Isotope Geochem Final

*⁴⁰Ar-³⁹Ar dating spectra excess argon*

First it can arise when minerals crystallize under a finite partial pressure of Ar. This is referred to as inherited Ar. Second, during a thermal event, ⁴⁰Ar diffusing out of some minerals may be taken up by other minerals. Can present a saddle shape on the release spectra A new technique uses LA-ICP-MS

*What 4 conditions must be met to obtain a meaningful isochron*

*1* The ratio of parent to daughter should be large *2* The parent/daughter should have have a large range *3* Deviations from closed system behavior must be minimal *4* The isotopic composition of the daughter must have been homogeneous

*How isotopic compositions of mantle-derived magmas have changed through time time*

*A* the continental crust is created in one pulse at 4.5 Ga. The mantle Sm/Nd ratio increase at this time and remains fixed thereafter. In this case εnd in the depleted mantle should increase linearly through time, since the half life of Sm is so long. *B* the crust grows continuously through time. The Sm/Nd ratio in the mantle therefore increases with time. In this case εnd follow a concave upward path *C* Crust is created at 4.5Ga but after that there is a net return of crust to the mantle, so that the Sm/Nd ratio in the mantle decreases with time, making a concave down path.

*Concordia diagram* has been developed to be applied to any system with high ²³⁸U/²⁰⁴Pb ratios. A concordia diagram is simply a plot of ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U In essence, the concordia diagram is a plot of ²³⁵U-²⁰⁶Pb age against the ²³⁵U-²⁰⁷Pb age.

*Concordia diagram*

*U-Th-Pb Concordia example Instruments*

*La-ICP-MS* problems with with elemental fractionation and isotopic mass discrimination. These problems require a standard such as zircon crystals. *EMP* 1: Predominantly of radiogenic origin 2: Uranium has normal isotopic composition 3: Concentrations of U,Th, Pb in the mineral have only changed by decay of U

*The branching ratio*

.117=λe/λb The fraction of K that decays to ⁴⁰Ar is given by λe/λβ

*K-Ar technique what 8 things must be satisfied*

1: Closed system 2: Closed system after its formation with rapid cooling 3: No argon incorporated into mineral 4: corrections are made 5: Isotopic composition of K is normal 6: K was closed through its lifetime 7: decay constants have not been affected 8: concentrations of ⁴⁰Ar and K are determined accurately

*Concordia Example*

A 4.0 Ga old zircon when it first formed would have plotted on the origin because the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ratios must be zero. Any zircon that has remained as a completely closed system since its crystallization must plot on the Concordia line

*Sm-Nd model ages and crustal residence time*

A general assumption about the earth is that the crust has been created from the mantle by magmatism. When a piece of crust is first created, it will have the ¹⁴³Nd/¹⁴⁴Nd ratio of the mantle, though its Sm/Nd ratio will be lower than that of the mantle (Nd being incompatible). Making the simplistic assumption that the mantle has the same Nd isotopic history as Chur. This means a piece of crust will have the same ¹⁴⁴Nd/¹⁴⁴Nd as the mantle and as CHUR. THis means that εNd=0, if we know the present day Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd ratio of this piece of crust we can estimate the age

*Rb-Sr Isochron disadvantage*

A serious disadvantage of the Rb-Sr system is mobility of these elements, particularly Rb. The slope is related to the age of the system.

*What happens when a zircon grain loses U or Pb?*

A zircon must lose ²⁰⁷Pb and ²⁰⁶Pb in exactly the proportions they exist in the zircon because the two are fundamentally chemically identical. In other words it will not lose preference to either ²⁰⁷Pb and ²⁰⁶Pb.

*The question of the rate of crust evolution will be of primary interest*

All three provinces formed between 1.8 and 1.65 Ga, though 1 might slightly older

*⁴⁰Ar-³⁹Ar dating inverse Isochrons*

An alternative is to use a plot of ³⁹Ar/⁴⁰Ar against ³⁹Ar/⁴⁰Ar often called an inverse isochron plot. We can think of the Ar in the sample as a mixture of a trapped component and a radiogenic component. The radiogeneic component has a ³⁶Ar/⁴⁰Ar ratio of 0 (because Ar³⁶ is not produced by radioactive decay), whereas the trapped, non-radiogenic components can be found by extrapolating to a ³⁹Ar/⁴⁰Ar ratio of 0, corresponding to a ³⁹K/⁴⁰Ar ratio of 0, since K³⁹ is proprtional to ⁴⁰K. Thus, the age may be computed from the ³⁹Ar/⁴⁰Ar ratio obtained by extrapolating the correlationg line to ³⁶Ar/⁴⁰Ar to 0

Inverse Argon plot with trapped and relic argon

An inverse isochron plot. The intercept of ³⁶Ar/⁴⁰Ar gives the trapped argon composition. The intercept with the ³⁹Ar/⁴⁰Ar axis gives the radiogenic argon composition, from which the inverse isochron age can be calculated If all ratios are 1/295.5 it is all atmospheric

*⁴⁰Ar-³⁹Ar dating*

Another problem is the production of ³⁹Ar and ⁴⁰Ar by other reactions, where corrections must be made. In conventional K-Ar dating, Ar is released from samples by fusing in a vacuum. Crystal rims experience Ar loss at a lower temperature than crystal interiors. The rims would record younger ages.

Ar-Ar Plot 4

Argon isotope correlation diagram that define a plateau

*U-Th-Pb Concordia example*

At the end of the short episode of metamorphism, all of the zircon grains resumed their evolution by decay of U isotopes and the accumulation of radiogenic Pb after the thermal event.

*Chemistry of Rb and Sr* Rb

Both Rb and Sr are trace elements in the earth, Rb is an alkali element with a valence state of +1. Like other alkalis, it is generally quite soluble in water and hydrous fluids. Rb substitutes readily for K in K-bearing minerals such as mica and K-feldspar.

*Continual Pb loss*

Continuous Pb loss do not define a straight line cord. but rather a slightly curved one.

*Sm-Nd Chur Model 2*

Depicts the isotopic evolution of Nd in a hypothetical crustal rock whose present ¹⁴³Nd/¹⁴⁴Nd ratio is .51 and whose initial ¹⁴³Nd/¹⁴⁴Nd was .5090 An extrapolation of the evolution line of this rock yields a model Nd age of 3.25 Ga. Demonstrates that if the magma that formed this rock originated from a mixture of old crust and CHUR Or could have originated not from CHUR but from a depleted mantle whose present ¹⁴³Nd/¹⁴⁴Nd ratio is about .51315

*Clay quantification and Ar-Ar dating*

Does this correspond to total Ar released even 39Ar and 40ar? Meaning the K39 converted to 39Ar by radiation, thus is a sum of 40Ar and 39Ar?

*The 4 Ga zircon that loses half Pb at 3 Ga*

During the Pb loss, the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U would have decreased by half, the point would have migrated. The line is straight because the loss of ²⁰⁷Pb is always directly proportional to the loss of ²⁰⁶Pb. The intercepts of this cord with the concordia give the ages of initial crystallization (4.0 Ga) and metamorphism (3.0) Ga. U gain would affect the position of zircons on the concordia diagram the same manner as Pb loss; the two processes are indistinguishable

*⁴⁰Ar-³⁹Ar dating Isochrons*

Hypothetical ⁴⁰Ar-³⁹Ar isochron diagram. The slope is proportional to the age and the intercept is gives the initial ⁴⁰Ar/³⁶Ar ratio, which is commonly atmospheric

Fall inverse isochrons

If argon loss occurred after long period of time after closure temperature will give false plateaus

If all samples have a high radiogenic argon component relative to their trapped argon component

If the ³⁶Ar component of the trapped argon is relatively small, then the data points will cluser close to ³⁹Ar/⁴⁰Ar

*K-Ar-Ca system omitting Ca*

In our equation, because while K decaying to Ca does not produce radiogenic Ar, it is no longer K available to produce ⁴⁰Ar production Good for young events

*U-Th-Pb*

In practice this means is that the age is independent of the parent/daughter ratio, we do not need to measure parent/daughter ratio. The U-Th-Pb system is composed of: ²³⁸U_²⁰⁶Pb, ²³⁵U-²⁰⁷Pb, ²³²Th-²⁰⁸Pb, and ²⁰⁷Pb-²⁰⁶Pb The first three are not particularly powerful, compared to Pb-Pb

*Inverse Argon* Non-radiogenic argon

Is argon that did not accumulate within the mineral of interest by radioactive decay of K (blank, trapped, cosmogenic)

*Inverse Argon* Trapped Argon

Is incorporated within the mineral and can consist of atmospheric argon with or without components of excess argon

*Inverse Argon* Blank Argon

Is unavoidable surficial argon introduced into the mass spectrometer sample (should be corrected for)

*Epsilon Nd*

It also leads to a useful notation for Nd isotope ratios. Because we know the bulk-earth ¹⁴³Nd/¹⁴⁴Nd at any instant in time it is useful to consider relative deviations of ¹⁴³Nd/¹⁴⁴Nd from the bulk-earth value

*Model age*

Moment in the past when the isotopic composition of the sample was identicla with that some of model reservoir

*K-Ar and being noble*

Much of what is special about K-Ar derives from Ar being a noble gas and its resulting refusal to be chemically bound in crystal lattice. Ar in rocks and minerals is simply trapped there. Thus during initial crystallization their is virtually no Ar (except pillow basalts) resulting in only one unknown, Time If minerals crystallize in the atmosphere, the correction is ⁴⁰Ar/³⁶Ar is 295.5

*⁴⁰Ar-³⁹Ar dating spectra excess argon*

Non-radiogenic argon is often called excess Ar. ⁴⁰Ar/³⁶Ar ratios used to calculate ages in release spectra are typically corrected for the presence of atmospheric Ar by measuring the ⁴⁰Ar/³⁶Ar ratio. Atmopheric has a contant ratio of ⁴⁰Ar/³⁶Ar anything in excess of this ratio is considered radiogenic. Nevertheless, some samples can have "initial" ⁴⁰Ar/³⁶Ar ratios greater than the atmospheric ratio; this will lead to older ages if not accounted for

*Sm-Nd advantage*

Perhaps the greatest advantage of Sm/Nd is the lack of mobility of these elements, good for mafic rocks and low grade metamorphism.

Not detecting relic argon

Recognition of excess ⁴⁰Ar but not relic ⁴⁰Ar and ³⁶Ar which is the isotope at the lowest concentration can produce large inaccuracies on both axes

*Rb-Sr*

Rubidium has two naturally occurring isotopes ⁸⁵Rb (72%) and ⁸⁷Rb (28%) ⁸⁷Rb is radioactive and decays to stable ⁸⁷Sr by emission of β- particle

Heterogenous argon loss

Shows an inverse isochron plot displaying effects of argon loss, but showing apparent excess ⁴⁰Ar. Spectrum age ould be the same as the isochron age

Heterogenous argon loss false isochron

Shows an inverse isochron plot with apparent age younger thant he real cooling age as a reult of heterogenously distributed excess 40Ar

Ar-Ar Plots

Significant plateaus indicate significant Ar loss

Ar-Ar Plot 1

Spectrum of dates calculated from ⁴⁰Ar/³⁹Ar ratios of gas fractions released by incremental heating *the plateau is not well established which suggests signficant loss of ⁴⁰Ar from the mineral*

Ar-Ar Plot 2

Spectrum of dates obtained by incremental heating, the sample was contaminated with ⁴⁰Ar during intrusion into the granite basement (produces a saddle shape)

*Chemistry of Rb and Sr & ratio* Sr

Sr is an alkaline earth element with a valence of +2. The alkaline earths are also reasonably soluble in water and hydrous fluids, but not as soluble as alkalis. The Rb/Sr in the earth as a whole is about .029, Rb/Sr ratios increase because Sr is removed.

Ar-Ar Plot 3

Stepwise Ar release pattern of biotite from granitic intrusion the anomalously old dates is attributed to excess ⁴⁰Ar (exceeds the age of the earth)

*Sm-Nd Chur Model*

The CHUR model can be used to date rocks of the continental crust by determining the time at which the Nd they contained separated from the chondritic reservoir

*Sm-ND Isochrons*

The Sm-Nd method is most frequently used to date cogenetic mafic igneous rocks. Rocks or minerals that formed at the same time and had the same initial ¹⁴³Nd/¹⁴⁴Nd ratio are represented by points that define a straight line. Good for plutonic and precambrian rocks

*Tera-Wasserburg Plot*

The Tera Wasserburg plot was devised a new concordia that does not require prior knowledge of the initial ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios

*Tera-Wasserburg Plot example*

The U, Th-Pb data reported by Terra and Wasserburg for lunar basalts define a discordia line that intersects the Y-axis ²³⁸U/²⁰⁶Pb at 1.46 The graph indicates a slope of -.88266 corresponds to a date of 3.91 Ga

*Sm-Nd Chur Model*

The calculation of Nd model dates illustrate that a hypothetical example of a syenite with present Sm/Nd ratio of .11 and a ¹⁴³Nd/¹⁴⁴Nd ratio is .5104 corrected to .7219, demonstrates that the Nd that now resides in the specimen could have originated from CHUR at 2.5 Ga

*K-Ar Method*

The decay of naturally occuring ⁴⁰K to stable ⁴⁰Ar by electron capture and by positron emission. About 10.48% of the ⁴⁰K atoms that decay from ⁴⁰Ar and the rest 89.52% to ⁴⁰Ca. The total decay is λ=λβ+λe

Trapped argon beyond atmospheric

The effect of excess ⁴⁰Ar on the isochron plot is depicted when the part of the trapped argon is excess of ⁴⁰Ar, analyses with a component of trapped argon will have a lower ³⁶Ar/⁴⁰Ar ratio than when all trapped is atmospheric

*Rb-Sr Thermal events*

The effect of thermal metamorphism of igneous rocks and their minerals are depicted. The crosses marked R1, R2, and R3 are whole rocks, while open circled marked M2 are minerals of R2. The rocks and minerals initially lay on an isochron of slope equal to zero and thus T=0. After an interval of time Ti-Tm the rocks were heated. The whole rock samples remained closed but radiogenic ⁸⁷Sr was redistributed among the minerals M2 until they had the same ⁸⁷Sr/⁸⁶Sr ratio as R2. Therefore, at a time t=tm years the minerals of R2 had realigned themselves on a new isochron having a slope equal to zero. The whole rock systems continued their evolution without interuption and formed an isochron whose slope corresponds to *ti* which is the time elapsed since initical crystallization, in the meantime the minerals evolved on their own isochron, which still includes R2 but whose slope represents Tm, the time elapsed since the minerals became closed systems after being requilibrated by thermal metamorphism

*Mantle Models*

The idea that the lower mantle was primitive gain favor with the acquisition of the Nd isotope data. The first Nd data obtained showed that Nd and Sr isotope ratios in oceanic basalts were well correlated and that Nd isotope ratios fell between typical MORB values. There are good arguments why the depleted reservoir should overlie a primitive one. sediment could contribute to incompatible elements when subducted.

*The question of the rate of crust evolution will be of primary interest*

The initial εnd values of the granites from the three provinces plotted as a function of crystallization age. Thus if a continental rock formed at Ga contained Nd derived in equal proportions from the mantle and 2 Ga crust, its crustal residence time would be 1.5 Ga.

*K-Ar Isochrons*

The isochron method of dating can avoid the problem caused by the presence of ⁴⁰Ar under favorable conditions the ⁴⁰Ar/³⁶Ar initial depends on the isotopic composition of the different Ar components present and may thus differ from that of atmospheric Ar The intercept of the straight line with the y-ais is the initial ⁴⁰Ar/³⁶Ar ratio Therefore K-Ar isochrons can be constructed only by combining data derived from suites of samples under the same thermal history

*Sm-ND* Model dates

The isotope evolution of Nd in the mantle of the earth is described in terms of a reservoir known as CHUR. The model assumes that the Nd in the mantle has evolved in a uniform reservoir whose Sm/Nd ratio is equal to that of chondritic meteorites

*Rb-Sr Sr or Rb loss

The leached coarse fine clay fractions define two straight lines that converge to this cluster of leachates and yield dates of 638 Ma

*K-Ar technique*

The number of ⁴⁰Ar and ⁴⁰K atoms in a unit weight of the sample must be measured. The K concentration is determined by dissolving in Hf in a vacuum system. It is introduced into a mass spectrometer ⁴⁰Ar/³⁸Ar and ³⁸Ar/³⁶Ar are measured. It must be corrected for a spike and atmospheric The number of ⁴⁰Ar and ⁴⁰K is used to solve the equation above

*K-Ar diffusion*

The point to made here is that relatively small increases in temperature result in large increases in the diffusion coefficient from 600 to 700 °C, resulting in two orders of magnitude. The result is losing ⁴⁰Ar at a higher rate than it is being created, the result is the ages measuring closure temperature not when the grain crystallized. Thus it may be rather easy for K-Ar minerals to undergo partial reset

*U-Th-Pb*

The ratio of Th/U is 3.8 The U-Th-Pb system is certainly the most powerful tool in the geochronologist tool chest. The reason there are three parent decaying to 3 isotopes of Pb, the two of isotopes of U which decay to Pb with very different half lives. This is important chemical process will not change the ratio of the two U isotopes to each other and will not change the ratio of the two Pb daughter isotopes to each other.

*⁴⁰Ar-³⁹Ar dating*

The reaction is produced by irradiating a sample with neutrons in a reactor. The amount of ³⁹Ar produced is then a function of the amount of ³⁹K present. Since the ⁴⁰K/³⁹K ratio in the earth is constant, the amount of ⁴⁰K can be calculated from ³⁹Ar. In practice is performed by simultaneously irradiating and analyzing a standard of known age.

*What does the U-Th-Pb age proportional to*

The slope on the plot of ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb is proportional

*Ar-Ar dating total argon release dates*

The total argon release dates they are subject to the same limitations as conventional K-Ar dates because they depend on the assumption that no radiogenic ⁴⁰Ar escaped from the sample and has no excess ⁴⁰Ar present. The stepwise degassing technique permits the determination of a date that approaches but may underestimate the original cooling rate

*K-Ar-Ca system*

The two lamdas are the decay constants for each mode, the branching ratio is .118. λe=.581E-10 and λbβ=4.962E-10 yr The total decay constant for ⁴⁰K is λ=λβ+λe=5.543E-10 yr

*Sm-Nd Chur epsilon*

The ¹⁴³Nd/¹⁴⁴Nd ratios of terrestrial rocks are evaluated by comparison to the present isotope ratio o Chur. *If an igneous rock derived depleted mantle sources have positive epsilon while rocks from continental crust have positive epsilon.*

*Ar-Ar dating*

The ⁴⁰Ar/³⁹Ar method of dating is based on the formation of ³⁹Ar by the irradiation of K-bearing samples with neutrons Argon-39 is unstable back to K-39 with a half life of 269 years. The isotopes are formed by several reactions of K, Ca, Cl The samples are irradiated for several days to allow ³⁹Ar to be produced, after the irradiation the Ar in the flux monitors is released by fusion in a vacuum system and their ⁴⁰Ar/³⁹Ar ratios are measured

*Rb-Sr Thermal events*

This rock formed at Ti by crystallization of a magma containing Sr of uniform isotopic composition. Consequently all the minerals in this rock had some initial ⁸⁷Sr/⁸⁶Sr ratio. Some time after crystallization the rock was subjected to an increase in temperature for a short lived interval at ∆T. The rock cool to ambient temperature Tm years ago and remained undisturbed. There are two meaningful dates in the history outlined above, Ti the time since crystallization and Tm the time since metamorphism at best can determine age since closure

*K-AR-Ca system*

Two aspects of this system make it special. First, it is branched decay a ⁴⁰K nucleus may decay to either a ⁴⁰Ca by β- or to a ⁴⁰Ar atom by electron capture (or positron emission rare). It is impossible to predict how a given ⁴⁰K atom will decay. We can predict what proportion of a large number of ⁴⁰K atoms will decay to each other, however. The ratio is called the branching ratio

*U Loss*

U loss moves these points away from the origin, in this case the zircons lie on an extension of a cord above. The upper intercept of the cord gives initial age and the lower intercept gives the age of U loss. U is happy in the zircon, Pb is not, Pb will occupy a site damaged by alpha decay.

*Rb-Sr*

Unlike the K-Ar system, in the Rb-Sr and Sm-Nd systems, both the initial ratio and the age are virtually unknown, meaning we must solve for both simultaneously through the isochron method

*U-Th-Pb*

Uranium has three naturally occurring isotopes ²³⁸U, ²³⁵U, ²³⁴U Thorium exists as ²³²Th The principle isotopes of U and Th are each the parent chain of radioactive daughters ending with stable isotopes of Pb by emission of eight alpha particles and six beta particles ²³⁸U = ²⁰⁶Pb ²³⁵U=²⁰⁷Pb ²³⁴U=²⁰⁸Pb

*Epsilon Nd*

We can calculate an εnd any point in time. For the present, the value of ¹⁴³Nd/¹⁴⁴Nd is .512638 when ¹⁴⁶Nd/¹⁴⁴Nd=.7219

*⁴⁰Ar-³⁹Ar dating Isochrons*

When ⁴⁰Ar/³⁶Ar data from a series of samples are plotted against ⁴⁰K/³⁶Ar, the slope of the resulting line will be proportional to age, and the intercept gives the inititial ⁴⁰Ar/³⁶Ar ratio. since the produce of ³⁹Ar from ³⁹K is the same ⁴⁰K/³⁹K ratios are constant

*Inverse Argon* Cosmogenic argon

Which is produced from cosmic ray interaction witi Ca, Ti, Fe involving spallation

*⁴⁰Ar-³⁹Ar dating*

You'll see ³⁹Ar has a half life of 269 years and does not occur naturally. The so called 40-39 method is actually ⁴⁰K-³⁹Ar dating but employing somewhat different analytical technique for the potassium. The key is the production of ³⁹Ar by a nuclear reaction on ³⁹K (most abundant K)

*K-Ar variation diagram showing Ar loss due to contact metamorphism*

basalt can have ⁴⁰Ar/³⁶Ar of 10000 You can use it to measure burial rates by dating illite at various depths, burial depth results in sheet silicates unerggoing tranformation and losing Ar

*Rb-Sr dating*

based off the equation D=D₀+N(e^λt-1)

*Inverse Argon* Inherited

both radiogenic and non-radiogenic argon introduced into the mineral by contamination of older minerals

What does heterogeneity cause during step wise heaitng

causes scattering on the isochron

*Changes in U-Th-Pb ratios*

parent/daughter ratios have changed hence might prefer the Pb-Pb approach to one involving parent-daughter ratios. The solubility of U under oxidizing conditions often leads to mobility. When melting occurs, the U and Pb isotope ratios in the magma will be identical to those in the source, but the U/Pb ratio (and Th/Pb) ratio will change, as the chemical behavior of U and Pb differ. ²³⁸U_²⁰⁶Pb, ²³⁵U-²⁰⁷Pb, ²³²Th-²⁰⁸Pb, and ²⁰⁷Pb-²⁰⁶Pb when used together

*Inverse Argon* Neutron-induced argon

produced from 39K by neutron

Heterogeneous argon loss false isochron

shows a hypothetical case in which excess ⁴⁰Ar does not result in the expected lowering of the isochron gradient whilte the interpcet with the ³⁹Ar/⁴⁰Ar axies remaines fixed

If all data points represent large amounts of trapped argon relative to radiogenic

then they will cluster close to ³⁶Ar/⁴⁰Ar

Loss of ³⁹Ar by recoil

when ³⁹K is irradiated the recoil produced by the emittion of the proton causes loss of ³⁹Ar

*Sm-Nd system*

¹⁴⁷Sm decays to ¹⁴³Nd by alpha decay with a half life of 106 Ga. Sm and Nd are both intermediate rare earth elements. The ionic radii of Sm and Nd only differ by .04A

*Sm-Nd system Ratios*

¹⁴⁷Sm/¹⁴⁴Nd ratio of earth is .1967 and Sm/Nd of about .32

*Rb-Sr*

•Record time of metamorphism rather initical crystallization •diagneeiss alterations etc may effect sed dating

*fault dating in the canadian rockies*

•States 40Ar-39Ar is good for small sample size •39Ar recoil •100% detrital may be a proxy for cooling age and the lower ages the authigenic ages

*Inverse Argon* Relic

⁴⁰Ar is radiogenic argon that remains following partial resetting event and only in favorable cases be distinguished from radiogenic argon accumulated after re-closure

Basic Ar-Ar concept

⁴⁰Ar/³⁹Ar geochronology is based on decay of ⁴⁰K to ⁴⁰Ar. An age can be derived from the ⁴⁰K/⁴⁰Ar ratio and therefore this ratio needs to be determined. In ⁴⁰Ar/³⁹Ar geochronology ³⁹Ar is created from ³⁹K, with knowledge of the abundance of ³⁹K and ⁴⁰K the amount of ⁴⁰K can be determined and the ⁴⁰K/⁴⁰Ar ratio can be determined