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.
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.
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.
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.
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.
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.
Several radioactive nuclides exist in nature with half-lives long enough to be useful for geologic dating.
Grand Canyon Lava Flows: A Survey of Isotope Dating Methods
With heat, daughter isotopes diffuse out of their host minerals but are incorporated into other minerals in the rock. When the rock again cools, the minerals close and again accumulate daughter products to record the time since the second event. Remarkably, the isotopes remain within the rock sample analyzed, and so a suite of whole rocks can still provide a valid primary age.
This situation is easily visualized on an isochron diagram, where a series of rocks plots on a steep line showing the primary age, but the minerals in each rock plot on a series of parallel lines that indicate the time since the heating event. If cooling is very slow, the minerals with the lowest blocking temperature, such as biotite mica, will fall below the upper end of the line. The rock itself gives the integrated , more gradual increase.
The study of the rubidium/strontium isotopic ratios in a set of meteorite samples shows the general approach to this kind of radioactive dating. The age consistent with the slope of this plot can be calculated in a way which makes use of the.
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. As the rock ages, rubidium decreases by changing to 87 Sr. This ratio is about 0. Values at this boundary have been given as 1. The age of the rock is determinable from the slope of the isochron. This slope is a monotonic function of the age of the rock and the older the rock, the steeper the line.
If the slope of the line is m, the age t is given by: Measuring rubidium and strontium concentrations in suitable minerals can be effected using standard chemical and mass spectrometric procedures, X-ray fluorescence or isotope dilution.
Rubidium/Strontium Dating of Meteorites
Roger C. Wiens has a PhD in Physics, with a minor in Geology. His PhD thesis was on isotope ratios in meteorites, including surface exposure dating. First edition ; revised version Radiometric dating–the process of determining the age of rocks from the decay of their radioactive elements–has been in widespread use for over half a century.
For example, with Rb/Sr isochron dating, any age less than a few The slope of the line is the ratio of enriched D to remaining P. It can be relation between the amount of the rubidium parent and the strontium daughter.
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. Time line of a simple event sequence from Earth’s origin event 1 until today event 3.
Let us examine the physical processes in more detail by considering a specific example. During cooling and crystallization of an intrusive magma body, differentiation will cause the mineral abundances to vary locally throughout the rock. Suppose that the rock consists of K-feldspar, plagioclase, quartz and biotite. Rubidium, which is geochemically coherent with potassium, enters readily into the biotite crystal structure, less into K-feldspar, and even less into plagioclase.
Quartz contains negligible Rb and Sr. Consequently the data points at crystallization event 2, Figure R9 are arrayed on a horizontal line. Davies et al.
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 the unreliability of the Rb-Sr method for dating is readily demonstrated, Diffusion of Sr occurs at the concentration gradient. Ringwood, A.E., Jagoutz, E. and Hofmann, A.W., Potassium, rubidium, and caesium in.
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. But there is no reason at all to suppose that there was no 87 Slope present initially. Rubidium we produced the formula for K-Ar dating , it was reasonable enough to think that there was little to no rubidium-strontium present in the historical state of the rock, because argon is an inert gas, does not take strontium in chemical processes, and so in particular does not take part in mineral formation.
Strontium, on the other hand, does take part in method reactions, and can substitute chemically for such elements as calcium, which is commonly found method igneous rocks.