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Indirect Dating Methods:

1. Dating Alteration Assemblages

Dating hydrothermal mineral assemblages (e.g, sericite; biotite) in

altered wall rocks adjacent to a mineral deposit is probably the most

common way to indirectly date mineralization.

In relatively simple cases this should be straightforward.

2. Dating Host Rocks for Mineralization

This approach is obviously only applicable in situations where the

mineralization can be tied temporally and genetically to the host

rocks, and is therefore highly model-dependent. This method is probably reasonably accurate for VMS and SEDEX

type deposits and for porphyry deposits, if the causative pluton can

be conclusively identified.


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3. Bracketing the Age of Mineralization

Obtaining precise and accurate crystallization ages for pre- and

post-ore intrusions is the only fool-proof method of dating

mineralization. This is particularly valuable when used in

conjunction with some of the other dating methods.


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Common methods of indirect dating

K-Ar and 40Ar/39Ar

U-PbPb isotopes


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K-Ar and 40Ar/39ArGeochronology

The conventional K-Ar dating methodand the derivative 40Ar/39Ar

methodare two of the most widely used geochronological tools for a

wide range of geological applications

Potassium is one of the most abundant elements and a major constituent

of the Earths crust; however the radioactive K isotope (40K) comprises

only a small fraction (0.017%) of the total K present in rock-forming

minerals

The relative proportion of naturally occurring K isotopes is fixed:39K 93.2581%

40K 0.0167%41K 6.7302%

40K decays to 40Ar with a half-life of 1250 m.y.


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40K in a rock or mineral undergoes branching decay a small proportion

(~11.0%) decays to 40Ar by electron capture and the remainder decays to40Ca by beta decay.


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Basis of K-Ar Dating Method

K is contained in the mineral lattice (or within volcanic glass)

Ar is inert, so is not present in rocks or minerals (or is present in very

small quantities)

With time, 40K decays to 40Ar*, which is trapped in the crystal lattice

The age is proportional to the relative abundances of40K and 40Ar*

To calculate an age, amounts of40K and 40Ar* must be determined

Age calculation:

T = 1/ (40Ar*/40K(/) + 1

Where = total decay constant of40K (=+)

= decay constant of40K to 40Ar*


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Mass spectrometer

Extraction line

Conventional K-Ar Extraction Line and Mass

Spectrometer


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Conventional K-Ar Dating Method:

40Ar* and 40K contents are measured on separate sample aliquots

K determination: Total K is measured and 40K content is calculated

using the constant ratio of K isotopes

Ar determination: Isotopic ratios are

measured using a 38Ar tracer (38Ar is the

least abundant naturally occurring Arisotope)


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Calculating a K-Ar age:

Age is proportional to 40K/40Ar*

Determination of40Ar* requires corrections for other sources of40Ar

40Ar in a sample is not all 40Ar*; but also includes some non-

radiogenic 40Ar, usually of atmospheric origin

40Ar* = 40Ar(m) 36Ar(m) [295.5]

(m = measured)

Sources of analytical uncertainty:

1. Homogeneous monomineralic sample? Sample preparation orsingle mineral phase

2. Atmospheric Ar correction Inherent in samples, but also lab

3. Precision of analysis Laboratory care


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K-Ar Isochrons

K-Ar data can also be portrayed and interpreted using an isochron approach

The age of the sample shouldbe proportional to the slope of the isochron

The lower intercept shouldcorrespond to the 40Ar/36Ar ratio of the

atmosphere (295.5)

The problem is that the non-radiogenic component of Ar present in the

different minerals (or whole rock samples) commonly does nothave exactly

the same isotopic composition; thus one of the basic premises of the isochronmethod is invalid


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K-Ar and 40Ar/39ArGeochronology40Ar/39Ar method is a variation on the conventional K-Ar method.

It is analytically more difficult and time-consuming

A great deal more information can be obtained from an analysis

ability to recognize thermal disturbance,mixed mineral phases,

the presence of excess Ar,

complex thermal history

sample impurities

etc.


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40Ar/39Ar Method

Based on measuring K and Ar in the same sample aliquot

K content is determined indirectly from conversion of39K to 39Ar bybombardment with fast neutrons in the core of a nuclear reactor


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Mass spectrometerExtraction line

Resistance furnace

Laser

A typical Ar-Ar extraction line and mass spectrometer

Ar isotopic measurement is similar to that used in the conventional K-Ar

method, but uses a more sensitive and more accurate mass spectrometer,

and a more precise sample heating method (to permit incremental

heating).

di i f b i i l h l k l b f


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Irradiation of a K-bearing mineral or whole rock sample by fast neutrons

in the core of a reactor converts a small proportion of39K to 39Ar

39Ar is unstable and decays back to 39K with a half-life of 269 yrs

(fortunately this is slow enough that the delay between sample irradiationand analysis is not a problem)

Several other interfering isotopes of Ar are also produced by the

irradiation process. This is especially a problem with phases such as

hornblende or plagioclase that contain significant amounts of Ca inaddition to K. The relevant reactions are:

40Ca 36Ar

40Ca 37Ar

42Ca 39Ar

37Ar does not occur naturally; therefore we can measure the amount of37Ar

in the irradiated sample and use this conversion factor to estimate how

much reactor-induced36

Ar and39

Ar are present, and subtract these


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We must accurately estimate the efficiency of conversion of39K to 39Ar in

the reactor. This will vary depending on the particular reactor that is used

and the actual position of the sample within the core region (neutron fluxwill vary to some extent within the reactor core)

This is done by usingflux monitors, which are natural mineral samples

whose K-content and age are very precisely known

Samples for irradiation are individually wrapped in Al foil, stacked in a

small sample can with interspersed flux monitors, and all irradiated

together. After irradiation, the flux monitors are removed from the sample

can and their Ar isotopic composition is measured on the mass

spectrometer. By knowing the age of the monitor mineral and its K-content we can back-calculate to get the conversion efficiency factor,

which is referred to as theJ-factor. This factor is then used in calculating

the age of unknown samples that were in close proximity to that particular

monitor in the sample can

40A /39A A C l l ti


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40Ar/39Ar Age Calculation

The age is calculated from:

t = 1\ [ln(40Ar*/39Ar) J +1]

Where: J = the efficiency of conversion factor, and

40Ar*/39Ar = 40Ar/39Armeasured 295.5(36Ar/39Ar)

The J factor for the flux monitor is calculated as follows:

J = [e tm 1]/[40Ar*/39Ar]

Several flux monitors are in widespread use. One is either biotite or

sanidine from the ~28 Ma Fish Canyon Tuff in Colorado. It has recently

been shown that the actual age of this unit is slightly older than the age

that is being used to calculate J factors in most labs; hence many Ar-Ar

ages that have been reported in the literature of as much as 2% too old. In

order to evaluate published 40Ar/39Ar ages one must know which flux

monitor was usedand what age was inferred for the monitor.


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Advantages of the 40Ar/39Ar Method over the Conventional K-

Ar Method

1. Avoids sample heterogeneity because only one sample aliquot is used

2. Age is more precise because the age is measured using only Ar

isotopes (conventional K analysis is relatively imprecise)

3. Can determine a total fusion age (which is analogous to a

conventional K-Ar age but more precise); or

4. Can analyze using step-heating methods, which helps detect episodic

Ar loss, contamination by non-atmospheric Ar, and the detailed

assessment of the thermal history of a sample


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Total fusion orstep-heatingof samples can be done either using a

furnace or a CO2 (or UV) laser


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One final problem!!!


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One final problem!!!

Observed that when heating hydrous minerals (micas, hornblendes, etc.) in

vacuum, the minerals undergo phase changes.

Suggested that the release of Ar from a mineral may reflect these phase

changes rather than diffusive loss of Ar simply related to the heating.

For example

Biotite experiences major dehydration reactions at relatively lowtemperatures

These reactions are strongly correlated with episodic Ar loss.

Because of this, unambiguous interpretation of biotite Ar release spectra issomewhat questionable.


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Hornblende undergoes a significant phase change at ~1000 degrees C

Hornblende structure begins to exsolve Fe oxides.

Another major phase change at 1030 degrees (usually the peaktemperature when most Ar loss occurs.

Hornblende structure decomposes into a fine-grained aggregate of cpx,

plagioclase and Fe-Ti oxides.

The release of Ar appears to be unaffected up to ~1000 degrees;

however the exact meaning of heating steps above this temperature are

somewhat uncertain.

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