title: Conditions of oceanic serpentinisation: Insights from microscale chemical and isotopic compositions of serpentine
Creators: name: Vesin, Coralie
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology

description: The deep-water cycle is a process that links the lithosphere of the Earth’s interior with the hydrosphere on the Earth’s surface. Aqueous fluids play a pivotal role in numerous fundamental processes, including the rheology and seismicity of the lithosphere, as well as the formation of volcanic domains. The key exchange of elements and water between the lithosphere and the hydrosphere occurs during the serpentinisation of mantle rocks when they are exposed at the subsurface. This process is principally observed in oceanic environments, such as the mid-ocean ridges (MOR), transform faults, and extended passive margins (PaMa). The serpentinisation process is known to be a significant hydration reaction within the oceanic lithosphere. However, there is a paucity of knowledge regarding the constraints of chemical and physical parameters, such as the different events of water uptake, the element exchange, and the varying temperature and fluid composition. Until recently, investigations into serpentinisation were conducted using bulk serpentinites, which did not permit an understanding of the complexity of the reaction in terms of the crystallising products and their textural and chemical heterogeneity.
A microscale study of the main reaction product, serpentine, enables a more comprehensive understanding of serpentinisation, given the textural and chemical complexity of the reaction. This research project identifies the range of conditions under which serpentinisation occurs, including temperature, fluid composition, and temporality. Serpentinite samples from MOR and PaMa drill cores provide constraints for the present-day serpentinisation process. Subsequently, the geochemistry of serpentine in ancient, obducted oceanic lithosphere, including those exposed in the Erro-Tobbio Massif (Western Alps), the Ligurian ophiolites (Northern Apennines) and the Oman ophiolites, provides insights into the ancient serpentinisation processes. Only after the present-day conditions of serpentinisation have been better understood that this process can be compared to the chemical characteristics of obducted ophiolites. Consequently, this comparison provides novel insights into the tectonic history of the past oceanic lithosphere.
The various textures in which serpentine crystallises, following the crystallisation of mantle olivine and orthopyroxene, are examined. The chemical composition of these serpentine textures is determined, including the major, minor and trace elements. Furthermore, the in situ isotopic composition of oxygen and boron is obtained. The systematic approach of obtaining the chemical and isotopic composition of the serpentine at the microscale allows for the control of spatial variation and the correlation of different chemical signals.
The redistribution of transition metals, as well as alumina, in serpentine textures demonstrates that the serpentinisation of olivine and orthopyroxene can occur simultaneously in certain instances, whereas in other cases the hydration of the two mantle minerals is more sequential. The fluid-mobile element (FME) composition of serpentine is a distinguishing factor where the fluid composition underwent alteration during the process of serpentinisation. In particular, Cl and B are employed as proxies to qualitatively describe the salinity of the fluid, while the Sr content indicates whether the fluid has reacted with sediments prior to, or during, the serpentinisation of mantle peridotites. The serpentine from MOR exhibits elevated Cl/B (> 25) and low Sr (< 10 μg · g−1) content relative to the serpentine from PaMa. The oxygen isotope composition provides insights into the temperature of serpentinisation. The correlation of FME compositions with d18O depicts where the change of the fluid composition influences the d18O. The case where the fluid composition remains unaltered with a constant Cl/B indicates that the observed change in d18O is due to temperature variation. Serpentinisation in the MOR setting typically occurs at elevated temperatures, ranging from 120 to 300 "C (± 15 to 60 "C), whereas it occurs in the range of 65−200 "C range (± 10−30 "C) in the PaMa setting. Moreover, the considerable disparity in B isotope composition observed in the MOR samples indicates a shift in pH during serpentinisation. Finally, the Sr isotope composition of serpentinite can be indicative of a fluid reacting with sediments in some cases, or of a serpentinisation system occurring in a water- or rock-dominated environment. The geochemical affinity of serpentinites from the Erro-Tobbio Massif and the External Ligurian domain is comparable to that of the PaMa setting. The Internal Ligurian serpentinites display chemical characteristics of both the PaMa and MOR settings, whereas the serpentinites from the Oman ophiolites are likely to have formed in a MOR setting.
This work contributes to the understanding of the conditions that prevail during the successive stages of serpentinisation in oceanic environments. It demonstrates the reliability of in situ geochemical tools in such settings. A parallel can be drawn between the serpentinisation observed in present-day and obducted seafloor environments, with regard to the specific chemical features. The new findings can be employed to investigate the tectonic history of past oceanic basins from a chemical point of view.