Serpentinisation-driven remagnetisation in ophiolites: insights from the troodos ophiolite, Cyprus
File(s)
Author(s)
Qi, Liang
Type
Thesis
Abstract
Serpentinisation of ultramafic rocks occurs in various tectonic settings, from seafloor hydration to subduction dehydration and ophiolite emplacement. It is known to promote magnetite formation, leading to remagnetisation events; however, serpentinisation-driven remagnetisation is often temporally and spatially complex due to the overprinting and mixing of multiple episodes of serpentinisation. This is particularly true in ophiolites that undergo prolonged alteration from ridge-axis formation to subduction-zone emplacement. To quantify serpentinisation-driven remagnetisation in ophiolites, I undertook a magnetic study in the Troodos ophiolite, Cyprus where the serpentinisation history was poorly constrained. A multi-scale magnetic approach was applied to resolve this, including mineral-scale magnetic imaging, bulk-sample-scale rock magnetic measurements, palaeomagnetic directional analysis and kilometre-scale magnetic modelling based on aeromagnetic data. My results highlight three stages of serpentinisation-driven remagnetisation events, from ridge-axis serpentinisation during seafloor spreading, through mantle-wedge serpentinisation in the subduction zone, to meteoric-associated serpentinisation after surface exposure of ultramafic rocks. Ridge-axis serpentinisation occurs in the high-temperature (> 200 °C), leading to highly magnetic serpentinite; mantle wedge and meteoric water-related serpentinisation processes are low-temperature (< 180-200 °C), giving rise to weakly magnetic serpentinite. These new insights support a serpentinite diapir emplacement of the Troodos mantle sections in the subduction zone, despite ridge-axis serpentinisation occurring in the lower-crust cumulate zone. This study also addresses how the rock magnetic properties of serpentinite respond to increasing deformation within an oceanic transform fault zone as preserved in the Troodos ophiolite. During deformation, the dominant serpentine mineral changes from lizardite to chrysotile and magnetite forms from iron released during this recrystallisation. Increasing deformation facilitates seawater circulation, which increases the water–rock ratio and promotes the growth of coarse magnetite grains. This work highlights the remagnetisation process that occurs during the progressive shearing of a transform fault, with a 90° rotation of palaeomagnetic directions preserved in variably deformed serpentinites.
Version
Open Access
Date Issued
2025-09-09
Date Awarded
01/11/2025
License URL
Advisor
Muxworthy, Adrian
Sponsor
The British Geophysical Association (Great Britain)
Publisher Department
Department of Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)