USING APATITE TO INVESTIGATE THE VOLATILE HISTORY OF TORFAJÖKULL CENTRAL VOLCANO, ICELAND

Apatite (Ca₁₀(PO₄)₆(F,Cl,OH)₂), a common accessory mineral in Icelandic silicic rocks, can be used to investigate the origin and evolution of the magma from which it crystallized because of its ability to incorporate major volatile elements (F, Cl, H, S, C), redox-sensitive elements (S, Mn, Fe) and many rare earth elements (REE). In this study, the first to use apatite for a focused Icelandic investigation, we use apatite to elucidate the volatile history of Torfajökull, a central volcano with infrequent but voluminous eruptions of high Si peralkaline rhyolites in the Pleistocene and frequent but smaller eruptions of lower Si metaluminous rhyolites in the Holocene. Apatites and volcanic matrix glasses from five rhyolitic eruptions spanning Torfajökull’s history (384 ka, 278 ka, 67 ka, 871 AD, and 1477 AD) were analyzed by EMPA. Matrix glasses and apatites indicate increasing volatile concentrations over time, especially in Cl (glass: 60 to 3170 µg/g; apatite: 0 to 0.4 wt%). The Cl-OH concentrations in apatites from the 67 ka eruption indicate a classical degassing pattern, with Cl/OH weight ratios increasing from 0.01 to 0.93 as OH contents decrease. The Cl-OH trends in apatite are remarkably similar to those observed in published melt inclusion data from the same eruption (Owen et al. 2013) and are interpreted to record the same degassing event. Measurements of S in glasses and modeled S solubility show increasing concentrations of S in the melt over time (from 80 to 590 µg/g). As expected in reduced rhyolitic magmas (e.g., Konecke et al. 2017), most apatites exhibit lower concentrations of S (20 to 100 µg/g S) than their associated melts. High sulfur contents in some apatites from the 67 ka eruption (20 to 570 µg/g S), however, surpass measurements in glasses and calculations of S in the melt. This is interpreted to indicate a change of S speciation at 67 ka during the transition from a sulfide- to sulfate-dominated melt. Degassing and separation of a sulfur-rich volatile phase may explain this oxidation at 67 ka, with contemporaneous apatite crystallization recording this process. This first study of Icelandic apatite highlights the mineral’s utility to uncover processes during volcanic degassing, establish longer-term evolution at volcanic centers, and contribute to understanding Icelandic magmatism more broadly.