The Far-Offset Active-Source Imaging of Mantle (FAIM) experiment was conducted along an 800-km-long transect in the Western Atlantic to study the evolution of 108-157 m.y. lithosphere. The main transect (Line 1) crosses a transition from slow (13-14 mm/yr in half rate) to ultra-slow (~8 mm/yr) paleo spreading rates, and thus represents an ideal setting to study spreading-rate dependent processes as expressed in preserved lithospheric structure. This thesis presents results of four analysis efforts along this transect. We present a crustal model based on seismic refraction and wide-angle traveltime modeling, we extend the crustal model to an upper lithosphere density model using gravity constraints, we constrain Poissons ratio in oceanic Layer 3 using converted shear-wave phases, and we consider regional lithospheric structure by analysis of geoid/topography ratios. The crustal model indicates that a transition in crustal thickness accompanies the spreading-rate change, with the crust produced at slow rates being 1.0-1.5 km thinner. The gravity modeling shows that a density model can be constructed that simultaneously satisfies observed gravity, seismic constraints on crustal thickness, and our expectation of isostacy if ~1.3 km of low-density material is distributed into the upper 30-60 km of the mantle. This amount of material (~1.3 km) roughly equals the difference in thickness between slow and ultra-slow spreading crust, suggesting that that the thinner crust formed during very slow spreading arises due to melt retention in the mantle rather than decreased mantle melting. Modeling of mode-converted S-wave phases reveals a uniform of Poissons ratio (~0.27) in the lower crust. Along with the observation of sharp crust/mantle boundary, this result suggests that crust along the FAIM transect is primarily melt-derived igneous crust. Geoid versus topography relationships along Line 1 and nearby parallel tracks show abrupt changes that may originate from lateral changes in mantle density, possibly in response to the transition from slow to ultra-slow spreading. This type of observation may enable us to extend our inferences to a more regional scale.

Iceland is the land-based expression of the Mid-Atlantic Ridge and is one of the most volcanically active regions of the world. Volcanic eruptions on Iceland are a source of geological hazard to humans and the environment due to the release of ash, gases and lava. The composition of the material released is determined by the chemical composition of the surrounding bedrock and the magma upwelling from the Earth’s crust. The effects of historical eruptions on Iceland have been locally devastating and of global impact. The eruption of Lakagígar in 1783-1784 is known to have been the largest eruption in historical time, and is responsible for the death of ~22 % of theIcelandic population. Skeletal fluorosis is a disease that is sometimes observed following large volcanic eruptions. Volcanic ash can travel great distances in the upper atmosphere and spread over vast areas far away from the erupting volcano. Volcanic ash can change incomposition in the atmosphere, and bring about climate-changing effects. Most notably in recent times, violent ash eruptions can also cause problems to the aviation industry, when ash enters and damages airplane engines. Iceland has many active volcanoes and needs to ensure plans for future eruptions are in place. One such measure is an evacuation plan that protects people living close to an active volcano, such as the most lively on Iceland: Hekla, Katla and Eyjafjallajökull.

At fast spreading ridges such as the East Pacific Rise (EPR) volcanic eruptions are predicted to occur on a decadal timescale. Due to the limited ability to observe submarine eruptions, little is known about the magmatic processes occurring in the underlying magma chamber leading up to a volcanic event, including differentiation and magma mixing. The recent 2005-06 eruption at 9˚46’N-9˚56’N along the EPR provides a unique opportunity to gain a better understanding of rates of magma transport and magma replenishment associated with a typical eruption. This study examines the geochemistry of phenocrysts from the 2005-06 eruption in order to determine if they are in equilibrium with their host melt, or if magma mixing occurred prior to eruption. A diffusion model is used to model those crystals which are out of equilibrium with their erupted host to determine timescales of magma mixing. The major and trace element contents of plagioclase and olivine phenocrysts provides evidence for melts both more evolved (> 3.5 wt % MgO) and more primitive (< 8.8 wt % MgO) than those found within the host lava (7.7-8.3 wt % MgO; Goss et al., 2010). Glomerocrysts and resorbed crystals in equilibrium with evolved melts (3.5-6.5 wt % MgO) suggests an origin in a roof mush zone, and were disrupted and entrained into their host melt within days of eruption. Modelling of the zoning profiles of phenocrysts suggest the 2005-06 eruption was likely triggered by an influx of hotter, more primitive melt (~ 9.0 wt % MgO) which was injected into the melt lens a few weeks to months prior to the eruption. With decreasing time before eruption, there is an overall increase in the number of crystals with modelled timescales representing mixing events in the magma chamber. This increase in modelled timescales appears to correlate with the increase in seismic activity recorded prior to the eruption (Tolstoy et al., 2006). This suggests magma mixing events within the underlying magma chamber may be linked to seismic activity at fast spreading ridges. / Graduate / 0372 / 0996

abstract: Mantle derived basalts along the entirety of the Earth’s Mid-Ocean Ridge (MOR) spreading centers are continuously altered by seawater, allowing the hydrosphere to subsume energy and exchange mass with the deep, slowly cooling Earth. Compositional heterogeneities inherent to these basalts—the result of innumerable geophysical and geochemical processes in the mantel and crust—generate spatial variation in the equilibrium states toward which these water-rock environments cascade. This alteration results in a unique distribution of precipitate assemblages, hydrothermal fluid chemistries, and energetic landscapes among ecosystems rooted within and above the seafloor. The equilibrium states for the full range of basalt compositional heterogeneity present today are calculated over all appropriate temperatures and extents of reaction with seawater, along with the non-equilibrium mixtures generated when hydrothermal fluids mix back into seawater. These mixes support ancient and diverse ecosystems fed not by the energy of the sun, but by the geochemical energy of the Earth. Facilitated by novel, high throughout code, this effort has yielded a high-resolution compositional database that is mapped back onto all ridge systems. By resolving the chemical and energetic consequences of basalt-seawater interaction to sub-ridge scales, alteration features that are globally homogeneous can be distinguished from those that are locally unique, guiding future field observations with testable geochemical and biochemical predictions. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020

The Hess Deep Rift in the East Pacific Rise is a mid-ocean ridge spreading center that produces melts which exhibit geochemical characteristics of evolved MORB. Using basaltic glass samples collected from multiple dive cruises that explored Hess Deep geology, volatile and chemical data were collected at USF using FTIR and EMPA, respectively. In addition, a data suite of samples of glass from Hess Deep were compiled from the EarthChem database. The intention was to use the data suite and models to compare the Hess Deep regime to analog models for mid-ocean ridge crystallization regimes and tectonic structures. The USF and EarthChem samples were then compared to various crystallization models generated in Petrolog3 (Danyushevsky and Plechov, 2011) and COMAGMAT (Ariskin and Barmina, 2004). The starting compositions using depleted, normal, and enriched MORB (Gale et al, 2013) were modeled at depths reflecting an upper and lower melt lens along the rift axis. The volatile components of the USF samples were compared to models for water and carbon dioxide behavior in basalt made using VolatileCalc (Newman and Lowenstern, 2002). Based on the comparison of the samples to the forward modeling in Petrolog3, it appears that the geochemical behavior of major and trace elements most closely resembles that of small amounts of fractional crystallization and re-assimilation of accessory minerals. The VolatileCalc models suggest that the USF samples most likely followed a degassing pathway at depths corresponding to the shallow melt lens. When considering the analog models for ophiolite sequences and melt flow beneath a fast-spreading ridge, it appears that the melt regime at Hess Deep deviates from both standing theories. Instead the most likely mechanisms are shallow crystallization, at depths equal to or less than an upper melt lens, and shallow dynamic degassing.

Le segment 16°N de la dorsale Est-Pacifique (EPR) interagit avec le point chaud des Mathématiciens (PCM). A partir de l’analyse géochimique de 120 échantillons de verres basaltiques prélevés par submersible Nautile (campagne à la mer PARISUB 2010) coulée par coulée, ce travail présente une étude à très petite échelle de l’hétérogénéité chimique du manteau sous quelques kilomètres de dorsale (15°37’N et 15°47’N). Les résultats obtenus témoignent d’une variabilité géochimique jamais observée à si petite échelle sous une dorsale. Le degré d’hétérogénéité du manteau dans cette zone est comparable à celui de l’ensemble de l’EPR. Cette diversité est le produit du mélange entre trois sources mantelliques principales, caractérisant l’influence et l’hétérogénéité du point chaud. La densité de l’échantillonnage offre une résolution spatiale en adéquation avec les données bathymétriques, ce qui a permis de coupler les deux approches et faire une reconstruction spatio-temporelle de l’évolution géochimique et morphologique du système EPR/PCM.Celle-ci commence il y a environ 600 ka par le gonflement du segment et un changement général dans la composition du manteau ambiant, suivi de deux sauts d’axe successifs en direction de la chaîne il y a 250 et 150 ka. Lors de cette phase de rapprochement de nouvelles signatures géochimiques émergent ponctuellement dans les MORB. Parmi elles,une signature relique de manteau appauvri régional, une autre d’hétérogénéité enrichie locale, et enfin celle de deux types d’hétérogénéités contenues dans la source du PCM.Cette dernière, n’apparaît dans les MORB qu’au cours des 100 dernières années autour de 15°44’N, et constitue une signature géochimique inédite pour des MORB. Ce nouveau composant a la particularité d’avoir du Pb très peu radiogénique ("Unradiogenic Lead Component", ULC) associé à des signatures isotopiques en Sr, Nd et Hf enrichies.Les compositions en éléments majeurs, traces et isotopes (Sr, Nd, Hf, Pb et He) suggèrent l’implication de matériel métagabbroique, ancien (>2Ga) à affinité continentale.La présence de sulfures dans la source permettrait d’expliquer le Pb peu radiogénique.Le recyclage dans le manteau supérieur de pyroxénites à sulfures, provenant de la partie profonde d’anciens arcs continentaux permettrait d’expliquer l’origine de ULC. Les basaltes ULC seraient donc les témoins volcaniques de la fusion de ce réservoir discret qui contribue à résoudre le paradoxe du Pb. / 120 Mid-Ocean-Ridge basaltic (MORB) glasses were collected on discrete lava flow (~200m sampling interval) during submersible dives along the East-Pacific-Rise (EPR), between 15°37’N and 14°47’N, precisely where the ridge intersects the Mathematicianshotspot track. The data display a geochemical variability that has never been observedalong a ridge at such a small spatial scale. The range of isotopic compositions along this 15 km segment is commensurable to that of the entire EPR. It can be accounted for by a mixture of three main components, representative of the hotspot heterogeneity.The dense sampling, along and across the ridge segment, matches the resolution of themicro-bathymetric data, which made the spatio-temporal reconstruction of the geochemical and morphological evolution of the EPR/Mathematician hotspot system possible.The latest starts 600 kya with the segment inflation and a global change in the ambient mantle composition, followed by two successive jumps of the ridge axis (250 and 150 kya) towards the seamounts chain. During this phase as the two systems are getting closer,new geochemical signatures emerge in MORB. Among them, relics of regional depleted mantle, small enriched local heterogeneities, and two types of heterogeneities belonging to the hotspot source. The last one become apparent only during the last 100 yearsaround 15°44’N, and constitutes a novel geochemical signature for MORB. This new component’s most noticeable property is its very unradiogenic Pb ("Unradiogenic Lead Component", ULC) associated with mostly enriched Sr, Nd and Hf isotopic signatures. Putted together, major, trace elements and isotopes (Sr, Nd, Hf, Pb and He) suggest anancient (>2Ga) lower continental metagabbroic origin for this material, while the involvement of sulfides is considered in order to explain the unradiogenic lead compositions. Overall, the preferred model for the formation of ULC is the recycling within the uppermantle of sulfide bearing pyroxenites coming from continental arc roots. ULC-influenced basalts represent magmatic witnesses of the melting of this cryptic reservoir that can contribute to solve the Pb paradox.

This dissertation combines mineralogical data and petrographic and field observations with geochemical analysis (major, trace and isotope) to provide new insights into the hydrology and geochemistry of mid-ocean ridge hydrothermal systems. Two study areas were chosen to study two different aspects of hydrothermal circulation: high-temperature on-axis hydrothermal systems were studied using samples from the Hess Deep Rift (Cocos Plate, Equatorial Pacific) and low-temperature off-axis hydrothermal systems were studied at the Troodos Ophiolite in Cyprus. Significant findings include the documentation of a previously unknown warm fluid that pervades the lavas leaching Li from newly formed crust. This finding corroborates a model of broad hydrothermal discharge in the sheeted dikes. In the off-axis low-temperature regime, lateral flow of warm fluid is documented in the lavas, advecting heat from the oceanic lithosphere, with minor geochemical changes to the lavas. The sedimentary cover was found to influence alteration in two ways. The longer an area remains unsedimented allowing the free ingress and egress of seawater, the deeper the enrichment of alkali metals is observed. The maximum enrichment in alkali metals (K, Rb, Cs) however, is similar in both locations. The sedimentary cover can also modify the seawater before it becomes impermeable to fluid flow; early metaliferrous oxide sediments react with seawater, creating a fluid that mobilizes and fractionates the REEs and Y. The fractionation results in negative Ce anomalies, positive Eu anomalies, and negative Y anomalies. Basalts altered under these conditions also lack the ubiquitous Fe-oxides and Fe-oxyhydroxides that are commonly associated with alkali metal uptake. In situ trace element analysis of alteration minerals formed at low-temperature confirmed that secondary phyllosilicates are strongly enriched in alkali metals (K, Rb, Cs and Li), Ba is found in adularia and zeolites, Sr is hosted in carbonates, and no phases were found to be enriched in U. The concentrations of K2O, Rb, Cs (as well as B) are highest in celadonites, whereas Li concentrations are highest in smectites (saponite, Al-saponite, beidellite) and smectite-chlorite mixtures, and much higher than previously reported. Alkalis are also taken up into palagonite, with Li having the highest concentrations, over 1000 ppm in one analysis. Crystal chemical factors were found to be the dominant control on trace element uptake, and for the phyllosilicates no correlation was found between the temperature, age of the crust, texture of the phyllosilicates. In phyllosilicates the K, Rb and Cs are adsorbed as exchange cations, with enrichment (Cs > Rb > K) increasing with decreasing hydration energy, whereas the uptake of Li and B does not correlate with the hydration energy. Lithium concentrations also do not correlate with the Mg content, suggesting substitution of Li for Mg is not the only mechanism of Li uptake into phyllosilicates as has been suggested. / Graduate

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Les dorsales océaniques sont caractérisées par une sismicité de faible magnitude, induite par une succession d’épisodes volcaniques et tectoniques. Le déploiement de réseaux d’hydrophones autonomes le long de grandes sections de dorsales permet d’enregistrer cette sismicité peu détectable par les stations terrestres. Dans l’océan Atlantique Nord, de nombreuses expériences hydroacoustiques ont été conduites depuis 1999, donnant accès à des séries d’observations temporelles sur plusieurs années. La présente thèse s’inscrit dans ce contexte, avec l’acquisition et le traitement de nouvelles données. L’analyse de catalogues télésismiques et hydroacoustiques, couplés à des données bathymétriques et gravimétriques, a contribué à la caractérisation des processus d’accrétion actifs le long de la dorsale médio-Atlantique Nord. Les travaux réalisés lors de cette thèse ont permis de : (1) rechercher des paramètres responsables d’une différence de sismicité entre des groupes de segments de la dorsale médio-Atlantique ; (2) proposer une nouvelle limite sud d’influence du point chaud des Açores ; (3) identifier et caractériser différents types de processus d’accrétion à partir de la recherche de crises sismiques ; (4) mettre en évidence une crise magmatique de grande ampleur et une intrusion magmatique à travers une discontinuité non-transformante ; et (5) proposer un schéma de répartition des processus d’accrétion dominants le long de l’axe de la dorsale médio-Atlantique Nord, à partir d’une analyse spatiotemporelle de la sismicité. L’ensemble de ces résultats montre l’importance des catalogues de sismicité à grande étendue spatiale et temporelle pour approfondir notre connaissance : sur la dynamique des segments de dorsale et les interactions entre l’axe de la dorsale et le point chaud, et d’autre part, sur la récurrence des processus d’accrétion. / The mid-ocean ridge is caracterised by a seismicity of low magnitude generated by multiple volcanic and tectonic episodes. Autonomous hydrophones arrays along large ridge sections are efficient to record low seismicity contrary to landbased stations. In the northern Atlantic Ocean, several hydroacoustic experiments have been realised since 1999 that supply long term records.This thesis is based on the acquisition and the treatment of new data. The analysis of teleseismic and hydroacoustic catalogues combined with bathymetric and gravimetric data, have contributed to the characterisation of accretionary processes along the northern Mid-Atlantic Ridge. This PhD work allowed : (1) to research parameters responsable of a seismicity difference between groups of Mid-Atlantic Ridge segments; (2) to propose a new southern limit of the Azores hotspot influence ; (3) to identify and define different types of seafloor spreading processes based on seismic crisis interpretations ; (4) to highlight a large magmatic crisis and a magmatic intrusion across a non-transform discontinuity ; and (5) to propose dominant seafloor spreading processes from a spatio-temporal seismicity analysis along the northern Mid-Atlantic Ridge. The entire study shows the value of large spatial and temporal seismicity catalogues to broaden our knowledge on : the dynamic of the ridge segments, the interactions between the ridge axis and the hotspot and the recurrence of the seafloor spreading processes.

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