In situ 40K-40Ca 'double-plus' SIMS dating resolves Klokken feldspar 40K-40Ar paradox

The K-40-Ca-40 decay system has not been widely utilized as a geochronometer because quantification of radiogenic daughter is difficult except in old, extremely high K/Ca domains. Even these environments have not heretofore been exploited by ion microprobe analysis due to the very high mass resolving power (MRP) of 25,000 required to separate K-40(+) from Ca-40(+). We introduce a method that utilizes doubly-charged K and Ca species which permits isotopic measurements to be made at relatively low MRP (similar to 5000). We used this K-Ca 'double-plus' approach to address an enduring controversy in 40Ar/39Ar thermochronology revolving around exsolved alkali feldspars from the 1166 Ma Klokken syenite (southern Greenland). Ion microprobe K-40-Ca-40 analysis of Klokken samples reveal both isochron and pseudoisochron behaviors that reflect episodic isotopic and chemical exchange of coarsely exsolved perthites and a near end-member K-feldspar until <= 719 Ma, and perhaps as late at similar to 400 Ma. Feldspar microtextures in the Klokken syenite evolved over a protracted interval by non-thermal processes (fluid-assisted recrystallization) and thus this sample makes a poor model from which to address the general validity of 40Ar/39Ar thermochronological methodologies. (C) 2010 Elsevier B.V. All rights reserved.