I have a question about isochron data. Below is some data that puzzles me.
______________________________________________computed Sr86 ppm
Rb87___Sr87____Rb87/Sr86_error__Sr87/Sr86_error____Rb87/_____Sr87 /
ppm____ppm____________________________________Rb87/Sr86_Sr87/Sr86
135.86__595.95__0.6606__0.0134__0.71990__0.00005__205.6615__827.8233
161.60__582.60__0.8040__0.0230__0.72240__0.00005__200.9950__806.4784
126.72__612.52__0.5995__0.0106__0.72022__0.00009__211.3761__850.4624
141.78__641.18__0.6413__0.0108__0.72005__0.00006__221.0822__890.4659
140.79__642.96__0.6345__0.0110__0.71984__0.00010__221.8913__893.1985
151.26__570.05__0.7690__0.0129__0.72105__0.00007__196.6970__790.5832
On the left are data that I found in an on-line paper [
SBGEO - Sociedade Brasileira de Geologia ]. (This data is typical of others that I found on the internet.) It gives the ppm of Rb87 and Sr87 for 6 samples. Then it gives the ratios of Rb87/Sr86 and Sr87/Sr86 for the same samples.
It occurred to me that one ought to be able to compute the ppm of SR86 for each of the samples. So I divided the Rb87 (ppm) by the ratio of Rb87/Sr86 and obtained what should be Sr86 (ppm) for each sample. Then I divided Sr87 (ppm) by the ratio of Sr87/Sr86 and obtaind what should be Sr86 (ppm) for the same samples.
HOWEVER! Sr86 (ppm) computed from Rb87 averages at 209 (ppm), while Sr86 (ppm) when computed from Sr87 averages at 843 (ppm)!!!!
Why? How can the same sample have 209 ppm and 843 ppm of Sr86? Is this typical of isotopic measurements?? Have I missed something?
The formula for computing Sr87 over time from Rb87 is:
SR87 = Sr87i + Rb87 ( e^t*lamda - 1)
For Isochrons the formula is modified by dividing by Sr86 (where Sr86 is a stable isotope that must be the same )
SR87 = Sr87i + Rb87(e^t*lamda - 1)
------ = -------__-----
SR86 = Sr86i__Sr86
Yet the computations above indicate that ther are different amounts for SR86 from the same sample.
Anybody know what is going on? Am I missing something?
I have Faure, 1986, Principles of Isotope Geology, 2nd ed.; Dalrymple, 1991, The Age of the Earth; and Attendorn, 1997, Radioactive and Stable Isotope Geology. None of them point this out, nor explain why this should be.
Allen
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