Rb sr dating ppt

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).

Because they are chemically indistinguishable, they will appear in the same ratio in every mineral at the time when it is formed: some minerals will have more strontium, some will have less, but all must necessarily have the same The initial state of the rock may therefore be schematically represented by the graph to the right, which shows the initial states of four minerals imaginatively named A, B, C, and D.A small deviation from a straight line tells us that there is some uncertainty about the date, and this degree of uncertainty can be calculated; and if we get something which is nothing like a straight line, then the method simply doesn't supply us with a date.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. 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.

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