St rb dating
However, this is worth it: it would, as I say, require a fluke for this to happen, so if we reject dates based on the mixing plot, then we will be throwing out a hundred bad dates for every good one.
Rb decays to stable strontium-87 by emission of a negative beta particle, i.e. During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase.
It is also easy to evaporatively cool, with a consistent strong mutual scattering.
There is also a strong supply of cheap uncoated diode lasers typically used in CD writers, which can operate at the correct wavelength.
Suppose that the original source of the rock was two different magmas (call them X and Y) imperfectly mixed together so that some parts of the rock will be all X, some all Y, some part X and part Y in varying proportions.
Then these different parts of the rock, when analyzed for their isotopic composition, will plot in a straight line on the isochron diagram; and the slope of this line, and the point at which it intercepts the vertical axis, will have nothing to do with the age of the rock, and everything to do with the compositions of X and Y.
As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotite, potassium feldspar, and hornblende.
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.
However, there is still a way to extract a date from the rock.
In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar method, although the two are not exactly alike. When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium.