FC SANIDINE AND OTHER K-AR AGE MONITORS

U-Pb dating is based on metrologically traceable reference materials. Therefore U-Pb ages obtained by TIMS analyses do not require indirect age monitors. K-Ar dating has not yet achieved metrological compliance in amounts-of-substance measurements of Ar and thus has to rely on natural samples as age monitors. This limits both precision and accuracy of K-Ar ages.

Even if measurements of Ar isotopic compositions have low in-run uncertainties, their translation into ages needs to propagate the very high systematic uncertainty on the monitor age. The general principle common to all measurements is that in order to reduce the total uncertainty to a desired level it is necessary to recognize and eliminate all systematic uncertainties to at least an order of magnitude lower level. This includes mass spectrometer performance (e.g. collector linearity, pressure-dependent mass fractionation, etc) and sample characterization (e.g. homogeneity, adherence to the "point-like history" of [1], etc).

The characterization of K-Ar age monitors is scarce in the literature up to now. Clayey alteration and mineralogical complexities in MMhb were documented by TEM [2]. A discontinued mo­nitor, B4M, is so far the only instance of extensive electron microprobe documentation of (lack of) ho­mogeneity [3]. Petrological heterogeneity of FCs was studied by [4].

A crucial discriminant to assess age monitors is Ca/Cl/K systematics combined with stepheating (and/or stepleaching). FCs contains at least three phases: 1-2 % very young, Cl-rich alteration phase, degassed at very low T; 95-97 % sanidine; 2-3 % older, restitic plagioclase, degassed above 1300 °C. According to the chosen heating protocol (preheating yes/no, complete fusion of plagioclase yes/no), the total gas age may be higher or lower than the Ca/Cl/K-defined sanidine "plateau" age. Systematic age differences exceed 0.1 Ma (0.4 %).

[1] Begemann et al, GCA 65 (2001) 111 [2] Villa et al, CMP 126 (1996) 67 [3] Heri et al, GSL Spec Pub 378 (2013) 69 [4] Bachmann et al, J Petrol 43 (2002) 1469