南海珊瑚礁铀系年龄及其地质意义

珊瑚礁是铀系方法测定年龄相当理想的对象,在国外已获得十分有价值的成果。主要用于讨论更新世以来海平面变化和构造运动上升速率,研究反映环境的珊瑚生长速度。本文珊瑚礁样品选取于海南岛鹿回头、雷州半岛、西沙和中沙群岛等地。成功地测试了铀系年龄,揭示出与¹⁴C 数据的不一致,初步讨论了构造上升速率和南海海面的变化。     

Dating fluvial terraces by Th/U on pedogenic carbonate, Wind River Basin, Wyoming

Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with ²³⁰Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm ²³²Th, with median (²³⁰Th/²³²Th) = 270, making them well suited for ²³⁰Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥10⁵ yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite ²³⁰Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum.     

洞穴地点骨化石铀系年龄可信度的讨论*

骨化石是铀不平衡系测年法广泛应用而又颇有争议的研究对象。文章通过对典型铀加入模式的计算,指出以α能谱的精度,二种铀系年龄差异的显著性表明近期内有较大量铀的迁移,但其在误差范围内的一致不能保证样品构成封闭体系。以往积累的数据表明,大多数洞穴地点骨化石给出了二法一致的铀系年龄,但钙板与下伏骨化石铀系年龄大多差异显著且与地层顺序矛盾,即使被次生碳酸盐岩包裹,多半骨化石的铀系年龄仍显著偏低于其包裹体。此外,相当一部分样品的U-Th同位素比难以用简单的铀后期加入或淋失来解释。基于对次生碳酸盐岩铀系年龄可信度的认识,骨化石总体上不构成封闭体系,所载铀系年代信息只具有限的分辨率。     

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.     

Precise radiocarbon dating of Late-Glacial cooling in mid-latitude South America

Variability of atmospheric ¹⁴C content often complicates radiocarbon-based chronologies; however, specific features such as periods of constant ¹⁴C age or steep changes in radiocarbon ages can be useful stratigraphic markers. The Younger Dryas event in the Northern Hemisphere is one of those periods, showing conspicuous ¹⁴C wiggles. Although the origin of those variations is not fully understood, we can make practical use of them and determine: (i) whether the Younger Dryas was global in extent; if so, (ii) were the initial cooling and the final warming synchronous worldwide; and (iii) what are the implications of these similarities/differences? Here we report high-resolution AMS ¹⁴C chronologies from the mid-latitudes of South America that pinpoint a cool episode between 11,400 and 10,200¹⁴C yr B.P. The onset of the final cool episode of the Late Glacial in the southern mid-latitudes, i.e., the Huelmo/Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by ∼550 calendar years. Both events ended during a radiocarbon-age plateau at ∼10,200¹⁴C yr B.P. Thus, the Huelmo/Mascardi Cold Reversal encompasses the Younger Dryas, as well as a couple of short-term cool/warm oscillations that immediately preceded its onset in the North Atlantic region.     

Suitability of biogenic carbonate of Lithospermum fruits for C dating

Lithospermum (Boraginaceae) belongs to a small group of plant taxa that accumulate biogenic carbonate in their fruits. In this genus, carbonate incrustations form in the cells of the epidermis and sclerenchyma of the pericarp. Fossil Lithospermum fruits (nutlets) with well-preserved calcified tissues commonly occur in Quaternary sediments and cultural layers. We tested the suitability of biogenic carbonate of Lithospermum fruits for radiocarbon dating using a total of 15 AMS measurement results from four modern and 11 fossil samples. The ¹⁴C data from modern samples suggest that Lithospermum utilises only atmospheric carbon to synthesise calcite in the nutlets. In general, the ages determined through ¹⁴C dating of fossil fruitscorresponded well with the absolute-age intervals for archaeological sites over the last 5000 yr. Biogenic carbonate of Lithospermum fruits, like that of Celtis, represents a new source of chronological information for late Quaternary studies.     

Late Quaternary speleothem pollen in the British Isles

As with many terrestrial areas, the British Quaternary sequence is characterised by incomplete, fragmentary records, whose correlation is based on stratigraphic or biostratigraphic techniques due to the lack of radiometric ages beyond the ～40 kyr limit of ¹⁴C dating. Speleothems (secondary cave calcite deposits) offer a significant advantage over many sources of palaeoenvironmental information; they can be dated to a high precision and accuracy by uranium‐thorium (²³⁸U‐²³⁰Th) thermal ionisation and inductively coupled plasma (ICP) mass spectrometry in the time period back to 500 kyr. They may also contain sufficient well‐preserved pollen representative of contemporary vegetation above the cave to allow palaeoenvironmental reconstruction. This study adopts the novel approach of combining pollen and thermal ionisation mass spectrometric (TIMS) U‐Th dating of British speleothems to produce well‐constrained palaeoenvironmental records. We report for the first time precisely dated records of pollen assemblages from speleothems suggesting the presence of thermophilous arboreal species in phases previously considered to have been consistently cool or cold and devoid of trees. Copyright © 2007 John Wiley & Sons, Ltd.     

C chronology for ice retreat and inception of Champlain Sea in the St. Lawrence Lowlands, Canada

AMS radiocarbon cross-dating of plant debris and marine shells trapped in a lake basin on Mount St. Hilaire (Québec, Canada) provides a direct assessment of a reservoir effect totaling ca. 1800 ¹⁴C years during the early stage of Champlain Sea. Pollen-based extrapolation of bottommost ages on terrestrial plant macrofossils in sediments of this lake, and of another lake nearby support an estimate of 11,100 ± 100 ¹⁴C yr B.P. for marine invasion in the Central St. Lawrence River Lowlands. Results indicate a 400–1000 years younger regional chronology of ice retreat, now congruent with the one inferred from the New England varve chronology. This is a summary of a longer paper to be published in French.     

40Ar/39Ar laser probe evidence concerning the age and associated hazards of the Lake Nyos Maar,Cameroon

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent ⁴⁰Ar/³⁹Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic ⁴⁰Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a ¹⁴C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria.     

Early diagenesis impact on precise U-series dating of deep-sea corals: Example of a 100-200-year old Lophelia pertusa sample from the northeast Atlantic

Deep-sea coral has proved useful for paleoceanographic reconstructions and for documenting ¹⁴C-ages of water masses using ²³⁰Th-ages. However, for precise and accurate U-series dating, further information on coral-age structure, growth rate and diagenetic evolution is still needed. To document such processes, we used U-Th-²²⁶Ra systematics in a 40 cm diameter, Lophelia pertusa specimen collected in 1912 from the Eastern Atlantic (Sea of the Hebrides). External parts of the specimen are thought to have been alive when collected whereas more internal parts were likely dead. The “live-collected” and “dead” parts of the skeleton were sampled and analyzed for their ²³⁰Th, ²³²Th, ²³⁴U, ²³⁸U, ²²⁶Ra and Ba contents by thermal ionization and multicollector inductively coupled plasma mass spectrometry. ²³⁰Th/²³⁴U ratios in the most recent parts yielded ages of 86 ± 6 a and 92 ± 9 a, in agreement with the date of recovery. The older parts yielded ²³⁰Th ages ranging from 169 ± 15 to 211 ± 10 a (n = 5), but had a 31% higher U content than more recent parts of the skeleton. This raises concerns about the possibility of secondary diagenetic U uptake, although an environmentally controlled U/Ca shift between coral growth stages cannot be ruled out. ²²⁶Ra/Ba measurements were made, and model- ²²⁶Ra/Ba ages averaging 250 ± 12 yr were calculated for the older part, assuming a constant initial ²²⁶Ra/Ba ratio in bottom waters. These ages are slightly older than ²³⁰Th-ages, suggesting either that ²²⁶Ra/Ba ratios of ambient-seawater changed over time or that a diagenetic phenomenon have affected the U-series system, or both. Scanning electron microscope observations revealed bioerosion and secondary biomineralization in the older part of the coral skeleton, supporting the hypothesis that diagenetic processes may have influenced the ages obtained by the U-series toolbox. Modeled U-series ages for such an open system are discussed. However, a comparison between ¹⁴C- and ²³⁰Th-ages performed on both pristine and bioeroded parts of the coral gives coherent values (ca 450 a) for the preindustrial ¹⁴C-reservoir age of North Atlantic waters. It remains to be determined, however, whether diagenesis occurs rapidly over a short period of time, or whether it continues for longer periods. In the latter case, diagenetic processes would hamper paleoceanographic interpretations as well as the precise calculation of ¹⁴C ages of deep-water masses, based on comparative U/Th- and ¹⁴C-chronologies.     

ESR and Th/U dating of speleothems from Aladağlar Mountain Range (AMR) in Turkey

Electron spin resonance (ESR) and ²³⁰Th/²³⁴U ages of speleothem samples collected from karstic caves located around 3000 m elevation in the Alada?lar Mountain Range (AMR), south-central Turkey, were determined in order to provide new insight and information regarding late Pleistocene climate. ESR ages were validated with the ²³⁰Th/²³⁴U ages of test samples. The ESR ages of 21 different layers of six speleothem samples were found to range mostly between about 59 and 4 ka, which cover the Marine Oxygen Isotope Stages (MIS) MIS 3 to MIS 1. Among all, only six layers appear to have deposited during MIS 8 and 5. Most of the samples dated were deposited during the late glacial stage (MIS 2). It appears that a cooler climate with a perennial and steady recharge was more conducive to speleothem development rather than a warmer climate with seasonal recharge in the AMR during the late Quaternary. This argument supports previous findings that suggest a two -fold increase in last glacial maximum mean precipitation in Turkey with respect to the present value.     

Development of high precision Rb-Sr phlogopite and biotite geochronology; an alternative to Ar/Ar tri-octahedral mica dating

Potassium-Ar and Rb-Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K-Ar and, in particular, ⁴⁰Ar/³⁹Ar dating of micas is still widely applied, Rb-Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb-Sr ages than the K-Ar or ⁴⁰Ar/³⁹Ar system. Moreover, a reduction of uncertainties typically reported for Rb-Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb-Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% (n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield ⁴⁰Ar/³⁹Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb-Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb-Sr and ⁴⁰Ar/³⁹Ar ages demonstrates the robustness of the Rb-Sr system in tri-octahedral micas and cautions against the sole use of ⁴⁰Ar/³⁹Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb-Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb-Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma.Combining the mica Rb-Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb-Sr system in 1-2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.     

Th/U/U and Pa/U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

The ²³⁰Th/²³⁴U/²³⁸U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system ²³⁰Th/²³⁴U/²³⁸U ages is elevated ²³⁴U/²³⁸U_initial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have ²³⁴U/²³⁸U_initial values close to seawater, there is a need for age validation. Redundant ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in fragments as small as 500 mg. We have obtained excellent agreement between ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the ²³⁰Th/²³⁴U/²³⁸U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby ²³⁸U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for ²³¹Pa and from undetermined sources currently limit the ²³¹Pa/²³⁵U age uncertainty to about ±2.5%. U isotope data and ²³⁰Th/²³⁴U/²³⁸U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.     

Uranium-Series Dating of Carbonate (Tufa) Deposits Associated with Quaternary Fluctuations of Pyramid Lake, Nevada

Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by²³⁰Th/²³⁴U ages of the cave samples,²³⁰Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous¹⁴C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported¹⁴C ages.     

Age of Saurashtra miliolites by U-Th decay series methods: possible implications to their origin

The miliolite deposits of Saurashtra have been dated by²³⁴U,²³⁰Th,²³¹Pa and¹⁴C methods. Concordant ages of 10⁵ years using the U decay series isotopes are obtained which agree with the ages of the coral reefs of Okha-Dwaraka coast suggesting a contemporaneous origin for both. The lower¹⁴C ages (≤40,000 years) may be due to a recent influx of seawater or ground water. Quartz and clay minerals together constitute only ≤10% by weight, as such the aeolian characteristics of quartz grains may not be relevant to the origin of the miliolites.     

The use of ‘bomb spike’ calibration and high-precision AMS C analyses to date salt-marsh sediments deposited during the past three centuries

A combination of ‘bomb spike’ calibration and conventional calibration of AMS ¹⁴C dating has been used to determine a detailed age-depth model for a 1-m sediment section collected from a salt marsh in Poole Harbour, southern England. These data were compared with the chronology obtained from ²¹⁰Pb analysis and ¹³⁷Cs age markers. We report post bomb values of over 1.46 F¹⁴C (> 146% modern ¹⁴C), and both the rising and falling limbs of the atmospheric ‘bomb spike’ are identified. Five pre-bomb samples were analysed using multi-target high-precision 2‰ AMS analysis, and after the replicates were combined the one-sigma uncertainty was as low as ± 9 ¹⁴C yr on some ages. These data, and an additional three normal-precision pre-bomb ¹⁴C samples, were calibrated using CALIB 5.0 and the chronology constrained using the ‘prior knowledge’ of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle (SCPs). No agreement was found between the ¹⁴C ‘bomb spike’ dates and the CRS ²¹⁰Pb chronology modelled for this sequence. In addition, poor agreement was found between the signal of the 1960s weapons test fallout indicated by the ¹⁴C ‘bomb spike’ dates and the timing suggested by the ¹³⁷Cs data. This disagreement is attributed to the influence of the local discharge of ¹³⁷Cs from the former UKAEA site at Winfrith. We use our new chronology to confirm the existence of an acceleration in sedimentation rates in Poole Harbour during the last 100 yr previously reported for this site by Long et al. (Long, A.J., Scaife, R.G., Edwards, R.J. 1999. Pine Pollen in intertidal sediments from Poole Harbour, UK; implications for late-Holocene sediment accretion rates and sea-level rise. Quaternary International, 55, 3–16.), and conclude that ‘bomb spike’ ¹⁴C calibration dating may offer a more robust alternative to the use of ²¹⁰Pb chronologies for dating sediment deposition in salt-marsh environments. In addition, we demonstrate how the use of high-precision AMS analysis has the potential for reducing some of the uncertainties involved in the high-resolution dating of recent salt-marsh sediments.     

An assessment and comparison of 230Th and AMS 14C ages for lacustrine sediments from Qarhan Salt Lake area in arid western China

The age framework of Qarhan Salt Lake in arid western China is still controversial due in part to (1) age discrepancy between conventional ¹⁴C and ²³⁰Th dating results, and (2) no AMS ¹⁴C ages of organic carbon from drilling cores in Qarhan Salt Lake were reported until now. In order to discuss these chronological problems, upper 54.50 m lacustrine sediments from a drilling core (ISL1A) recovered from Qarhan Salt Lake were dated based on ²³⁰Th and AMS ¹⁴C dating techniques. Results show that (1) AMS ¹⁴C ages of total organic carbon (TOC) from 4.65 to 30.29 m are almost in stratigraphic order and consistent with ²³⁰Th ages of halite in the corresponding layers; (2) AMS ¹⁴C ages of TOC from 30.29 to 54.50 m are younger with increasing depth. This phenomenon was also found in Shell Bar in the study area, suggesting that AMS ¹⁴C ages from upper 30.29 m are more reliable while those from lower 24.21 m in ISL1A may be underestimated; (3) ²³⁰Th ages of halite from lower 24 m lacustrine sediments are obviously older than AMS ¹⁴C ages of TOC in the corresponding layers, which results into different age framework of salt lake sediments in Qarhan Salt Lake; (4) if extrapolating these reliable AMS ¹⁴C ages in ISL1A, similar age framework with ²³⁰Th ages in this core confirms that ²³⁰Th ages are much close to the true ages of these sediments, which suggests that the forming timing of the bottom salt layer is ~50 ka.     

Age of the Rockland tephra, western USA

The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by ⁴⁰Ar/³⁹Ar and U-Pb dating. ⁴⁰Ar/³⁹Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has ⁴⁰Ar/³⁹Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.     

U-Pb ages of secondary silica at Yucca Mountain,Nevada: implications for the paleohydrology of the unsaturated zone

Uranium, Th and Pb isotopes were analyzed in layers of opal and chalcedony from individual mm- to cm-thick calcite and silica coatings at Yucca Mountain, Nevada, USA, a site that is being evaluated for a potential high-level nuclear waste repository. These calcite and silica coatings on fractures and in lithophysal cavities in Miocene-age tuffs in the unsaturated zone (UZ) precipitated from descending water and record a long history of percolation through the UZ. Opal and chalcedony have high concentrations of U (10 to 780 ppm) and low concentrations of common Pb as indicated by large values of ²⁰⁶Pb/²⁰⁴Pb (up to 53,806), thus making them suitable for U-Pb age determinations. Interpretations of U-Pb isotope systems in opal samples at Yucca Mountain are complicated by the incorporation of excess ²³⁴U at the time of mineral formation, resulting in reverse discordance of U-Pb ages. However, the ²⁰⁷Pb/²³⁵U ages are much less affected by deviation from initial secular equilibrium and provide reliable ages of most silica deposits between 0.6 and 9.8 Ma. For chalcedony subsamples showing normal age discordance, these ages may represent minimum times of deposition. Typically, ²⁰⁷Pb/²³⁵U ages are consistent with the microstratigraphy in the mineral coating samples, such that the youngest ages are for subsamples from outer layers, intermediate ages are from inner layers, and oldest ages are from innermost layers. ²³⁴U and ²³⁰Th in most silica layers deeper in the coatings are in secular equilibrium with ²³⁸U, which is consistent with their old age and closed system behavior during the past ∼0.5 Ma. The ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from lithophysal cavities in the welded part of the Topopah Spring Tuff yield slow long-term average growth rates of 1 to 5 mm/Ma. These data imply that the deeper parts of the UZ at Yucca Mountain maintained long-term hydrologic stability over the past 10 Ma. despite significant climate variations. U-Pb ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from fractures in the shallower part of the UZ (welded part of the overlying Tiva Canyon Tuff) indicate larger long-term average growth rates up to 23 mm/Ma and an absence of recently deposited materials (ages of outermost layers are 3–5 Ma.). These differences between the characteristics of the coatings for samples from the shallower and deeper parts of the UZ may indicate that the nonwelded tuffs (PTn), located between the welded parts of the Tiva Canyon and Topopah Spring Tuffs, play an important role in moderating UZ flow.     

A comparative U–Th–Pb (zircon–monazite) and 40Ar–39Ar (muscovite–biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen

Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U–Th–Pb system in zircon and monazite and of the ⁴⁰Ar–³⁹Ar system in micas during ductile deformation. Meso‐ and micro‐structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top‐to‐the‐SW and ‐NE, respectively) and developed under upper greenschist facies conditions. In situ U–Pb dating by laser‐ablation inductively coupled plasma‐mass spectrometry of zircon areas with well‐preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross–Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U–Th–Pb ages of c. 440 Ma, in agreement with in‐situ Ar laserprobe ages of syn‐shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal‐chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U–Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ‘ghost’ structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U–Pb ages of c. 450 and 410 Ma. The c. 450‐Ma date matches within errors most monazite U–Th–Pb ages and in‐situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician–Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear‐zone system formed in response to ～NE–SW‐directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician–Late Devonian Lachlan Fold Belt.