Chemical controls on lithium mobility in granitic pegmatites: Constraints from experiments on spodumene stability

The dependence of the stability of spodumene (LiAlSi₂O₆) on fluid composition and pegmatite-forming mineral assemblage was investigated by in situ experiments using a hydrothermal diamond-anvil cell and Raman spectroscopy at 550–700 °C and 400–750 MPa. Our results show that Li-rich weakly alkaline (pH) conditions are favorable for spodumene crystallization. Spodumene formation from Li-rich systems is suppressed by high levels of Na and/or K and, thus, is controlled by alkali activity. This implies that the formation of hydrothermal and magmatic spodumene requires earlier removal of Na and K from the fluid or melt. Experimental metasomatism shows that spodumene is susceptible to hydrothermal alteration and is readily converted to alkali feldspar in alkaline to neutral sodic/potassic hydrothermal fluids and to muscovite in acid and potassic fluids. This implies that intense post-magmatic hydrothermal overprint in open systems can completely destroy spodumene mineralization in pegmatites. The results of our study confirm earlier speculation that the alkalinity of hydrous melts or fluids has a major influence on lithium mobility in granitic pegmatites.