Six samples of granodiorite from a pluton in British Columbia, Canada, have strontium and rubidium isotopic compositions as follows. 97sr/86sr 97Rbia6sr XXXXXXXXXX XXXXXXXXXX XXXXXXXXXX XXXXXXXXXX (a)...

Six samples of granodiorite from a pluton in British Columbia, Canada, have strontium and rubidium isotopic compositions as follows.
97sr/86sr 97Rbia6sr 0.7117 3.65 0.7095 1.80 0.7092 1.48 0.7083 0.82 0.7083 0.66 0.7082 0.74
(a) Find the age of the intrusion. (b) Find the initial 87Sr/s6Sr ratio of the magma at the time of the intrusion. (c) Assuming an r/Sr/66Sr ratio of 0.699 and an 87Rb/86Sr ratio of 0.1 for the undif-ferentiated Earth 4550 Ma ago, comment on the possibility that this batholith originated in the mantle. 2. Whole-rock rubidium—strontium isochrons for two plutons are shown in Fig. 6.10. Calculate the age of each pluton and comment on the some of the magma.
3. Figure 6.1 I shows a rubidium-strontium isochron for some unmetamorphosed sedi-ments and clay minerals, namely illite and chlorite, found in the sediment. Calculate an age from this isochron and comment on its meaning. 4. (a) Given that N = where N is the number of surviving radioactive atoms at time t, No the initial number and A the decay constant, and given that today 235U/238L1 = 0.007 257 in the Earth, Moon and meteorites, estimate the date of sudden nucleosynthesis for the two estimated production ratios of 235U/23/U =1.5 and 2.0. Show all steps in your work clearly. Note: 42351.1) = 9.8485 x I0-16 yet and A(238U) = 1.55125 x 10-10 yr-I. (b) Given that the solar system had an 87Sr/86Sr ratio of 0.699 4.6Ga ago, what should the initial 87 Sr/"Sr ratio of the earliest continental crust that formed at 3.8 Ga be. given that the mantle had an 87Rb/86Sr ratio of 0.09 at that time and that .).("Rb) = 1.42 x yr-I? Show all steps in your work clearly. (Cambridge University Natural Science Tripos IB, 1980.) 5. A granite was extracted from the mantle 2500 Ma ago. Later, at 500 Ma, this granite was remelted. What is the present-day strontium isotope ratio (87Sr/86Sr) in the young pluton? Assume that the granites have gross rubidium-strontium ratios of 0.5 and 0.7, respectively.