mclean: Predict disequilibrium concordia compositions

In IsoplotR: Statistical Toolbox for Radiometric Geochronology

Predict disequilibrium concordia compositions

Description

Returns the predicted {}^{206}Pb/{}^{238}U and {}^{207}Pb/{}^{235}U ratios for any given time with or without initial U-series disequilibrium.

Usage

mclean(tt = 0, d = diseq(), exterr = FALSE)

Arguments

tt	the age of the sample
d	an object of class diseq
exterr	propagate the uncertainties associated with decay constants and the {}^{238}U/{}^{235}U-ratio.

Details

U decays to Pb in 14 (for {}^{238}U) or 11/12 (for {}^{235}U) steps. Conventional U-Pb geochronology assumes that secular equilibrium between all the short lived intermediate daughters was established at the time of isotopic closure. Under this assumption, the relative abundances of those intermediate daughters can be neglected and the age equation reduces to a simple function of the measured Pb/U ratios. In reality, however, the assumption of initial secular equilibrium is rarely met. Accounting for disequilibrium requires a more complex set of age equations, which are based on a coupled system of differetial equations. The solution to this system of equations is given by a matrix exponential. IsoplotR solves this matrix exponential for any given time, using either the assumed initial activity ratios, or (for young samples) the measured activity ratios of the longest lived intermediate daughters. Based on a Matlab script by Noah McLean.

Value

a list containing the initial and present-day atomic abundances of the {}^{238}U-{}^{206}Pb and {}^{235}U-{}^{207}Pb decay chains; the {}^{206}Pb/{}^{238}U, {}^{207}Pb/{}^{235}U and {}^{207}Pb/{}^{206}Pb ratios at time tt; the derivatives of the {}^{206}Pb/{}^{238}U, {}^{207}Pb/{}^{235}U and {}^{207}Pb/{}^{206}Pb ratios with respect to time; and the derivatives of the {}^{206}Pb/{}^{238}U, {}^{207}Pb/{}^{235}U and {}^{207}Pb/{}^{206}Pb ratios with respect to the intermediate decay constants and {}^{238}U/{}^{235}U-ratio.

Author(s)

Noah McLean (algorithm) and Pieter Vermeesch (code)

See Also

diseq

Examples

d <- diseq(U48=list(x=0,option=1),ThU=list(x=2,option=1),
           RaU=list(x=2,option=1),PaU=list(x=2,option=1))
mclean(tt=2,d=d)

IsoplotR documentation built on Nov. 10, 2023, 9:08 a.m.