Mathematics – Logic
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010e%26psl.299..426h&link_type=abstract
Earth and Planetary Science Letters, Volume 299, Issue 3-4, p. 426-433.
Mathematics
Logic
Scientific paper
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. 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 40K+ from 40Ca+. We introduce a method that utilizes doubly-charged K and Ca species which permits isotopic measurements to be made at relatively low MRP (~ 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 40K-40Ca 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 ~ 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.
Harrison Mark T.
Heizler Matthew T.
McKeegan Kevin D.
Schmitt Axel K.
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