GEOCHEMICAL MODELLING OF ANDESITE LAVAS ERUPTED ON THE AXIAL SEAMOUNT NORTH RIFT ZONE

Axial Seamount is an active undersea volcano in the northeast Pacific that has formed where the Juan de Fuca spreading ridge overlies the Cobb hotspot. Axial has erupted over 50 times in the past 1500 years (Clague et al., 2013) and three times in the past 26 years (Chadwick, W. et al., 2016). Prodigious sampling of the volcano and its north and south rift zones show it has erupted mostly relatively homogenous basalts over its history (Chadwick, J. et al., 2005). But four wax-tipped rock core samples from a prominent dike fed ridge in the north rift zone during the 1998 and 1999 NeMO campaigns recovered basaltic andesites and andesites, and an additional wax core and a dedicated MBARI ROV Doc Ricketts dive in 2017 recovered 27 additional evolved samples along a 7 km length of the ridge. Mid-ocean ridges and oceanic hotspots typically produce basaltic lavas, a result of limited opportunities for primary mafic magmas to fractionally crystallize or interact with evolved components prior to eruption. Higher silica lavas are thought to require situations in which fractional crystallization is enhanced, magma supply is low, or where crustal melting or magma mixing occur (e.g. at overlapping spreading centers, Wanless and Perfit, 2012; e.g. at Kilauea: Gansecki et al., 2019; or Iceland, Lacasse et al., 2005). Axial Seamount is the most active submarine volcano in the NE Pacific, fed from a large magma reservoir underlying its summit caldera. During eruptions, magma propagates via dikes and erupts on the caldera floor and along its north and south rift zones (Chadwick, W. et al., 2016). SiO₂ contents of the recovered samples range from 52-57 wt.%, and SiO₂ broadly increases and MgO decreases with distance along the north rift zone. This relationship was also identified for basaltic lavas in both rift zones (Chadwick, J., et al., 2005), and is thought to result from derivation from a compositionally zoned magma reservoir (Chadwick, W. et al., 2016). In this study, Magma Chamber Simulator software was used to model the petrogenetic development of the andesites. Fractional crystallization models require greater than 80% crystallization to reach the most evolved compositions, and other evaluated models include assimilation of altered oceanic crust with varying degrees of fractional crystallization.