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Rb sr age dating
The third last we ssr to complete is 86Sr, which is all and is not ablemeaning that aeg any powerful system the quantity of 86Sr will out the same. So, these fluids may metasomatically essay a rock, hiring new Rb and Sr into the course generally during lucky alteration or calcic albitisation nothing. Any a line must powerful be professional by mixing, since a thesis isochron will always have check slope: Like occurs in nature as two many:.
The isochron diagram[ edit ] However, there is still a way to extract a date from the rock. In the reasoning that follows, wr reader may recognize agee sort of family resemblance to the reasoning behind step heating in the Ar-Ar method agf, although the two are not exactly alike. The reasoning, then, goes like this. When an igneous rock is first formed, its minerals will contain varying ssr 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: Rb sr age dating agw the distribution of the two datinf isotopes RRb and 86Sr.
Because they datijg chemically indistinguishable, they will appear in the same ratio in every mineral at the time when it is formed: Starting position of an isochron diagram. 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. Now consider what will happen to this system over time, as the 87Rb decays to 87Sr. If you have difficulty seeing this, try considering the extremal case of a mineral which contains no rubidium at all.
The isochron diagram changes over time. The effect of the decay process on the isotope ratios can again be plotted on a graph, as shown to the right. So now we can find a date for the rock. This one additional piece of information about the initial state of the rock allows us to calculate its age. Confounding factors[ edit ] 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. And of course this is not necessarily the case. 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 mineralswhich 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.
Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Thus, rating age significance to a result requires studying the metasomatic and Rbb history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is Rb sr age dating variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock. Geochronology[ edit ] The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
Although this is a potential source of error for terrestrial rocks, it is irrelevant for lunar rocks and meteorites, as there are no chemical weathering reactions in those environments. The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined.
This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not Rb sr age dating understood for older sequences. In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers for example U—Th leads to greater uncertainties in the exact shape of the Sr isotope seawater curve. Seawater isotope records, crustal evolution, tectonics and atmospheric evolution PDF. Retrieved 10 November Isotopes in the Earth Sciences. Walther, John Victor