INITIAL PETROGENETIC MODELING OF CARLISLE VOLCANO, AK

Mt. Carlisle volcano is one of six volcanic centers in the Islands of Four Mountains (IFM), Alaska, suspected to have Holocene eruptive activity. This study is the first look into the petrology of Mt. Carlisle. The high density of contemporaneous volcanic centers in the IFM is unusual for the Aleutian arc, and the explanation remains unknown. This study aims to use geochemical data in a magma chamber modelling program, MELTS, to constrain the pre-eruptive conditions of a suite of 24 samples from Mt. Carlisle. A better understanding of the petrologic history of the volcano may unveil clues to the anomalous density of similarly aged volcanic centers. The Mt. Carlisle stratovolcano rises 1,620m above sea level with a 7.5km basal radius. The samples collected had compositions ranging from basaltic andesite to nearly dacite (56-62 wt% SiO₂) with a picritic enclave (44 wt% SiO₂). The samples all have alkali compositions ranging from 4.1 - 5.3 wt% total alkalis. For modelling, we used the most mafic Carlisle sample (SiO₂: 56.23 wt%; FeO: 6.54 wt%; MgO: 5.55 wt%) and a more mafic composition from a Holocene cinder cone on adjacent Chuginadak Island. The constraints were the starting composition, Ni-NiO redox conditions, and a percolation threshold of 0.1. The variable parameters used were: pressure (kb: 0, 3, 12) and water content (wt% H₂O: 0, 2, 4). Through varying the pressure and volatile content we hoped to be able to match the compositional trend of the Carlisle suite. Using the mafic Carlisle starting composition, the Carlisle samples correlated with the 3kb, anhydrous model for FeO and MgO vs silica, but not for K₂O and Al₂O₃. For K₂O, the best fit was with the hydrous (2 and 4 wt% H₂O) at 3kb. For Al₂O₃, the best fit was with 2 wt% H₂O at 12kb. Neither the mafic enclave composition nor the mafic cinder cone composition produced a model that closely matched the sample suite. We conclude that either open system processes affected the resident magma and/ or the best parental magmatic composition is not represented in our sample suite.