IGPP Seminar Series

Feldspar Thermochronometry and The End of History

by Prof. T. Mark Harrison
UCLA, IGPP & ESS

Abstract

Feldspar has the distinction of being both the most common mineral in the crust and the most structurally complex. The high K content of some alkali feldspars make them very suitable for dating using the 40Ar/39Ar variant of the 40K-40Ar decay system. By coupling this method with diffusion theory, we developed an interpretive model 20 years ago that permits continuous thermal history data to be obtained from feldspar 40Ar/39Ar age spectra. Despite the manifest success of this model, an enduring controversy revolving around exsolved alkali feldspars from the 1,166 Ma Klokken syenite (Southern Greenland) has limited its broad acceptance. In essence, feldspars predicted by the model to leak Ar readily at moderately elevated temperatures were found to yield older ages than apparently co-existing phases that have greater intrinsic retentivity. Although 40K experiences branched decay to both 40Ar and 40Ca, the 40K-40Ca 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. We have developed a novel method that utilizes doubly-charged K and Ca species permitting isotopic measurements to be made using an ion microprobe at relatively low mass resolution. ‘Double-plus’ 40K-40Ca measurements of Klokken feldspars reveal both isochron and pseudo-isochron behavior that indicate episodic isotopic and chemical exchange until ~400 Ma. Thus feldspar microtextures in the Klokken syenite did not form immediately upon emplacement as previously assumed but instead evolved over a protracted interval by non-thermal processes (fluid assisted recrystallization). Inferring Ar retentivity without first determining and taking account the age of the crystal microtextures has proven to be a distraction to appreciation of an important and versatile thermochronometer. Our data convincingly put to rest a controversy that has persisted for more than 20 years.
Tuesday, 28 September 2010
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM