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Using Iron Stable Isotopes as Tracers of Subsurface Processes in Seafloor Hydrothermal Systems

DOEI Project Funded: 2004

Proposed Research

What are the primary questions you are trying to address with this research?
Past studies have highlighted the importance of sub seafloor environments in controlling the diversity of seafloor hydrothermal vents. It is also well known that these environments harbor a diverse and unique biological community capable of using dissolved chemical species and minerals for energy metabolism. However, the mechanisms and extent of subseafloor metal precipitation, remobilization and bacterial cycling are still poorly constrained and new approaches are required. Here we propose to undertake a combination of Fe isotope analysis of field samples, including hydrothermal fluids an associated sulfides, altered rocks and experimental work to develop a conceptual model for using Fe isotopes as new tracers of subsurface environments.

What is the significance of this research for society?
Variations in the isotopic ratios of light elements such as H, C, N, O and S have been widely studied over the last five decades and provided major advances in the knowledge of natural and anthropogenic processes directly affecting the society. For example, these stable isotope systems have been applied to a range of problems such as planetary geology, the origin and evolution of life, climate change, and water-rock interactions. However, much less attention has been paid to the stable isotope variations of heavier elements, such as Iron, mainly due to analytical challenges. With the recent advent of multi-collector inductively-coupled plasma mass spectrometry (MC-ICPMS), the study of Fe stable isotopes is now accessible and should lead to unprecedented discoveries in biogeochemical cycles.

What is the significance of this research for others working in this field of inquiry and for the broader scientific community?
Traditionally, the behavior of metals in seafloor hydrothermal systems have been investigated by integrating results from laboratory studies, theoretical models, mineralogy and fluid and mineral chemistry. Our new approach consists of using Fe isotopes as tracers of the interactions between hydrothermal fluid, altered rocks, sulfide deposits and biologic systems in subsurface environments.

This work overlaps with different research fields and requires expertise in analytical geochemistry, petrography, modeling, and experimental investigation. Through the characterization of this new isotopic tool, this study will increase our understanding of the fate of Iron in seafloor hydrothermal systems and other environments on Earth surface. This approach will allow us to contribute to the deep biosphere initiative as the identification of Fe stable isotope biosignatures requires a good understanding of the variability of Fe isotopes generated by abiotic processes solely from hydrothermal activity.

When and where will this investigation be conducted?
The first work period (early 2005), which will be undertaken during Rouxel postdoc, will be dedicated to acquiring Fe isotope data from hydrothermal samples (fluids and sulfides) and altered rocks recovered during former Alvin and Ocean Drilling Program expeditions. Experimental simulation of hydrothermal basalt alteration and mineral (sulfide) precipitation will be undertaken in collaboration with Jeff Seewald to provide important understanding of Fe isotope fractionation in natural samples.

The second work period (through 2006) will include the participation of the PIs in a 42-day cruise using ROV Jason 2 at PACMANUS field (Wolfgang Bach Co-chief scientist). Subsequent extensive on-shore work will focus on Fe-isotope analysis of hydrothermal fluids, chimneys and altered rocks.
What are the key tools or instruments needed to conduct this research?

An important aspect of the project is the strong analytical commitment involving the use of the Multi-Collector ICPMS (inductively coupled plasma mass spectrometry) at WHOI. While we have experience in analyzing the isotopic composition of various metals and metalloids, we will adapt and further refine analytical techniques for to achieve our scientific goal.

Is this research part of a larger project or program?
The proposed project will provide the opportunity to explore natural variations of other metal and metalloid stable isotopes, such as Copper and Germanium isotopes, having distinct behaviors in seafloor hydrothermal systems. Ultimately, this larger project should open the way for new approaches in the study of chemical and bacterial subsurface processes in seafloor hydrothermal systems.

Publications
O. Rouxel, N. Dobbek, J. Ludden and Y. Fouquet, Iron Isotope Fractionation During Oceanic Crust Alteration (Site ODP 801), Chemical Geology 202, 155-182, 2003.

O. Rouxel, Y. Fouquet and J.N. Ludden, Subsurface processes at the Lucky Strike hydrothermal field, mid-atlantic ridge: Evidence from sulfur, selenium and iron isotopes, Geochim. Cosmochim. Acta 68, 2295-2311, 2004.

O. Rouxel, Y. Fouquet and J.N. Ludden, Copper Isotope systematics of the Lucky Strike, Rainbow and Logatchev Seafloor Hydrothermal Fields on the Mid Atlantic Ridge, Economic Geology 99, 585-600, 2004.

Originally published: January 1, 2004