The Orifice Correction for K–Ar Dating: Reply
Gale and Beckinsale (1974), in a discussion of our recent paper (Baksi and Farrar 1973), suggest ways of avoiding the use of the orifice correction, except for the case where a small amount ( 90% atmospheric argon content can yield useful geochronological information (Baksi 1974~). Assume that 10% of the available gas (A,, = 0.10, Baksi and Farrar 1973) is used for the mass spectrometric analysis; apply a simple root mass correction to the measured isotopic ratios, as per Gale & Beckinsale's (1974, p. 730) suggestion. Let E(40/38) and E(36/38) be the errors in the calculated values of 40Ar/38Ar and 36Ar/38Ar respectively, and E(40*) the resulting error in the calculated value of 40Ar*. Then E(40/38) E(36/38) 1. 0.13%; for gas samples showing 90% and 95% atmospheric argon, E(40*) 1. 2.5% and 5.1%, respectively. Even more serious problems could arise using Gale and Beckinsale's method, if the activated charcoal finger does not collect 100% of the argon in the extraction line. With care, 100% gas collection can be achieved in K-Ar dating (Dalrymple 1969; Baksi 1973). However, incomplete gas collection often occurs, and can be due to at least three different causes (see Armstrong 1966; Wampler and Yanase 1974; Baksi 19743). "A straightforward [manner] to establish a safe limit for the amount of sample leaked into the mass spectrometer from the 38Ar [spike] ion-beam intensity" (Gale and Beckinsale 1974, p. 730) would not detect incomplete gas collection. If only 50% gas collection has resulted, A,, will be 0.20, when it is thought to be 0.10. Application of a simple root mass correction yields E(40/38) cr: E(36/ 38) 0.27%. For gas samples showing 90% and 95% atmospheric argon, E(40*) 5.2% and 10.7% respectively; note that systematic errors are involved, all the calculated values of 40Ar* being too high. (Samples showing 95% atmospheric contamination yield ages with a 1 o (random) error of about 5% (Baksi 1974c).) It is evident that Gale and Beckinsale's (1974) method can give fairly accurate results only when large amounts of gas (> 5 x lo-' cm3NTP of Ar) are available, and that too, only if the samples show low to moderate atmospheric contamination. The 'pipetting method' of gas analysis, currently in use at many laboratories, eliminates the need to apply the orifice correction. This technique seems well suited for mass spectrometers showing high memory effect. However, it must often be difficult to (a priori) determine what fraction of the total gas available should be pipetted into the mass spectrometer. In replicate runs, some basaltic samples show total argon content (per gram of rock) varying by two orders of magnitude (Baksi 1974~); even in less unusual cases, incorrect choice of the fraction of gas to be pipetted must lead to occasional problems; i.e., (1) 40Ar and/or 38Ar ion-beam off-scale on the least sensitive range of the recorder, or (2) 36Ar ion-beam too small to be measured with optimum precision. In