Strontium isotopes initial ratios

Another precise radiochronometric tool is initial Sr/ Sr, especially in objects with low bulk rubidium. Gray et al. (1973) found one Allende CAl to have the lowest initial ratio yet seen in any solar system material ( Sr/ Sr)o = 0.69877. This value has endured as a solar system benchmark for strontium isotopes, and was given the name ALL. Later work by Podosek et al. (1991) on a different set of Allende CAIs also found very low initial ratios 0.698793-0.698865, which are, however, not quite as low as ALL. An interesting result of the study of Podosek et al. (1991) is that they reanalyzed the same CAI (D7 USNM 3898 of Podosek et al.) on which ALL is based but obtained a significantly higher initial ratio than did the Caltech group. This discrepancy remains unresolved. Nevertheless, the low initial strontium ratios of CAIs are consistent with their being the oldest objects formed in the solar system. 

The first neodymium-isotopic analyses aimed at characterizing the oceans closely followed the initial development of neodymium isotopes as a chronometer and tracer (Richard et al., 1976 DePaolo and Wasserburg, 1976a O Nions et al., 1977). O Nions et al. (1978) was the first to report neodymium (along with lead and strontium) isotopes in manganese nodules and hydrothermal sediments. They confirmed the distinction between continental and mantle provenances of lead in hydrogenous and hydrothermal manganese sediments, respectively. Consistent with the previous studies, they found that strontium in these deposits is derived from seawater. All of the neodymium-isotope ratios in their samples from the Pacific were similar and lower than the bulk... 

The starting point for the evolution of strontium isotopes is the Sr/ Sr ratio at the formation of the Earth. This is taken to be the isotopic composition of basaltic achondrite meteorites, thought to have a composition approximating to that of the solar nebula at the time of planetary formation. It is usually referred to as BAEl (Basaltic Achondrite Best Initial) and the measured value is 0.69897 0.000003. 

A number of authors (e.g. Faure, 1977) have proposed on the basis of measured initial ratios that the growth of Sr/ Sr in the mantle with time defines a curvilinear path (cf. Figure 6.14, curves 1 and 2) and that this reflects the irreversible loss of Rb from the mantle into the crust during the formation of the continental crust. The loss of Rb from the mantle and its enrichment in the continental crust lead to very different patterns of strontium isotope evolution in the two reservoirs as a consequence of their different Rb/Sr ratios (Figure 6.15). The high Rb/Sr ratios found in the continental crust give rise to an accelerated... 

There are many examples in the literature of authors who have used mantle evolution diagrams of the type illustrated in Figure 6.15 to plot the initial strontium isotope ratios of measured samples relative to mantle and crustal evolution curves, in order to determine their likely source region. For example, it is easy to see from Figure 6.15 that a suite of rocks produced by partial melting of the mantie at 1.0 Ga wUl have a very different initial ratio ( Sr/ Sr = 0.7034) from rocks produced by partial melting of the crust at this time ( Sr/ Sr = 0.7140). It is this principle which may be used to identify the source of magmatic rocks of known age. 

Fig. 12.33 The initial Sr/ Sr ratios of the Kirkpatrick Basalt on Storm Peak and Mt. Falla in the Queen Alexandra Range at 170 Ma (X= 1.42 x 10 year ) range widely but cluster between 0.7100 and 0.7130 with an average of 0.7117 0.0004 (2d) for 27 specimens. This value resembles the initial ratio of the Granite Harbor Intrusives in the basement of the Transantarctic Mountains and is not compatible with basalt magma derived from the mantle. The isotope composition of strontium in sills of Ferrar Dolerite was not included in this data set by Bowman (1971) and Pace (1977) reported by Faure et al. (1974, 1982). The data of Hill (1969) were included in the report by Faure et al. (1972)...

Ma of the flows and dikes of Plogen and Basen range widely from 0.70401 to 0.70794 and from 0.511596 to 0.51259, respectively. These isotope ratios are not correlated with the reciprocals of the concentrations for strontium and neodymium, respectively, which effectively rules out magma contamination. In addition, secondary alteration of the flows is unlikely because neodymium is not a mobile element in weathering environments. Although the data points do not form mixing lines. Fig. 14.20 demonstrates that the initial ratios (at 180 Ma) and the 1/Nd... 

Fig. 3. If the minerals are closed to strontium and rubidium and all have the same initial strontium isotopic composition, but different Rb/Sr ratios, they will all initially lie on a horizontal straight line. As Rb decays to Sr each mineral will move along a straight line trajectory of slope ...

A wide variety of rock types can be dated by the 87Rb-87Sr system, provided that the samples satisfy the assumptions that the system was initially isotopically homogeneous (had a uniform 87Sr/86Sr ratio) and did not gain or lose rubidium or strontium after if formed. Both... 

The whole-rock Rb-Sr date of 1789 141 Ma (Paleoproterozoic) is the time when these rocks were recrystallized during regional metamorphism to amphibolite grade and the strontium they contained was isotopically homogenized so that all three specimens had the same initial Sr/ Sr ratio of 0.7059 0.0041. This value indicates that their protoliths contained strontium derived from the lithospheric mantle in the form of volcanic rocks or their weathering products. 

Fig. 12.15 The low-Ti flows of the Kirkpatrick Basalt in Mills Valley of Pain Mesa yield a whole-rock Rb-Sr date of 169 7 Ma (Middle Jurassic) and an initial Sr/ Sr ratio of 0.71041 0.00017, whereas the high-Ti flows that cap the mesa yield a much lower date of 82 9 Ma (Late Cretaceous) and a higher initial Sr/ Sr ratio of 0.71162 0.00016. This unexpected result means either the high-Ti flows were erupted much later than the low-Ti flows (contrary to evidence in the field) or that the isotopic composition of strontium of the high-Ti flows was homogenized by a thermal pulse during the Late Cretaceous as first suggested by Mensing (1987, 1991)...

Fig. 12.20 The initial Sr ratios of the 24 basalt flows on Solo Nunatak in the Mesa Range vary systematically within narrow limits between 0.70985 and 0.71207. These ratios are typical of the isotopic composition of strontium in the low-Ti flows of the Kirkpatrick Basalt of the Mesa Range. The high initial Sr/ Sr ratios indicate that the magma was either derived from a heterogeneous source in the mantle or that it was contaminated by rocks of the continental crust. The measured Sr/ Sr ratios were corrected for decay of Rb to t = 175 Ma and 1.42 x 10" year (Mensing et al. 1984)...

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