Modeling Deep-Seated Reactive Fluid Flow within Active Hawaiian Volcanoes

Hawaiian volcanoes commonly exhibit gravitationally-driven seaward spreading accommodated along a weak sub-volcanic décollement, and shallow slump displacements and detachments, facilitated by high pore-fluid pressures. New evidence of deep hydrothermal processes is now provided by a suite of alteration minerals recognized within the distal submarine flanks of Mauna Loa, interpreted to consist of an imbricate stack of volcaniclastic thrust sheets. The degree of mineral alteration in Mauna Loa's offshore bench increases up-section, with interstitial smectite, zeolites, and actinolite occuring in fine grained sediments near the base of the bench, and higher temperature phases including chlorite, epidote, and quartz recognized in courser samples near the bench top.

We constructed reactive flow models of the lower flanks of Mauna Loa to characterize the style of density-driven fluid convection that might occur in this setting. Large convection cells are predicted to develop in the old oceanic crust and young basaltic seamount; hot fluids leak upward into the volcaniclastic aquifer unit that crops out on the seafloor. The reactive flow code RST2D was used to predict solute transport and mineralization in this hydrothermal profile. The system is modeled with 18 chemical components, 40 secondary species and complexes, and 38 mineral phases representative of volcanic rocks and marine sediments. For this test, we ran the RST2D simulation for 100 years. The patterns of geochemical alteration indicate a small amount of plagioclase feldspar and Mg-chlorite mineralization within the flow field, focused within the permeable volcaniclastic sediments that channel hydrothermal discharge towards the seafloor. Remarkably, this reactive flow field results in a pattern of chloritization similar to that observed on the submarine flank of Mauna Loa, where the highest grades occur near the present top of the bench, although the processes by which these rocks were exhumed is not considered.