Tracking Water‐Rock Interaction at the Atlantis Massif (MAR, 30°N) Using Sulfur Geochemistry. (21st November 2018)

• Record Type: Journal Article
• Authors: Liebmann, J.; Schwarzenbach, E. M.; Früh‐Green, G. L.; Boschi, C.; Rouméjon, S.; Strauss, H.; Wiechert, U.; John, T.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 19:Number 11(2018)
• Page Start: 4561
• Page End: 4583
• Publication Date: 2018-11-21
• Subjects: multiple sulfur isotope geochemistry -- oceanic core complexes -- water‐rock interaction -- serpentinization -- sulfur cycle -- Atlantis Massif; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2018GC007813
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Hydrothermal alteration and serpentinization of ultramafic rocks at oceanic core complexes involve extensive element exchange between fluid and rock and a wide range of biogeochemical processes. These processes influence the global sulfur cycle due to both biogenic and abiogenic removal of seawater sulfate. Hence, ocean floor serpentinization connects the hydrosphere, biosphere, and lithosphere. This work presents a study of the sulfur geochemistry of highly altered mafic and ultramafic samples from the Atlantis Massif located at 30°N along the Mid‐Atlantic Ridge. The analyzed samples were drilled during International Ocean Discovery Program Expedition 357 and collected during Alvin dives in 2000, 2003, and 2005. Multiple sulfur isotope analyses of sulfide and sulfate phases indicate that several processes took place during progressive hydrothermal alteration: (1) incorporation of seawater sulfate, (2) thermochemical sulfate reduction during interaction with high‐temperature (high‐T; 350–400 °C), low‐pH fluids and input of H2 S derived from leaching gabbroic intrusions, (3) microbial sulfate reduction, and (4) oxidation of sulfides at high water‐rock ratios. Petrological examinations show that high‐T fluids mostly postdated the bulk serpentinization and that these fluid pulses were relatively localized (<1 dm scale) resulting in a highly heterogeneous mineralogy. Locally, high‐T fluid influx took place subsequent to microbial sulfate reduction and oxidation as indicated by geochemical modeling. Overall, this study documents the complex interplay of magmatic processes, fluid‐rock interaction, and microbial activity that take place during the formation of oceanic core complexes and where mantle rocks are exposed to seawater. Key Points: Multiple sulfur isotope signatures reveal a wide range of biogeochemical processes that accompany hydrothermal alteration of mantle rock Localized high‐T fluid pulses resulted in a highly heterogeneous mineralogy at the southern wall of the Atlantis Massif This study provides new constraints on the hydrothermal evolution of the Atlantis Massif