# Geochemistry of Radioactive Isotopes

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GSA Bulletin ; : — The Wufeng and Longmaxi organic-rich shales host the largest shale gas plays in China. The fractures hosted in the Longmaxi Formation are mineralized with quartz as the predominant fracture cement, and calcite as an intracementation phase postdating the earlier quartz cement. In contrast, the fractures hosted in the Wufeng Formation are dominantly mineralized by calcite, which occurs either as the only cement present or as a cement phase predating later quartz cement. REE data equally indicate that the distinguishable Eu anomalies 6. The Sm-Nd isochron ages and fluid inclusion data of fracture cements suggest that fracture opening and calcite precipitation in composite veins within the Wufeng and Longmaxi Formations were triggered by gas generation overpressurization.

## Dating age range calculation

Radiometric dating is a means of determining the “age” of a mineral specimen by determining the relative amounts present of certain radioactive elements. By “age” we mean the elapsed time from when the mineral specimen was formed. Radioactive elements “decay” that is, change into other elements by “half lives.

It must be emphasized that 87Rb/86Sr ratio in equation , which we will call this A line whose slope depends only on t is known as an isochron. the age we obtain will depend on the mineral we use for dating (since the.

Below you will find a text describing the basic principles behind the Planetary Science Institute system of utilizing crater counts and isochron diagrams in order to estimate crater retention ages of surfaces on Mars. As discussed by Hartmann the crater numbers can date the actual formation age of a surface in an ideal case, such as a broad lava flow which forms a one-time eruptive event. The flow accumulates craters and the crater numbers date the time of formation. In other cases, not uncommon on Mars, the story is more complex.

For example a surface may be covered by a few hundred meters of mobile sand dunes; the numbers of craters of diameter D and depth d would give a mean characteristic age of topographic features of the scale of D,d. Smaller craters in mobile dunes would disappear faster and have lower mean ages. In this sense, the derived age is a size-dependent Acrater retention age B the survival time of craters of given size.

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Another example is exhumation, which is common on Mars Malin and Edgett , A surface could be formed and covered by sediments at some later time, degrading any craters on the original surface. Much later, the surface may be exhumed, as documented in various cases by Malin and Edgett. In an ideal case, such a surface might then show vestiges of the degraded original craters indicating the duration of exposure of the first surface and a second population of fresh, small, sharp-rimmed craters formed since the recent exhumation event.

Malin and Edgett , , thinking in terms of dating the underlying process, asserted that such processes render Martian crater counts more or less useless for dating.

## Isotope of carbon used in carbon dating

The Bible is quite clear about the origin and timeframe for the creation of Earth and the cosmos. If Scripture is inaccurate in this, then how can it be trusted in anything else? Some evolutionists throw out theistic evolution God using evolution as His creative process as a philosophical panacea, with the goal of leading people to conclude that Genesis is a myth. Like Nimrod of ancient times, they know they must provide an alternative i.

One of the indirect evidences that evolutionists universally appeal to is radioactive dating because it appears to supply the deep time their evolutionary models demand. But how accurate is their model, and how scientific is their approach?

The foundations of these so-called isotopic or radiometric dating methods were laid This equation forms the foundation of most geochronological meth- ods. ages is by analysing several minerals in a rock and create an isochron, similar to.

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. There is no way of anticipating what the 87 Sr is at the time of melt, but if there is 87 Rb present then it will increase with time as the rubidium isotope decays.

## The uncertainty of the half-life

The lead-lead isochron method for determining the age of ancient rocks including meteorites is generally thought to be the most reliable and precise method for such dating. Dalrymple calls the lead method “the hourglass of the solar system”. Many years of painstaking research has gone into establishing what is commonly called the Holmes-Houtermans System. The two series proceed to different final lead isotopes: and there is a third lead isotope, Pb, that is not formed in any of the radioactive processes and can therefore be used as a reference.

Different growth curves are formed with different amounts of U in the mineral at the time of crystallization. But the points on the different growth curves of the mineral constituents of an undisturbed rock will lie along a straight line, an “isochron”.

Mathematics has then given us a simple equation to describe this decay: To solve for this problem, the isochron dating method was created, which measures.

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. In many cases, there are independent cues such as geologic setting or the chemistry of the specimen which can suggest that such assumptions are entirely reasonable.

However, the methods must be used with care — and one should be cautious about investing much confidence in the resulting age Isochron methods avoid the problems which can potentially result from both of the above assumptions.

## Dating equation age

Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists. These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. In this case, it is a big advantage to present the data in a form in which the abundance of both the parent and daughter isotopes are given with respect to the abundance of the initial background daughter.

The incremental additions of the daughter type can then be viewed in proportion to the abundance of parent atoms. In mathematical terms this is achieved as follows.

Calculation details of Rb-Sr method of dating was report- ed in Vanhaecke et al. (). To fit an isochron from one bulk granite sample for age determination.

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## Calculating Rb-Sr Isochrons

An oversight in a radioisotope dating technique used to date everything from meteorites to geologic samples means that scientists have likely overestimated the age of many samples, according to new research from North Carolina State University. To conduct radioisotope dating, scientists evaluate the concentration of isotopes in a material. The number of protons in an atom determines which element it is, while the number of neutrons determines which isotope it is.

For dating. Neodymium isotope concentrations by time. Senior scholar, we obtain. Dating equations are an isochron dating with samarium-neodymium ratios are.

This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are. This is a spreadsheet that I use in petrology to walk students through calculating Rb-Sr isochrons and talking about isotope heterogeneity and sampling at various scales. The example exercise is the Tuolumne intrusive series.

The file, available for download below, contains workbooks see different tabs with example isochrons and a spreadsheet of data for the homework problems. Students have successfully met the goals of this exercise if they answer the homework problem set completely and accurately. Teaching Notes to Accompany Calculating Rb-Sr Isochrons Microsoft Word 75kB Oct21 08 — directions on how to calculate slopes and intercepts from the Excel spreadsheet, and how to use the slope to calculate an age.

Each example and homework data set is a separate tab in this Excel spreadsheet. Get Involved Archive. Your Account. This activity was selected for the On the Cutting Edge Reviewed Teaching Collection This activity has received positive reviews in a peer review process involving five review categories.