Ultramafic-hosted hydrothermal systems at mid-ocean ridges: Chemical and physical controls on pH, redox and carbon reduction reactions

William E Seyfried, D. I. Foustoukos, D. E. Allen

Research output: Chapter in Book/Report/Conference proceedingChapter

49 Scopus citations

Abstract

Experimental, theoretical and field investigations of hydrothermal alteration processes in ultramafic systems at mid-ocean ridges, indicate that these systems have the capacity to buffer pH at surprisingly low values (pHT,P= 4.9-5.2), which profoundly affects fluid chemistry. Sluggish reaction kinetics of olivine at elevated temperatures and pressures, (e. g., 400°C, 500 bars), together with SiO2 and Ca dissolution from coexisting pyroxene minerals, enhance the stability of tremolite and talc accounting for the observed acidity. Moreover, oxidation of ferrous silicate components in unstable minerals, especially pyroxenes, generates high H2(aq) concentrations, which together with the relatively low pH, increase Fe solubility, consistent with the Fe-rich nature of vents fluids issuing from ultramafic-hosted hydrothermal systems at Rainbow and Logatchev at 36°N and 14°N, respectively, on the Mid-Atlantic Ridge. The high dissolved Cu and Ni concentrations, and low H2S(aq) of these vent fluids, indicate redox buffering by magnetite-bornite-chalcocite- heazelwoodite (Ni2S3)-fluid equilibria, as indicated by experimental and theoretical data. Data show that dissolved Cu is particularly sensitive to temperature change, while H2S(aq) and Fe are affected less by this, although Fe is highly sensitive to pH and dissolved chloride. Dissolved chloride concentrations observed for both the Rainbow and Logatchev hydrothermal systems depart significantly from seawater and suggest supercritical phase separation in subseafloor reaction zones. The relatively high temperatures required for this, together with the high rates of fluid flow at Rainbow, indicate a magmatic heat source. The most unusual feature of fluids issuing from the Rainbow and Logatchev hydrothermal systems, however, involves high dissolved concentrations of methane and other hydrocarbon species, and detectable carbon monoxide. Experimental data indicate that reducing conditions and mineral catalytic effects may account for this, although the reported CO(aq) at Rainbow is well below predicted levels, suggesting re-equilibration at lower temperatures.

Original languageEnglish (US)
Title of host publicationMid-Ocean Ridges
Subtitle of host publicationHydrothermal Interactions Between the Lithosphere and Oceans, 2004
EditorsC.R. German, J. Lin, L.M. Parson
PublisherBlackwell Publishing Ltd
Pages267-284
Number of pages18
ISBN (Electronic)9781118665879
ISBN (Print)9780875904139
DOIs
StatePublished - 2004

Publication series

NameGeophysical Monograph Series
Volume148
ISSN (Print)0065-8448
ISSN (Electronic)2328-8779

Bibliographical note

Funding Information:
This manuscript has benefited greatly from thoughtful reviews by Jeff Alt, Jean-Luc Charlou, Marv Lilley and Chris German, which significantly improved the final version. The paper also benefited from many discussions with attendees at the InterRidge Theoretical Institute, Pavia, Italy. The authors would also like to thank Ms. Sharon Kressler, Department of Geology and Geophysics, University of Minnesota, for assisting with preparation of the final layout and format of the paper. The senior author is appreciative of funding from IRTI. Funding from the U.S. National Science Foundation through research grants OCE-0117117, National Science Foundation OCE-9911471 and National Science Foundation OCE-9818908 played key roles in the development of concepts expressed in the paper.

Funding Information:
Acknowledgments: This manuscript has benefited greatly from thoughtful reviews by Jeff Alt, Jean-Luc Charlou, Marv Lilley and Chris German, which significantly improved the final version. The paper also benefited from many discussions with attendees at the InterRidge Theoretical Institute, Pavia, Italy. The authors would also like to thank Ms. Sharon Kressler, Department of Geology and Geophysics, University of Minnesota, for assisting with preparation of the final layout and format of the paper. The senior author is appreciative of funding from IRTI. Funding from the U.S. National Science Foundation through research grants OCE-0117117, OCE-9911471 and OCE-9818908 played key roles in the development of concepts expressed in the paper.

Publisher Copyright:
© 2004 by the American Geophysical Union.

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