Diffusion artifacts in dating by stepwise thermal release of rare gases     

J.C. Huneke 《Earth and Planetary Science Letters》1976,28(3):407-417

Age plateaux and isochrons in the ⁴⁰Ar-³⁹Ar and similar dating techniques can be severely altered by processes changing the geometric distribution of one isotope relative to the other. Age plateaux and isochrons can even be generated entirely as experimental artifacts. Alterations of ⁴⁰Ar-³⁹Ar plateau ages by recoil redistribution of ³⁹Ar, incorporation of trapped ⁴⁰Ar and prior ⁴⁰Ar loss provide significant examples.10% shifts in isotopic ratios are very easily obtained and would result in errors in ⁴⁰Ar-³⁹Ar plateau ages of 4 AE old samples of ～100 m.y., which is comparable to the age differences which must be resolved to develop early lunar and solar system chronology. The possible occurrence of diffusion artifacts must be evaluated in every case to establish that ages and age differences obtained by stepwise thermal release analyses are real.All studies involving the stepwise thermal extraction of multiple isotopic components may show similar diffusion artifacts. Constant isotopic compositions may be obtained during thermal release which do not represent the actual compositions of sample reservoirs.  相似文献   

³⁹Ar recoil losses and presolar ages in Allende inclusions     

I.M. Villa  J.C. Huneke  G.J. Wasserburg 《Earth and Planetary Science Letters》1983,63(1):1-12

Ar analyses are reported for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in individual evacuated ampoules, and the Ar gas in the ampoules as well as the samples was analyzed. A large fraction (up to 60%) of the³⁹Ar from³⁹K (n, p) reactions was lost out of the inclusions into the ampoules. The³⁹Ar losses resulted in substantial increases in the apparent⁴⁰Ar-³⁹Ar ages of the samples.³⁹Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in⁴⁰Ar-³⁹Ar dating. All of the Allende inclusions studied contained substantial trapped³⁶Ar. The origin of the trapped³⁶Ar is unknown, and the possible presence of trapped⁴⁰Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of³⁹Ar recoil loss and possible contributions of trapped⁴⁰Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages.  相似文献   

Geochronological study of the Monte Amiata Lavas (Central Italy)     

G. Bigazzi  O. Giuliani  F. P. Bonadonna  C. Ghezzo  F. Rita  F. Radicati  Di Brozolo 《Bulletin of Volcanology》1981,44(3):455-465

K/Ar and fission track age measurements were performed on lava samples of Monte Amiata volcano (Central Italy) which is formed by a sequence of lava flows and lava domes ranging in composition from trachybasalts to high-SiO₂ quartzlatites. The ages obtained, except for the oldest volcanic products, range from 290,000 to 180,000 years. Excess⁴⁰Ar was found in some mineral separates, especially biotite, causing apparent rather old ages. The correct ages were calculated by means of K/Ar isochrons, and were found to be very similar to the ages calculated on sanidine separates. The volcanic activity that built up the outcropping Monte Amiata units seems to have lasted a relatively short time.  相似文献   

The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: Time for a change?   总被引：1，自引：0，他引：1  

Paul R. Renne  William S. Cassata  Leah E. Morgan 《Quaternary Geochronology》2009,4(4):288-298

A redetermination of the isotopic composition of atmospheric argon by Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S. [2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochimica et Cosmochimica Acta 70, 4507–4512] represents the first refinement since the work of Nier [1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Physical Reviews 77, 789–793]. The new ⁴⁰Ar:³⁸Ar:³⁶Ar proportions imply <1% adjustments to ⁴⁰Ar/³⁹Ar ages in all but exceptional cases of very young and/or K-poor and/or Ca-rich samples, or cases in which samples are grossly under- or over-irradiated. Analytical protocols employing atmospheric argon to determine mass discrimination corrections are insensitive to the effects of revision on the air correction, but are subject to non-negligible adjustments arising from expanded heavy to light isotope ratios attending the increased mass discrimination correction. The competing effects of increased ⁴⁰Ar/³⁹Ar and ⁴⁰Ar/³⁷Ar ratios render the adjustments a function of sample chemistry and neutron irradiation parameters. The improved precision of atmospheric ⁴⁰Ar/³⁶Ar and ³⁸Ar/³⁶Ar permits increasingly sensitive detection of departures from atmospheric values. Non-atmospheric initial ⁴⁰Ar/³⁶Ar values are increasingly well-documented in volcanic materials, including subatmospheric values correlated with ³⁸Ar/³⁶Ar in a trend consistent with kinetic mass fractionation whereby incomplete equilibration between magma and atmosphere favors light isotope enrichment in the magma. The detailed mechanism(s) of such fractionation are unclear and must be clarified by further study. A detectable increase in atmospheric ⁴⁰Ar/³⁶Ar in the past 800 ka [Bender, M.L., Barnett, B., Dreyfus, G., Jouzel, J., Porcelli, D., 2008. The contemporary degassing rate of ⁴⁰Ar from the Earth. Proceedings of the National Academy of Sciences 105, 8232–8237] suggests that ages of late Quaternary (e.g., <100 ka) materials incorporating large amounts of atmospheric argon such as biotite may be underestimated by as much as 100% if a modern atmospheric ⁴⁰Ar/³⁶Ar value is erroneously assumed, unless air argon is used to determine mass discrimination. Further evaluation of the evolution of paleoatmospheric ⁴⁰Ar/³⁶Ar, and the fidelity with which argon trapped in igneous materials reflects this, would be very productive. The use of isochrons rather than model (e.g., plateau) ages mitigates the vagaries associated with uncertain trapped argon isotope ratios, and the importance of strategies to derive statistically valid isochrons is underscored.  相似文献   

Reconciling discrepant chronologies for the geomagnetic excursion in the Mono Basin,California: Insights from new 40Ar/39Ar dating experiments and a revised relative paleointensity correlation     

William S. Cassata  Brad S. Singer  Joseph C. Liddicoat  Robert S. Coe 《Quaternary Geochronology》2010,5(5):533-543

Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by ¹⁴C and ⁴⁰Ar/³⁹Ar geochronology to have occurred within the last 50 ka. However, ¹⁴C and ⁴⁰Ar/³⁹Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New ⁴⁰Ar/³⁹Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous ¹⁴C and ⁴⁰Ar/³⁹Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined ⁴⁰Ar/³⁹Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than ¹⁴C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.  相似文献   

The interpretation of inverse isochron diagrams in Ar/Ar geochronology     

Yvette D. Kuiper 《Earth and Planetary Science Letters》2002,203(1):499-506

The concepts involved in the construction and interpretation of inverse isochron diagrams used in ⁴⁰Ar/³⁹Ar geochronology are reviewed. The diagrams can be useful as a means of recognising atmospheric argon and excess ⁴⁰Ar, incorporated in the mineral lattice, which cannot be recognised from ⁴⁰Ar/³⁹Ar spectra. The age established using an inverse isochron plot, unlike that yielded by a spectrum, is not affected by trapped argon ⁴⁰Ar/³⁶Ar ratios that are different from the atmospheric argon ratio (e.g. due to excess ⁴⁰Ar), and may contribute to a better age interpretation. However, a heterogeneous distribution of excess ⁴⁰Ar or heterogeneous argon loss can cause ‘false’ isochrons, with axial intercepts indicating an incorrect age and an incorrect trapped argon composition. Inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons. However, it is possible that both the spectrum and inverse isochron yield the same incorrect age. The importance of considering all possible interpretations before assigning an age to a specimen is stressed.  相似文献   

Age of the Hawaiian-Emperor bend     

G. Brent Dalrymple  D.A. Clague 《Earth and Planetary Science Letters》1976,31(3):313-329

⁴⁰Ar/³⁹Ar age data on alkalic and tholeiitic basalts from Diakakuji and Kinmei Seamounts in the vicinity of the Hawaiian-Emperor bend indicate that these volcanoes are about 41 and 39 m.y. old, respectively. Combined with previously published age data on Yuryaku and Ko¯ko Seamounts, the new data indicate that the best age for the bend is 42.0 ± 1.4 m.y.Petrochemical data indicate that the volcanic rocks recovered from bend seamounts are indistinguishable from Hawaiian volcanic rocks, strengthening the hypothesis that the Hawaiian-Emperor bend is part of the Hawaiian volcanic chain.⁴⁰Ar/³⁹Ar total fusion ages on altered whole-rock basalt samples are consistent with feldspar ages and with⁴⁰Ar/³⁹Ar incremental heating data and appear to reflect the crystallization ages of the samples even though conventional K-Ar ages are significantly younger. The cause of this effect is not known but it may be due to low-temperature loss of³⁹Ar from nonretentive montmorillonite clays that have also lost⁴⁰Ar.  相似文献   

⁴⁰Ar/ ³⁹Ar and K–Ar geochronological age constraints for the inception and early evolution of the Izu–Bonin – Mariana arc system     

MICHAELA.  COSCA  RICHARDJ.  ARCULUS  JULIANA.  PEARCE & JOHNG.  MITCHELL 《Island Arc》1998,7(3):579-595

K–Ar and ⁴⁰Ar/³⁹Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the ⁴⁰Ar/³⁹Ar plateau and ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by ⁴⁰Ar/³⁹Ar analysis; (ii) ⁴⁰Ar/³⁹Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and ⁴⁰Ar/³⁹Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ～ 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.  相似文献   

Dating young MORB of the Central Indian Ridge (19°S): Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method     

《Quaternary Geochronology》2017

We have applied the unspiked K-Ar and the ⁴⁰Ar/³⁹Ar methods to samples precisely collected and localised, on both Central Indian Ridge flanks, to test their effectiveness and reliability when applied to the dating of recent (i.e. less than 1 Ma) MORBs. Twenty six samples) from the sixty five samples collected every ∼500 m up to the Brunhes-Matuyama boundary on both ridge flanks, were selected based on their distance from the ridge axis. Therefore, we can evaluate whether the isotopic ages are a good indicator of the crystallisation age by considering their geographic position with respect to the ridge axis (zero age) and the B/M magnetic boundary. Direct comparison of the isotopic and model ages shows that only 9 out of 26 samples were successfully dated. The GIMNAUT – MORB's test case amply demonstrates that the unspiked K-Ar technique, when applied to submerged volcanic samples, is subject to potentially defective assumptions of trapped atmospheric argon, excess/fractionated argon and extremely sensitive to alteration. Although the unspiked K-Ar technique is theoretically capable to produce high precision ages, the comparison with the ⁴⁰Ar/³⁹Ar techniques reveals that only 15% (i.e. 4 samples out of 26) of the ages obtained here are geologically meaningful. Five of the seven ⁴⁰Ar/³⁹Ar incremental heating experiments provide meaningful ages. Because potential sources of systematic errors such as excess ⁴⁰Ar*, recoil of ³⁹Ar_K and ³⁷Ar_Ca can be identified and because effects of alteration are significantly reduced by the pre-heating of the samples up to 500–600°c, the ⁴⁰Ar/³⁹Ar incremental heating method appears to be the method of choice to date MORBs.  相似文献   

New 40Ar/39Ar ages for selected young (<1 Ma) basalt flows of the Newer Volcanic Province,southeastern Australia     

E. Matchan  D. Phillips 《Quaternary Geochronology》2011,6(3-4):356-368

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision ⁴⁰Ar/³⁹Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for ⁴⁰Ar/³⁹Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma).⁴⁰Ar/³⁹Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant ⁴⁰Ar/³⁹Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average ⁴⁰Ar/³⁹Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean ⁴⁰Ar/³⁹Ar ages, suggesting the presence of extraneous ⁴⁰Ar. This study demonstrates the suitability of the ⁴⁰Ar/³⁹Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP.  相似文献   

Episodic mesozoic volcanism in Namibia and Brazil: A K—Ar Isochron study bearing on the opening of the south atlantic     

Gerard Siedner  John G. Mitchell 《Earth and Planetary Science Letters》1976,30(2):292-302

An attempt is made to find a more objective and precise basis for the correlation of volcanics from southwestern Africa and South America than is possible by frequency diagrams of individual K—Ar ages. This leads to a critical appraisal of conventionally calculated K—Ar ages with the conclusion thata priori assumption regarding the isotopic composition of non-radiogenic argon and, hence, the standard atmospheric correction, are no longer tenable.K—Ar isotoopic data on Mesozoic basalts and dolerites from Namibia and Brazil are presented in terms of an isochron model. Plots for cogenetic rocks are unacceptably scattered on a “radiogenic”⁴⁰Ar vs. K diagram, but show a high degree of collinearity on⁴⁰Ar/³⁶Ar diagrams0K/³⁶Ar diagrams. Using the latter plots, a number of isochrons are generated which indicate that Mesozoic volcanism in these regions occured as several discrete episodes of fairly short duration. Effusion of the extensive Serra Geral basalts of Brazil and the Kaoko basalts of Namibia is shown to have occured simultaneeously at 121 m.y.B.P. Basalts from a series of boreholes along the central Parana Basin, as well as a group of dykes from Sao Paulo, yield isochrons of 128 m.y., which coincides with the postulated onset of separation of Africa and South America based on marine magnetic anomalies. Linear dyke swarms along the Namibian seaboard, interpreted as an expression of the earliest rift phase, have an isochron age of 134 m.y. Sills and dykes, mainly from southern Namibia, with isochron ages of 183 m.y. are considered to be the westernmost manifestation of Stormberg volcanism, not necessarily related to rifting. Most of the igneous suites examined have initial⁴⁰Ar/³⁶Ar ratios significantly different from the modern atmospheric value.  相似文献   

⁴⁰Ar/³⁹Ar dating on volcanic rocks of the Deccan Traps,India     

Ichiro Kaneoka 《Earth and Planetary Science Letters》1980,46(2):233-243

⁴⁰Ar/³⁹Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas.In the Igat Puri area, the uppermost flow shows an⁴⁰Ar/³⁹Ar age of 63 Ma, whereas a lower flow has an age of around 82–84 Ma.⁴⁰Ar/³⁹Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabaleshwar area indicate a total⁴⁰Ar/³⁹Ar age of around 63–64 Ma, though they show the effect of secondary disturbance in the age spectra.The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.  相似文献   

Ar/Ar geochronology of Neogene phreatomagmatic volcanism in the western Pannonian Basin,Hungary     

Jan Wijbrans  Károly Németh  Ulrike Martin  Kadosa Balogh 《Journal of Volcanology and Geothermal Research》2007

Neogene alkaline basaltic volcanic fields in the western Pannonian Basin, Hungary, including the Bakony–Balaton Highland and the Little Hungarian Plain volcanic fields are the erosional remnants of clusters of small-volume, possibly monogenetic volcanoes. Moderately to strongly eroded maars, tuff rings, scoria cones, and associated lava flows span an age range of ca. 6 Myr as previously determined by the K/Ar method. High resolution ⁴⁰Ar/³⁹Ar plateau ages on 18 samples have been obtained to determine the age range for the western Pannonian Basin Neogene intracontinental volcanic province. The new ⁴⁰Ar/³⁹Ar age determinations confirm the previously obtained K/Ar ages in the sense that no systematic biases were found between the two data sets. However, our study also serves to illustrate the inherent advantages of the ⁴⁰Ar/³⁹Ar technique: greater analytical precision, and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field. Periods of increased activity with multiple eruptions occurred at ca. 7.95 Ma, 4.10 Ma, 3.80 Ma and 3.00 Ma.  相似文献   

40Ar/39Ar geochronology and exhumation of mylonitized metamorphic complex in Changle-Nanao ductile shear zone     

Zhihong Wang  Huafu Lu 《中国科学D辑(英文版)》1997,40(6):641-647

New⁴⁰Ar/³⁹Ar plateau ages from rocks of Changle-Nanao ductile shear zone are 107.9 Ma(Mus), 108.2 Ma(Bi), 107.1 Ma(Bi), 109.2 Ma(Hb) and 117.9 Ma(Bi) respectively, which are concordant with their isochron ages and record the formation age of the ductile shear zone. The similarity and apparent overlap of the cooling ages with respective closure temperatures of 5 minerals document initial rapid uplift during 107–118 Ma following the collision between the Min-Tai microcontinent and the Min-Zhe Mesozoic volcanic arc. The⁴⁰Ar/³⁹ Ar plateau ages, K-Ar date of K-feldspar and other geochronologic information suggest that the exhumation rate of the ductile shear zone is about 0.18–1.12 mm/a in the range of 107–70 Ma, which is mainly influenced by tectonic extension.  相似文献   

一种新的K-Ar和~(40)Ar-~(39)Ar等时线——~(40)Ar/~(40)K-~(36)Ar/~(40)K和~(40)Ar/~(39)Ar-~(36)Ar/~(39)Ar等时线          下载免费PDF全文

李大明  李齐  王瑜 《地震地质》2001,23(1):79

通过理论推导 ,提出了一种新的K -Ar、4 0 Ar - 39Ar等时线 :4 0 Ar/ 40 K - 36Ar/ 40 K、4 0 Ar/ 39Ar - 36Ar/ 39Ar等时线。通过实例证明它与其它等时线方法可以进行相互检验。在一些不能利用其它等时线方法的情况下 ,新的4 0 Ar/ 40 K - 36Ar/ 40 K和4 0 Ar/ 39Ar- 36Ar/ 39Ar等时线方法可以充分显示它的使用价值和存在意义  相似文献   

一种新的K-Ar和40Ar-30Ar等时线          下载免费PDF全文

李大明  李齐  王瑜 《地震地质》2001,23(1)

通过理论推导，提出了一种新的K-Ar、40Ar-39Ar等时线：40Ar/40K-36Ar/40K、40Ar/39Ar－36Ar/40Ar等时线。通过实例证明它与其它等时线方法可以进行相互检验。在一些不能利用其它等时线方法的情况下，新的40Ar/40K－36Ar/40K和40Ar/39Ar－36Ar/40Ar等时线方法可以充分显示它的使用价值和存在意义。  相似文献   

Investigation of excess argon in ultramafic rocks from the Kola Peninsula by the⁴⁰Ar/³⁹Ar method     

Ichiro Kaneoka 《Earth and Planetary Science Letters》1974,22(2):145-156

In several xenolithic ultramafic rocks from the Kola Peninsula, including a magnetic separate, abnormally high⁴⁰Ar/³⁹Ar ratios persisted at low and high temperatures. The lowest⁴⁰Ar/³⁹Ar ratio was consistently observed at intermediate temperatures (900–1100°C), indicating an apparent age of 2.8–3.1 b.y.; however, this may not indicate the formation age.The quantity of excess⁴⁰Ar was estimated at each temperature fraction, adopting ages inferred from published Rb-Sr ages or the minimum⁴⁰Ar/³⁹Ar age. Excess⁴⁰Ar is abundantly trapped both in mineral lattices and nonretentive trapping sites, but the trapping sites are different from those of in-situ radiogenic⁴⁰Ar. The high temperature component of excess⁴⁰Ar is considered to represent Ar dissolved during mineral formation in the upper mantle or the lower crust.A correlation between the amount of high temperature excess⁴⁰Ar and³⁶Ar exists for some samples. The⁴⁰Ar_excess/³⁶Ar ratios of the rocks of probable upper mantle or lower crust origin vary from about 10 000 to 35 000, which may suggest large fluctuations of this ratio in the deep interior of the earth. The high value implies that most³⁶Ar was already degassed from the earth's interior at least 2 or 3 b.y. ago.  相似文献   

Combined unspiked K–Ar and 40Ar/39Ar dating applied to the dating of 80–260 ka old Icelandic sub-glacial rhyolites     

《Quaternary Geochronology》2023

We have used two techniques (i.e. K–Ar and ⁴⁰Ar/³⁹Ar) on Icelandic obsidian samples to produce and more specially to estimate the quality and accuracy of the ages that can be obtained. Following a meticulous protocol, we were able to date six rhyolitic eruptions with an accuracy 7 to 40 times better than those obtained previously. Among these six rhyolites are the first published K–Ar and ⁴⁰Ar/³⁹Ar ages of Krafla.The combined K–Ar and ⁴⁰Ar/³⁹Ar approach produces not only highly precise but also accurate ages. Such high precision makes it possible to produce accurate reconstructions of ice thickness at a given location and time, to test whether there was a possible link between deglaciation and rhyolitic volcanism onset in Iceland, and to explore other possible applications of the ⁴⁰Ar/³⁹Ar dating method to paleo-environmental and paleo-climatic reconstruction at Iceland's latitude.Then, we investigate, by combining geochemistry (i.e. determination of major and trace element composition) and geochronology (i.e. dating of rhyolitic eruptions via K–Ar and ⁴⁰Ar/³⁹Ar dating) for a number of Icelandic rhyolitic volcanoes whose activity could be recorded in North Atlantic sedimentary cores as well as in Arctic ice. The aim of this approach is to provide new independent anchors and correlations between climate records. Of the six dated eruptions, we propose that one is record in North Atlantic sediments, the Loðmundur eruption that constitutes one of the Kerlingarfjöll tuyas, which we date at 189.9 ± 1.1 ka and assume to be the source of the tephra recognized in core MD04-2822 at a depth of 3630–3631 cm.  相似文献   

Rare gas isotopes and mass fractionation: An indicator of gas transport into or from a magma     

Ichiro Kaneoka 《Earth and Planetary Science Letters》1980,48(2):284-292

A simple model of mass fractionation may explain the isotopic ratios of rare gases in volcanic materials. Single-stage mass fractionation of atmospheric rare gases predicts an upper limit for²⁰Ne/²²Ne of 10.3 and a lower limit for⁴⁰Ar/³⁶Ar of 280. The rare gas data in volcanic materials seem to support this interpretation.Relatively low⁴⁰Ar/³⁶Ar ratios, as low as 282, have been observed in recent Japanese volcanic rocks. Such a low⁴⁰Ar/³⁶Ar ratio may be explained by mass fractionation of the atmospheric value if the rare gases represent those which were transported into the magma chamber with other volatile elements.Both the amounts and the fractionated rare gas abundance pattern of lighter elements which are observed in pumices from the recent eruption of Mt. Usu, Southern Hokkaido, Japan, suggest the possibility of air injection into its magma chamber. Thus, the fractionation of rare gases in volcanic materials may be a common occurrence, and it must be considered in models for the origin of isotopic differences between rare gases in volcanic materials and the atmosphere.  相似文献   

The evolution of excess argon in alpine biotites — A⁴⁰Ar-³⁹Ar analysis     

J.C. Roddick  R.A. Cliff  D.C. Rex 《Earth and Planetary Science Letters》1980,48(1):185-208

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.  相似文献