PPT - Rb-Sr and Sm-Nd Dating 8/30/12 PowerPoint Presentation - ID PPT - Rb-Sr and Sm-Nd Dating 8/30/12 PowerPoint Presentation - ID

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Walther, John Victor 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.

About half the time this will produce a straight line with negative slope: If these form a straight line then the samples are consistent, and the age probably reliable. The reasoning, then, goes like this. The quantity will be small because there is much more potassium than rubidium in the Universe.

Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.

The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole how to flirt with your husband christian and their component minerals will be markedly different.

Rubidium–strontium dating - Wikipedia

Starting position of an isochron diagram. A Rb-Sr date which is at 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.

And rb sr dating ppt viewer course this is not necessarily the case.

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Wikimedia Commons has media related to Rb-Sr dating. Most of the presentations and slideshows on PowerShow. The isochron diagram changes over time.

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Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock. Geochronology[ edit ] The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites. Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock generally during potassic alteration or calcic albitisation alteration.

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The isochron diagram[ edit ] However, there is still a way to extract a date from the rock. So we have every reason to think that rocks when they form do incorporate strontium, and 87Sr in particular. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.

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. We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: Such a line must necessarily be produced by mixing, since a real isochron will always have positive slope: The third isotope we need to consider is 86Sr, which is stable and is not radiogenicmeaning that in any closed system the quantity of 86Sr will remain the same.

The ideal scenario according to Bowen's reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende i.

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But there is no reason at all to suppose that there was no 87Sr present initially. The effect of the decay process on the isotope ratios can again be plotted on a graph, as shown to the right.

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.

Calculating the age[ edit ] The age of a sample is determined by analysing several minerals within the sample. Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement.

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. 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.

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Retrieved 10 November 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. As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotitepotassium feldsparand hornblende.

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The Wikibook Historical Geology has a page on the topic of: If you have difficulty seeing this, try considering the extremal case of a mineral which contains no rubidium at all. 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.

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In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar methodalthough the two are not exactly alike. 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.

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.