Ophiolites as archives of hydration, carbonation and metasomatic processes in the oceanic lithosphere: insights from halogen and noble gas systematics

Abstract

Subduction of oceanic lithosphere constitutes a major flux of volatile elements (water, carbon, halogens) to the Earth's interior. Net fluxes from the surface to the mantle represent a mass balance, with volatile inputs to subduction, in sedimentary pore fluid, altered oceanic crust and lithospheric serpentinites, offset by volatile losses to the overlying forearc and sub-arc mantle by sediment compaction and dehydration reactions. To quantify these fluxes thus requires an understanding of the volatile compositions of the inputs and the magnitudes of losses. The volatile inventory of oceanic lithosphere entering subduction zones is uncertain due to a lack of recovery of in situ samples from depths >2 km in the oceanic crust. Estimating the fluxes of volatiles from the slab based on the magmatic products of arc volcanism is likewise problematic, due to possible contributions from the mantle wedge and anatexis of arc crust. Furthermore, an unknown proportion of the volatile outfluxes from the slab may enter the forearc and remain unsampled by arc volcanism. This results in hidden fluxes and overestimation of net volatile subduction to the mantle. Ophiolites offer an important opportunity to quantify volatile cycles at subduction zones. They consist of oceanic crust and lithospheric upper mantle, formed at oceanic spreading centres and subsequently obducted onto continental margins. They therefore allow direct sampling of otherwise inaccessible material from deep within the oceanic lithosphere. A potential caveat is that the majority of ophiolites have experienced processing above a subduction zone prior to or during their emplacement. As a result many show evidence for both seafloor alteration processes and supra-subduction zone magmatic and fluid metasomatism. Deconvoluting the geochemical signatures of these different processes thus offers an opportunity to interrogate both inputs and outputs of volatiles during subduction. This thesis presents an exploration of the use of halogen and noble gas geochemistry to critically evaluate and deconvolute the mineral-scale record of oceanic and supra-subduction zone metasomatic processes in ophiolites. In doing so, the aim has been to gain further insight into the behaviour of fluids and volatiles over a range of temperatures from spreading centre to subduction zone settings. This has been achieved through three detailed case-studies carried out on two ophiolites: the ~96 Ma Oman Ophiolite and the ~497 Ma Leka Ophiolite Complex, Norway. A combined study of heavy halogen abundances and noble gas isotopes in a suite of gabbros from the sheeted-dyke to gabbro transition and from the lower oceanic crust of the Oman Ophiolite indicates marked differences between the styles of alteration in the upper and lower crust. Halogens are principally hosted in amphibole, serpentine/iddingsite and chlorite, and were incorporated over a range of temperatures from >700°C to

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Raymond Burgess (Supervisor), Patricia Clay (Supervisor) & Brian O'Driscoll (Supervisor)