IsoplotR (version 6.8)

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)