Step-heating Data Research Articles

Discussion of ‘the Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/ 39Ar geochronology of eclogite garnet fluid inclusions by Qiu Hua-Ning and Wijbrans J.R.’

The in vacuo crushing procedure employed by Qiu and Wijbrans (2008) is identical to that of Qiu and Wijbrans (2006) and has produced a similar result. The authors provide no description of the sample material analysed and the display of data is highly selective (see also Kendrick, 2007). The new step-heating data produced for crushed garnets are readily explained in terms of degassing from mineral inclusions (Fig. 1) and the argon isotope systematics preclude the ~ 450 Ma 'plateau' or 'isochron' ages being related to primary metamorphic fluid inclusions. The range of apparent ages determined for garnets, phengite and biotite by Qiu and Wijbrans (2006, 2008) extend from ~ 150 Ma to over 1000 Ma and only a subset of garnets yield in vacuo crushing apparent ages of ~ 450 Ma (see Fig. 2). The range of apparent Ar/Ar ages reported by Qiu and Wijbrans (2006, 2008) is similar to that determined for Dabie Shan phengite previously (Giorgis et al., 2000). Such variability is typical of UHP metamorphic rocks where it is well understood that argon has limited mobility and that the prevalence of extraneous Ar makes application of the Ar/Ar technique challenging (Arnaud and Kelley, 1995; Scaillet, 1996; Boundy et al., 1997; Giorgis et al., 2000). In summary, the in vacuo crushing data presented by Qiu and Wijbrans (2006, 2008) appear to add little to our understanding of Dabieshan geochronology, the prevalence of extraneous Ar in UHP terranes, or the in vacuo crushing technique. The data are poorly constrained and remain compatible with the majority of published data, that constrain Dabieshan UHP metamorphism to a Triassic age of 210-230 Ma (see Ames et al., 1993; Chavagnac and Jahn, 1996; Zheng et al., 2003; Chen et al., 2006; Zhao et al., 2007).

Isotopic and petrographic evidence for young Martian basalts

Radiometric age data for shergottites yield ages of 4.0 Ga and 180–575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new 39Ar– 40Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of 40Ar/ 36Ar versus 39Ar/ 36Ar, the more strongly shocked ALH 77005 (45–55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding 40Ar/ 36Ar ratios of 1600–2026. Apparent ages calculated from these extractions range from 374–8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30–44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest 40Ar/ 36Ar ratios (1260–1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess 40Ar in the shock melts. The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martian surface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be ⩾4.0 Ga; we contend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (Late Amazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We show that although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasively altered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.

Age constraints for the thermal evolution and erosional history of the central European Variscan belt: new data from the sediments and basement of the Carboniferous foreland basin in western Poland

Three Carboniferous-age detrital muscovites from the Variscan foreland basin of SW Poland and two muscovites from phyllites underlying the basement have been dated by the 40 Ar/ 39 Ar step-heating and single-crystal laser fusion method. 40 Ar/ 39 Ar analysis of the detrital micas defines step-heating preferred ages of 370.7 ± 1.4, 363.0 ± 1.9 and 355.0 ± 1.3 Ma. Single-crystal laser fusion data indicate little dispersion for the first of three samples, with an integrated age that closely matches the step-heating data, but the latter two describe inhomogeneous populations. The white mica concentrate from one phyllite yields a step-heating preferred age of 358.6 ± 1.8 Ma. The second phyllite sample displays an incremental discordant apparent age spectrum representing a mixture of white mica grains of varying ages. Our most important finding is that the Variscan foreland basin was supplied by source rocks that were exhumed and cooled in the Late Devonian, probably as a result of an early Variscan collisional event, previously largely undocumented. Although accessible exposures of the Variscan basement in SW Poland exhibit only a minor component of rocks exhumed before the Carboniferous, our work suggests that large tracts of rocks with a Devonian cooling signature are preserved at depth beneath the foreland basin.

Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on 40Ar/ 39Ar step heating data

New 40Ar/ 39Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhön and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhön ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously. The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhön area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.

Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N): age, source, and relationship to the Iceland and putative Jan Mayen plumes

The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope compositions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr=0.70344–0.70352, 143Nd/144Nd=0.51283–0.51288, 206Pb/204Pb=18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb=18.95) together with relatively radiogenic Nd (143Nd/144Nd=0.51295) and low-radiogenic Sr (87Sr/86Sr=0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6°N, apparently reflecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge.

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773

Abstract— The 40Ar‐39Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 ± 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of 39Ar, or the presence of pre‐shock 40Ar incorporated into shock‐melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61–2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ˜2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.

Re‐interpretation of the existence of a primitive plume under Australia based on neon isotope fractionation during step heating

ABSTRACT The origin of the volcanism in south‐eastern Australia is poorly understood. Matsumoto and collaborators found near solar neon isotope values in measurements obtained by step heating apatites in ultramafic xenoliths from Bullenmerri (SE Australia), and proposed that these values result from the presence of a primitive plume. However, the results of simple diffusion models indicate that the neon isotopic signature observed by Matsumoto et al. can be generated by mass fractionation during step heating. If such mass fractionation does occur during step heating, then neither neon nor helium isotopes require the presence of primitive mantle, and the rare gas observations are consistent with the involvement of MORB‐source mantle in the generation of south‐eastern Australian volcanics. These results show that both the acquisition and the interpretation of step heating data should be carefully controlled and that a mineral crushing technique may yield more reliable analyses.

Cooling of a Late-Syn Orogenic Pluton: Evidence from Laser K-feldspar Modelling of the Carion Granite, Madagascar

Since the late 1980s, it has been hypothesized that the wide range of apparent argon ages seen within single K-feldspar samples might be due to a distribution of diffusion domain sizes within the mineral. To test and apply this idea, an analytical technique that combines conventional laboratory degassing experiments (resistance heating) with numerical inversion procedures has been developed to extract cooling history information from feldspars. A key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples. In our study, we have K-feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments. Our step-heating data are based on using a continuous argon-ion laser with no direct control on temperature. However, with only a single added free parameter in the model, we show that it is possible to analyze this data in the multi-domain style, and make some simple inferences on the nature of the cooling history of the Carion pluton in central Madagascar. The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 ± 5.2 Ma followed by relatively slow cooling as constrained by 40Ar/ 39Ar ages on hornblende, biotite and K-feldspar. Four hornblende samples yielded a mean 40Ar/ 39Ar age of 512.7 ± 2.6 Ma. A biotite sample yielded an age of 478.9 ± 1.0 Ma and modeled K-feldspar ages show cooling from 350° C at 466 Ma to 100° C by 410 Ma. Collectively, the data suggest that the pluton cooled from 850° C at 532.1 ± 5.2 (U-Pb zircon) Ma to 500° C at 512.7 ± 2.6 Ma ( 40Ar/ 39Ar hornblende), or approximately 18 °C/Ma slowing to ∼4 °C/Ma between 512 Ma and 478 Ma and finally to about 3°C/Ma between 478 and 410 Ma.

Direct measurement of Ar diffusion profiles in a gem-quality Madagascar K-feldspar using the ultra-violet laser ablation microprobe (UVLAMP)

Controversy surrounding the mechanisms and controls on argon diffusion in K-feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures, over a range of temperatures. Measurements of argon diffusion profiles in a gem-quality iron-rich orthoclase heated in a cold seal apparatus, have been undertaken in situ using an ultra-violet laser ablation microprobe (UVLAMP) technique. The results agree very closely with the previously determined bulk values for Benson Mines orthoclase (activation energy ( E)=43.8±1 kcal mol −1) and vacuum furnace cycle-heating studies of K-feldspars ( E=46±6 kcal mol −1). However, instead of defining a single activation energy ( E) and diffusion coefficient ( D o), the data yield two sets of parameters: a low-temperature (550–720°C) array with an E of 47.2±2.5 kcal mol −1 (198.2±10.5 kJ mol −1) and a D o of 0.0374 +0.1123 −0.0281 cm 2 s −1, and a high-temperature (725–1019°C) array with an E of 63.8±3.4 kcal mol −1 (268.0±14.3 kJ mol −1) and a D o of 55.0 +225.5 −44.2 cm 2 s −1. The new results closely reproduce two sets of apparent activation energies previously measured in cycle-heating studies of Madagascar K-feldspar (40±3 and 57±3 kcal mol −1). Previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion. However, the UV depth profile analyses indicate simple diffusion to the grain surface and importantly, diffusion radii calculated by combining the UVLAMP and cycle-heating data, are the same as the physical grain sizes used in the experiments, around 1 mm. Vacuum furnace stepped heating experiments on slowly cooled K-feldspars have been interpreted as showing diffusion radii of around 6 μm and indicate complex populations of sub-grains. This study indicates that Madagascar K-feldspar and thus probably all gem-quality K-feldspars act as single diffusion domains and that short-circuit (or pipe) diffusion was not an important loss mechanism. An apparent diffusion compensation relationship in the stepped heating data for Madagascar K-feldspar implies that similar relationships seen in other K-feldspars are a result of a range of diffusion mechanisms.

CO2, 13C/12C and H2O variability in natural basaltic glasses: a study comparing stepped heating and ftir spectroscopic techniques

A comparison of two independent techniques was used to assess the homogeneity of CO2 and H2O concentrations in a number of natural basaltic glasses. Variations in carbon concentration and isotopic ratio were determined by comparison of stepped heating data obtained in two different laboratories. Dissolved volatile concentrations were also obtained by stepped heating and Fourier Transform Infrared (FTIR) spectroscopy. Replicate stepped heating analyses of a mid-ocean ridge basaltic glass show that the concentration and 13C/12C of bulk magmatic and dissolved CO2 vary by less than ±10% and ±0.5‰, respectively. A similar degree of correlation is observed for replicate stepped heating analyses of Mariana Trough glasses conducted in two different laboratories. Dissolved CO2 concentrations determined by stepped heating also correlate well with concentrations measured by FTIR spectroscopy. The correspondence of results obtained in these experiments provide an upper limit to the degree of natural variation in concentrations and isotopic ratios of these volatiles in basaltic glasses and suggest that intrinsic, magmatic carbon has a relatively homogeneous distribution in these glasses. Water concentrations determined through extraction by heating and FTIR also show excellent agreement.

On the age and tectonic significance of Permo-Triassic dikes in the Bergen-Sunnhordland region, southwestern Norway

New 40 ArP9 Ar step heating data for amphiboles from three N-S to NNW -SSE-trending alkaline dikes in western Norway yield remarkably consistent plateau ages at about 221 Ma. Together with most previously published amphibole and whole-rock Kl Ar ages from Sunnhordland, which cluster around 220 Ma, the new data provide convincing evidence for a Late Triassic main pulse of dike intrusion. We conclude that most (but not all) of the K-Ar data are reliable and that Permian K-Ar and fission-track ages from four dikes in the region suggest an earlier pulse of intrusion at around 250-260 Ma. However, it is argued that the previously published Jurassic whole-rock age of one of the dikes is caused by hydrothermal alteration of the groundmass. A �220 Ma40 ArP9 Ar plateau (and isochron) age of unaltered homblende is believed to refiect more closely the age of intrusion, in agreement with other ages of late dikes in the region. The Permian (250-260 Ma) and Triassic (220-230 Ma) pulses of dike injection are correlated with Permian and Triassic phases of extension in the North Sea basin to the west. The strong inftuence of Permo- Triassic stretching at the eastem side of the Viking Graben and only mild rejuvenation of Permo- Triassic structures in the late Jurassic help to explain the apparent absence of Jurassic rnagmatism in southwestem Norway.

Emerald dating through 40Ar/ 39Ar step-heating and laser spot analysis of syngenetic phlogopite

Emerald, occurring in K-metasomatic rocks developed at the contact of the Carnaíba leucogranite with serpentinite (Bahia State, Brazil), has been dated using an original 40Ar/ 39Ar procedure. It combines step heating and spot fusion experiments on two types of phlogopite crystals: (1) bulk samples and individual grains extracted from the enclosing K-metasomatic host rocks; and (2) syngenetic solid inclusions precipitated along growing zones of the emerald host crystals. The second procedure uses in situ laser probe experiments on rock sections. In spite of the huge amounts of excess 40Ar detected in adjacent emerald, we could measure reliable ages of 1951 ± 8 Ma and 1934 ± 8 Ma for the Trecho Velho and Braulia occurrences, respectively. Spot fusion data had higher discrepancy than the step heating data, but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon. A muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 ± 8 Ma, which may correspond to a higher closure temperature of the K Ar system during the cooling of the whole pluton and associated hydrothermal halo. These accurate measurements lead to the following conclusions: (1) direct emerald dating is possible; (2) in spite of a polyphase history during the Transamazonian orogenesis (2 Ga), combined step heating and spot fusion experiments give a better precision for granite-related emerald mineralization than the scattered ages obtained by Rb-Sr and K-Ar methods; (3) the late-Transamazonian tectonothermal retrograde event which probably caused the dispersion of previous Rb-Sr and K-Ar data is not revealed by our procedure; (4) the emerald mineralization and K-metamorphism appear to be linked with the thermal history of the leucogranite; (5) in addition to its use in polyphase crustal domains, accurate 40Ar/ 39Ar dating is of major interest in the field of metallogenic models, even, for instance, for mineralizations characterized by disturbed isotopic systems, which record effects as excess argon.

Isotopic composition of CO 2 and dissolved carbon species in basalt glass

The isotopic composition of coexisting fluid and dissolved carbon species in MORB glass have been studied using a combination of replicate stepped heating experiments, vacuum crushing and grain-size analysis. Stepped heating carbon release profiles demonstrate the presence of isotopically light carbon released by combustion below 600°C (LTC) and isotopically heavier carbon released above 600°C (HTC). Up to 95% of the LTC component, which mostly resides on the surface of the glass, can be removed by extraction in dichloromethane; combustion at 400 and 600°C ensures its complete removal prior to extraction of HTC species above 600°C. Assuming that HTC consists of fluid (i.e. CO 2 trapped within vesicles) and dissolved ( i.e. carbonate ion complexes) species, limits on the net difference between the isotopic composition of coexisting fluid and dissolved carbon have been estimated from stepped heating data. The apparent magnitude of carbon isotopic fractionation between CO 2 vapour and basaltic melt is greater than 1%. but significantly less than 4:5%. and localised fractionation processes associated with magma degassing are associated with Δco 2-basait men values of ≈2%.

40Ar/ 39Ar thermochronometry of the Imataca Complex, Venezuela

The Imataca Complex of Venezuela, an Archean metamorphic belt located on the northern margin of the Amazonian Craton, underwent upper amphibolite to granulite facies metamorphism during the 2.0 Ga Trans-Amazonian Orogeny. 40Ar/ 39Ar plateau dates on hornblende separates from the Imataca Complex vary from 1972 to 1760 Ma and record post Trans-Amazonian Orogeny cooling. Biotite and feldspar pairs, which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories, yield distinct results. The biotite 40Ar/ 39Ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 Ma, whereas perthitic potassium feldspar yield stair-step 40Ar/ 39Ar spectra and integrated dates ranging from 1395 to 1166 Ma. The internally discordant perthite spectra appear to reflect cooling at rates of 0.1–0.2°C Ma −1, consistent with the cooling rates estimated from the blocking temperature and age differences between biotite-feldspar pairs from the same sample. Two plagioclase samples yield saddle-shaped 40Ar/ 39Ar spectra with 1235 and 1221 Ma minimum dates. These internally discordant spectra are caused by degassing of non-atmospheric, trapped Ar from the plagioclase and of radiogenic Ar sited in K-rich inclusion phases. The 2.0–1.7 Ga biotite dates are confined to the margins of the Imataca Complex and record the waning stages of Trans-Amazonian Orogeny uplift and cooling. The 1.4-1.2 Ga biotite dates, which are confined to the center of the Imataca Complex, are correlated with their 200–350°C Ar blocking temperatures as estimated from their average grain size and Fe/(Fe + Mg) composition. The 1.3-1.1 Ga feldspar ages are correlated with their 200–300°C Ar blocking temperatures as determined from analysis of the 40Ar/ 39Ar step-heating data. The extremely slow cooling rates implied by the biotite-feldspar pairs suggest that between 1.7 and 1.1 Ga, the Imataca Complex cooled isobarically at intermediate crustal levels. All Ar mineral systems closed by 1.1 Ga, perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the Nickorie Episode.

On the significance of argon release from biotite and amphibole during 40Ar/ 39Ar vacuum heating

The nature of Ar release from biotite and amphibole is addressed via vacuum-heating experiments followed by measurements of 40Ar loss and unit cell parameters and scanning electron microscope observations of the heated specimens. 40Ar diffusion during hydrothermal heating was previously reported for the studied biotite and amphibole specimens allowing direct comparison to the Ar loss kinetics from vacuum heating. The results demonstrate conclusively that volume diffusion is not the mechanism controlling Ar loss during normal 40Ar/ 39Ar measurements. These hydrous minerals also suffer weight loss and irreversible structural changes in the range 600 to 800°C. Moreover, biotite suffers morphological modifications dramatically expressed by splitting along the basal cleavage. A few experiments on muscovite suggest similar weight loss and splitting effects. The behavior of these minerals has important implications for the interpretation of 40Ar/ 39Ar step-heating data. The Ar-loss kinetics of 40Ar/ 39Ar experiments cannot be used to calculate diffusion coefficients and it is inappropriate to estimate closure temperatures from such data. The morphological changes will not permit the meaningful resolution of “fossil” Ar gradients which may have been present in the initial grains.

Formula omitted] and K/Ar dating of the bishop and fish canyon tuffs: Calibration ages for fission-track dating standards

Ambiguous values of the 238U spontaneous fission decay constant and complexities of absolute neutron dosimetry underline the dependency of fission-track dating upon age standards. New K/Ar and 40 Ar 39 Ar step-heating data are presented for two putative standards: the Bishop Tuff, California and the Fish Canyon Tuff, Colorado. Measured K/Ar ages of 0.741 ± 0.014 Ma for Bishop Tuff sanidine and 27.4 ± 0.4 Ma for the Fish Canyon Tuff (mean of concordant results on biotite, sanidine, hornblende and plagioclase) show good agreement with previous results. 40 Ar 39 Ar plateaus for Bishop Tuff sanidine (plateau age 0.734 ±0.024 Ma) and Fish Canyon Tuff biotite (plateau age 27.8 ± 0.2 Ma) suggest minimal thermal disturbance since deposition of the tuffs, supporting the underlying assumption of equality of their fission-track and argon ages.