragneiss and the tonalite, were emplaced 2660 ± 20 m.y. ago, after the major metamorphism in the region. The youngest rock in the area, a cross-cutting pegmatitic granite, was emplaced 2560 ± 40 m.y. ago.

New hominid fossils from the Koobi Fora formation in northern Kenya.

A WELL preserved partial innominate KNM-ER 3228, and a relatively complete cranium KNM-ER 3733 provide further support for the presence of Homo during the Plio–Pleistocene in eastern Africa1–5. These hominid specimens and others (Table 1) were collected during the 1974 and 1975 field programme in the Koobi Fora Formation, lying to the east of Lake Turkana (formerly Lake Rudolf) in northern Kenya. (This area was previously known as East Rudolf and has been studied since 1968; ref. 6.) The material reported here has been referred to either the Upper or Lower Member of the Koobi Fora Formation. Recent work on the dating of tuffs by Curtis et al.7 has raised important questions that will require time to resolve and may necessitate a complete re-examination of the reported chronology of the Koobi Fora succession and all other East African Plio–Pleistocene localities. Alternative dating methods such as fission track will be given close attention in the future and may offer, together with studies of certain taxa, useful indications about the reliability of isotopic age determinations.

Isotopic age and metamorphic history of the banded gneiss at Danmarkshavn, East Greenland

Along the northern part of the East Greenland coast the Caledonian structures are superimposed on an older fold system called the “Carolinidian”. Traces of this Carolinidian belt are preserved in a few isolated fragments within the Caledonian fold belt. According to Haller (1970) one of these fragments exhibiting the typical Carolinidian NNW to NW-trending infrastructural folds is the peninsula of Germania Land which is accessible near the Danish weather station Danmarkshavn. The rock sampled there is a banded gneiss of granodioritic composition with steeply inclined, NNW-trending layers. Isotopic age determinations yielded essentially two groups of ages: 1) 3,000±150 m.y. (zircon suite and Rb/Sr whole rock analyses of layers) and 2) 320–380 m.y. (Rb/Sr mineral isochrons, U-Th-Pb on sphene, K/Ar on hornblende and biotite). The egg-shaped zircons support a sedimentary origin of the banded gneiss and in conjunction with the Rb/Sr whole rock ages determine the age of formation of the banded gneiss (or its last high grade metamorphism) some 3,000 m.y. ago. No other Precambrian metamorphism or orogeny is recorded in the rock. The ages between 320–380 m.y. date a thermal event of lower amphibolite facies grade related to a late Caledonian spasm.

Palynological evidence for the age of the synorogenic Brewer Conglomerate, Amadeus Basin, central Australia

An assemblage of spores recovered from the Undandita Member of the Brewer Conglomerate, the uppermost unit of the Pertnjara Group in the Amadeus Basin of central Australia, establishes the age of that formation as Late Devonian. Comparison with spore assemblages from sedimentary basins in Western Australia indicates that the central Australian assemblage is dateable within the post-early Frasnian to pre-late Famennian interval. Fish remains and spores previously described from the Parke Siltstone at the base of the Pertnjara Group are of probable early Frasnian age, so it now appears that the entire group was deposited during the Late Devonian. The Brewer Conglomerate is of synorogenic origin, and was deposited during uplift associated with the Alice Springs Orogeny; the spore data thus indicate that this tectonic event began in the Late Devonian. Isotopic age determinations in the Arltunga Nappe Complex and the Strangways Range of the Arunta Block have yielded Early Carboniferous dates. These possibly re...

Revised Cenozoic polarity time scale

The polarity sequence of Heirtzler and his co-workers for Neogene time is modified by compromise solutions between the polarity record in deep-sea sediments and oceanic magnetic anomalies. We have also revised the entire chronology by use of more recent isotopic age determinations of the stratigraphic ages of sediments overlying identified anomalies. The main features of the revised time scale are a general reduction of the length of polarity periods during Eocene and later times, and an expansion of Paleocene polarity periods.

Paleosalinities within a Pliocene Bay, Kettleman Hills, California: A Study of the Resolving Power of Isotopic and Faunal Techniques

Pliocene sediments of the Kettleman Hills, California, were deposited in a marine embayment that occupied part of the San Joaquin Valley. Faunal and sedimentologic data from the upper Pliocene Pecten zone indicate a decreasing salinity trend south and southwestward across the Kettleman Hills. In general, the least diverse faunas and coarsest sediment are in the west-central part of the area, near an apparent small delta and are relatively distant from the entrance of the embayment. Salinity and substrate are probably the main factors controlling the composition of the eight fossil communities recognized by Q-mode cluster analysis. Oxygen and carbon isotopic age determinations were made on specimens of Pecten , Aequipecten , Ostrea , and the fresh-water clam, Anodonta , in order to determine paleosalinities. Assuming that Anodonta grew in water that was representative of the fresh water that flowed into the embayment, that the shells with the heaviest isotopic composition grew in normal marine water, and that the temperature was constant throughout the area at that time, the salinity at which the shells grew can be calculated from their oxygen isotopic composition. The salinities calculated by this procedure range from normal marine (35 ‰) in the north and east parts of the Kettleman Hills to brackish (about 10 to 15 ‰) in the west-central part. These results are in good agreement with those suggested by faunal composition and diversity. A similar but more limited study of the stratigraphically higher upper Mya zone indicates that the salinity during this time was reduced but did not vary in a systematic pattern as during Pecten zone time. Comparison of fossil and modern communities provides the means for detailed paleoenvironmental reconstruction by establishing analogous modern settings. Environmental interpretation, based on isotopic analysis, is similar to interpretations based on faunal analyses but, in addition, provides more quantitative estimates of the paleotemperature and salinity.

K—Ar isotopic age determinations from some Lake District mineral localities

SummaryForty-seven new K—Ar analyses are reported from 15 mines in the Lake District. Analyses of clay mineral concentrates from vein gouges and altered wallrock support a hypothesis of at least 3 distinct isotopic events. Caledonian ages (388 Ma) are reported from the Coniston Copper deposits, while the extensive mineralization in and around the Vale of Newlands indicates a metasomatic event occurring at around 360 Ma. The ages of the sporadic mineral localities to the E of the area (i.e. Greenside, Threlkeld, etc.) are grouped around 325 Ma while those of the Caldbeck Fell deposits are close to 230 Ma. An epidosic model may be advanced for the Carrock Fell Tungsten deposits with a range from 197 to 282 Ma. The geochronological significance of these and other previously published results is considered.

Relationships between transform directions and alkaline igneous rock lineaments in Africa and South America

Alkaline igneous complexes form distinct lineaments in Angola, South West Africa, Brazil, and Uruguay. Each of these lineaments lie along small circles centered on the Cretaceous and present poles of rotation for the South Atlantic and can be correlated with a unique fracture zone/transform fault that offsets the Mid-Atlantic ridge. It is suggested that the lineaments are continental expressions of these fracture zones. Isotopic age determinations from complexes in the lineaments lie in the range 120–135 my and 50–80 my and are consistent with the current model for the evolution of the South Atlantic and the origin of the lineaments as suggested here.

A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations

A simple procedure for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations has been developed and tested ( Krogh, 1971a,b). Samples are decomposed at 220°C with 48 per cent HF in a Teflon® capsule confined by a self-sealing stainless steel jacket. Uranium and lead are isolated on a Teflon® anion exchange column using Dowex 1 resin. Measured lead contamination levels range from 0.5 to 5.0 ng. In contrast, lead blanks for the borax fusion technique used in all previous zircon analyses are typically 0.2 to 1.0 μg. Eight small samples weighing 0.3 to 7 mg contained 30 to 260 ng of Pb 206. The average value for the total amount of common lead present in the lead isotopic composition analysis, contributed from both the sample and the chemical procedures, was 1.4 ng. The highest Pb 206 Pb 204 ratio measured to date (126,000) was obtained on a 0.1-g sample that contained 50 ppm of Pb 206. An exact determination of the amount of common lead in zircons is now possible. A maximum value of 0.3 ppm was found for fourteen nonmagnetic zircon fractions from granites and rhyolites. Higher values reported in the literature suggest that lead contamination levels are often underestimated in the analysis of zircons by the borax fusion technique. The silica-gel loading technique for lead provides stable emission for small samples as well as limited isotopic fractionation in the mass spectrometer. These features, combined with the low levels of lead contamination and the high precision of mass spectrometric analyses, make possible an average reproducibility (for duplicate decompositions of the same finely ground sample) of 0.3 m.y. for lead 207 206 ages of 2750 m.y. The new method requires fewer reagents and is much easier than the borax fusion technique.