Syn- and post-fragmentation textures in submarine pyroclasts from Lō`ihi Seamount, Hawai`i

The extent and timing of vesiculation and microlite formation in ascending basaltic melts are relatively well understood for subaerial eruptions, but they have not been studied thoroughly for submarine eruptions. This is largely due to a lack of samples of appropriate volume and grain size. We present a quantitative textural evaluation of coarse lapilli from two cone-forming deposits at ∼ 1 km depth on Lō`ihi Seamount, Hawai`i, collected during a sampling-intensive submersible dive series in 2006. Lapilli from the deposits, named the northern and southern cones, have modal bulk vesicularities of 55 and 42%, respectively, and vesicle populations dominated by sub-spherical, non-interconnected vesicles, with number densities on the order of 104 to 105 cm− 3. Clasts of modal vesicularity have margin-parallel zonation, with seawater-quenched rims interpreted to preserve “syn-fragmentation” magmatic textures in microlite-free sideromelane glass enclosing “post-fragmentation” tachylitic interiors with vesicle and microlite textures that progressively coarsen from rim-to-core. Degassing scenarios are linked to syn-fragmentation vesicle textures to demonstrate that both magmas degassed in dominantly closed systems. Diffusion-controlled cooling rates of basaltic pyroclasts in contact with water are linked to post-fragmentation evolution of vesicle and microlite textures to make inferences about transportation and dispersal of the pyroclasts in low submarine jets. The textural analysis shows that the northern and southern cone eruptions were not strictly analogous to any classical subaerial eruption type. They were uniquely submarine, with ambient seawater playing a crucial role in the processes of degassing, vesiculation, crystallization, and dispersal.