Rb sr dating ppt
Whole-rock samples from different parts of the same body generally differ in rubidium content and the Sr ratio changes in each part of the rock, the slope of the isochron increases progressively, providing a measure of the age of the crystallization.
The intercept of the isochron at the ordinate indicates the isotopic composition of common strontium at the beginning of the process.
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.
(The quantity will be small because there is much more potassium than rubidium in the Universe.) This means that if we wanted to date a rock, and if there was no Sr present initially.
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.
Interestingly, such Rb-contents are typically achieved in lepidolites, found in countless pegmatites.We shall omit the math, but it happens to work out so that after any given period of time, the minerals will still lie on a straight line on the graph, as the diagram shows, and, crucially, the point at which this line intersects the vertical axis is still the initial value of Sr. What we have to do is take samples from the rock consisting of different minerals, or at least of different mineral composition, so that our samples will all have different Sr ratio.This one additional piece of information about the initial state of the rock allows us to calculate its age.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.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.
When we produced the formula for K-Ar dating, it was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in mineral formation.