The statistical assessment of Rb-Sr isochrons

McIntyre, G. A.; Brooks, Christopher; Compston, W.; Turek, A.

doi:10.1029/jz071i022p05459

Cited by 472 publications

(104 citation statements)

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“…Following McIntyre et al (42), the MSWD is equivalent to the 2 statistic divided by the number of degrees of freedom and compares the observed variability within the sample population to the expected variability defined by the analytical uncertainties of the measurements. An MSWD value of 1.0 indicates that analytical errors are as expected, whereas a value less than 1.0 indicates that the analytical error may be overestimated.…”

Section: Methodsmentioning

confidence: 99%

Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali : Mio-Pliocene hominids from Chad

Lebatard

Bourlès

Duringer

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

194

112

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Ages were determined at two hominid localities from the Chad Basin in the Djurab Desert (Northern Chad). In the Koro Toro fossiliferous area, KT 12 locality (16°00N, 18°53E) was the site of discovery of Australopithecus bahrelghazali (Abel) and in the Toros-Menalla fossiliferous area, TM 266 locality (16°15N, 17°29E) was the site of discovery of Sahelanthropus tchadensis (Toumaï). At both localities, the evolutive degree of the associated fossil mammal assemblages allowed a biochronological estimation of the hominid remains: early Pliocene (3-3.5 Ma) at KT 12 and late Miocene (Ϸ7 Ma) at TM 266. Atmospheric 10 Be, a cosmogenic nuclide, was used to quasicontinuously date these sedimentary units. The authigenic 10 Be/ 9 Be dating of a pelite relic within the sedimentary level containing Abel yields an age of 3.58 ؎ 0.27 Ma that points to the contemporaneity of Australopithecus bahrelghazali (Abel) with Australopithecus afarensis (Lucy). The 28 10 Be/ 9 Be ages obtained within the anthracotheriid unit containing Toumaï bracket, by absolute dating, the age of Sahelanthropus tchadensis to lie between 6.8 and 7.2 Ma. This chronological constraint is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock.beryllium-10 ͉ absolute dating ͉ hominid site ͉ Abel ͉ Toumaï

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Section: Methodsmentioning

confidence: 99%

Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali : Mio-Pliocene hominids from Chad

Lebatard

Bourlès

Duringer

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

194

112

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“…Regression of isochrons is based on the least-square method with 2 error notation (McIntyre et al, 1966;York, 1969;Titterington and Halliday, 1979). All other errors cited in the text are at the 2 level.…”

Section: Analytical Proceduresmentioning

confidence: 99%

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

Heumann

Davies

Elliott

2002

Geochimica et Cosmochimica Acta

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Abstract-In this study, we present 87 Rb/ 86 Sr and 230 Th/ 238 U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. Both isotope systems indicate complex magma evolution with preeruptive mineral crystallization and magma fractionation, followed by extended storage in a silicic magma reservoir. Glass analyses yield a Rb-Sr isochron of 257 Ϯ 39 ka, which can be explained by a feldspar-fractionation event ϳ150 ky before eruption. Individual feldspar-glass pairs confirm this age result. A mineral 230 Th-238 U isochron in a low-silica rhyolite from the Deer Mountain Dome defines an age of 236 Ϯ 1 ka, but the glass and whole rock do not lie on the isochron. U-Th fractionation of the rocks is controlled by the accessory minerals zircon and probably allanite, which crystallized at 250 Ϯ 3 ka and 187 Ϯ 9 ka, respectively. All major mineral phases contain accessory mineral phases; therefore, the mineral isochron represents a mixture of zircon and allanite populations. A precision of Ϯ1 ka for the mixing array implies that the minor phases must have crystallized within this timescale. Longer periods of crystal growth would cause the mixing array to be less well defined. U-series data from other lowand high-silica rhyolites indicate younger accessory mineral crystallization events at ϳ200 and 140 ka, probably related to the thermal evolution of the magma reservoir. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. We interpret the indistinguishable age results from both isotope systems (ϳ250 ka) to record the fractionation of small magma batches by filter pressing from a much larger underlying magma volume, followed by physical isolation and extended storage at the top of the magma reservoir for up to 150 ky.

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“…If one or more of these conditions is not met, then the analyses will not conform to the regression (within reasonable expectation; cf. Brooks et al 1972, Mclntyre et al 1966, in which case statistical treatment and analysis of the data may be applied to evaluate the reliability of the "age", to infer the geological basis of the uncertainty, and to investigate the disturbance of the system.…”

Section: (8)mentioning

confidence: 99%