Host Rock Composition and Subsequent Alteration as Primary Constraints on Biogeochemical Processes in Seafloor Hydrothermal Systems

P. Canovas¹* & E. Shock¹’²

Corresponding author: pcanovas@asu.edu
¹GEOPIG, School of Earth & Space Exploration, Arizona State University, Tempe, AZ, 85287
²Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ, 85287

Abstract:
The alteration of igneous basement rocks produces disequilibria that lead to microbial habitat generation in seafloor hydrothermal systems. The evolution of fluid compositions and mineral alteration assemblages constrains the availability of energy for microbes and influences the viability of various metabolic pathways. Reaction path modeling shows only slight differences among MORB alteration assemblages and the resulting fluid compositions regardless of moderate differences in equilibration temperature, or fairly wide ranges of water-to-rock ratios. As examples, calculated pHs, silica activities and hydrogen concentrations are quite similar even when using MORB compositions that span the range of observations. In contrast, ultramafic assemblages yield far more variability, consistent with experiments and observations of natural systems. In order to investigate this phenomenon, reaction path modeling was done for host rock assemblages with Mg-end member compositions, as well as those with representative Mg-Fe solid solution compositions of olivine (90% forsterite, 10% fayalite), orthopyroxene (92% enstatite, 8% ferrosilite) and clinopyroxene (85% diopside, 15% hedenbergite). Comparisons between the two sets of calculations reveal large contrasts between working with pure minerals versus more realistic solid-solution compositions. As an example, pH values calculated at 100°C for the solid-solution assemblages do not reach the highly basic values found for the pure endmembers, and the highest pH values occur in host-rock systems rich in olivine and clinopyroxene. Silica activities are also less variable in the results for calculations using solid solutions. Dissolved H2 concentrations at 100°C and 300°C also differ by several orders of magnitude between the two sets of calculations. At 100°C, calculated H2 concentrations change dramatically in response to small changes in the initial mineral assemblages, with large variations centered around host rocks dominated by olivine. These results illustrate the concept of tipping points during hydrothermal alteration. These dynamic systems are extremely sensitive to subtle shifts in the composition of ultramafic rocks that are rich in olivine, which is common for the majority of seafloor ultramafic rocks. We anticipate many dramatic shifts in fluid composition owing to comparatively minor shifts in the primary mineralogy of ultramafic rocks that host hydrothermal systems. These shifts, in turn, will dictate how disequilibria are generated and how microbes can take advantage of geochemically provided energy supplies.

Keywords:
ultramafic alteration, hydrogen generation, hydrothermal fluids

Contributions to Integration and Synthesis:
The present work is part of an integrated approach for quantifying water-rock-microbe interactions that accounts for the variability in known hydrothermal systems and those that may exist, but have not yet been discovered. It directly uses ridge data including, but not limited to, that of hydrothermal fluids, host rock assemblages and microbial investigations, and incorporates data from all integrated study sites. Major foci include identifying tipping points that can cascade a hydrothermal system from one extreme to another with respect to enhancement or inhibition of biogeochemical processes. These tipping points may include physical, chemical or reaction parameters depending on the system in question and their quantification will elucidate the energetic potential of these diverse systems.