New geochemical constraints on hydrothermal fluid fluxes at mid-ocean ridge axes and ridge flanks

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

This research project addresses a very fundamental issue of geochemistry - how much water is processed through and expelled from the sub-seafloor hydrothermal systems that are found throughout the world's oceans? This is an important question because hydrothermal processes have a profound effect on the global geochemical budgets of numerous elements (including the greenhouse gas carbon dioxide) and they thus exert a strong influence on the composition of the oceans, the atmosphere and, through subduction of altered ocean crust, the upper mantle. Estimates of hydrothermal power outputs that are derived from thermal models of the oceanic crust are generally thought to be more reliable than direct geochemical estimates of hydrothermal fluid fluxes, many of which are based on the poorly constrained marine mass balances of individual elements and isotope systems (e.g., Mg and 87Sr/86Sr). The conversion of power outputs into hydrothermal water fluxes is fraught with difficulties, however, primarily due to the unknown partitioning of heat between fluids of different temperature (and hence different chemical reactivity) and uncertainties about the extent to which the available heat is used to drive hydrothermal circulation at mid-ocean ridge axes. This is unfortunate because the geochemical impact of hydrothermal systems is directly related to the water fluxes rather than the power outputs. In this study, we will use a novel geochemical approach to obtain precise constraints on the global water fluxes that are expelled into the oceans from both the hot sub-seafloor hydrothermal systems of mid-ocean ridge axes and the cooler hydrothermal systems that exist at ridge flanks. The method utilizes concentration and isotope composition data for the element thallium (Tl) in relevant samples. A recent pilot study demonstrated that the new approach is significantly more robust than other geochemical methods. The results of the pilot study are only of limited value, however, because they are based on only a small number of analytical data. The present investigation will address this shortcoming, through a thorough characterization of the Tl concentrations and isotopic compositions of altered oceanic basement rocks, using representative samples from different tectonic settings and spreading rates, and a suite of globally representative hydrothermal fluids. Due to the unique properties of the Tl isotope system, these analyses will provide the most reliable geochemical estimates of hydrothermal fluid fluxes available to date.

Publications

10 25 50
 
Description Hydrothermal circulation at mid-ocean ridge axes and flanks is a fundamental process in the formation and aging of the ocean crust. It provides a major control on the compositions of the oceans, including several elements that are used to decipher past climate variations, and it has an important impact on the atmospheric concentration of the greenhouse gas carbon dioxide.

The hydrothermal contribution to ocean chemical budgets depends on the amount of water passing through the ocean crust, and the composition and temperature of the fluids. Heat flow studies suggest that two-thirds of hydrothermal heat removal occurs on the ridge flanks. However, the volumes of hydrothermal fluid required to remove this heat are difficult to estimate because the variation in fluid temperature with crustal age is not known.

In this study we have exploited the unique behaviour of the element thallium (Tl) to place new geochemical constraints on both axial and ridge flank hydrothermal circulation. A pilot study had previously demonstrated that on-axis, Tl is leached from the sheeted dikes by high-temperature hydrothermal fluids, with no associated fractionation in Tl isotope composition. In contrast, isotopically light Tl is added to the upper ocean crust from seawater during low temperature off-axis circulation.

The Tl concentrations and isotopic compositions of ocean crust were determined by multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS), to achieve the primary objectives of this project:

(1) To thoroughly characterize the behaviour of Tl during hydrothermal alteration, ~140 representative rock samples from the various stratigraphic levels of the ocean crust that span the range of alteration styles were analysed. In particular, this included sections of ocean crust produced at slow- (Macquarie Island), intermediate- (ODP Holes 504B and 896A and the Juan de Fuca Ridge), and fast- (ODP Hole 1256D) spreading rates. Additional trace element and sulphur isotope/concentration data were acquired for a subset of ~25 samples.

(2) The Tl concentration results for whole rock samples that exhibit different styles of alteration were combined with petrological investigations and trace element analyses, to determine which minerals are responsible for the Tl uptake from seawater at low temperature in the upper ocean crust. Surprisingly, the Tl concentrations were found to correlate with S contents, but not Rb or Cs. This indicates that Tl uptake is associated with the formation of secondary pyrite rather than clay minerals, as was previously thought.

(3) Thallium isotope and concentration profiles through a drilled in-situ section of upper ocean crust (ODP Site 1256) and a unique complete ocean crustal section sub-aerially exposed on Macquarie Island were obtained, to evaluate the depth to which Tl mobility occurs in the ocean crust at slow and fast spreading rates. The data reveal that high temperature leaching occurs through the sheeted dikes and the uppermost gabbros. They also indicate, however, that there is a mixing zone between the lavas and sheeted dikes where some of this Tl is re-deposited. This suggests that there is some limited recycling of Tl within the ocean crust.

(4) The unique behaviour of Tl during axial and ridge flank hydrothermal circulation was exploited to estimate the hydrothermal fluid fluxes through both regimes, using innovative mass balance models. The results support the original hypothesis that the axial hydrothermal fluid flux is an order of magnitude smaller than previous estimates, which are based on traditional seawater chemistry mass balance techniques. Isotopically light Tl was added to all the studied sections of upper ocean crust. Variations in the extent of the isotopic shift between the different crustal sections studied suggest that the ridge flank hydrothermal fluid flux is a factor of five greater through ocean crust produced at intermediate compared to fast spreading rates.
Exploitation Route They can be used to obtain a better understanding of the impact of hydrothermal fluxes and processes on marine element budgets and processes.
For example, Co-I Coggon is now investigating C sequestration in the ocean crust. These C sequestration studies profit from the understanding that Coggon gained about the alteration of ocean crust in this investigation.
Sectors Education,Energy,Environment

 
Description The findings provide an improved understanding of (i) thallium cycling in hydrothermal systems and (ii) the water fluxes and power outputs discharged by sub-seafloor hydyrothermal systems.
First Year Of Impact 2010
Sector Education,Energy,Environment
Impact Types Societal