Relationships of vent deposit morphology and composition to geologic setting (including vent fluid chemistry) and to vent fauna

M.K. Tivey¹*, R. Haymon², D.S. Kelley³, M.D. Hannington⁴, & J. Jamieson⁴

Corresponding author: mktivey@whoi.edu
¹WHOI, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, 02543 USA
²Dept. of Earth Science, University of Santa Barbara, Santa Barbara, CA 93106 USA
³School of Oceanography, University of Washington, Seattle, WA 98195 USA
⁴University of Ottawa, Dept. of Earth Sciences, Ottawa, Ontario K1N 6N5 Canada

Abstract:
Studies over 30 years have documented that vent deposit morphology, chemical composition, and mineral content vary with geologic setting and are correlated to site-specific parameters: e.g. vent fluid composition (different due to substrate composition, presence of magmatic acid volatiles, phase separation, time since magmatic event, etc.); also local factors affecting fluid velocities and mixing styles; and presence/absence of specific biota that affect mixing and/or fluid composition. At the East Pacific Rise (EPR) Integrated Study Site (ISS), time-series collections of vent deposit samples (Haymon, Ency. Geology, 2005), fluids, biota, micro-seismicity data, vent temperatures, and geological observations reveal the effects of volcanic events on vent deposit formation. Here lava flows interact directly with chimney growth. Eruptions also trigger changes in physical and chemical properties of hydrothermal fluids, and in crustal thermal structure, permeability, and pressure. These changes alter fluid flow, mixing, and fluid composition, particularly redox and pH. At the EPR ISS, the composition and morphology of deposit exteriors are significantly affected by the presence of Alvinella pompejana (Haymon and Kastner, EPSL 53, 1981). Magmatic events have also impacted chimney formation at the Endeavour ISS, affecting vent fluid temperatures and compositions, and biota. In addition, Endeavour vent fluids differ from EPR ISS fluid compositions in that they are enriched in ammonium. This results in a high fluid pH at <300°C. Also, barite is saturated in the vent fluids and present in many of the deposits. The higher pH of Endeavour fluids affects both the composition of the deposits (they are Zn-and Si-rich) and their morphology, with flanges dominating many of the deposits (e.g., Tivey et al., JGR 104, 1999; Kristall et al., G^3 7, 2006). The presence of different biota (e.g., Paralvinella) than those along the EPR also influences morphology and composition of the Endeavour deposits (e.g., Juniper et al., Geology 20, 1992). At the Lau ISS, along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR), deposit morphology and composition at six vent fields are similarly influenced by differences in vent fluid composition (reflecting substrate composition ranging from basalt in the north to more felsic in the south, temperatures of water-rock reaction, and presence of magmatic acid volatiles at the two southernmost vent fields). As at the Endeavour ISS, higher pH vent fluids correlate with the presence of flanges and Zn-rich deposits. In addition, more felsic substrates correlate with Ba-, Pb-, and As–rich deposits, and the presence of magmatic acid volatiles correlates with low pH and Cu-rich, flange-poor deposits. In contrast to mineralization at EPR and Endeavour ISS, at the Lau ISS there appears to be less influence on morphology and composition of exteriors of deposits from megafauna, with shrimp, scale worms, snails and mussels dominating instead of Alvinella and Paralvinella.

Contributions to Integration and Synthesis:
Hydrothermal deposits record processes transpiring on and beneath the seafloor. The differences in the geologic settings of the three ISS allow us to identify how setting-specific geologic/geochemical processes and characteristics are reflected and revealed by petrologic, compositional, and morphologic differences of the vent deposits, and are encoded in the geologic record. The deposits are sites of significant transfer of energy and mass from the Earth to the ocean, and record how geologic setting can influence these transfers. Furthermore, the documented variation in vent fauna among the different ISSs offers an excellent opportunity to examine the extent to which biota influence the structure and composition of deposits, and leave a fossil record indicative of geologic setting.