stepHeat {astrochron}

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.

See Also

wtMean

Examples

 ## Not run: 
# 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)

}
 
## End(Not run)