Rb-Sr isochron method

These problems are avoided by the whole-rock Rb-Sr isochron method of dating because the radiogenic Sr that may diffuse out of Rb-rich minerals is quantitatively retained in most cases within the grain boundaries and by other minerals in a sample of the whole rock. When Eq. 3.9 is applied to a suite of rock samples all of which have the same age t, had the same initial Sr/ Sr ratio at the time of formation, and neither gained nor lost radiogenic Sr, their measured " Sr/ Sr and Rb/ Sr ratios form points on a straight line. 

The Rb-Sr isochron method was used by Jones and Faure (1967) in Section 3.4.2 to date whole-rock samples and separated alkali feldspars of two Vanda porphyry dikes in Wright Valley. A least-squares regression of the data points in Fig. 3.14 yields a slope (m) ... 

The basement rocks of the Nilsen Plateau were dated by Eastin (1970) who used the whole-rock Rb-Sr isochron method. The resulting dates in Table 6.3 have been recalculated to A, = 1.42 x 10 year". All of the rock specimens dated by Eastin (1970) were collected by Douglas McLelland or other members of the group that mapped the Nilsen Plateau during the 1963/64 field season and were provided to Eastin by M.J. Hibbard, University of Nevada in Reno. The dates were calculated by the method of York (1966, 1969). The analytical data are tabulated in Appendix 6.7.1 and are interpreted there in Figs. 6.8-6.11. 

Any sample of rock or nfineral that does not satisfy the preconditions stated above does not plot on the isochron defined by a suite of cogenetic rocks that had the same initial Sr/ Sr ratio when they formed. Therefore, the goodness of fit of data points that define a Rb-Sr isochron is indicative of how well the assumptions of dating by this method are satisfied. 

Unfortunately, the basalt flows on the southern nunataks of Vestfjella (Fig. 14.10) cannot be dated by the whole-rock Rb-Sr method. The samples that were analyzed by Fumes et al. (1987) scatter on the Rb-Sr isochron diagram in Fig. 14.14a and do not define a unique straight line. Instead, we have drawn a reference line based on an assumed age of 170 Ma and an initial ratio of 0.70416 0.00021 (2a) which... 

Hirt, Tilton, Herr und Hoffmeister (29) haben auf diese Weise die Halb-wertszeit von Rhenium-187 zu (43 5) 109 bestimmt. Die Alterswerte, die mit der Re/Os-Methode erhalten wurden, sind deshalb mit einem groBeren Fehler behaftet als die nach der Rb/Sr-Methode bzw. nach der Pb/Pb-Methode erhaltenen Werte. Tab. 5 zeigt die Ergebnisse der Mes-sungen. Diese Werte sind in Abb. 4 eingetragen die nach der Methode der kleinsten Grundrate bestimmte Isochrone hefert ein Alter von (4,0 ... 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

This equation is used for dating suites of Sm-bearing rocks by the isochron method used also for dating comagmatic igneous rocks by the Rb-Sr method. 

The whole-rock Rb-Sr method has had limited success for dating the Admiralty Intrusives because data points scatter on the isochron diagrams. For example, only four of the nine whole-rock samples of the Yule Bay Batholith (Pennell Coast) in Fig. 4.22a analyzed by Kreuzer et al. (1987) define a straight line that yields a date of 364 5 Ma and an initial Sr/ Sr ratio of 0.71495 0.00015. The other five rock samples scatter above and below the line because their initial Sr/ Sr ratios at 360 Ma range from 0.7125 to 0.7184. 

Fig. 6.8 Rb-Sr age determinations of metasedimentary rocks of the LaGorce Formation from the valley of the Blackwall Glacier, Nilsen Plateau (Fig. 6.7). The dates and initial Sr/ Sr ratios were calculated by Eastin (1970) by the methods of York (1966, 1969) and have been adjusted to = 1.42 x 10" year" Isochron A is based on four colinear data points (472, 474, 475. and 476). Line B is defined by samples 471 and 473 which have the highest Rb/ Sr ratios and have lost some in situ produced radiogenic Sr. Plotted from data by Eastin (1970)...

The data listed below are previously unpublished because they scatter on the isochron diagram and were judged to be unsuitable for dating the Wyatt Formation by the whole-rock Rb-Sr method. Faure et al. (1979) used this data set only to set an upper limit of 788 Ma for the age of the Wyatt Formation. 

Determination of ages by the Sm/Nd method entails analyzing either individual minerals or cogenetic rock suites in which the ratios between the two are sufficiently different to define the slope of an isochron in coordinates of [ Nd/ Nd] and ] Sm/ Nd]. The method is especially suitable for mafic and ultramafic rocks, c the Rb/Sr method, which is best suited for acidic and intermediate igneous rocks enriched in rubidium and depleted in strontium. Since the rare earth elements are less mobile than the alkali metals and the alkaline earths, phenomena such as regional metamorphism have less effect on them. Hence, suitable rocks can be dated by the Sm/Nd method even if they have lost or gained rubidium and strontium and this makes the Sm/Nd method a useful complement to the R/Sr method. 

The equation above obviously describes a straight line in a diagram of ( Sr/ Sr)t versus Rb/ Sr, with the slope of the line dependent on the age of the sample, and the intercept with the y-axis the value for ( Sr/ Sr)o - sometimes also called ( Sr/ Sr)i where i refers to the initial ratio. Hence if we have two or more materials, with different Rb/ Sr ratios, which formed at the same time and in equilibrium with each other, so they both have the same ( Sr/ Sr)o, we can determine the age at which they formed, and ( Sr/ Sr)o. This is called the isochron method of dating (Figure 9.1a). 

Sr is present as a trace element in most minerals when they form. This contrasts with Ar which escapes easily from most molten rocks. Accirrate accirmirlation ages can be calculated from Eq. (7) only for rare minerals that contain large amounts of Rb and a negligible amoimt of initial Sr. However, most rocks contain measurable amounts of initial Sr. An equation like (7) is of little use because the amount of initial Sr cannot be determined. Rb-Sr dating is done using a graphical method, commonly called the isochron method, that eliminates the problem of initial Sr. 

The isochron diagram was described above in the section about the Rb-Sr method. The U-Pb concordia method differs from simple isochron methods in that it utilizes the simultaneous decay and accumulation of two parent-daughter pairs 2 U- ° Pb and U- ° Pb. If we rewrite Eq. (5) in terms of U and its final daughter product... 

Common techniques found to yield useful solidification ages include the Pb-Pb method Sm (t,/2 = 106 Ga) - Nd "Rb (t,/2 = 48 Ga) - Sr and Re (ti/2 = 41 Ga) Os (cf, 1, 2, II, IS). Generally, methods used to determine solidification ages depend upon data depicted in isochron diagrams, for example, in which enrichment of radiogenic Sr is proportional to the amount of Rb, with Sr being taken for normalization. The slope of such a line yields an "internal isochron" for a meteorite or a single inclusion of a... 

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