Every Andesite Tells a Story: From Maps to Microprobes, the Origin of Andesites on Axial Volcano, Juan de Fuca Ridge

Abstract:

Andesitic hummocky flows, some over 200 m high form a northeasterly trending ~ 11km long ridge ~3 km east of Axial’s North Rift neovolcanic zone (NRZ). The ridge is morphologically similar to numerous ridges up to 15 km long and 150 m high that parallel the modern NRZ and likely reflect past diking events. Meter-scale AUV mapping was completed along the andesite ridge and detailed observations and sampling were done with MBARI’s ROV Doc Ricketts on the southernmost 4 km. The steep-sided volcanic ridge is comprised of pillow lavas up to 2-3 m in diameter and smaller elongated pillow tubes. Radiocarbon ages of sediment on top of the ridge indicate the andesites are at least ~1390 years old. This minimum age is much younger than the ~56 Ka age estimated from its distance from the rift axis, indicating it was likely an off-axis eruption. The total volume of the andesite cones is 0.73 km³ covering 3.2 km².

Twenty-six andesitic lavas with SiO₂ contents from 52 to 57 wt. % with <3 wt% MgO and relatively high FeO (12.8-15.7 wt%) were recovered. They are compositionally similar to other mid-ocean ridge (MOR) andesites. They differ in being highly vesicular (<5 – 30 vol. %), more phyric (although still very glassy) and more rarely, subtrachytic. Incompatible trace element abundances are ~4-6 times more enriched than in typical Axial Seamount transitional-MORB. Incompatible element abundances are in general comparable to those of high-silica lavas from other MORs having significant positive U anomalies, large negative Sr anomalies, small negative Eu anomalies, and slight positive Zr-Hf anomalies relative to primitive mantle values. However Axial andesites have greater abundances of the most highly incompatible elements (e.g. Ba, Rb, Nb, Ta). Common to other high-silica MOR suites, they also have unusually high Cl (~0.3-0.6 wt%) and H₂O (~1.60-1.71 wt%) contents. Major and incompatible element trends are can be modeled with ~76% to 86% fractional crystallization of phases present in the lavas (plagioclase> clinopyroxene> olivine>FeTi oxides) at low pressures. The elevated Cl, H₂O and U indicate some crustal or brine assimilation also occurred. Andesite Sr, Nd and Pb isotopic values comparable to those of Axial basalts are consistent with their petrogenesis from parental basaltic magmas in a shallow, upper crustal magma reservoir.