Using trace element correlation patterns to decipher a sanidine crystal growth chronology : An example from Taapaca volcano, Central Andes

DOI: https://doi.org/10.1016/j.jvolgeores.2006.03.004

Sanidine megacrysts occur in a number of dacite lava domes of young volcanic edifices in the Central Andes of Northern Chile. Trace element variations within a sanidine crystal erupted in a 14.1 ± 1.4 ka old dacite lava dome from Taapaca volcano are studied here to decipher its crystal growth chronology using trace element diffusion systematics. The trace element concentration profiles were determined by ion microprobe at 80 µm stepwidth across a total core to rim profile length of ~ 2.5 cm. The crystal displays large variations in Sr and Ba content, with wavelengths of trace element variations down to < 160 µm. Clear growth zone boundaries outbound of the distinct crystal core are absent at this sampling density. Strong correlation of Sr with Ca and the virtual lack of correlation of Ba with Ca are shown to be consistent with intracrystalline diffusion of Sr subsequent to crystal growth. Despite the complicated crystal growth history, correlation analysis across the crystal yields 4 diffusion ages of individual growth segments at a magmatic temperature of 875 °C, ranging from ~ 1300 years close to the core to ~ 550 years at the crystal rim, implying an effective crystal growth rate of ~ 1.2 × 10- 10 cm s- 1. The short residence time at high temperature indicates that the magma from which the crystal grew was stored in a small and ephemeral upper crustal reservoir, and was likely remobilized just prior to its eruption by intrusion of hot, more mafic magma, as suggested by the abundance of mafic enclaves within the dacite lava dome.