- 1University of Leeds, Institute of Geodynamics and Tectonics, School of Earth and Environment, Leeds, United Kingdom
- 2Istituto di Geoscienze e Georisorse - Consiglio Nazionale delle Ricerche, Pisa, Italy
- 3Géosciences Montpellier, CNRS, Université de Montpellier, France
- *A full list of authors appears at the end of the abstract
The use of fluid-mobile elements and their isotopes to track fluid-mediated subduction zone processes requires an accurate estimate of the volatile element systematics of subducting oceanic crust. Near-ridge hydrothermal circulation represents the primary means by which seawater can penetrate the oceanic crust and produce enrichments in fluid-mobile elements (B, Sr, Li, U, Cl etc.), particularly at slow-spreading ridges where hydrated mantle peridotite (ie. serpentinite) is commonly exposed at the seafloor. However no previous drilling has penetrated abyssal serpentinite deeper than 200.8m below seafloor, where late-stage alteration and intense fault-controlled circulation during exhumation might produce anomalous fluid-mobile element signatures. While ophiolites provide a valuable analogue, it is often hard to distinguish geochemical signatures related to interaction with seawater-derived fluid from those acquired during subsequent interaction with subduction-related and/or meteoric fluids.
We present new data from IODP Expedition 399, which recovered 1268m of serpentinized depleted mantle peridotite and variably altered gabbroic rocks (Hole U1601C) from the southern wall of the Atlantis Massif (30°N; Mid-Atlantic Ridge). Peridotites are generally highly serpentinized (80-90%) and display complex pseudomorphic, mesh and vein textures, attesting to a multistage history of alteration. Gabbros range from fresh to completely altered and exhibit a diversity of secondary mineral assemblages (±amphibole ±serpentine ±talc ±chlorite ±sulphides ±prehnite ±secondary plagioclase ±zeolite ±saponite ±carbonate). Our downcore fluid-mobile trace element and B and Sr isotopic profiles provide a comprehensive framework in which to understand physicochemical conditions during serpentinization and metasomatism of the actively metamorphosing basement of the massif, and their relation to current seafloor venting at the Lost City Hydrothermal Field.
B concentrations in serpentinites decrease by an order of magnitude downcore, which we interpret in terms of B depletion of alteration fluid through the serpentinization process. Substantial downcore variation in the B isotopic composition of serpentinite (δ11B of +12‰ to +40‰) reflects local T and pH conditions as well as isotopic evolution of the alteration fluid along the flow path. Serpentinite Sr isotopic compositions vary between seawater and near mantle values (87/86Sr of 0.704 to 0.709); likely reflecting considerable elemental exchange between alteration fluid and gabbroic intrusions. Our results also shed new light on the geochemical influence of late-stage alteration processes (carbonation, oxidation, infilling of reaction porosity etc.) postdating serpentinization.
In addition, we present new B isotope data from (olivine-bearing) gabbroic rocks of the central massif (Hole U1309D) and detachment-proximal serpentinites from the south wall drilled during IODP Expedition 357. Together, these data represent an important step towards quantifying the fluid-mobile element makeup and specifically the B and 11/10 B content of the lower oceanic crust.
Andrew McCaig(1). Susan Q. Lang; Department of Geology and Geophysics, Woods Hole Oceanographic Institution, USA. Peter Blum; International Ocean Discovery Program, Texas A&M University, USA. Natsue Abe; Agency for Marine-Earth Science and Technology, Japan. William Brazelton; School of Biological Sciences, University of Utah, USA. Rémi Coltat; Geosciences Department, Ecole Normale Supérieure, Paris, France and Instituto Andaluz de Ciencias de la Tierra, CSIC-UGR, Spain. Jeremy R. Deans; School of Biological, Environmental, and Earth Sciences, University of Southern Mississippi, USA. Kristin L. Dickerson; Department of Earth and Planetary Sciences, University of California, Santa Cruz, USA. Marguerite Godard; Department of Geosciences, University of Montpellier, France. Barbara E. John; Department of Geology and Geophysics, University of Wyoming, USA. Frieder Klein; Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, USA. Rebecca Kuehn; Institute of Geosciences and Geography, Martin-Luther-University Halle-Wittenberg, Germany. Kuan-Yu Lin; Department of Earth Sciences, University of Delaware, USA. C. Johan Lissenberg; School of Earth and Environmental Sciences, Cardiff University, United Kingdom. Haiyang Liu; Institute of Oceanology, Chinese Academy of Sciences, China. Ethan L. Lopes; Department of Geophysics, Stanford University, USA. Toshio Nozaka; Department of Earth Sciences, Okayama University, Japan. Andrew J. Parsons; School of Geography, Earth, and Environmental Sciences, University of Plymouth, United Kingdom. Vamdev Pathak; Department of Geology, Central University of Punjab, India. Mark K. Reagan; Department of Earth and Environmental Sciences, University of Iowa, USA. Jordyn A. Robare; School of Molecular Sciences, Arizona State University, USA. Ivan Savov(1). Esther Schwarzenbach; University of Fribourg, Switzerland. Olivier J. Sissmann; 1 et 4 avenue de Bois-Preau 75005 Paris, France. Gordon Southam; Earth and Environmental Sciences, The University of Queensland, Australia. Fengping Wang; International Center for Deep Life Investigation (IC-DLI), Shanghai, Jiao Tong University, China. C. Geoffrey Wheat; College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, USA. Lesley Anderson; United States Antarctic Program, USA. Sarah N.R. Treadwell; Department of Communication, University of North Dakota and Blue Marble Space Institute, USA.
How to cite: Osborne, W., Savov, I., McCaig, A., Agostini, S., and Godard, M. and the the International Ocean Discovery Program Expedition 399 Sci Party: The Boron Isotope Record of Fluid-Rock Interaction in Abyssal Serpentinite: Insights from IODP Expedition 399, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1339, https://doi.org/10.5194/egusphere-egu25-1339, 2025.
