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The obsidian from Quiron (Salta Province, Argentina): a new reference glass for fission-track dating
Radiation Measurements 39 (2005) 613 – 616 www.elsevier.com/locate/radmeas
The obsidian from Quiron (Salta Province, Argentina): a new reference glass for fission-track dating G. Bigazzia,∗ , M.A. Laurenzia , J.G. Viramonteb a Istituto di Geoscienze e Georisorse, C.N.R., Area della Ricerca di Pisa, V.G. Moruzzi, 1, 56124 Pisa, Italy b Instituto Geonorte, Universidad Nacional de Salta and Conicet, Buenos Aires 177, 4400 Salta, Argentina
Received 13 February 2004; accepted 13 July 2004
Abstract In the course of a geochronological study of the volcanic activity in the Andean Cordillera in northern Argentina, we have found in the El Quevar volcanic complex (24◦ 19 S/66◦ 43 W, 6180 m) a phenocryst poor obsidian (Quiron obsidian) showing an unusually high spontaneous track density. Defects which might produce “spurious” tracks are virtually absent. Application of fission-track dating using an absolute approach, based on the IRMM-540 standard glass for neutron fluence measurements, yielded an apparent age of 7.27 ± 0.29 Ma (1) and a plateau age of 8.99 ± 0.31 Ma (1). A 40Ar–39Ar isochron age on biotite of 8.61 ± 0.04 Ma (1) was already available for the Quiron rhyolite. We determined further 40Ar–39Ar ages on several chips of the glass itself using two analytical approaches: total fusion with a focussed laser beam (LTFA) and a step-heating approach using a de-focussed laser beam (LSHA). We have obtained a weighted average of 8.77 ± 0.09 Ma, an isochron age of 8.71 ± 0.12 Ma and an integrated age of 8.77 ± 0.09 Ma for LTF analyses, and a w.a. of 8.75 ± 0.09 Ma, an iso.a. of 8.77 ± 0.09 Ma and an int.a. of 8.77 ± 0.09 Ma for LSH analyses (all age errors are 2). The Quiron obsidian is very easy to analyse for its high spontaneous track density and because microlites which might produce spurious tracks are very rare. Independent reference 40Ar–39Ar ages determined in different laboratories are available. For these reasons we believe that this glass may be very useful for testing fission-track system calibrations and apparent age correction procedures. Splits of obsidian Quiron will be distributed upon request to colleagues who intend to test it. © 2005 Elsevier Ltd. All rights reserved. Keywords: Obsidian; Fission-track dating; 40Ar–39Ar dating; Reference glass
1. Introduction Reference materials play an important part in fission-track dating whether the calibration or an absolute approach is adopted. One can attain a reliable fission-track calibration system only through these materials. The only glass enclosed in the list of the age standards by the I.U.G.S. Subcommis∗ Corresponding author. Tel.: +39 050 315 2283; fax: +39 050 315 3280. E-mail address: [email protected] (G. Bigazzi).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.12.005
sion on Geochronology is Moldavite, the central European tektite (Hurford, 1990). Other glasses, namely: obsidian JAS-G1 (Japan), Macusanite (Peru) and the rhyolitic glass Roccastrada (Italy) have also been proposed as putative age standards and/or useful materials for standardization of analytical procedures (Balestrieri et al., 1998, and references therein). Recently, Latin American obsidian that was being analysed using the fission-track dating method revealed an unusually high spontaneous track density (around 100, 000 cm−2 ) due to a relatively high U content
614
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
(28.5 ppm). Natural glasses with similar track densities are very rare. Considering, for example, the glasses mentioned above, the spontaneous track density ratio between this obsidian and these glasses is > 6, > 30, > 3 and > 4, respectively. A relatively high track density makes track counting procedures easier and may be a useful requisite when a sample is used for testing experimental methodologies. For these reasons we decided to share this glass with the fission-track community and to propose it as a useful reference material. For this purpose an independent 40Ar–39Ar age was also determined.
2. The Quiron obsidian The El Quevar volcanic complex (24◦ 19 S/66◦ 43 W, 6180 m) rises on about 50 km southwest of San Antonio de los Cobres (Salta Province), in the Andean Cordillera of northern Argentina. Its volcanic activity developed during Late Miocene. The evolution and the volcanic stratigraphy of this complex have recently been studied in detail by Goddard et al. (1999). One of the products of the El Quevar complex is a rhyolite lava dome (Quiron rhyolite), erupted from a source to the west of Cerro Azufre (about 5 km south–southwest of Cerro Quevar). This dome outcrops in Quebrada Quiron and Quebrada Incahuasi. The Quiron rhyolite consists of a 15–30 m body of fractured green perlite, bonded at the top and at the base by a rapidly cooled shell of < 2 m phenocryst poor (5%) obsidian. This is the Quiron obsidian subject of this study.
3. Fission-track dating The obsidian of Quiron was analysed using the techniques described in previous papers (Balestrieri et al., 1998, and references therein). One split of glass was irradiated in the Lazy Susan facility (Cd ratio 6.4 for Au and 48 for Co) of the LENA Triga Mark II reactor of the University of Pavia, Italy. The neutron fluence was determined using the NRM IRMM-540 standard glass (De Corte et al., 1998).
Fig. 1. The reduced spontaneous to induced track-size ratio (DS /DI =0.88) indicates that the spontaneous tracks of the Quiron obsidian suffered a moderate degree of thermal annealing. After the thermal treatment imposed for the plateau age determination the DS /DI ratio is ∼ 1, indicating that the plateau condition was attained.
After irradiation, two fractions of sample for spontaneous and induced (the irradiated one) track density determination, respectively, were mounted in epoxy resin, grounded and polished in order to expose an internal surface, and then etched for 120 s in 20% HF at 40 ◦ C for track development. Tracks were counted in transmitted light under a Leica microscope at a magnification of 500×. Track sizes were measured using Leica Microvid equipment at a magnification of 1000×. The results of the measurements are shown in Table 1. The thermal stability of fission tracks in natural glasses over geological times is relatively poor. A certain amount of annealing of spontaneous tracks occurs in many glasses even at room temperature. Partially annealed tracks are revealed with reduced efficiency compared to the “fresh” induced tracks produced by the irradiation. Therefore, a fission-track age on glass is commonly a reduced age (“apparent age”). Storzer and Wagner (1969) have shown that the annealing amount can be estimated in glass by track-size (the major axis of the etch pit) measurements. The obsidian Quiron shows a reduced (< 1) spontaneous to induced track-size ratio (the induced tracks are assumed here as undisturbed reference tracks), DS /DI =0.88 (Fig. 1), which corresponds
Table 1 Fission-track dating of the obsidian from Quiron, Salta Province (Argentina) Sample
(×1015 ) (cm−2 )
S (cm−2 )
NS
I (cm−2 )
NI
p(2 ) (%)
DS /DI
Age ± 1 (Ma)
App. age 4 h 220 ◦ C
3.15
93,400 75,900
1349 2248
2,014,600 1,324,800
1217 2102
98 53
0.88 1.00
7.27 ± 0.29 8.99 ± 0.31
App. age: apparent age; 4 h 220 ◦ C: thermal treatment imposed for the plateau age determination; : neutron fluence; S (I ): spontaneous (induced) track density; NS (NI ): spontaneous (induced) tracks counted; p(2 ): probability of obtaining 2 value testing induced track counts against a Poisson distribution; DS /DI : spontaneous to induced track-size ratio. Parameters used for age calculation: =1.55125×10−10 a−1 ; F =8.46×10−17 a−1 ; =5.802×10−22 cm2 ; 238 U/235 U isotopic ratio= 137.88.
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
to a moderate degree of track annealing and of rejuvenation of the fission-track age. The plateau method proposed by Storzer and Poupeau (1973) for correcting thermally lowered fission-track ages was used in this work. This technique consists in reestablishing by laboratory thermal treatments an identical etching efficiency of spontaneous and induced tracks. The plateau method is considered to yield a reliable estimate of the formation age of a glass, at least in case of a simple thermal history such as an annealing process at ambient temperature (see Wagner and Van den Haute, 1992, and references therein). Following the common practice, only one heating step was used here (Arias et al., 1981; Westgate, 1989). After the thermal treatment imposed for the plateau age determination, the DS /DI ratio became ∼ 1. This proves that the plateau condition—an identical etching efficiency of spontaneous and induced tracks—was attained (Table 1, Fig. 1). 4. 40Ar–39Ar dating A 40Ar/39Ar age (Willson et al., 1999) on biotite had been determined by incremental heating experiment, carried out at the Oregon State University (USA). Sample was irradiated with fast neutrons at the OSU TRIGA reactor for 6 h at 1 MW power. The sample was heated under high vacuum in a resistance furnace at increasing temperatures. After each step-heating the gas composition was analysed and an age determined. Step ages were calculated assuming an atmospheric initial Ar composition (40 Ar/36 Ar = 295.5). A biotite isochron age of 8.61±0.04 Ma (1) was obtained with a Y-intercept of 293.08 ± 0.04 and a correlation coefficient of 0.999 (Willson et al., 1999). In this work 40Ar–39Ar dating was applied to the glass itself. Clear, unaltered and inclusions-free chips of obsidian were carefully handpicked under a binocular microscope for the 40Ar–39Ar analysis. The sample was irradiated for 10 h, along with FCT#3 biotite as a flux monitor, in the core of the 250 kW TRIGA reactor at Pavia (see Laurenzi et al., 2003, for details). The reference age for the standard used in this paper is 27.95 Ma (Baksi et al., 1996), and the K decay constant (− + E.C.) is 5.543 × 10−10 a−1 (Steiger and Jäger, 1977). Errors quoted in the following discussion are at the ±2 analytical level, unless otherwise stated.
615
Fig. 2. Age spectrum of the Quiron obsidian. The abscissa represents the cumulative percentage of 39Ar released during the analysis. Error-boxes represent the ±2 analytical error. The error on the plateau age (81% cumulative 39Ar released) comprises the uncertainty in the irradiation parameter J .
Two complementary analytical approaches have been used: 10 glass fragments were completely melted in a unique solution with a focussed laser beam (LTFA), and another set of chips was step-heated using a de-focussed laser beam (LSHA). Raw data were corrected for blanks, mass discrimination, and nuclear interferences. The radiogenic yields are very high, comprised between 97% and 99% for LTFA, around 98–99% on the majority of steps in LSHA with lower values in low- and high-temperature steps. The K/Ca ratio, obtained through the K-derived 39Ar and the Ca-derived 37Ar, is equal within error to the one calculated by the chemical analyses. The 10 single fusion ages give a mean apparent age of 8.77 ± 0.04 Ma (MSWD = 1.34), obtained by weighting each age by the inverse of its variance, and assuming an atmospheric (i.e. 295.5) initial 40Ar/36Ar. Both these last assumptions have been used to calculate the plateau age obtained in the step-heating experiment: 8.75 ± 0.09 Ma (13 steps, 80.7% of 39Ar release, MSWD = 1.01) (Fig. 2). Ages calculated with the isochron approach have higher uncertainties, due to the low dispersion of data points: 8.71 ± 0.12 Ma (MSWD = 1.16) for LTFA and 8.77 ± 0.08 Ma (MSWD = 0.84) for LSHA. All data are concordant within analytical error; the final uncertainties on the obtained ages are shown in Table 2, where the error on the irradiation parameter J is also considered.
Table 2 40Ar/39Ar age data of the obsidian from Quiron Analysis
Weighted average ± 2 (Ma)
MSWD
Isochron age ± 2 (Ma)
MSWD
Integrated age ± 2 (Ma)
LTFA LSHA
8.77 ± 0.09 8.75 ± 0.09(∗)
1.34 1.01
8.71 ± 0.12 8.77 ± 0.09
1.16 1.01
8.77 ± 0.09 8.77 ± 0.09
LTFA: laser total fusion analysis; LSHA: laser step-heating analysis; integrated age: obtained from the sum of 39Ar(K) and 40Ar (radiogenic) of the sample; (*) plateau age, which is the weighted average of consecutive steps with the same age within the analytical error, and contains 81% of 39Ar(K) release.
616
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
5. Conclusions The fission-track plateau age and 40Ar–39Ar age determined on obsidian Quiron are in agreement within experimental errors. This glass shows a relatively high spontaneous track density. Defects which may produce “spurious” tracks, such as microlites or bubbles, are virtually absent. For these reasons we propose it as a useful additional reference material for interlaboratory comparisons and analytical procedures standardization, including correction techniques of thermally lowered fission-track ages. It has to be pointed out that the obsidian Quiron cannot be considered an age standard, as it does not fulfill one of the characteristics that the fission-track community had requested for an age standard: no corrections should be necessary in obtaining the fission-track age (Hurford and Green, 1981). Although this requirement appears very judicious, considering that a general consensus on the reliability of age correction techniques does not exist, it might be only a hypothetical one. A natural glass with a reasonable track density certainly unaffected by spontaneous track annealing has not been identified yet. Splits of obsidian Quiron will be distributed upon request (contact: G.B.) to colleagues who intend to test it.
References Arias, C., Bigazzi, G., Bonadonna, F.P., 1981. Size correction and plateau age in glass shards. Nucl. Tracks Radiat. Meas. 5, 129–136. Baksi, A.J., Archibald, D.A., Farrar, E., 1996. Intercalibration of 40Ar/39Ar dating standards. Chem. Geol. 129, 307–324. Balestrieri, M.L., Bigazzi, G., Bouška, V., Labrin, E., Hadler, N.J.C., Kitada, N., Osorio, A.A.M., Poupeau, G., Wadatsumi, K., Zúñiga, A., 1998. Potential glass age standards for fission-track dating: an overview. In: Van den Haute, P., De Corte, F. (Eds.),
Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Dordrecht, pp. 287–304. De Corte, F., Bellemans, F., Van den Haute, P., Ingelbrecht, C., Nichol, C., 1998. A new U doped glass certified by the European Commission for the calibration of fission-track dating. In: Van den Haute, P., De Corte, F. (Eds.), Advances in FissionTrack Geochronology. Kluwer Academic Publishers, Dordrecht, pp. 67–78. Goddard, P., Willson, J.J., Couch, S., Viramonte, J.G., 1999. The evolution of the El Quevar volcanic complex, Salta Province, Argentina. XIV Congreso Geológico Argentino, Salta 1999. Actas II, 225–227. Hurford, A.J., 1990. Standardization of fission track dating calibration: recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology. Chem. Geol. (Isot. Geosci. Sec.) 80, 171–178. Hurford, A.J., Green, P.F., 1981. Standards dosimetry and the uranium-238 f decay constant: a discussion. Nucl. Tracks Radiat. Meas. 5, 73–75. Laurenzi, M.A., Bigazzi, G., Balestrieri, M.L., Bouška, V., 2003. 40Ar/39Ar laser probe dating of the Central European tektiteproducing impact event. Meteorit. Planet. Sci. 38, 887–894. Steiger, R.H., Jäger, E., 1977. Subcommission on geochronology: convention on the use of decay constant in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Storzer, D., Poupeau, G., 1973. Ages-plateau de minéraux et verres par la méthode des traces de fission. C.R. Acad. Sci. Paris Série D 276, 137–139. Storzer, D., Wagner, G.A., 1969. Correction of thermally lowered fission-track ages of tektites. Earth Planet. Sci. Lett. 5, 463–468. Wagner, G.A., Van den Haute, P., 1992. Fission Track Dating. Kluwer Academic Publishers, Dordrecht, 285pp. Westgate, J.A., 1989. Isothermal plateau fission track ages of hydrated glass shards from silicic tephra beds. Earth Planet. Sci. Lett. 95, 226–234. Willson, J.J., Goddard, P.R., Couch, S., 1999. Evolution of a Volcano, “El Quevar ‘98” University of Bristol–Universidad Nacional de Salta Geological Expedition. In: Smart, P. (Ed.), University of Bristol Expedition Reports Series, Bristol, 116pp, 5 appendices.
The obsidian from Quiron (Salta Province, Argentina): a new reference glass for fission-track dating G. Bigazzia,∗ , M.A. Laurenzia , J.G. Viramonteb a Istituto di Geoscienze e Georisorse, C.N.R., Area della Ricerca di Pisa, V.G. Moruzzi, 1, 56124 Pisa, Italy b Instituto Geonorte, Universidad Nacional de Salta and Conicet, Buenos Aires 177, 4400 Salta, Argentina
Received 13 February 2004; accepted 13 July 2004
Abstract In the course of a geochronological study of the volcanic activity in the Andean Cordillera in northern Argentina, we have found in the El Quevar volcanic complex (24◦ 19 S/66◦ 43 W, 6180 m) a phenocryst poor obsidian (Quiron obsidian) showing an unusually high spontaneous track density. Defects which might produce “spurious” tracks are virtually absent. Application of fission-track dating using an absolute approach, based on the IRMM-540 standard glass for neutron fluence measurements, yielded an apparent age of 7.27 ± 0.29 Ma (1) and a plateau age of 8.99 ± 0.31 Ma (1). A 40Ar–39Ar isochron age on biotite of 8.61 ± 0.04 Ma (1) was already available for the Quiron rhyolite. We determined further 40Ar–39Ar ages on several chips of the glass itself using two analytical approaches: total fusion with a focussed laser beam (LTFA) and a step-heating approach using a de-focussed laser beam (LSHA). We have obtained a weighted average of 8.77 ± 0.09 Ma, an isochron age of 8.71 ± 0.12 Ma and an integrated age of 8.77 ± 0.09 Ma for LTF analyses, and a w.a. of 8.75 ± 0.09 Ma, an iso.a. of 8.77 ± 0.09 Ma and an int.a. of 8.77 ± 0.09 Ma for LSH analyses (all age errors are 2). The Quiron obsidian is very easy to analyse for its high spontaneous track density and because microlites which might produce spurious tracks are very rare. Independent reference 40Ar–39Ar ages determined in different laboratories are available. For these reasons we believe that this glass may be very useful for testing fission-track system calibrations and apparent age correction procedures. Splits of obsidian Quiron will be distributed upon request to colleagues who intend to test it. © 2005 Elsevier Ltd. All rights reserved. Keywords: Obsidian; Fission-track dating; 40Ar–39Ar dating; Reference glass
1. Introduction Reference materials play an important part in fission-track dating whether the calibration or an absolute approach is adopted. One can attain a reliable fission-track calibration system only through these materials. The only glass enclosed in the list of the age standards by the I.U.G.S. Subcommis∗ Corresponding author. Tel.: +39 050 315 2283; fax: +39 050 315 3280. E-mail address: [email protected] (G. Bigazzi).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.12.005
sion on Geochronology is Moldavite, the central European tektite (Hurford, 1990). Other glasses, namely: obsidian JAS-G1 (Japan), Macusanite (Peru) and the rhyolitic glass Roccastrada (Italy) have also been proposed as putative age standards and/or useful materials for standardization of analytical procedures (Balestrieri et al., 1998, and references therein). Recently, Latin American obsidian that was being analysed using the fission-track dating method revealed an unusually high spontaneous track density (around 100, 000 cm−2 ) due to a relatively high U content
614
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
(28.5 ppm). Natural glasses with similar track densities are very rare. Considering, for example, the glasses mentioned above, the spontaneous track density ratio between this obsidian and these glasses is > 6, > 30, > 3 and > 4, respectively. A relatively high track density makes track counting procedures easier and may be a useful requisite when a sample is used for testing experimental methodologies. For these reasons we decided to share this glass with the fission-track community and to propose it as a useful reference material. For this purpose an independent 40Ar–39Ar age was also determined.
2. The Quiron obsidian The El Quevar volcanic complex (24◦ 19 S/66◦ 43 W, 6180 m) rises on about 50 km southwest of San Antonio de los Cobres (Salta Province), in the Andean Cordillera of northern Argentina. Its volcanic activity developed during Late Miocene. The evolution and the volcanic stratigraphy of this complex have recently been studied in detail by Goddard et al. (1999). One of the products of the El Quevar complex is a rhyolite lava dome (Quiron rhyolite), erupted from a source to the west of Cerro Azufre (about 5 km south–southwest of Cerro Quevar). This dome outcrops in Quebrada Quiron and Quebrada Incahuasi. The Quiron rhyolite consists of a 15–30 m body of fractured green perlite, bonded at the top and at the base by a rapidly cooled shell of < 2 m phenocryst poor (5%) obsidian. This is the Quiron obsidian subject of this study.
3. Fission-track dating The obsidian of Quiron was analysed using the techniques described in previous papers (Balestrieri et al., 1998, and references therein). One split of glass was irradiated in the Lazy Susan facility (Cd ratio 6.4 for Au and 48 for Co) of the LENA Triga Mark II reactor of the University of Pavia, Italy. The neutron fluence was determined using the NRM IRMM-540 standard glass (De Corte et al., 1998).
Fig. 1. The reduced spontaneous to induced track-size ratio (DS /DI =0.88) indicates that the spontaneous tracks of the Quiron obsidian suffered a moderate degree of thermal annealing. After the thermal treatment imposed for the plateau age determination the DS /DI ratio is ∼ 1, indicating that the plateau condition was attained.
After irradiation, two fractions of sample for spontaneous and induced (the irradiated one) track density determination, respectively, were mounted in epoxy resin, grounded and polished in order to expose an internal surface, and then etched for 120 s in 20% HF at 40 ◦ C for track development. Tracks were counted in transmitted light under a Leica microscope at a magnification of 500×. Track sizes were measured using Leica Microvid equipment at a magnification of 1000×. The results of the measurements are shown in Table 1. The thermal stability of fission tracks in natural glasses over geological times is relatively poor. A certain amount of annealing of spontaneous tracks occurs in many glasses even at room temperature. Partially annealed tracks are revealed with reduced efficiency compared to the “fresh” induced tracks produced by the irradiation. Therefore, a fission-track age on glass is commonly a reduced age (“apparent age”). Storzer and Wagner (1969) have shown that the annealing amount can be estimated in glass by track-size (the major axis of the etch pit) measurements. The obsidian Quiron shows a reduced (< 1) spontaneous to induced track-size ratio (the induced tracks are assumed here as undisturbed reference tracks), DS /DI =0.88 (Fig. 1), which corresponds
Table 1 Fission-track dating of the obsidian from Quiron, Salta Province (Argentina) Sample
(×1015 ) (cm−2 )
S (cm−2 )
NS
I (cm−2 )
NI
p(2 ) (%)
DS /DI
Age ± 1 (Ma)
App. age 4 h 220 ◦ C
3.15
93,400 75,900
1349 2248
2,014,600 1,324,800
1217 2102
98 53
0.88 1.00
7.27 ± 0.29 8.99 ± 0.31
App. age: apparent age; 4 h 220 ◦ C: thermal treatment imposed for the plateau age determination; : neutron fluence; S (I ): spontaneous (induced) track density; NS (NI ): spontaneous (induced) tracks counted; p(2 ): probability of obtaining 2 value testing induced track counts against a Poisson distribution; DS /DI : spontaneous to induced track-size ratio. Parameters used for age calculation: =1.55125×10−10 a−1 ; F =8.46×10−17 a−1 ; =5.802×10−22 cm2 ; 238 U/235 U isotopic ratio= 137.88.
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
to a moderate degree of track annealing and of rejuvenation of the fission-track age. The plateau method proposed by Storzer and Poupeau (1973) for correcting thermally lowered fission-track ages was used in this work. This technique consists in reestablishing by laboratory thermal treatments an identical etching efficiency of spontaneous and induced tracks. The plateau method is considered to yield a reliable estimate of the formation age of a glass, at least in case of a simple thermal history such as an annealing process at ambient temperature (see Wagner and Van den Haute, 1992, and references therein). Following the common practice, only one heating step was used here (Arias et al., 1981; Westgate, 1989). After the thermal treatment imposed for the plateau age determination, the DS /DI ratio became ∼ 1. This proves that the plateau condition—an identical etching efficiency of spontaneous and induced tracks—was attained (Table 1, Fig. 1). 4. 40Ar–39Ar dating A 40Ar/39Ar age (Willson et al., 1999) on biotite had been determined by incremental heating experiment, carried out at the Oregon State University (USA). Sample was irradiated with fast neutrons at the OSU TRIGA reactor for 6 h at 1 MW power. The sample was heated under high vacuum in a resistance furnace at increasing temperatures. After each step-heating the gas composition was analysed and an age determined. Step ages were calculated assuming an atmospheric initial Ar composition (40 Ar/36 Ar = 295.5). A biotite isochron age of 8.61±0.04 Ma (1) was obtained with a Y-intercept of 293.08 ± 0.04 and a correlation coefficient of 0.999 (Willson et al., 1999). In this work 40Ar–39Ar dating was applied to the glass itself. Clear, unaltered and inclusions-free chips of obsidian were carefully handpicked under a binocular microscope for the 40Ar–39Ar analysis. The sample was irradiated for 10 h, along with FCT#3 biotite as a flux monitor, in the core of the 250 kW TRIGA reactor at Pavia (see Laurenzi et al., 2003, for details). The reference age for the standard used in this paper is 27.95 Ma (Baksi et al., 1996), and the K decay constant (− + E.C.) is 5.543 × 10−10 a−1 (Steiger and Jäger, 1977). Errors quoted in the following discussion are at the ±2 analytical level, unless otherwise stated.
615
Fig. 2. Age spectrum of the Quiron obsidian. The abscissa represents the cumulative percentage of 39Ar released during the analysis. Error-boxes represent the ±2 analytical error. The error on the plateau age (81% cumulative 39Ar released) comprises the uncertainty in the irradiation parameter J .
Two complementary analytical approaches have been used: 10 glass fragments were completely melted in a unique solution with a focussed laser beam (LTFA), and another set of chips was step-heated using a de-focussed laser beam (LSHA). Raw data were corrected for blanks, mass discrimination, and nuclear interferences. The radiogenic yields are very high, comprised between 97% and 99% for LTFA, around 98–99% on the majority of steps in LSHA with lower values in low- and high-temperature steps. The K/Ca ratio, obtained through the K-derived 39Ar and the Ca-derived 37Ar, is equal within error to the one calculated by the chemical analyses. The 10 single fusion ages give a mean apparent age of 8.77 ± 0.04 Ma (MSWD = 1.34), obtained by weighting each age by the inverse of its variance, and assuming an atmospheric (i.e. 295.5) initial 40Ar/36Ar. Both these last assumptions have been used to calculate the plateau age obtained in the step-heating experiment: 8.75 ± 0.09 Ma (13 steps, 80.7% of 39Ar release, MSWD = 1.01) (Fig. 2). Ages calculated with the isochron approach have higher uncertainties, due to the low dispersion of data points: 8.71 ± 0.12 Ma (MSWD = 1.16) for LTFA and 8.77 ± 0.08 Ma (MSWD = 0.84) for LSHA. All data are concordant within analytical error; the final uncertainties on the obtained ages are shown in Table 2, where the error on the irradiation parameter J is also considered.
Table 2 40Ar/39Ar age data of the obsidian from Quiron Analysis
Weighted average ± 2 (Ma)
MSWD
Isochron age ± 2 (Ma)
MSWD
Integrated age ± 2 (Ma)
LTFA LSHA
8.77 ± 0.09 8.75 ± 0.09(∗)
1.34 1.01
8.71 ± 0.12 8.77 ± 0.09
1.16 1.01
8.77 ± 0.09 8.77 ± 0.09
LTFA: laser total fusion analysis; LSHA: laser step-heating analysis; integrated age: obtained from the sum of 39Ar(K) and 40Ar (radiogenic) of the sample; (*) plateau age, which is the weighted average of consecutive steps with the same age within the analytical error, and contains 81% of 39Ar(K) release.
616
G. Bigazzi et al. / Radiation Measurements 39 (2005) 613 – 616
5. Conclusions The fission-track plateau age and 40Ar–39Ar age determined on obsidian Quiron are in agreement within experimental errors. This glass shows a relatively high spontaneous track density. Defects which may produce “spurious” tracks, such as microlites or bubbles, are virtually absent. For these reasons we propose it as a useful additional reference material for interlaboratory comparisons and analytical procedures standardization, including correction techniques of thermally lowered fission-track ages. It has to be pointed out that the obsidian Quiron cannot be considered an age standard, as it does not fulfill one of the characteristics that the fission-track community had requested for an age standard: no corrections should be necessary in obtaining the fission-track age (Hurford and Green, 1981). Although this requirement appears very judicious, considering that a general consensus on the reliability of age correction techniques does not exist, it might be only a hypothetical one. A natural glass with a reasonable track density certainly unaffected by spontaneous track annealing has not been identified yet. Splits of obsidian Quiron will be distributed upon request (contact: G.B.) to colleagues who intend to test it.
References Arias, C., Bigazzi, G., Bonadonna, F.P., 1981. Size correction and plateau age in glass shards. Nucl. Tracks Radiat. Meas. 5, 129–136. Baksi, A.J., Archibald, D.A., Farrar, E., 1996. Intercalibration of 40Ar/39Ar dating standards. Chem. Geol. 129, 307–324. Balestrieri, M.L., Bigazzi, G., Bouška, V., Labrin, E., Hadler, N.J.C., Kitada, N., Osorio, A.A.M., Poupeau, G., Wadatsumi, K., Zúñiga, A., 1998. Potential glass age standards for fission-track dating: an overview. In: Van den Haute, P., De Corte, F. (Eds.),
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