Strontium crust/mantle ratios

We start with another set of isotope signatures. The rate of erosion in the distant past can be estimated by measuring the ratio of strontium isotopes in marine carbonates. Two stable isotopes of strontium — strontium-86 and strontium-87 — differ in their distribution between the Earth s crust and the mantle underneath it. The mantle is rich in strontium-86, whereas the crust is more richly endowed with strontium-87. The major source of strontium-86 in the oceans is the igneous rock basalt. This rock is extruded continuously from the mantle at the mid-ocean ridges, from where it spreads slowly across the ocean floor before diving back into the mantle beneath the ocean trenches. A little strontium dissolves from the basalt into seawater. The speed of dissolution is more or less constant. The gradual build-up of dissolved strontium-86 in the oceans is balanced by a steady uptake of strontium by marine carbonates, such as limestone (calcium carbonate). This is because strontium can displace its sister element, calcium, in the crystalline structure of limestone. As each of these processes takes place at a steady rate, we would not expect the relative amount of strontium-86 in limestone to fluctuate a great deal. In fact it varies quite a lot. Strontium-87 is to blame. 

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. 

The Sm/Nd ratio of the condnental crust is highly fractionated (<1.0) relative to CHUR and shows retarded Nd/ Nd evoludon with time (Figure 6.16). This is the opposite of the Rb-Sr system where the strontium-isotopic evolution of the crust shows accelerated evoludon with dme reladve to the mantle. 

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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