Six samples of granodiorite from a pluton in British Columbia, Canada, have strontium and rubidium...

Question

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) 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 XXXXXXXXXXCalculate 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 = XXXXXXXXXXin 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: XXXXXXXXXX) = XXXXXXXXXXx I0-16 yet and A(238U) = XXXXXXXXXXx 10-10 yr-I. (b) Given that the solar system had an 87Sr/86Sr ratio of XXXXXXXXXX6Ga 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, XXXXXXXXXXA 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.

David · Accepted Answer

Problem 1. 
87Rb/86Sr 87Sr/86Sr 
3.65 0.7117 
1.8 0.7095 
1.48 0.7092 
0.82 0.7083 
0.66 0.7083 
0.74 0.7082
From curve, equation developed is y=0.001x+0.707  which is in the form y=mx+c which is good fit as 
R2=0.996 
Compare with standard equation 
 
0
86
0
87
86
87
86
87
1
)(
)(
)(
)(
Sr
Sr
e
tSr
tRb
tSr
tSr t  
Thus 
a) Age of intrusion is given by 
me t 1  
001.01 te  
y = 0.0012x + 0.7074 
R² = 0.9969 
0.708
0.7085
0.709
0.7095
0.71
0.7105
0.711
0.7115
0.712
0 1 2 3 4
8
7
 S
r/
8
6
Sr
 
87Rb/86Sr 
Curve normalize both axis with 
reference to Sr 86 
Series1
Linear (Series1)
1001.0 te  
001.1te  
Taking ln both side 
)001.1ln(t  
t=9.995x10-4/  
For rubidium to Strontium system,

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)...

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