Relationship between feldspar alteration and the precise post-crystallization movement of rubidium and strontium isotopes in a granite

Abstract

Preparation of high-prurity plagioclase separates from a coarse-grained granite (Heemskirk granite, western Tasmania) allows, for the first time, the exact delineation of the amounts of radiogenic Sr, Rb, and common Sr migrating between cogenetic K-feldspar and plagioclase in a typical post-crystallation granite environment. Further, the isotope relocation is demonstrated to occur in the alteration products of one of these feldspars (plagioclase) rather than in the crystal structure. Relative to their calculated, unaltered present-day compositions the most altered plagioclase has gained 15.8 times the radiogenic Sr, 8.8 times the Rb87, and 1.7 times the Sr86, whereas the K-feldspar has lost 19, 14, and 34% of these isotopes, respectively. Whereas the alteration isochron for the plagioclase is governed by the radiogenic Sr and Rb87 gain, the K-feldspar alteration isochron reflects the relatively high loss of common Sr. As a result, the calculated present-day composition of the unaltered K-feldspar in this granite occupies a surprising position on the reference isochron. The normal interpretation of discordant K-feldspars in terms of radiogenic Sr loss only must now be viewed as a gross simplification of the isotopic migration pattern. The larger amounts of isotopes recognized to be involved in this migration must also be taken as firm indication of the possibility of open system total-rock behavior in granites displaying feldspar alteration.