APATITE AS AN INDICATOR OF PRE-ERUPTIVE DESTABILIZATION AT ICELANDIC VOLCANOES

Apatite is a petrologically valuable mineral due to its ability to incorporate a variety of magmatic tracers into its crystalline structure, including volatiles (F, Cl, S, OH) and the light rare earth elements (LREE). Despite apatite’s utility and ubiquity as an accessory mineral in igneous rocks, studies of Icelandic apatite are extremely recent and rare (Connors et al., 2020). Here we establish baseline geochemical signatures of Icelandic apatite using samples from Krafla and Askja (Northern Volcanic Zone), Kerlingarfjöll (Western Volcanic Zone), Öræfajökull (Öræfi Volcanic Belt), Hekla (Eastern Volcanic Zone), and five detrital river samples. Volcanic samples represent the variety of petrogenetic settings in modern Iceland (on-rift, propagating-rift, off-rift), while detrital samples represent Iceland more broadly.

Apatite compositions (measured by EMPA) from these Icelandic samples are dominantly fluorapatite (F [wt%]: 0.56 to 1.06), with low Cl (wt%: 0.0 to 0.11) and variable OH (0 to 0.43 APFU).

Apatite from Krafla (~100 ka) and Hekla (1158 C.E.) show low S (Kr: <100 ppm; Hk: <200 ppm), OH (0 to 0.27 APFU) and Cl (Kr: 0.21 to 0.30 wt%; Hk: 0.06 to 0.18 wt%). These compositions are similar to those of detrital samples (S: < 300 ppm; OH: 0.02 to 0.43 APFU; Cl: 0.07 to 0.37 wt%).

Volatile contents of apatite separated from pumice from the 1875 C.E. Plinian eruption of Askja are distinct compared to other apatites considered in this study, with high and highly variable S (<1295 ppm), OH (0.27 to 0.40 APFU), and Cl (0.21 to 0.50 wt%). Connors et al. (2020) observed a similar range of high and variable S from a 67 ka eruption of rhyolite at Torfajökull (0 to 571 ppm). They attributed this S anomaly at Torfajökull to a magmatic degassing event, an interpretation consistent with a published investigation of melt inclusions (Owens et al. 2013). High and variable volatiles measured in Askja apatites likely represent a similar degassing event preceding the Plinian, caldera-forming, eruption of Askja—perhaps associated with the Sveinagjá fissure eruption in the months leading up to the explosive event. This study shows that apatite can be used to place better constraints on degassing and pre-eruptive destabilization of magmas, and thus, help us better understand Icelandic volcanism.