Faulkner, C. E.2025-07-232025-07-232022https://hdl.handle.net/2440/146268This item is only available electronically. Whole thesis (as available).The Semail Ophiolite in Oman records a transect through the oceanic lithosphere, including the crust and upper mantle, which obducted onto the Arabian Peninsula during the Late Cretaceous. This thesis explores the formation and alteration history of the Semail Ophiolite using coupled chromium (Cr) and strontium (Sr) isotope geochemistry. As standard seafloor drilling cannot reach typical upper-mantle depths, the results provide unique insights into the geochemical processes that form oceanic lithosphere. The measured redox-sensitive Cr isotopes (expressed as δ53Cr‰, relative to NIST 979) help determine (i) the Cr isotope fractionation effects related to oceanic lithosphere formation; and (ii) the secondary fluid alteration effects. The Cr isotope data supports heterogenous δ53Cr composition of the upper-mantle and oceanic crust, indicating that partial melting of the upper-mantle and fractional crystallisation may impart detectable Cr isotope fractionation effects (up to ~0.5 per mil, ‰). However, samples collected through a profile of the ophiolite show various degrees of alteration related to a complex tectonic history. Mineralogy (XRD and SEM-EDS) shows evidence for serpentinisation of upper-mantle and lower crustal rocks, which may cause Cr isotope fractionation. Radiogenic Sr isotope ratios (87Sr/86Sr) paired with loss on ignition (LOI) data reflect the source and extent of fluid alteration processes, including serpentinisation. Fluids have raised the LOI and 87Sr/86Sr values of most ophiolite samples. Some rocks have higher 87Sr/86Sr signatures than Cenomanian seawater from when the ophiolite was forming, indicating post-obduction alteration by non-marine fluids (crustal/meteoric waters). The findings suggest that δ53Cr varies throughout the upper-mantle and oceanic crust. Thus, Cr isotopes can be modified by high-temperature processes such as partial melting and fractional crystallisation, as well as via low-temperature alteration involving serpentinisation. Further studies on Cr isotopes at other locations and ophiolite sections are needed to corroborate these findings and to better constrain δ53Cr variability in the Earth’s upper-mantle and oceanic crust.enHonoursGeologychromium isotopesstrontium isotopesOmanSemail Ophioliteoceanic lithospherepartial meltingserpentinisationThesis