CALCULATION OF (230)TH U ISOCHRONS, AGES, AND ERRORS/

If analytical errors are responsible for the scatter of points on a Th -230-U-234-U-238 isochron diagram, the isochron should be fitted by a technique that (1) weights the points according to their analytical er rors and error correlations, and (2) either takes into account the pre sence of some of the same data in two coupled XY isochrons or (equival ently) uses a single, three-dimensional XYZ isochron. A method based o n maximum-likelihood estimation is presented that fulfills these requi rements, and the relevant equations for errors in age and initial U-23 4/U-238 are given. Equations for estimating the necessary isotope-rati o errors and error-correlations for both alpha-spectrometric and mass- spectrometric data are also developed.