Posted: Dec 29, 2012 12:32 am
by Pulsar
Atheistoclast wrote:Did it every occur to you that radioactive decay may have occurred in the immediate aftermath of the creation of these atoms in the furnaces of stars? :plot:

Apparently it never occurred to you to properly read Cali's OP. So, let's do this again, shall we?

The ages of meteorites can be determined from the decay of Rubidium (87Rb) into Strontium (87Sr), which has a half-life of about 49 billion years. So, some of the 87Sr in a meteorite sample is the result of decayed 87Rb. However, the sample will also contain some 87Sr that was already present when the meteorite formed. In other words, the amount of 87Sr is

87Srnow = (87Rboriginal - 87Rbnow) + 87Sroriginal

using the exponential decay law,

87Rboriginal = 87Rbnow*(elt),

with l the decay rate (which can be derived from the half-life) and t the elapsed time, the equation can be written as

87Srnow = 87Rbnow*(elt-1) + 87Sroriginal

It is more convenient to measure isotope ratios, in particular the ratio with the stable isotope 86Srnow (which is not a decay product, so its amount stays constant over time). Thus,

87Srnow/86Srnow = 87Rbnow/86Srnow*(elt-1) + 87Sroriginal/86Srnow

This is the equation of a straight line:

y = mx + b


x = 87Rbnow/86Srnow
y = 87Srnow/86Srnow
m = elt-1
b = 87Sroriginal/86Srnow

x and y can be measured with a mass spectrometer. But now comes the important bit: different parts of the same meterorite will yield different values of x and y. However, these values will all lie on the same straight line with slope m and intercept b. By taking several samples of a meteorite, one can therefore determine m, and thus the age t.