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About half the time this will produce a straight line with negative slope: that is, it will slope down from left to right instead of up.
Such a line must necessarily be produced by mixing, since a real isochron will always have positive slope: the rarity of such an occurrence tells us that mixing of this type must itself be rare.
So just as step heating in Ar-Ar dating protects us from error, so too does the isochron method in Rb-Sr dating: it may not always lead us to the right date, but it is a good safeguard against our accepting one that is wrong.
There is, however, one potential source of error which will not show up on the isochron diagram, since it is expected to produce a straight line.
Isochron dating can be further separated into mineral isochron dating and whole rock isochron dating; both techniques are applied frequently to date terrestrial and also extraterrestrial rocks (meteorites).
The advantage of isochron dating as compared to simple radiometric dating techniques is that no assumptions are needed about the initial amount of the daughter nuclide in the radioactive decay sequence.
, hence the same long term evolution, and hence form an isochron.
It can happen that if we produce a mixing plot for a perfectly good isochron, it will by some statistical fluke produce a straight line on the mixing plot; we would then be throwing out a perfectly good date.
Thus, the ratio of the daughter to non-radiogenic isotope will become larger with time, while the ratio of parent to daughter will become smaller, for rocks that start out with a small concentration of the parent, the daughter/non-radiogenic ratio will not change quickly as compared to rocks starting with a large concentration of the parent.
To perform dating, a rock is crushed to a fine powder, and minerals are separated by various physical and magnetic means, each mineral has different ratios between parent and daughter concentrations.
This one additional piece of information about the initial state of the rock allows us to calculate its age.
As with the other methods we've discussed so far, the Rb-Sr method will only work if nothing but the passage of time has affected the distribution of the key isotopes within the rock. Hydrothermal or metasomatic events may have added or subtracted rubidium and strontium to or from the rocks since their formation; or a metamorphic event may have redistributed the rubidium or strontium among its constituent minerals, which would also interfere with the method.
However, barring an extraordinary coincidence, the result of such events will be that when we draw the isochron diagram, the minerals will no longer lie on a straight line.