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Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the Rubidium-87 is an isotope of rubidium.Rubidium-87 was the first and the most popular atom for making Bose–Einstein condensates in dilute atomic gases.Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in igneous rocks.So we have every reason to think that rocks when they form do incorporate strontium, and Sr in particular.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.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.It is also easy to evaporatively cool, with a consistent strong mutual scattering.
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.We can also test for mixing using what is known as a mixing plot: if we draw up a graph of the composition of our sample in which the Sr ratio is the vertical axis (as in the isochron diagram) but the horizontal axis represents 1/Sr (the reciprocal of the quantity of both isotopes of strontium taken together) then if the rock was produced by this mixing process, then the points on this graph will lie along a straight line.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.U gaat akkoord met dit cookiegebruik door hieronder op “akkoord” te klikken of door verder gebruik te blijven maken van MSN.com/nl-nl.
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.