IsoplotR (version 6.1)

discfilter: Set up a discordance filter

Description

Define a discordance cutoff to filter U--Pb data.

Usage

discfilter(option = 0, before = TRUE, cutoff)

Value

a list with the input parameters. Default values for

cutoff are

c(-48,140) if option=='t';

c(-5,15) if option=='r';

c(-0.36,0.96) if option=='sk';

c(-1.6,4.7) if option=='a'; and

c(-2,5.8) if option=='c'.

Arguments

option

one of five options:

0: do not apply a discordance filter

1 or 't': the absolute age difference (Ma) between the \(^{206}\)Pb/\(^{238}\)U and \(^{207}\)Pb/\(^{206}\)Pb ages.

2 or 'r': the relative age difference (%) between the \(^{206}\)Pb/\(^{238}\)U and \(^{207}\)Pb/\(^{206}\)Pb ages.

3 or 'sk': percentage of common Pb measured along a mixing line connecting the measured composition and the Stacey-Kramers mantle composition in Tera-Wasserburg space.

4 or 'a': logratio distance (%) measured along a perpendicular line connecting Tera-Wasserburg concordia and the measured composition.

5 or 'c': logratio distance (%) measured along a line connecting the measured composition and the corresponding single grain concordia age composition.

Further details in Vermeesch (2021).

before

logical flag indicating whether the discordance filter should be applied before (TRUE) or after (FALSE) the common-Pb correction.

cutoff

a two-element vector with the minimum (negative) and maximum (positive) allowed discordance. Default values vary between the different options. To view them, enter discfilter(option) at the R command line.

Details

The most reliable U--Pb age constraints are obtained from (zircon) grains whose \(^{206}\)Pb/\(^{238}\)U and \(^{207}\)Pb/\(^{206}\)Pb ages are statistically indistinguishable from each other. U--Pb compositions that fulfil this requirements are called `concordant'. Those that violate it are called `discordant'. The discordance of the \(^{206}\)Pb/\(^{238}\)U and \(^{207}\)Pb/\(^{206}\)Pb systems can be defined in five different ways. By setting a cutoff for any of these criteria, U--Pb data can be filtered for data quality.

References

Vermeesch (2021) ``On the treatment of discordant data in detrital zircon U--Pb geochronology'', Geochronology.

See Also

cad, kde, radialplot

Examples

dscf <- discfilter(option='c',before=TRUE,cutoff=c(-1,1))
weightedmean(x=examples$UPb,exterr=FALSE,sigdig=2,
             cutoff.disc=dscf,common.Pb=3)