Novel paleomagnetic insights into the timing of serpentinization of peridotites in the Troodos ophiolite
Authors: Hepworth James , Morris Antony , Harris Michelle , Brenner Alec , Fu Roger , Harrison Richard , Schwarzenbach Esther 
Published: March 2024

Large volumes of mantle peridotite are exposed at the Earth&#8217;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 records the direction of the Earth&#8217;s magnetic field at the time of its formation, allowing paleomagnetism to be used as a tool to investigate this process.&#160;Many ophiolites also experienced large-scale tectonic rotations during their evolution. In some cases, the timing of these rotations is well-documented paleomagnetically, 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. Here we present the first attempt to combine paleomagnetic analysis with Quantum Diamond Microscopy (QDM) observations of individual magnetic grain assemblages to test whether sequential phases of alteration can be dated in serpentinized peridotites in this way.&#160;We focus on the Late Cretaceous Troodos ophiolite of Cyprus that underwent a ~90 CCW tectonic rotation that began shortly after it formed by seafloor spreading. The timing of this rotation is well-constrained by paleomagnetic 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.Magnetization directions within the Troodos serpentinites are shown to be highly variable, and include: (i) samples carrying W-directed, high-coercivity components, inferred to be acquired in the Late Cretaceous, and NNW-N-directed, low-coercivity overprints, inferred to be acquired post-rotation; (ii) samples with single component, NW-directed magnetizations, inferred to have been acquired partway through the rotation; (iii) samples with single component, N-directed magnetizations inferred to have been acquired post-rotation; and (iv) one site where samples exhibit antipodal normal and reversed polarity, N-S-directed magnetizations inferred to have been acquired post-rotation but before the Brunhes-Matuyama reversal (780 ka). Overall, these data demonstrate that serpentinization occurred throughout the entire history of the Troodos Ophiolite, with six sites showing evidence of serpentinization in the Late Cretaceous or partway through the rotation and six sites where serpentinization occurred post-rotation from the Eocene to the present day. QDM data from one site where samples exhibit a W-directed ChRM and a NNW-N-directed overprint confirm that all observed magnetite associated with serpentine veins carries the early ChRM, inferred to be acquired during the Late Cretaceous. The source of the low stability component in these samples was not observed in QDM images and remains elusive, but is interpreted to be a modern magnetic overprint.