Geochronological Problems Research Articles

New Late Pleistocene age for the Homo sapiens skeleton from Liujiang southern China

The emergence of Homo sapiens in Eastern Asia is a topic of significant research interest. However, well-preserved human fossils in secure, dateable contexts in this region are extremely rare, and often the subject of intense debate owing to stratigraphic and geochronological problems. Tongtianyan cave, in Liujiang District of Liuzhou City, southern China is one of the most important fossils finds of H. sapiens, though its age has been debated, with chronometric dates ranging from the late Middle Pleistocene to the early Late Pleistocene. Here we provide new age estimates and revised provenience information for the Liujiang human fossils, which represent one of the most complete fossil skeletons of H. sapiens in China. U-series dating on the human fossils and radiocarbon and optically stimulated luminescence dating on the fossil-bearing sediments provided ages ranging from ~33,000 to 23,000 years ago (ka). The revised age estimates correspond with the dates of other human fossils in northern China, at Tianyuan Cave (~40.8–38.1 ka) and Zhoukoudian Upper Cave (39.0–36.3 ka), indicating the geographically widespread presence of H. sapiens across Eastern Asia in the Late Pleistocene, which is significant for better understanding human dispersals and adaptations in the region.

INTERDISCIPLINARY SHARED RESEARCH FACILITIES "GEOANALYST" UB RAS: OPERATING EXPERIENCE AND PROSPECTS

The article presents general information about the interdisciplinary Shared Research Facilities "Geoanalyst" of the Ural Branch of the Russian Academy of Sciences, the main directions of its work, analytical equipment and types of work that allow obtaining quantitative information on the chemical (elemental and isotopic) and phase composition, parameters of the crystal and electronic structure, type and concentration of the lattice defects, optical properties of samples including minerals, rocks, bottom sediments, ores, products of their enrichment and processing, natural and drinking water, as well as synthetic compounds, techno- and biogenic objects. Consideration is being given to the techniques involved in the Shared Research Facilities such as electron probe microanalysis and electron microscopy; X-ray fluorescence, atomic emission and atomic absorption analysis; X-ray structural and thermal analysis; microelement mass spectrometric analysis; isotope mass spectrometric analysis and geochronology; physics and spectroscopy of minerals. Emphasis has been placed on the fundamental and applied sciences based on the Shared Research Facilities, its scientific and educational activities and the experience of working with users. Details have been provided for the formulation, testing and use of microanalytical methods for in situ study of the composition, structure and properties of minerals-concentrators of transition, rare-earth and radioactive d- and f-elements (zircon, titanite, etc.) with a spatial resolution from units to tens of micrometers. There have been analyzed the application and development of research methods; their metrological parameters; the procedure for selection and certification of internal laboratory reference samples; the possibilities for solving problems of fundamental and applied physics and chemistry of mineral matter, as well as geochronological problems.

U-Pb (ID-TIMS) geochronological method and “in-situ” SIMS zircon dating: possibilities and constrains

Research subject and methods . The work is based on a comparative study of the data obtained by the authors and those published in literature on U-Pb ID-TIMS and “in-situ” isotopic examination of zircons extracted from a number of Ural geological objects (volcanogenic rocks of the Mashak Formation of the Bashkir Meganticlinorium, granitoid massifs of the Northern part of the Uraltau Ridge, Kumba gabbro-norite massif Platinum-bearing belt), as well as Australian diorites and South African mafic xenoliths. Results. Since the methods of ID-TIMS (ID-MC-ICP/MS) and SIMS dating of zircon are fundamentally different, their advantages and disadvantages should be taken into account when solving the corresponding geochronological problems. Conclusions . U-Pb ID-TIMS (ID-MC-ICP/MS) methods allow the analysis of individual crystals, as well as their fragments, with an unprecedented level of uncertainty concerning age dating (up to 0.05%). The main drawback of the U-Pb SIMS method consists in a significant error when dating U-Pb (2–5%), which might disguise possible Pb losses (and / or U-contribution) and lead to appearance of artifacts when interpreting U-Pb zircon data. This, in turn, leads to the identification of non-existent stages of magmatism, metamorphism, false ideas about the length of evolutionary processes, etc. The use of U-Pb SIMS methods should be limited to solving geological problems that do not require a high accuracy of dating (i.e., allowing rough estimation) but involving the need to study a large number of samples and corresponding zircon grains. Such problems arise during examination of clastic material in the process of reconstructing sources and geological conditions of sedimentary basin formation. U-Pb SIMS methods can be used for studying the heterogeneity of zircon crystals having a complex structure under preliminary selection of materials for subsequent high-precision dating using U-Pb ID-TIMS and / or ID-MC-ICP/MS methods. The expansion of the number of laboratories using these high-precision methods seems to be the most important direction in the development of Russian isotope geochronology.

On the feasibility of determining the 230Th activity in minerals without the addition of a Th radiotracer

The necessity of measuring the radioactivity of the 230Тh nuclide arises when attempting to solve a number of geochemical or geochronological problems based on the effects of radioactive disequilibrium in the 238U series. It is the authors’ opinion that the use of the traditional α- and β-emitters of thorium as a radiotracer in order to determine the activity of 230Тh leads to various inconveniences and difficulties of a technical nature. The authors recommend using the 234Тh isotope contained in the study sample as the simplest and most natural way of measuring the chemical yield of the targeted nuclide (230Тh). A sequence of radio-analytical procedures is proposed that will provide a correct assessment of the radioactivity of 230Тh. In addition, a specific example and an evaluation of the accuracy of the proposed methodology are presented, using actual experimental data.

Development of CA-ID-TIMS zircon U–Pb dating technique at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

U–Pb geochronology using isotope dilution-thermal ionization mass spectrometry (ID-TIMS) is by far the most precise analytical technique. It affords the opportunity to determine 206Pb/238U dates with unprecedented precision often <1‰ on single analyses or weighted mean dates derived from multiple analyses.Pretreatment protocols for zircon analysis, and community-wide innovations spurred on by EARTHTIME tends to be at the leading edge of technological development. Also, more and more Chinese researchers perform lots of laboratory work in improving the technique in China. Here we present the running condition, procedural blanks and zircon standards measurement results in the clean laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). The results show the total procedural blanks (Pbc) are below ∼1 pg mostly, suitable for the sample pretreatment and operation. The 206Pb/238U dates of R33 and 500 Ma solution are 419.88 ± 0.94 Ma (n = 7; MSWD = 6.8) and 500.50 ± 0.19 Ma (n = 9; MSWD = 0.84), respectively. These results demonstrate that the CA-ID-TIMS procedure is well established at GIGCAS. Our efforts are beneficial to the development of EARTHTIME-CN program. In particular, when this technique is combined with the SIMS technique, it provides an efficient way to solve a number of geochronological problems encountered in China and worldwide.

Age and substance sources of the Ilmeno-Vishnevogorsky Alkaline Complex (South Urals): Rb-Sr, Sm-Nd, U-Pb, and Lu-Hf isotope data

) ofmiaskites, fenites, and carbonatites with REE–Zr–Nb mineralization located in the Uralian foldbelt.Although the IVC is well studied, a number of important genesis and geochronological problems are currently debatable. Thus, because of the wide dispersionof datings obtained by the different isotope–geochronological methods on various rocks and minerals, ofcritical concern are the problems of geochronology ofalkaline rocks and carbonatites of the IVC. The keyquestion of the IVC genesis is the role of mantle andcrustal sources for rock and ore formation. Theseproblems may be solved on the basis of the complexand systematic study of the Rb–Sr, Sm–Nd, U–Pb,and Lu–Hf isotope systems of rocks and minerals,which currently have become an important and indispensable instrument for reconstruction of the sourceof magmatic rocks and solution of the problems oftheir origin.We applied the isochron methods based on theSm–Nd and Rb–Sr isotope systems of bulk samplesand rockforming minerals of miaskites and carbonatites (TIMS method, Institute of Geology andGeochemistry, Yekaterinburg; Institute of Precambrian Geology and Geochronology, Saint Petersburg;Institute of Geology, Apatity), as well as the local U–Pb isotope dating of zircons from carbonatites andmiaskites from Vishnevogorsky and Buldym massifs(ion microprobe SHRIMPII, Russian GeologicalResearch Institute, Saint Petersburg; laser ablationLaICPMS, GEMOC, Sydney) for dating of themain rock types of the IVC. The U–Pb isotope systems of pyrochlores from the IVC (TIMS, AllRussianGeological Research Institute, SaintPetersburg) wereused for dating as well. The initial isotope characteristics of rocks and minerals of the IVC were obtained inorder to establish the sources of substances for magmatic rocks and ores and the role of mantle in theirformation.The Ilmeno–Vishnevogorsky Complex is locatedat the boundary between the South and Middle Uralsin the Sysertsko–Ilmenogorsky block of Precambrianrocks occurring in the Uralian paleooceanic complexes. The core of this block is composed of granuliticgneisses and migmatites of the Selyankinskaya Series,as well as plagiogneisses and amphibolites of the Vishnevogorskaya Series (

A method for determining argon isotopes in a continuous helium flow for K/Ar geochronology

A new method for the measurement of the argon isotope composition in a continuous helium flow using isotope mass spectrometers is described for potassium-argon geochronology. Argon was extracted from the samples in a chamber with a multiple-sample holder by successively heating the samples with a continuous infrared laser. Argon extracted in the chamber was preconcentrated in a capillary, separated on a chromatographic capillary column in a helium flow, and then injected into the ion source of a mass spectrometer through an open splitter. Measurements of the 36Ar, 38Ar, and 40Ar isotopes were carried out on isotope mass spectrometers in a dynamic mode using a triple-collector ion detector and three electrometric amplifiers. Preliminary experiments on the developed device for measuring argon isotopes in a continuous helium flow have shown that the proposed method can be used for the determination of radiogenic argon in an amount of n × 10−12 g with the accuracy satisfying the solution of many geochronological problems. In sensitivity and reproducibility, the proposed method is highly competitive to the isotope dilution method for measuring argon and simpler than the conventional methods for measuring radiogenic argon.

A continuous flow mass spectrometry technique of argon isotope measurement for K/Ar geochronology

A new method for the measurement of argon isotope composition in a continuous flow of helium for potassium/argon geochronology is described. Extraction of argon from geological samples in multiple-sample holders was carried out in a chamber by heating with a continuous Nd-YAG laser. The extracted and pre-concentrated argon is passed through a chromatographic capillary column in a flow of helium. Argon is separated from possible contaminants in the column and is injected through an open split into the ion source of an isotope ratio mass spectrometer. Measurement of the (36)Ar, (38)Ar and (40)Ar isotopes was carried out in dynamic mode, using a triple-collector ion detector. These experiments have shown that continuous flow mass spectrometry can be used for the analysis of radiogenic argon in picogram quantities with an accuracy that is satisfactory for the solution of many geochronological problems. The method of argon isotope measurement in a continuous flow of helium is an alternative to the measurement of argon isotopes in the static mode. The sensitivity and accuracy of argon measurement by this method are comparable with those provided by the classical static method. The measurement of argon isotopes in a continuous flow of helium is simpler and more reliable than measurement in the static mode.

Geochronological problems related to polymetamorphism in the Limpopo Complex, South Africa

The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature ( P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D 3/M 3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D 2/M 2 event, characterized by SW-plunging sheath folds. These major D 2/M 2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D 3/M 3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating ( P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.

Exploring the potential of allanite as a geochronometer of high-grade crustal processes

The REE-rich accessory mineral allanite plays a key role in the storage and mobility of geochemically important trace elements (LREE, Th) in magmatic and high-grade rocks. Allanite occurs in a wide range of rock types, but of particular interest is its common presence in mafic, migmatitic and high-pressure rocks. We report on the response of allanite trace element chemistry and its U-Th-Pb isotopic system to magmatism, partial melting and eclogite-facies metamorphism. In situ U, Th–Pb geochronology of allanite has been carried out using SHRIMP ion microprobe in conjunction with LA ICP-MS analysis. We analyse allanite with a variety of FeO and trace element compositions from anatectic rocks, granodioritic to tonalitic plutons and high-pressure mafic rocks. Allanite Th– Pb ages are capable of reliably addressing geochronological problems with a precision of 1–2% and 2–5% (2r) for magmatic and metamorphic rocks, respectively. This study allows preliminary conclusions to be made for the closure temperature of allanite. In the case studies presented, no indication of inheritance has been observed for allanite. The trace element composition of allanite records changes in paragenesis. Allanite HREE content relative to LREE provides an indicator for the co-crystallization of garnet. Where allanite formed in migmatites display a small negative Eu anomaly, eclogitic allanite lacks a Eu anomaly, which is related to crystallization in the absence of plagioclase. In addition, the Sr content in allanite can be used as an indicator of crystallization above the stability field of plagioclase. This study demonstrates that allanite can be correlated to major metamorphic and rock-forming minerals and therefore the P-T conditions of crystallization.

Sr isotope systematics of K-feldspars in plutonic rocks revealed by the Rb–Sr microdrilling technique

87Sr / 86Sr isotope variation was studied in subareas of K-feldspar megacrysts from Late-Palaeozoic granodiorite, diorite and syenite of the Bavarian Forest. Most samples were collected close to the Bavarian Pfahl shear zone and feldspars from these sites reveal remarkably constant initial 87Sr / 86Sr isotope ratios. These ratios are similar to those of the bulk groundmass or feldspar crystals from the groundmass. Individual feldspar grains however, define isochron ages that are considerably younger (by ∼15 to ∼30 Ma) than the crystallisation age of the host rocks as defined by U–Pb and Pb–Pb zircon geochronology. The Rb–Sr feldspar data probably reflect diffusion controlled isotopic homogenisation under hydrous conditions between 290–310 Ma. A pink feldspar crystal from a hydrothermally altered syenite reveals comparatively large degree of isotopic variation, indicating local disturbance of the Rb–Sr system caused by hydrothermal alteration during Permian–Triassic Pfahl quartz precipitation. A diorite from the Fürstenstein pluton displays some isotopic scatter and slight differences in the initial 87Sr / 86Sr ratios between the more radiogenic feldspar–megacryst and the less radiogenic bulk groundmass. This rock type was subsequently intruded by the Saldenburg granite and the more radiogenic 87Sr / 86Sr isotope signature of the feldspar relative to the groundmass is interpreted as being consistent with the feldspar having exchanged with the fluids derived from the granite. This study shows that K-feldspar crystals from slowly cooled plutonic rocks do not necessarily preserve their original Sr isotope composition. However, Sr isotope traverses through single crystals can give robust records of the mm-scale isotope system behaviour also in older plutonic rocks relevant for solving petrological or geochronological problems.

High-pressure/ultrahigh-pressure eclogites from the Hong’an Block, East-Central China: geochemical characterization, isotope disequilibrium and geochronological controversy

The Hong’an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist–greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The available age data are inconclusive that hinder our understanding of the tectonic evolution of the Block. The metamorphic temperatures in the Hong’an Block (Tmeta ≈ 700 to 500°C) are lower by 50–150°C than that of the Dabie and Sulu terranes. In this work, we undertook new trace element and Sr–Nd–O isotopic analyses on minerals in order to gain more insight into the geochronological problems. The results are as follows: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g., epidote) in garnet and omphacite has contributed to isotope disequilibrium. (2) Sm–Nd isotope analyses yielded no isochron ages for the Hong’an eclogites. (3) Rb–Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established, yielding ages from 210 Ma to 225 Ma. The pattern of Rb–Sr isotope disequilibrium appears to be independent of the petrological and O-isotope temperatures. (4) In contrast to the unequilibrated Sm–Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature due to mass balance constraints. (5) Whole-rock δ18O values show a large variation from +10‰ to −8‰, suggesting that their protoliths have undergone very different processes of water–rock interaction. In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong’an Block took place in the Triassic at about 220–230 Ma, as observed in the Dabie and Sulu terranes. The significance of published Paleozoic dates (450–300 Ma) for the Xiongdian eclogite is not clear. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data.

Ion microprobe U–Pb dating of hydrothermal xenotime from an episyenite: evidence for rift-related reactivation

Hydrothermally intergrown xenotime–monazite from an episyenite in the Bohus granite, southwest Sweden, has been analyzed in situ by ion microprobe. Although the analysed xenotime crystals are extremely small, sometimes less than the ion beam diameter (∼17 μm), they retain relatively undisturbed U–Pb systems, which yielded a 206Pb/ 238U age of 252±8 Ma. The evolution of this episyenite involved an initial stage of quartz dissolution accompanied by albitization, followed by a second stage of vug-infilling and replacement of igneous Fe–Ti phases. The obtained age shows that the second alteration stage, during which xenotime was precipitated, was coeval with the thermal anomaly associated with Permo-Carboniferous crustal thinning and the development of the nearby Oslo Rift. Moreover, the development of the permeable episyenite structure must be ascribed to a preceding high-temperature event, most likely the post-magmatic cooling of the host granite at 920 Ma. This study shows that U–Pb dating of young xenotimes may address geochronological problems, though further work is required to characterize possible matrix effects which presently limit the accuracy of the method.

Introduction

This issue contains a group of papers selected from those presented at the 1st workshop of the SCAR-GLOCHANT and IGBP-PAGES cosponsored programme on the Late Quaternary Sedimentary Record of the Antarctic Ice Margin Evolution (ANTIME), held in Hobart, 6-1 1 July 1997. ANTIME is focused on the circurn Antarctic reconstruction of palaeoclimate, palaeoenvironment, and ice sheet palaeogeography throughout the last glacial cycle. There were 65 participants from Australia, USA, UK, Italy, Spain, Japan, Sweden, Germany and Russia at the workshop. The participants included representatives of PAGES, IMAGES, and INQUA. The workshop included three scientific sessions on: Extent, timing and regional differences during Glacial Stage 2 (10–30 kyr BP) in Antarctica, from the terrestrial and marine records;Climatic, environmental and glacial events during the Holocene;Late Quaternary geochronological problems in Antarctica.

Revised reconstruction of Middle and Late Pleistocene sea-level changes based on new chronologic and morphologic investigations in Barbados, West Indies: Radtke, Ulrich and Rainer Grün, 1990. J. coast. Res., 6(3):699–708. Geograph. Inst., Heinrich Heine Univ. Dusseldorf, Universitatsstr. 1, D-4000 Dusseldorf 1, FRG

In an application of the relatively new technique of ESR (Electron Spin Resonance) dating, an extensive chronological and morphometric study has been carried out on coral reef tracts in Barbados. These sequences have been used in previous studies (Bender et al ., 1979) to reconstruct paleo sea-level elevations for the middle and upper Pleistocene, concluding in the so called Barbados-Model. The new data set implies that this model is subject to a far larger uncertainty than previously claimed due to geochronological problems and local variation in the uplift rate.

Diffusion in silicate minerals and glasses: A data digest and guide to the literature

The rates of diffusion of cations, oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems. In order that the Earth Scientist may readily apply the available experimentally-determined diffusion rates to such problems, a compilation of data for diffusion in silicate minerals and glasses is presented along with a “working guide” for its use. Published data and experimental conditions are listed in tables, and these are preceded by an outline of diffusion mechanisms, terminology, equations and units, diffusion regimes, factors controlling diffusion rates, measurement techniques, and diffusional behaviour.

Geochronological problems and radioisotopic dating in the Gregory Rift Valley

Geochronology is the science of geological time and has a wider connotation than radioisotopic dating. The two major tools of geochronology: stratigraphical palaeontology and radioisotopic dating are equally fallible and are best used in conjunction. Basic large-scale geological mapping and the erection of local rock-stratigraphies are necessary pre-requisites for the detailed geochronological analysis of an area. In the Gregory Rift Valley these requirements are beginning to be met at such places as Olduvai, Baringo, East Lake Turkana and Omo. Conventional K-Ar age determination of volcanic rocks and minerals has been used widely in the dating of East African Cenozoic successions. It is important that the theoretical and practical limitations of this otherwise very successful technique are fully appreciated. K-Ar and 40 Ar 39 Ar isochron dating are developments of the K-Ar technique that can be used to resolve some of the difficulties inherent in the conventional method. Isochron dating of rocks from Olduvai, the plateau phonolites of Kenya, East Lake Turkana and the Omo illustrates this point. Arguments are presented for rejecting discrepant apparent ages of less than 2.0 m.y. from the KBS Tuff, East Lake Turkana. It is suggested that fission track dating of volcanic zircons will resolve this controversy and provide independent time-scales for many East African and other volcanic successions in future.

Electromagnetic effects in the range of separation of chemically combined water from the crystal lattice of muscovite

In order to discover ways of improving the insulating properties of dielectrics and new methods of solving geochronological problems, the electrical and electromagnetic phenomena arising in muscovite samples as a result of the separation of chemically combined water are analyzed. Anomalous changes in electrical conductivity and the intensity of electromagnetic pulses in the radiofrequency range suggest the separation of a quartz phase in muscovite subjected to the action of thermal and electric fields.

General compilation of isotopic work on rocks from Greenland

1972 has seen a further development of the trend noted in Report of Activities, 1971 towards detailed isotopic studies of particular petrological and geochronological problems rather than the regional programmes of K/Ar mineral age determinations carried out previously. As the main picture of Greenland geochronology emerges so the problems to be solved become more complex and require close cooperation and mutual understanding between field and the laboratory investigators. In many studies the limits of the currently available field and laboratory techniques have been reached and in several cases now under investigation interpretation of the results involves a much more exact knowledge of the conditions under which the various rock and mineral systems become closed to loss of radiogenic daughter elements.