Novel Insights into the Palaeomagnetism of Serpentinized Peridotites from the Troodos and Oman Ophiolites
Large volumes of mantle peridotite are exposed at the Earth’s surface in ophiolites, becoming vulnerable to the concurrent chemical alteration processes of serpentinization and carbonation. Serpentinization frequently results in the production of secondary magnetite that can record the direction of the Earth’s magnetic field at the time of its formation, allowing palaeomagnetism to be used as a tool to investigate this process. Many ophiolites also experienced large-scale tectonic rotations during their evolution. In some cases, the timing of these rotations is well-documented palaeomagnetically, potentially allowing the timing of different phases of serpentinization to be constrained if the magnetization directions of secondary magnetite assemblages can be determined and compared to known rotation histories. This research represents the first attempt to combine paleomagnetic and rock magnetic analyses of serpentinized mantle peridotites with state-of-the-art Quantum Diamond Microscopy (QDM), a recently developed analytical tool that allows two-dimensional images of the magnetic fields in a sample arising from distributions of magnetic sources. Two ophiolites are the focus of this thesis. The Late Cretaceous Troodos ophiolite of Cyprus that underwent a ~90° anticlockwise tectonic rotation that began shortly after it formed by seafloor spreading. The timing of this rotation is well-constrained by palaeomagnetic analysis of the sedimentary rocks that were deposited continuously while the underlying oceanic crust rotated. In this context, different magnetization directions would be expected to be carried by magnetite assemblages produced by serpentinization during: (i) early exposure on the seafloor or deep fluid circulation during Late Cretaceous seafloor spreading; (ii) subsequent progressive tectonic rotation; and (iii) Plio-Quaternary to Recent tectonic uplift and/or reaction with modern meteoric water. The Late Cretaceous Samail/Semail ophiolite in Oman and the UAE (hereafter referred to as the Oman ophiolite) underwent a large tectonic rotation of ~120° after it formed. Serpentinization of the mantle peridotites of this ophiolite could potentially have happened during any or all of the following stages in its evolution: (i) via deep fluid circulation during Late Cretaceous seafloor spreading; (ii) during tectonic rotation after cessation of seafloor spreading; (iii) during Late Cretaceous emplacement onto the Arabian continental margin; and (iv) via reactions with modern meteoric water (via alkaline springs that demonstrably relate to ongoing serpentinization). Results demonstrate that magnetization directions within the Troodos serpentinites are highly variable and include: (i) WNW-directed magnetizations that are inferred to have been acquired during early serpentinization on the seafloor during the Late Cretaceous, that QDM and associated rock magnetic analyses definitely show are carried by magnetite distributed along serpentinite veins; (ii) NW-directed magnetizations inferred to have been acquired partway through the rotation during the Maastrichtian-Paleogene; and (iii) N- and S-directed magnetizations inferred to have been acquired post-rotation in different polarity chrons. Results from the Oman serpentinites (accompanied by results from Oman Fizh, Salahi and Haylayn lavas) demonstrate that that magnetization directions are also highly variable and indicate serpentinization either: (i) during the later stages of rotation but before emplacement; (ii) during emplacement of the ophiolite, due to upwards migration of orogenic fluids from the base of the ophiolite at ~ 75Ma (Feinberg et al., 1999); and (iii) due to interactions with meteoric water while exposed at the surface after emplacement until the present day.
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