stepHeat: Ar/Ar Geochronology: Generate an Ar/Ar age spectrum and calculate step-heating plateau age.

Description

The stepHeat function will evaluate data from stepwise heating experiments, producing an Ar/Ar age spectrum, a weighted mean age with uncertainty, and other helpful statistics/plots (with interactive graphics for data culling). The function includes the option to generate results using the approach of IsoPlot 3.70 (Ludwig, 2008) or ArArCALC (Koppers, 2002).

Usage

stepHeat(dat,unc=1,lambda=5.463e-10,J=NULL,Jsd=NULL,CI=2,cull=-1,del=NULL,output=F,
         idPts=T,size=NULL,unit=1,setAr=95,color="black",genplot=T,verbose=T)

Arguments

dat: dat must be a data frame with seven columns, as follows: (1) %Ar39 released, (2) date, (3) date uncertainty (one or two sigma), (4) K/Ca, (5) %Ar40*, (6) F, and (7) F uncertainty (one or two sigma). NOTE: F is the ratio Ar40*/Ar39K (see Koppers, 2002).
unc: What is the uncertainty on your input dates? (1) one sigma, or (2) two sigma. DEFAULT is one sigma. This also applies to the F uncertainty, and the J-value uncertainty (if specified)
lambda: Total decay constant of K40, in units of 1/year. The default value is 5.463e-10/year (Min et al., 2000).
J: Neutron fluence parameter
Jsd: Uncertainty for J-value (neutron fluence parameter; as one or two sigma)
CI: Which convention would you like to use for the 95% confidence intervals? (1) ISOPLOT (Ludwig, 2008), (2) ArArCALC (Koppers, 2002)
cull: Would you like select dates with a graphical interface? (0=no, 1=select points to retain, -1=select points to remove)
del: A vector of indices indicating dates to remove from weighted mean calculation. If specified, this takes precedence over cull.
output: Return weighted mean results as new data frame? (T or F)
idPts: Identify datum number on each point? (T or F)
size: Multiplicative factor to increase or decrease size of symbols and fonts. The default is 1.4
unit: The time unit for your results. (1) = Ma, (2) = Ka
setAr: Set the %Ar40* level to be illustrated on the plot. The default is 95%.
color: Color to use for symbols. Default is black.
genplot: Generate summary plots? (T or F)
verbose: Verbose output? (T or F)

Details

This function performs weighted mean age calculations for step-heating data, including estimation of age uncertainties, mean square weighted deviation, and probability of fit.

The following plots are produced:

(1) %Ar40* versus %Ar39 released

(2) K/Ca versus %Ar39 released

(3) Ar/Ar age spectrum, with 2 sigma uncertainties for each step, and weighted mean with 95% confidence interval (in red)

If the J-value and its uncertainty are input, stepHeat will calculate and include the uncertainty associated with J. The uncertainty is calculated and propagated following equation 18 of Koppers (2002).

A NOTE regarding confidence intervals: There are two conventions that can be used to calculate the confidence intervals, selected with the option 'CI':

(1) ISOPLOT convention (Ludwig, 2008). When the probability of fit is >= 0.15, the confidence interval is based on 1.96*sigma. When the probability of fit is < 0.15, the confidence interval is based on t*sigma*sqrt(MSWD).

(2) ArArCALC convention (Koppers, 2002). When MSWD <=1, the confidence interval is based on 1.96*sigma. When MSWD > 1, the confidence interval is based on 1.96*sigma*sqrt(MSWD).

ADDITIONAL ADVICE: Use the function readMatrix to load your data in R (rather than the function read).

References

A.A.P. Koppers, 2002, ArArCALC- software for 40Ar/39Ar age calculations: Computers & Geosciences, v. 28, p. 605-619.

K.R. Ludwig, 2008, User's Manual for Isoplot 3.70: A Geochronological Toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication No. 4, Berkeley, 77 p.

I. McDougall and T.M. Harrison, 1991, Geochronology and Thermochronology by the 40Ar/39Ar Method: Oxford University Press, New York, 269 pp.

K. Min, R. Mundil, P.R. Renne, and K. Ludwig, 2000, A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite: Geochimica et Cosmochimica Acta, v. 64, p. 73-98.

I. Wendt and C. Carl, 1991, The statistical distribution of the mean squared weighted deviation: Chemical Geology, v. 86, p. 275-285.

Examples

Run this code

 if (FALSE) {
# Check to see if this is an interactive R session, for compliance with CRAN standards.
# YOU CAN SKIP THE FOLLOWING LINE IF YOU ARE USING AN INTERACTIVE SESSION.
if(interactive()) {

# Sample MT-09-09 incremental heating Ar/Ar data from Sageman et al. (2014).
perAr39 <- c(4.96,27.58,19.68,39.9,6.25,1.02,0.42,0.19)
age <- c(90.08,89.77,89.92,89.95,89.89,89.55,87.71,86.13)
sd <- c(0.18,0.11,0.08,0.06,0.14,0.64,1.5,3.22)
KCa <- c(113,138,101,195,307,27,17,24)
perAr40 <- c(93.42,99.42,99.64,99.79,99.61,97.99,94.64,90.35)
Fval <- c(2.148234,2.140643,2.144197,2.145006,2.143627,2.135163,2.090196,2.051682)
Fsd <- c(0.00439,0.00270,0.00192,0.00149,0.00331,0.01557,0.03664,0.07846)
ex <- data.frame(cbind(perAr39,age,sd,KCa,perAr40,Fval,Fsd))

stepHeat(ex)

# plot without points identified
stepHeat(ex,size=0,idPts=FALSE,cull=0)

}
 }