Evolution in magma storage conditions beneath Kick-’em-Jenny and Kick-’em-Jack submarine volcanoes, Lesser Antilles arc

Kick-’em-Jenny and Kick-’em-Jack are two submarine volcanoes located in the southern Lesser Antilles arc. We present data on the temporal and spatial compositional variations in lavas and melt inclusions from Kick-’em-Jenny and four lavas from Kick-’em-Jack. Analyses of crater samples from Kick-’em-Jenny show an evolution in magma compositions between 1972 and 2013/14. Rock types have progressed from predominantly basalts in 1972 to basaltic andesites in 2013/14, while melt inclusions have also become more silicic over time, ranging basaltic to andesitic in 1972 and basaltic andesite to dacitic in 2013/14. Hornblende-phyric basalts are unusually rich in MgO (≤13 wt%) and Ni (≤230 ppm) and contain rare olivine phenocryst cores of Fo _90–92 , suggestive of a primitive character. Other phenocryst phases include spinel, clinopyroxene and plagioclase. Conversely, 2013/14 landslide deposit samples contain orthopyroxene as an additional phenocryst phase and are distinct from crater deposits in terms of phenocryst and bulk rock chemistry. We propose that 1972 and 2013/14 crater deposits sample Kick-’em-Jenny's current edifice, whereas 2013/14 landslide deposits likely represent “proto Kick-’em-Jenny” (an edifice existing pre 43.5 kyr BP). Geochemical data for Kick-’em-Jack support the notion that it is part of the same volcanic complex as Kick-’em-Jenny and probably represents part of the now collapsed proto Kick-’em-Jenny. Low eruption temperatures (~1000 °C), estimated from geothermometry, coupled with whole-rock magma chemistry suggest an origin of the most mafic lavas of Kick-’em-Jenny by dehydration melting of hornblende-peridotite just below the Moho. Subsequent injection of these cool, oxidised, wet primitive basalts into the deep crust was accompanied early fractionation of hornblende-dominated assemblages. Large, zoned hornblende phenocrysts track rapid ascent of magmas through the crust along a trajectory sub-parallel to the hornblende liquidus curve. Melt inclusion data provide clear evidence of volatile-rich (≤5 wt% H ₂ O and <3200 ppm CO ₂ ) pre-eruptive magma storage 7–12 km below Kick-’em-Jenny. This new understanding of the architecture of Kick-’em-Jenny's sub-volcanic system will be useful for interpreting future seismic signals and mitigating volcanic hazards.