Calculate U-Th-He and fission track central ages and compositions
Computes the geometric mean composition of a continuous mixture of fission track or U-Th-He data and returns the corresponding age and fitting parameters.
central(x, ...) ## Default S3 method: central(x, alpha = 0.05, ...) ## S3 method for class 'UThHe' central(x, alpha = 0.05, model = 1, ...) ## S3 method for class 'fissiontracks' central(x, mineral = NA, alpha = 0.05, exterr = FALSE, ...)
x |
an object of class |
... |
optional arguments |
alpha |
cutoff value for confidence intervals |
model |
if the scatter between the data points is solely caused by the analytical uncertainty, then the MSWD value should be approximately equal to one. There are three strategies to deal with the case where MSWD>1.choose one of the following statistical models:
|
mineral |
setting this parameter to either |
exterr |
include the zeta or decay constant uncertainty into the error propagation for the central age? |
The central age assumes that the observed age distribution is the combination of two sources of scatter: analytical uncertainty and true geological dispersion.
For fission track data, the analytical uncertainty is assumed to obey Poisson counting statistics and the geological dispersion is assumed to follow a lognormal distribution.
For U-Th-He data, the U-Th-(Sm)-He compositions and uncertainties are assumed to follow a logistic normal distribution.
For all other data types, both the analytical uncertainties and the true ages are assumed to follow lognormal distributions.
The difference between the central age and the weighted mean age is usually small unless the data are imprecise and/or strongly overdispersed.
If x has class UThHe, returns a list containing the
following items:
(if the input data table contains Sm) or uv (if it does not): the mean log[U/He], log[Th/He] (, and log[Sm/He]) composition.
the covariance matrix of uvw or uv.
the reduced Chi-square statistic of data concordance, i.e. mswd=SS/df, where SS is the sum of squares of the log[U/He]-log[Th/He] compositions.
the fitting model.
the degrees of freedom (2n-2) of the fit (only
reported if model=1).
the p-value of a Chi-square test with df
degrees of freedom (only reported if model=1.)
a three- or four-element vector with:t: the central age.s[t]: the standard error of t.ci[t]: the width of a 100(1-α)\% confidence
interval for t.disp[t]: the studentised 100(1-α)\% confidence
interval enhanced by a factor of √{mswd} (only reported
if model=1).
the geological overdispersion term. If model=3,
this is a three-element vector with the standard deviation of the
(assumedly) Normal dispersion and the lower and upper half-widths
of its 100(1-α)\% confidence interval. w=0 if
model<3.
OR, otherwise:
a three-element vector with:
t: the central age.
s[t]: the standard error of t.
ci[t]: the width of a 100(1-α)\% confidence
interval for t.
a three-element vector with the overdispersion (standard deviation) of the excess scatter, and the upper and lower half-widths of its 100(1-α)\% confidence interval.
the reduced Chi-square statistic of data concordance, i.e. mswd=X^2/df, where X^2 is a Chi-square statistic of the EDM data or ages
the degrees of freedom (n-2)
the p-value of a Chi-square test with df
degrees of freedom
Galbraith, R.F. and Laslett, G.M., 1993. Statistical models for mixed fission track ages. Nuclear Tracks and Radiation Measurements, 21(4), pp.459-470.
Vermeesch, P., 2008. Three new ways to calculate average (U-Th)/He ages. Chemical Geology, 249(3), pp.339-347.
data(examples) print(central(examples$UThHe)$age)
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