Serpentinisation of the Troodos Massif, Cyprus

Abstract

The interaction between ultramafic rocks and water, a process termed serpentinisation, plays a critical role within the wider Earth system, aiding plate tectonics, influencing geochemical cycles, and may be instrumental in the origin of life. Recovered serpentinised mantle rocks, both on land and in the oceans, typically reflect multiple overprinting serpentinisation events. To date, the conditions of distinct serpentinisation episodes are generally poorly constrained since mantle rocks generally reveal complex geological histories. This thesis integrates field observations, physical properties, elemental concentrations and isotope compositions of sampled waters, precipitates, and variably serpentinised mantle rocks to unravel the progressive sequence of overprinting fluid rock interaction events that have resulted in the occurrence of mantle rocks forming the highest elevations of the Troodos ophiolite, Cyprus.
In the Troodos Mantle Sequence, different serpentinisation styles have resulted in contrasting nested serpentinite diapirs formed due to differing extents of serpentinisation and density reduction. Mechanistic isostatic and differential erosional modelling show that the Troodos Mountains can form within the geologically constrained uplift time frame (~5.5 Myr) exclusively through partial serpentinisation reactions. Serpentinisation-induced differential uplift and exhumation have decoupled the two serpentinite diapirs from the originally overlying ocean crustal rocks. Incursion of meteoric water resulted in further serpentinisation, generating a second decoupling between the
contrasting serpentinite diapirs resulting in localised differential uplift and exhumation and the extrusion of deep materials.
Sr, B, O, and H isotopes and equilibrium modelling coupled with a Bayesian inversion engine are consistent with a progressive sequence of overprinting serpentinisation events: (i) A localised serpentinisation episode by Cretaceous hydrothermal fluids, limited to the crust-mantle transition zone soon after formation at the spreading ridge; (ii) A pervasive serpentinisation event by Cyprus
slab-derived fluids focussed underneath the differentially uplifted Mount Olympus, inducing serpentinisation of the Troodos Mantle Sequence by up to 66 % reaction extent; (iii) A pervasive serpentinisation event by meteoric waters that interacted with previously overlying submarine sediments and Messinian evaporites, resulting in further serpentinisation of up to 33 %; (iv) Once at
high altitudes continued incursion of meteoric water locally reinforced serpentine alteration through coupled deformation-alteration-recrystallisation processes that manifest as a low density serpentinite breccia end member. The alteration of serpentine forms hyperalkaline high pH >11.5 meteoric-derived waters emanating from portions of the Troodos Massif. In addition to these high
pH waters, contrasting meteoric-derived lower pH <10 surficial waters reflect variable proportions of remobilised geochemical and isotopic signals from previous serpentinisation events.
Some, but not all, of the observed change in the physical properties of variably serpentinised mantle rocks can be explained by serpentinisation hydration reactions. To reconcile this difference, ongoing serpentine alteration processes are proposed to modify mantle rock physical properties
further, resulting in contrasting but parallel mantle rock alteration pathways. Consequently, in addition to serpentinisation hydration reactions, ongoing serpentine alteration, carbonation, and oxidation are also instrumental processes that can induce serpentinite diapirism, generate hyperalkaline waters, and produce significant changes to physical and hydraulic properties of
serpentinised mantle rocks.

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ORCID for Damon Teagle: orcid.org/0000-0002-4416-8409

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