Hydrothermal vent chemistry time series through an eruptive cycle at the Ridge 2000 ISS 9-10°N EPR focus site

M.F. Meana-Prado¹*, J.M. McDermott¹, K.L. Von Damm¹, & J.G. Bryce¹

Corresponding author: mfmprado@gmail.com
¹Department of Earth Sciences, University of New Hampshire, Durham, NH 03824

Abstract:
A persistent challenge to quantifying mass and heat flux to the oceans from mid ocean ridge hydrothermal systems lies in the translation of snapshots of fluid chemistry from transient systems to longer timescales and the application of chemical fingerprints to deconvolve subsurface fluid flow and decipher relative contributions from mineral equilibration reactions and more direct magmatic inputs, such as from degassing. The hydrothermal field at 9°50’ N in the East Pacific Rise provides an ideal natural laboratory to address the significant influence of an eruptive cycle on these processes. At this Ridge 2000 Integrated Study Site, approximately twenty vents have been sampled for high-temperature fluids in the last sixteen years. Extending earlier studies [1], we have used fluid chloride and silica contents, coupled with thermodynamic data [e.g., 2, 3] to place constraints on the T, P conditions of fluid circulation. The fluid collection includes samples with chloride contents greater than 1 molkg-¹ and silica contents greater than 20 mmolkg-¹. Generally, Cl and SiO2 contents in all vents behave non-monotonically with time. Every vent is unique and contributes to a high resolution model of the spatially and temporally variable magma chamber dynamics. For example, in the case of M-vent, the thermobarometric model supports a shoaling of the P, T equilibration with time until its extinction. H2S contents peak immediately following an eruption, and attain a maximum of 30 mmolkg-¹ in several vents. These post-eruptive fluid compositions are indicative of a shallow heat source driving vigorous rates of fluid circulation. After two years the heat source has deepened, causing the extinction of some vents, while active vent chemistries are beginning to return to a state of equilibrium.

Contributions to Integration and Synthesis:
This unique data set contains chemical signatures of fluids and processes connecting key components of the mid-ocean ridge system. These data are necessary for ground truthing fluid circulation and residence time models and may provide important baseline data for understanding the unique chemosynthetic biological communities sustained by hydrothermal fluids. Links between fluid chloride-silica contents, their thermobarometric implications, and seismic data, will be especially helpful in providing insight into temporal changes in mid oceanic ridge processes.

References:
Von Damm, Geophys. Monogr. Ser., 2004.
Von Damm et al., A. J. Science, 1991.
Foustoukos and Seyfried, Geochim. Cosmochim. Acta, 2006.

Figures:
Figure 1. Sixteen year record of fluid exit temperatures, end member chlorinities, and silica contents of hydrothermal fluids from the 950’N area. These data are reported for the full eruptive cycle and provide an opportunity to compare the hydrothermal system following two eruptions. Note the axis is nonlinear to emphasize the two post-eruptive periods. Meana-Prado_fig1.jpg