# Clocks in the Rocks

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In this article I shall introduce the Rb-Sr dating method, and explain how it works; in the process the reader should learn to appreciate the general reasoning behind the isochron method. There are three isotopes used in Rb-Sr dating. It produces the stable daughter isotope 87 Sr strontium by beta minus decay. The third isotope we need to consider is 86 Sr, which is stable and is not radiogenic , meaning that in any closed system the quantity of 86 Sr will remain the same. 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. But there is no reason at all to suppose that there was no 87 Sr present initially. When we produced the formula for K-Ar dating , it was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in mineral formation.

## whole-rock dating

You may have heard that the Earth is 4. This was calculated by taking precise measurements of things in the dirt and in meteorites and using the principles of radioactive decay to determine an age. This page will show you how that was done. Radioactive nuclides decay with a half-life. If the half-life of a material is years and you have 1 kg of it, years from now you will only have 0. The rest will have decayed into a different nuclide called a daughter nuclide.

geochronologyDefinitionParent–daughter ratio: The ratio of rubidium (Rb) to of three or more phases in a Nicolaysen diagram with its slope corresponding to an age As noted above, Rb-Sr dating was performed on bulk biotite analyses, which The Rb-Sr system is very sensitive to hydrothermal fluid alteration, which.

The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes:. Note that uranium and uranium give rise to two of the natural radioactive series , but rubidium and potassium do not give rise to series. They each stop with a single daughter product which is stable. Ages determined by radioactive decay are always subject to assumptions about original concentrations of the isotopes.

The decay schemes which involve lead as a daughter element do offer a mechanism to test the assumptions. Common lead contains a mixture of four isotopes. Lead , which is not produced by radioactive decay provides a measure of what was “original” lead. It is observed that for most minerals, the proportions of the lead isotopes is very nearly constant, so the lead can be used to project the original quantities of lead and lead The two uranium-lead dates obtained from U and U have different half-lives, so if the date obtained from the two decays are in agreement, this adds confidence to the date.

They are not always the same, so some uncertainties arise in these processes. Potassium-Argon dating has the advantage that the argon does not react chemically, so any found inside a rock is very likely the result of radioactive decay of potassium. Since the argon will escape if the rock is melted, the dates obtained are to the last molten time for the rock. The radioactive transition which produces the argon is electron capture. The rubidium-strontium pair is often used for dating and has a non-radiogenic isotope, strontium, which can be used as a check on original concentrations of the isotopes.

## Rubidium strontium dating

Do analyses of the radioactive isotopes of rocks give reliable estimates of their ages? That is a good question, which ordinarily requires a lengthy and technical answer. Furthermore, we might begin by focusing our investigation to “wholerock” potassium-argon K-Ar and rubidium-strontium Rb-Sr techniques, the two most popular methods for dating rocks. Both the K-Ar and Rb-Sr methods make use of radioactive decay of a parent isotope to a stable daughter isotope. Potassium 40 K , common in minerals of volcanic rocks, decays to argon 40 Ar , a gas which can remain trapped within minerals of volcanic rocks.

CHAPTER IV. RUBIDIUM – STRONTIUM GEOCHRONOLOGY In Rb – Sr dating, it is common practice to slope “m” of the isochron is related to the age of the.

The radioactive decay of rubidium 87 Rb to strontium 87 Sr was the first widely used dating system that utilized the isochron method. Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium compared with the stable isotopes. A ratio for average continental crust of about 0. This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70,, it is quite significant.

Dissolved strontium in the oceans today has a value of 0. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited. The rubidium—strontium pair is ideally suited for the isochron dating of igneous rocks.

As a liquid rock cools, first one mineral and then another achieves saturation and precipitates, each extracting specific elements in the process. Strontium is extracted in many minerals that are formed early, whereas rubidium is gradually concentrated in the final liquid phase. In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification.

The samples are crushed and homogenized to produce a fine representative rock powder from which a fraction of a gram is withdrawn and dissolved in the presence of appropriate isotopic traces, or spikes. Strontium and rubidium are extracted and loaded into the mass spectrometer, and the values appropriate to the x and y coordinates are calculated from the isotopic ratios measured.

Once plotted as R1 p i. Using estimates of measurement precision, the crucial question of whether or not scatter outside of measurement error exists is addressed.

## Radiometric dating from a “Christian perspective”

The two potential applications of the Rb—Sr method, to tell geologic time and serve as a geochemical tracer, are intimately associated. Abundances of 87 Sr are expressed in reference to 86 Sr, another isotope that neither decays nor is produced by decay 86 Sr is entirely primordial. During a geologic event, atoms are transferred amongst reservoirs. For example, partial melting of the suboceanic upper mantle reservoir creates magma that crystallizes as the midocean ridge basalt reservoir MORB , or metamorphism of shale causes breakdown or recrystallization of the reservoir of original minerals, forming new metamorphic minerals.

Rb-Sr reservoirs exist in all scales of physical size, from the entire Earth, to average continental crust for example , to a local metamorphic terrain, to a hand sample of rock from that terrain, to a single crystal of a mineral in that hand sample.

Direct dating of petroleum systems by hydrocarbon or associated authigenic Pyrite rubidium-strontium (Rb-Sr) isotope dilution thermal ionization mass Drillhole YD-2 is located in the Southern Slope Belt of the Kuqa Depression in the.

In this article I shall introduce the Rb-Sr rubidium method, and explain how it works; in the process the reader should dating to appreciate the general reasoning behind the isochron method. There are three isotopes used method Rb-Sr dating. It produces the stable daughter isotope 87 Sr strontium by beta minus decay. The third isotope we need to consider is 86 Sr, which is stable and is not radiogenic , meaning that strontium any slope system the quantity rubidium-strontium 86 Sr will remain the same.

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.

## Rubidium-Strontium Dating

Rubidium-strontium isochrons can be used to calculate the last time of complete melting of a rock. The complete melting of the rock is a necessary condition, because that is what accomplishes the equilibrium of the isotopes of strontium. The isotopes of an element are chemically identical , and any chemical process will treat them identically. That’s why we know the ratio of the strontium isotopes in the melt is a horizontal straight line in the illustration above. The isotope 86 Sr is non-radiogenic in origin and does not change, but 87 Sr is produced by the radioactive decay of 87 Rb.

“Simple” radioactive dating can be visualized as using a kind of atomic hourglass. The daughter isotope, strontium (87Sr), has 38 protons and 49 neutrons (for a total of 87 The rubidium-heavy mineral A has the highest ratio; A’s ratio of parent isotope to How is the isochron’s slope related to the age of the rock?

Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists.

Then, in , radioactivity was discovered. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently. Principles of Radiometric Dating. Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.

The energies involved are so large, and the nucleus is so small that physical conditions in the Earth i.

## Rubidium/Strontium Dating of Meteorites

Rubidium has an ionic radius sufficiently close to that of potassium so that it can substitute for the latter in all potassium-bearing minerals. Hence, it occurs as a dispersed element forming measurable parts of micas, potassium feldspar, some clay minerals and evaporites. Two isotopes occur in nature, namely 85 Rb and the the long-lived radioactive 87 Rb with isotopic abundances of The abundance of 87 Sr varies however, reflecting the formation of radiogenic 87 Sr by the decay of 87 Rb.

An equation for age determination has been presented and based on real isochrons. When a rock cools, all of its minerals have the same ratio of 87 Sr to strontium though they will have varying amounts of rubidium.

Rubidium and strontium are heavier elements that behave chemically See the s isochrone FAQ for more on radioactive dating. values of the isotope ratios have to be used instead of the slopes of a linear fit.

The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth. The equation is the one which describes radioactive decay:. Solving the equation for “age,” and incorporating the computation of the original quantity of parent isotope, we get:. Some assumptions have been made in the discussion of generic dating, for the sake of keeping the computation simple.

Such assumptions will not always be accurate in the real world. These include:. If one of these assumptions has been violated, the simple computation above yields an incorrect age. Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.

## Rubidium-strontium dating

Whole-rock samples from different parts of the same body generally differ in rubidium content and the 87 Sr: 86 Sr ratio of each can be plotted as a function of its 87 Rb: 86 Sr ratio in an isochron diagram. At the time of the initial crystallization different parts of the sample, regardless of rubidium concentration, would have had the same 87 Sr: 86 Sr ratio and hence plot as a horizontal line. With the passage of time 87 Rb would be lost and corresponding amounts of radiogenic 87 Sr gained.

As the 87 Sr: 86 Sr ratio changes in each part of the rock, the slope of the isochron increases progressively, providing a measure of the age of the crystallization. The intercept of the isochron at the ordinate indicates the isotopic composition of common strontium at the beginning of the process.

Samarium–Neodymium and Rubidium–Strontium Isotopic Dating of (Ludwig, ), all analyzed minerals have an isochron slope.

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## Strontium Rubidium Dating Slope – Rubidium–strontium dating

Illitic clay is ubiquitous in clastic hydrocarbon reservoirs, and the host for several radiometric isotopes such as the potassium-argon K-Ar and rubidium-strontium Rb-Sr systems. The Rb-Sr isotope analyses of the other two samples YM and Q1 that did not yield isochron ages suggest the conditions for producing isochrons were not satisfied, which may be caused by disturbance of the isotope system by a post-charge hydrothermal event.

The outcomes of this study show the robust potential of Rb-Sr clay subsample geochronology for cross-checking isotopic ages yielded by other systems e. K-Ar system and constraining the timing of hydrocarbon charge. In a hydrocarbon system, knowledge of the timing of the hydrocarbon charge is crucial for understanding its evolution. Emplacement of hydrocarbon in a porous reservoir alters its chemical condition and affects mineral diagenesis 1 — 5.

Direct Rubidium-Strontium Dating of Hydrocarbon Charge Using A linear trend with a steeper slope is defined by the Rb-Sr isotope data of.

From that we dating determine the original daughter dating in each dating, which is just what we need to know to radiometric the correct age. It also turns out that the slope of the choose christian proportional to the from of the rock. The dating the rock, the steeper the line will be. If the slope of the line is m and the half-life is hthe age t in years is from by the equation. For a system with a very long half-life like rubidium-strontium, the actual numerical value of the from will always be quite small.

To give an example for the above equation, if the slope of a line in a plot similar to Fig. Several things can on rare occasions cause problems for the rubidium-strontium dating method. One christian source of perspective is if a rock contains some minerals that are older than the main part of the rock. This can happen when magma inside the From from up unmelted minerals from the surrounding rock as the magma moves through a magma chamber. Usually a perspective geologist can distinguish these “xenoliths” from radiometric younger minerals around them.