Diffusion artifacts in dating by stepwise thermal release of rare gases

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.     

40Ar-39Ar and Rb-Sr age determinations on Quaternary volcanic rocks

⁴⁰Ar-³⁹Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides⁴⁰K and⁸⁷Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the⁴⁰Ar and the³⁹Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×10⁵ years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the⁴⁰Ar/³⁶Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic⁴⁰Ar from trapped⁴⁰Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in⁸⁷Sr/⁸⁶Sr as large as 16 parts in 10⁴ and⁸⁷Rb/⁸⁶Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×10⁵to3.3±0.2×10⁵ years and are in good agreement with the⁴⁰Ar-³⁹Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial⁸⁷Sr/⁸⁶Sr. However, different tuffs show small differences in initial⁸⁷Sr/⁸⁶Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the⁴⁰Ar-³⁹Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.     

Temperature and pressure effects on40Ar39Ar systematics

The effects of thermal and compressional treatment on⁴⁰Ar-³⁹Ar systematics have been investigated on three artificially heated biotite samples (heated for 1 hour at 700°C and 860°C in air and 700°C in vacuum respectively) and uniaxially compressed granite (p = 1400bar) and basalt samples (p = 1650bar). The⁴⁰Ar-³⁹Ar results for the disturbed samples are compared with those for undisturbed samples. Except for the vacuum-heating case, the effects of the disturbances may be interpreted as the combined effect of a partial loss of radiogenic⁴⁰Ar from the sample and an incorporation of air Ar into the sample. Common diagnostic effects are (1) reduction of the total fusion age, (2) distortion of the age spectrum and, if the degree of the partial Ar loss is small, (3) approximate preservation of the isochron age, and (4) reduction of the intercept value (⁴⁰Ar/³⁶Ar) in the isochron plot.The features observed in the age spectra of artificially disturbed samples are rather common in geologically disturbed samples, suggesting that the artificial disturbances simulate the effects of geological disturbances on⁴⁰Ar-³⁹Ar systematics.     

Archaeological age constraints from extrusion ages of obsidian: Examples from the Middle Awash,Ethiopia

Extrusion ages of archaeological obsidian, especially as determined by the ⁴⁰Ar/³⁹Ar method, can provide reliable maximum ages for tool manufacture. In at least one case in the Middle Awash of Ethiopia, freshly extruded obsidian was used for tool making, resulting in useful maximum ages for site occupation. Hydration resulting in mobility of K and/or Ar in glass, and recoil artifacts produced by neutron irradiation, fatally affect most glass shards from volcanic ashes. The much lower surface area to volume ratio of most archaeological obsidian, however, indicates that the affected areas can be manually removed prior to analysis and the recoil and hydration problems can be easily overcome. A more important issue in dating obsidian is that of possible mass-dependent kinetic isotope fractionation during or subsequent to quenching of volcanic glasses. This is evidenced in some cases by sub-atmospheric initial ⁴⁰Ar/³⁶Ar ratios, and more generally in sub-atmospheric ³⁸Ar/³⁶Ar. Resulting bias can be avoided through the use of isochron ages, which do not entail the assumption of an initial value of ⁴⁰Ar/³⁶Ar as is required for plateau ages. Since step heating of glasses often yields limited variability in ⁴⁰Ar:³⁹Ar:³⁶Ar (and therefore little spread on isochrons), another approach is to use an average value for initial ⁴⁰Ar/³⁶Ar, with concomitantly larger uncertainty than is associated with atmospheric ⁴⁰Ar/³⁶Ar, when calculating a plateau age. The ³⁸Ar/³⁶Ar of an un-irradiated subset of our samples validates the inference of kinetic fractionation, and potentially provides a basis for determining initial ⁴⁰Ar/³⁶Ar in samples that fail to yield isochrons, but only in samples lacking magmatic excess ⁴⁰Ar. These approaches allow us to reliably apply the ⁴⁰Ar/³⁹Ar method to volcanic glasses, which has resulted in maximum ages for archaeological sites that are not amenable to traditional geochronological methods. ⁴⁰Ar/³⁹Ar geochronology can also provide information on the geological provenance of the raw material used for tool making, especially when combined with geochemical data.     

39Ar-40Ar ages of samples from the Apollo 17 station 7 boulder and implications for its formation

³⁹Ar-⁴⁰Ar ages and³⁷Ar-³⁸Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by³⁹Ar-⁴⁰Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of3.99 ± 0.02AE. A sample of the noritic clast 77215 gave4.04 ± 0.03AE, the highest age found so far in this boulder. The³⁹Ar-⁴⁰Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence.Taking into account the shielding by the boulder itself, an average³⁷Ar-³⁸Ar exposure age of(27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder.     

Argon in Apollo 15 green glass spherules (15426):40Ar39Ar age and trapped argon

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.     

几种全岩样品40Ar-39Ar年龄谱的探讨

探讨了玄武岩、辉绿岩和断层泥中粘土矿物3种全岩样品在40Ar-39Ar分析中的某些问题。利用快中子照射过程中不同矿物发生不均匀Ar丟失的模式解释了玄武岩40Ar-39Ar全熔年龄与K-Ar年龄间的差异。对辉绿岩样品的40Ar-39Ar分析确定了辉绿岩脉的侵入时代,并说明该地区其它辉绿岩样品K-Ar年龄的离散是受后期变质作用的影响。对于断层泥中粘土矿物,快中子照射中39Ar的反冲丢失导致<1μm粒级部分的40Ar-39Ar全熔年龄偏高。<1μm和2～10μm粒级样品的40Ar-39Ar年龄谱中,低温区较低的阶段年龄可能是断层后期较强烈活动的结果。     

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

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.     

39Ar-40Ar systematics of two millimeter-sized rock fragments from Mare Crisium

Two small fragments, L24B, a glass-rich agglutinate (1.9 mg) and L24A, a fine-grained lithic fragment (9.4 mg), from the Luna 24 landing site have been neutron irradiated for the purpose of³⁹Ar-⁴⁰Ar dating. A fairly well-defined³⁹Ar-⁴⁰Ar plateau age of 3.65 ± 0.12 AE was found for the larger fragment. After appropriate corrections the composition of the trapped and spallogenic Ar could be deciphered. The evolution of³⁸Ar_sp/³⁷Ar showed that 660 m.y. and 500 m.y. were the most reliable exposure ages for L24A and L24B, respectively. The Ti contents of ≤0.6% determined by gamma-counting prior to the Ar analysis indicate both fragments being associated with the group of low-Ti or even very low-Ti basalts.     

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

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.     

The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: Time for a change?

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.     

Thermal history of the nakhlites by the40Ar-39Ar method

This paper reports the results of thermal-release argon analyses of neutron-irradiated samples of the two nakhlite meteorites, Lafayette and Nakhla. The initiation of retention of radiogenic⁴⁰Ar in Lafayette appears to have been a reasonably well-defined event which occurred (1.33 ± 0.03) × 10⁹ yr ago, as determined by the⁴⁰Ar-³⁹Ar method. Nakhla also appears to have been retaining argon no longer than 1.3 × 10⁹ yr, but its gas-retention age cannot be considered well-defined because its apparently most-retentive sites have nominal gas-retention ages shorter than those of the less-retentive sites which contain most of its potassium.     

The application of isochron diagrams in40Ar-39Ar dating: A discussion

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.     

Age of the Hawaiian-Emperor bend

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.     

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

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.     

40Ar/39Ar incremental-release ages of biotite from a progressively remetamorphosed Archean basement terrane in southwestern Labrador

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.     

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

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.     

Comment on “The July 9 and 23, 1905, Mongolian earthquakes,a surface-wave investigation” by Emile Okal

K-Ar ages have been determined for sulfide minerals for the first time. The occurrence of adequate amounts of potassium-bearing sulfides with ideal compositions K₃Fe₁₀S₁₄ (～10 wt.% K) and KFe₂S₃ (～16 wt.% K) in samples from a mafic alkalic diatreme at Coyote Peak, California, prompted an attempt to date these materials. K₃Fe₁₀S₁₄, a massive mineral with conchoidal fracture, gives an age of 29.4 ± 0.5m.y.(⁴⁰Ar/³⁹Ar), indistinguishable from the 28.3 ± 0.4m.y.(⁴⁰Ar/³⁹Ar) and 30.2 ± 1.0m.y.8 (conventional K-Ar) ages obtained for associated phlogopite (8.7 wt.% K). KFe₂S₃, a bladed, fibrous sulfide, gives a younger age, 26.5 ± 0.5m.y.(⁴⁰Ar/³⁹Ar), presumably owing to Ar loss.     

Potassium argon dating of gamma-irradiated minerals

A method for measuring potassium-argon ages making use of the reaction³⁹K(λ, n)³⁸K^β+→³⁸Ar to indirectly determine potassium is discussed. In principle, it is closely analogous to the⁴⁰Ar/³⁹Ar dating method and should possess all of the attributes of that technique. It is demonstrated that precise dating of mica samples with ages between 15 my and 1000 my can be carried out, and a discussion of potentially interfering reactions suggests that it may be possible to extend the method to the problem of induced argon isotopic dating of calcium-rich minerals.     

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

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.