Thesis advisor: Ethan Baxter / Serpentinites assume a critical role in geochemical and geophysical cycles, from recycling fluid into the sub-arc mantle to facilitating exhumation within subduction zones. Rodingite dikes can be used as a lens to investigate the hydration of the oceanic lithosphere as their development is synchronous with serpentinization, and while serpentinites lack sufficient mineral phases for geochronology, rodingite dikes are rich in andradite and grossular garnet which are potentially amenable for geochronology. This research seeks to constrain the timescales and duration of hydration of the oceanic lithosphere within the Alpine Tethys ocean basin, and associated serpentinization, by examining Apennines rodingites from the Internal Ligurides (Italy). These rodingites experienced seafloor hydrothermal alteration and were obducted onto the continental margin during Alpine orogenesis. As a result, they are ideal for studying seafloor metasomatism as they were not affected by prograde subduction zone metamorphism and dehydration. Sr isotopic and trace element profiles were constructed across two rodingite-serpentinite transects, revealing a complex, multi-stage hydration history consisting of 1) Widespread serpentinization, 2) Gabbroic intrusions, 3) Rodingitization, and 4) Localized, late-stage advective fluid flow. Serpentinizing fluids locally display strong continental crustal isotopic signatures, while rodingitization fluids are characterized by seawater-like values. U-Pb geochronology on rodingite garnets produced an age of 96.1 ± 8.9 Ma, which could represent either the main rodingitization phase or the late-stage advective alteration. / Thesis (MS) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Les parties distales des marges passives pauvres en magma représentent la transition complexe entre les domaines continentaux et océaniques. Ces zones encore peu étudiées sont pourtant des endroits clefs pour comprendre les processus impliqués durant les premiers stades de l’accrétion océanique, et plus particulièrement ceux du magmatisme et du métasomatisme. Durant ces premiers stades, ces deux processus sont gouvernés par l’exhumation mantellique. L’interaction entre les liquides magmatiques, les roches du manteau et les fluides marins vont affecter le régime thermique de la marge. De par le monde, seulement deux Transitions Océan-Continent (TOC) ont pu bénéficier d’investigations scientifiques poussées et constituent naturellement les deux sites d’études de cette thèse, à savoir, les marges actuelles conjuguées d’Ibérie-Terre Neuve du sud de l’Atlantique Nord ainsi que les marges fossiles de la Platta et de Tasna, fragments de TOCs de la Téthys Alpine Jurassique. En combinant les études de terrain ainsi que les investigations minéralogiques, pétrologiques et géochimiques, nous avons pu contraindre trois processus clefs se déroulant dans les TOCs. (i) La percolation de liquide magmatique imprégnant le manteau sous-continental hérité dans les marges Ibérie-Terre Neuve permet une refertilisation de ces marges distales. (ii) La transition géochimique visible entre les basaltes des TOCs et les basaltes de dorsales océaniques peut s’appréhender par la fusion partielle du manteau sous-continental refertilisé. (iii) Le rôle des fluides hydrothermaux, ayant des températures comprises entre 60°C et 190°C, joue un rôle sur le métasomatisme de la lithosphère en produisant une intense serpentinisation et rodingitisation, respectivement du manteau sous-continental en exhumation et des dykes basaltiques. Ces températures étant cohérentes avec une exhumation mantellique au niveau du plancher océanique. / Distal parts of magma-poor rifted margins represent a complex transition between continental and oceanic domains. These areas remain poorly understood while being a key-place to unravel magmatic and metasomatic processes involved during the first stages of oceanization. At this time, these processes are enhanced by mantle exhumation, and the interaction between melts, mantle rocks and fluids affect the thermal regime of the margin. So far, only two Ocean-Continent Transitions (OCT) have been particularly investigated, namely the present-day Iberia Newfoundland conjugate margins and the fossil analog Platta-Tasna nappes, remnants of the Jurassic Alpine-Tethys OCTs. Studies presented in this Ph.D. thesis have been focused on these two margins. Here, by combining field-works, petrological, mineralogical and geochemical investigations, we have unraveled in OCTs three key-points: (i) The deep porous-flow melt percolation impregnating the long-lived inherited subcontinental mantle in Iberia-Newfoundland margins allow the refertilization of these distal domains; (ii) The geochemical transition depicted from OCT-basalts towards MOR-basalts can be explained by the partial melting of the refertilized subcontinental mantle; (iii) The role of active hydrothermal fluids, on both the exhumed mantle and basalt dikes, lead to the serpentinisation and the rodingitization respectively, at temperature ranging between 60°C and 190°C. These temperatures being consistent with the ongoing mantle exhumation towards near-seafloor conditions.