Argon isotopic evolution of upper mantle (reply)

Abstract

WE thank Alexander and Schwartzman (AS)¹ for bringing to light a significant error in data point 18 (Table 1) of our paper². Because of a misinterpretation of a verbal communication from Mueller, we omitted sample 30 in our regression of the data from Baadsgard and Mueller³. We confirm that there is no reason for omitting this sample, and that the data indicate that the non-radiogenic argon present cannot be distinguished from atmospheric contamination. We concur with Alexander and Schwartzman, however, on little else, and consider as still valid the four principal points summarised at the end of our paper²: first, the initial ⁴⁰Ar/³⁶Ar ratios of mantle-derived igneous rocks have increased steadily over the past 800 Myr. Second, the present day ⁴⁰Ar/³⁶Ar ratios in the upper mantle and atmosphere are very similar. Third, the majority of argon at present in the atmosphere has been derived from the mantle rather than the crust, and fourth, initial ⁴⁰Ar/³⁶Ar ratios from igneous rocks are potentially more sensitive petrogenetic indicators than strontium or lead isotopes.