FELDSPAR THERMOMETRY IN PEGMATITES: TRUTH AND CONSEQUENCES

Feldspar solvus thermometry is not commonly utilized for granites, pegmatites, aplites, and porphyries because of a widely held opinion that the feldspars lose their primary compositions during subsolidus recrystallization. Pegmatites, which are viewed as the most hydrous of granitic magmas, might be most prone to recrystallization and, hence, least likely to preserve their original feldspar compositions. Our investigations of feldspar composition, Al-Si order, and texture from granitic pegmatites at the Little Three mine, Ramona, California, reveal that solvus feldspars in thin granitic pegmatites preserve their original compositions to the extent that they can be used reliably for geothermometry. In the Little Three pegmatites, primary plagioclase shows a steady decrease in An content from the margins (An_8-9) to dike center (An₁) that is indicative of fractional crystallization from melt. Pearly-white Afs at the dike margins exhibits braid orthoclase cryptoperthite texture. Albite exsolution lamellae coarsen inward (except in layered aplite, where Afs is non-perthitic), culminating in the development of turbid, beige-colored deuteric macroperthite near the dike center. Al-Si order in Afs was calculated from peak positions of the (060) and (-204) Bravais planes measured by powder XRD. Cryptoperthites along dike margins possess a degree of structural order intermediate between orthoclase and maximum microcline (all Al in t₁, with a predominance of Al in T_om). Ordering of Al in t_om sites increases to maximum microcline at the dike center. Fractionation indices of Afs increase smoothly from margin to center: K/Rb decreases from 299 to 74, and K/Cs drops from 11717 to 2137. Although recrystallization of Afs increased from margin to dike center, the Afs remained as closed systems at the scale of individual perthite crystals, because the K/Rb and especially the K/Cs ratios would rise sharply upon recrystallization in an open hydrothermal system. Using re-integrated compositions of Afs from perthite, primary feldspar crystallization in the Little Three dike followed a 430° ± 30°C isotherm inward; exsolution occurred down to ~280°C. In conjunction with conductive cooling models, the primary growth of feldspars and their ensuing exsolution should have been essentially complete within one to two weeks.