Three streams are draining an area of the Canadian shield, which is completely c

Question

Three streams are draining an area of the Canadian shield, which is completely covered in muskeg and glacial till, so no bedrock is exposed.    The Rb and Sr content of several potential bedrock sources is tabulated below, and the uncertainties on the measurements are ~5% (incidentally, these are real data, not made up!). In addition, geochemical analyses of the waters are also indicated below for different scenarios. Assume that the waters draining the bedrock accurately reflect the abundance and ratio of both Rb and Sr coming from the bedrock –in other words, you can use the Rb and Sr data from the bedrock as indicative of the contribution of each to the stream(s). Also assume that there is no contribution from the muskeg or glacial till –those are just there to blind you to the bedrock

Rb A Rb B Rb C Sr A Sr B Sr C 125 ppm 12 ppm 147 142 ppm 954 ppm 439 ppm Scenario 1. (3) The three streams yield Rb/Sr as follows: 0.60, 0.62, 0.59. What, if anything, can you say about the nature of the underlying bedrock in the area based on these measurements? Scenario 2. (3) The three streams yield Rb/Sr as follows: 0.025, 0.205, 0.50. What, if anything, can you say about the nature of the underlying bedrock in the area based on these measurements? Scenario 3. (3) The three streams yield Rb/Sr as follows: 0.88, 0.89, 0.92. What, if anything, can you say about the nature of the underlying bedrock in the area based on these measurements?

Explanation / Answer

Rb/Sr(a)=.88

(b)=.0125

(c)=.334

so for scenario 1:

we can say that the streams are affected by mainly bedrock (a) and(b) as only by the contribution of them the ratio found in the stream .60 , .62 , .59 can be justified as (.88+.334)/2=.605

for scenario 2 : stream 1 and 2 has a prominent effect of rock b and minor effect of rock C while stream 3 has a prominent effect of rock A and a minor effect from rock B

in scenario 3: main effect of rock A can be seen

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