K-Feldspar Microtexture and Argon Transport

To enable us to better understand the relationship between Ar loss in Nature and laboratory step heating, and crystal microtexture in complex alkali feldspars, we have revisited microtexturally well-characterized feldspars from the 1166 Ma Klokken intrusion in South Greenland. We have imaged small (100-500 micron) cleavage fragments with BSE and SEM prior to irradiation for argon analysis, and obtained step-heating spectra from single fragments. Considering that the diffusion length is much smaller than fragment size our methods remain crude, but are a step in the right direction compared to bulk analysis of fragments with mixed microtextures. The new single fragment argon data define end-member apparent ages of Klokken microtextural types. Understanding of microtextures has improved significantly in the past 20 years. A recent discovery is that microcline patches in coarse ‘patch perthite' crystals are themselves cryptoperthitic, with straight albite lamellae with ‘nanotunnels' (fluid-enlarged dislocations) on their surfaces. Similar nanotunnel meshes are common in alkali feldspars from granites. The likelihood that they provide argon transport pathways is being tested but there remains a discrepancy between predicted temperature of formation of albite lamellae, argon apparent ages and thermal history. Specifically, argon apparent ages of ca. 450 Ma for fragments of patch perthite that suggest either argon loss related to microtextural modification or growth/reprecipitation of new K-feldspar do not conform with regionally low temperatures since pluton emplacement or models based on diffusion rates of alkali ions that require 10's of Ma to coarsen film perthites below 300°C. Based upon heating experiments prior to irradiation, it is unlikely that laboratory derived D/r2 values are grossly (i.e. 3 to 5 orders of magnitude) modified during step-heating thereby eliminating that the discrepancy between suggested thermal history and MDD analysis can be solely attributed to inaccurate closure temperature estimates of the Klokken patch perthites.