COMPOSITIONAL EVOLUTION OF TORFAJÖKULL CENTRAL VOLCANO, ICELAND: PERSPECTIVES FROM THE APATITE RECORD

The contributions that apatite can make to studies of magmatic and volcanic processes in Iceland are numerous due to its unique ability to incorporate rare earth elements (REE), volatile elements (F, Cl, H, S, C), and redox-sensitive elements (S, Mn, Fe) in its structure. We use apatite from Torfajökull volcano to learn about Icelandic apatite generally and the trace element and volatile evolution of the Torfajökull magmatic system specifically. Apatites and volcanic matrix glasses from five rhyolitic eruptions spanning Torfajökull’s history (384 ka – 1477 AD; higher Si peralkaline to lower Si metaluminous) were determined by electron probe micro-analysis. Apatites from the oldest, coolest, peralkaline eruptions have exceptionally high light REE (~5 to 19 wt% oxides); those from younger, warmer, metaluminous eruptions contain lower amounts of light REE (0.5 to 3 wt% oxides). This trend is consistent with experimental apatite-melt partition coefficients, which decrease with increasing temperature, and with decreasing silica activity (Watson & Green 1981). As with REE, Torfajökull apatites also capture a record of volatile change with time. Consistent with published melt inclusion data for Torfajökull (Owen et al. 2013), 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 apatite exhibit a degassing pattern, with Cl/OH wt ratios increasing from 0.01 to 0.93 with decreasing OH. The Cl-OH trends in apatite are very similar to melt inclusion data (Owen et al. 2013) and likely record the same degassing event. As expected in reduced rhyolitic magmas (e.g., Konecke et al. 2017), S concentrations are low in apatites from the oldest and youngest units (20 to 100 µg/g S). However, elevated S contents in some apatites from a 67 ka eruption (20 to 570 µg/g S) indicate a change in the S speciation of the melt, from sulfide- to sulfate-dominated. Degassing and separation of a sulfur-rich volatile phase, prior to or during apatite crystallization, may explain this oxidation. This first study of Icelandic apatites reveals their power to illuminate volcanic processes in individual eruptions, to elucidate longer-term evolution at volcanic centers, and to broadly contribute to understanding Icelandic magmatism.