首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
40Ar/39Ar incremental heating experiments were applied to hornblendes, coarse-grained biotites and K-feldspars from 1400 m.y. old rocks near the contact with the ～60 m.y. old Eldora stock in the Front Range of Colorado. The aim was to distinguish, on the basis of argon isotopic data alone, a partially re-set K-Ar date from an undisturbed or a completely overprinted K-Ar date. In the laboratory heating of biotites the radiogenic argon (40Ar*) and potassium-derived39Ar (39Ar*) were released in two stages — in the range ～600–900°C and above ～900°C. The two biotites furthest from the contact and the one adjacent to the contact give well-defined apparent-age plateaus at ～1230 m.y. and 63 m.y. respectively for all argon released above ～600°C. The 1230-m.y. date may represent a thermal event or the end of a long cooling while the 63-m.y. date essentially represents the time of reheating. Partially overprinted biotites at intermediate distances have significantly anomalous plunges in apparent ages for argon released above ～900°C, thus distinguishing them from undisturbed and completely outgassed biotites.The bulk of the40Ar* and39Ar* in the hornblendes was released in the range ～950–1100°C. The hornblende furthest from the contact gives a well-defined plateau at 1400 m.y. for the 98% of the argon that was released above ～950°C. A partially overprinted hornblende from near the contact gives an apparent plateau at ～1050 m.y. The existence of such a false plateau precludes the distinction of partially overprinted K-Ar hornblende dates from undisturbed K-Ar hornblende dates without independent evidence. Reasonable estimates of the time of reheating are not recovered in the age spectra for partially overprinted hornblende and biotites.For the feldspars the bulk of the40Ar* and39Ar* was released in the laboratory heating between about 900°C and 1200°C, probably reflecting phase changes near these temperatures. The argon released below about 900°C records reasonable maximum dates for the time of the thermal overprinting. For the microcline 22500 (the sample number specifies the distance, in feet, from the contact) this effect is slight — a minimum date of 147 m.y. occurs in 2.3% of the total39Ar*. For samples 2400, 1070, and 85 the respective minimum dates are similar at 72, 81, and 68 m.y. and dramatically improve on the total or integrated dates of 238, 358 and 211 m.y. The high-temperature (>900°C) apparent ages for these three feldspars do not define plateaus and are geologically meaningless. The high-temperature apparent ages for the last 50% of the39Ar* released from 22500 do define a plateau, but the 1060-m.y. date is also probably geologically meaningless.  相似文献   

2.
Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the K/Ar method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the 40Ar/39Ar method on single sanidine and plagioclase minerals. The K/Ar isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the K/Ar method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.  相似文献   

3.
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 39ArK and 37ArCa 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   

4.
The concentrations and isotopic compositions of argon, krypton and xenon have been determined in a grain size suite of zircons separated from pyroxene syenite of the Botnavatn Igneous Complex, southwestern Norway. The UPb systematics of these zircons has been studied previously.Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of238U and a component with atmospheric isotopic composition. From correlation diagrams the fissiogenic component is determined to be:83Kr :84Kr :86Kr = (4.6 ± 1.3) : (11.0 ± 2.0) : 100 and129Xe :131Xe :132Xe :134Xe :136Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100. The fissiogenic136Xe/86Kr is 6.0 ± 0.4.The Ar isotopic composition shows radiogenic40Ar and a small excess of38Ar. The excess38Ar of about 1 × 10−11 cm3 STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of238U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed38Ar concentrations.UXe ages are (1.19 ± 0.07) Ga, in agreement with UPb ages. However, if the recoil loss of fissiogenic Xe is considered the UXe ages of these zircons are about 1.53 Ga, which is comparable with the KAr ages and some RbSr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of238U prevents to decide which age is the true crystallization age.  相似文献   

5.
We report an 39Ar–40Ar age determination of a whole rock sample of the olivine-rich, martian meteorite Northwest Africa (NWA) 2737. Those extractions releasing 0–48% of the 39Ar define an 39Ar–40Ar isochron age of 160–190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm–Nd age of 1.42 Ga. At least part of this excess 40Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar–Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that 40Ar was only partially degassed by an impact event ~ 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures > 600 °C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~ 11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~ 300–500 °C and buried in ejecta to a depth of ~ 1–20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~ 11 Ma ago probably heated NWA 2737 to no more than ~ 400 °C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.  相似文献   

6.
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   

7.
We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the40Ar39Ar method, is (3.38 ± 0.06) X 109yr. This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with40Ar/39Ar > 30, and a deeper (2 μ) component with 40Ar/36Ar= 2.9. The ratio of trapped40Ar to36Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules' history. The cosmic-ray exposure age is 300 my.  相似文献   

8.
Seventeen K/Ar dates were obtained on illitic clays within Valles caldera (1.13 Ma) to investigate the impact of hydrothermal alteration on Quaternary to Precambrian intracaldera and pre-caldera rocks in a large, long-lived hydrothermal system ( 1.0 Ma to present). Clay samples came from scientific core hole VC-2B (295°C at 1762 m) which was spudded in the Sulphur Springs thermal area and drilled into the boundary between the central resurgent dome and the western ring-fracture zone. Six illitic clays within Quaternary caldera-fill debris flow, tuffaceous sediment, and ash-flow tuff (48 to 587 m depth) yield ages from 0.35 to 1.09 Ma. Illite from Miocene pre-caldera sandstone (765 m) gives an age of 6.74 Ma. Two dates on illite from sandstones in Permian red beds (1008 and 1187 m) are 4.33 and 4.07 Ma, respectively. Surprisingly, three dates on illites from altered andesite pebbles within the red beds (1010–1014 m) are 0.95 to 1.06 Ma. Four illite dates on variably altered Precambrian quartz monzonite (1615–1762 m) range from 2.90 to 276 Ma.Post-Valles age illite is not correlated with alteration style (argillic to propylitic). Rather, post-Valles ages are uniformly obtained from illites in highly fractured, intensely altered, caldera-fill rocks and the Permian volcanic clasts. Generally, finer clay fractions from identical samples yield younger ages. Plots of 40Ar/36Ar versus 40K/36Ar and 40Ar* versus 40K for the illites in caldera-fill rocks lie close to a 1-Ma isochron. Most illite dates older than Valles caldera are difficult to interpret because they correspond to the ages of pre-Valles volcanic and hydrothermal episodes in the Jemez volcanic field ( 13 Ma). In addition, older dates may be caused by co-mingling of different illites during sample preparation, or by inherited argon or lost argon in illites from rocks with potentially complex hydrothermal histories. However, the range of ages obtained from illites in Permian sands and pebbles and from Precambrian crystalline rocks indicates that Valles hydrothermal activity is overwhelming illite produced by earlier geologic events.  相似文献   

9.
The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses.40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported.Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT.The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.  相似文献   

10.
Five samples from a biotite-hornblende granodiorite phase of the 42.5 Ma Quxu pluton, Gangdese batholith, southern Tibet, have been collected at 250 m vertical intervals. Biotite from these rocks yields monotonically decreasing40Ar/39Ar isochron ages with decreasing elevation of 26.8 ± 0.2, 23.3 ± 0.5, 19.7 ± 0.3, 18.4 ± 0.4,and17.8 ± 0.1Ma (Tc = 335°C). Coexisting K-feldspars have virtually identical minimum apparent40Ar/39Ar ages of 17.0 ± 0.4Ma (Tc = 285°C). These data indicate parts of southern Tibet experienced a pulse of uplift in the early Miocene with the rate of uplift rising from 0.07 to 4.4 mm/year in the interval 20 to 17 Ma. An apatite fission track age of 9.9 ± 0.9Ma from this locality constrains the average uplift rate at this site to about 0.81 mm/year between 17 and 9.9 Ma and 0.30 mm/year from 9.9 Ma to present. K-feldspar from the Dagze granite, 30 km to the east, near Lhasa, yields a minimum apparent40Ar/39Ar age of 35.9 ± 0.9Ma (Tc = 227°C) which indicates an average uplift rate there of 0.21 mm/year since then. The marked pulse of uplift of the Quxu granodiorite and the difference in uplift history between the Dagze and Quxu plutons suggests southern Tibet has experienced discrete pulses of uplift variable in both space and time. These data are not consistent with models which require a large proportion of uplift of the Tibetan plateau to have occurred in the last 2 Ma. The data support the suggestion that convergence between India and Asia was largely accommodated by tectonic escape during the opening of the South China Sea 32 to 17 Ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the Tibetan plateau subsequent to 20 Ma.  相似文献   

11.
The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield40Ar/39Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.40Ar/39Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The40Ar/39Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from38Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.  相似文献   

12.
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   

13.
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   

14.
Analysis of nitrogen and light noble gases in a large sample of glass (lithology C) from the antarctic shergottite EETA 79001 yields a minimumδ15N > +300‰ for the isotopic composition of nitrogen trapped in the glass. The new data fall on the mixing line through the martian atmospheric composition defined byδ15N vs.40Ar/14N for two smaller samples analyzed previously. The results from all three samples are consistent with a two-component nitrogen system in which 84 ppb of trapped martian atmospheric N is mixed in variable proportions with another, more thermally labile N component during stepped heating. This second component, which appears to be indigenous to the glass rather than adsorbed from air and is present in amounts that vary by more than a factor of 3 from sample to sample, may represent volatiles from the martian interior. Data from crystalline phases of several SNC meteorites indicate that the indigenous gas may haveδ15N < −35‰ and36Ar/14N 3 × 10−6, similar to the enstatite chondrites.Neon compositions in EETA 79001 glass samples suggest an earth-like value of 10.1 ± 0.7 for the unknown20Ne/22Ne ratio in the martian atmosphere. The nitrogen-argon correlation systematics yield trapped40Ar/36Ar= 2260 ± 200, within error of the Viking value. There is evidence that36Ar/38Ar in the martian atmosphere is4.1 ± 0.2, strikingly different from terrestrial or typical chondritic ratios near 5.3. Attribution of this low value to excess38Ar generated over martian history by galactic cosmic-ray (GCR) spallation of surface materials would be difficult for a number of reasons, among them the excessive GCR fluences required and the absence of a corresponding21Ne excess.  相似文献   

15.
The guidelines set out by Renne et al. (2009) offer a start to potentially removing numerous problems often associated with the reporting of 40Ar39Ar ages in the literature. Herein, some recommendations are made concerning aspects of how, in what form, and with what “corrections”, supporting argon isotopic data should be published.  相似文献   

16.
Tetsumaru  Itaya  Hironobu  Hyodo  Tatsuki  Tsujimori  Simon  Wallis  Mutsuki  Aoya  Tetsuo  Kawakami  Chitaro  Gouzu 《Island Arc》2009,18(2):293-305
Laser step heating 40Ar/39Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.  相似文献   

17.
Gneisses within an Archean basement terrane adjacent to the southwestern portion of the Labrador Trough were variably retrograded during a regional metamorphism of Grenville age (ca. 1000 Ma). Biotites from non-retrograded segments of the gneiss terrane record40Ar/39Ar plateau and isochron ages which date times of cooling following an episode of the Kenoran orogeny (2376–2391 Ma). A suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the Grenville metamorphic gradient. Biotites in these samples show no petrographic evidence of retrograde alteration, however they do record internally discordant40Ar/39Ar age spectra. Although the extent of internal discordance is variable, the overall character of the release patterns is similar with younger apparent ages recorded in intermediate-temperature gas fractions. The total-gas dates range from 2257±27 Ma (northwest) to 1751±23 Ma (southeast), suggesting that variable quantities of radiogenic argon were lost from the Archean biotites during Grenville metamorphism. The “saddle-shaped” nature of the discordant spectra indicates that argon loss was not accomplished through single-stage, volume diffusion processes.Biotites in portions of the gneiss terrane which were completely recrystallized during Grenville metamorphism are petrographically and texturally distinct. A representative of this phase records a40Ar/39Ar plateau age of 2674±28 Ma. This date is markedly inconsistent with regional constraints on the timing of Grenville metamorphism, and indicates the presence of extraneous argon components. Both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well-defined40Ar/36Ar vs.39Ar/36Ar isochron corresponding to an age (2658±23 Ma) similar to that defined by the plateau portion of the spectrum.The40Ar/39Ar biotite dates suggest that the effects of Grenville metamorphism extent 15–20 km northward into the Superior Province. The limit of this overprint is approximately coincident with the northernmost development of Grenville age thrust faults in the Archean terrane. Therefore, it is proposed that the northern margin of the Grenville Province in southwestern Labrador should be located along the northernmost Grenville thrust fault because this represents both a structural and a thermal discontinuity.  相似文献   

18.
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   

19.
We present new 40Ar/39Ar data for sanidine and biotite derived from volcanic ash layers that are intercalated in Pliocene and late Miocene astronomically dated sequences in the Mediterranean with the aim to solve existing inconsistencies in the intercalibration between the two independent absolute dating methods. 40Ar/39Ar sanidine ages are systematically younger by 0.7-2.3% than the astronomical ages for the same ash layers. The significance of the discrepancy disappears except for the upper Ptolemais ashes, which reveal the largest difference, if an improved full error propagation method is applied to calculate the absolute error in the 40Ar/39Ar ages. The total variance is dominated by that of the activity of the decay of 40K to 40Ar (∼70%) and that the amount of radiogenic 40Arp in the primary standard GA1550 biotite (∼15%). If the 40Ar/39Ar ages are calculated relative to an astronomically dated standard, the influence of these parameters is greatly reduced, resulting in a more reliable age and in a significant reduction of the error in 40Ar/39Ar dating.Astronomically calibrated ages for Taylor Creek Rhyolite (TCR) and Fish Canyon Tuff (FCT) sanidine are 28.53±0.02 and 28.21±0.04 Ma (±1 S.E.), respectively, if we start from the more reliable results of the Cretan A1 ash layer. The most likely explanation for the large discrepancy found for the younger Ptolemais ash layers (equivalent to FCT of 28.61 Ma) is an error in the tuning of this part of the sequence.  相似文献   

20.
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. The40Arexcess/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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号