Rb sr dating ppt
Pegmatites are ideal test cases as they often form late in an orogeny and close to the surface, therefore exhibit simple cooling histories.
Furthermore, several U-bearing minerals (e.g., zircon, columbite-tantalite) allow comparison of Rb-Sr with U-Pb ages.
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.
We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: as we have noted, rubidium substitutes for potassium, and strontium for calcium.
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.
(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.
Finally, pegmatite veins are important time-markers in orgens due to their cross-cutting relationships.
It needs to be stressed that the daughter isotope 87Sr can be determined simultaneously with the parent isotope 87Rb (via 85Rb analysis, assuming a constant 87Rb/85Rb ratio).
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