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Over recent years geological, geochemical and geophysical surveys of mid-ocean ridges have revealed a significant degree of along-axis variability not only in seabed morphology, but also in crustal structure, particularly Numerous geophysical surveys of the Valu Fa Ridge, southwest Pacific, have mapped the extent of an axial mid-crustal reflector. This reflector has been interpreted as representing the top of a sill-like melt lens, comprising a high percentage of partial melt, lying at the top of a crustal magma chamber. In 1995, a controlled-source, wide-angle seismic dataset was acquired at the Valu Fa Ridge during RN Maurice Ewing cruise EW9512, to investigate the mid-deep crustal structure at this ridge, and particularly the crustal magma chamber associated with the melt lens beneath the ridge axis. The EW9512 acquisition geometry was primarily two-dimensional in design, and modelling of these 2-D profiles revealed the presence of an axial low velocity zone beneath the melt lens. This low velocity zone is thought to represent a region of crystal mush comprising a much lower percentage of partial melt than is present in the overlying melt lens. Similar structures have been modelled beneath a number of other mid-ocean ridges. The primary aim of this study was to build on this 2-D interpretation by taking advantage of three-dimensional ray coverage in the axial region in order to assess the along-axis continuity of the magmatic system, correlate this to any ridge segmentation apparent in the seabed morphology, and determine if ridge segmentation is related to the magma supply. The 3-D data were analysed using a tomographic inversion technique. The inversion results suggest that the axial low velocity zone may be segmented on a scale of 5-10 km, which correlates with the morphological segmentation of the ridge crest and is believed to reflect episodic magma supply with different ridge segments at different stages of a cycle of magmatic and amagmatic extension. However, three- dimensional ray coverage is not ideal owing to the dominantly 2-D acquisition geometry. Therefore a detailed assessment of data uncertainty and resolution was undertaken to enable a meaningful interpretation of the inversion results in terms of which features have a geological origin and which are artefacts of the inversion process. P-S mode converted arrivals arising from mid-crustal interfaces were also modelled in order to obtain improved geological constraints on the crustal structure than is possible from P-wave studies alone. This modelling indicates that the uppermost crust is pervaded by thin cracks. In addition, techniques were developed for modeling the polarisation of 5-wave arrivals with low signal strength. Application of these methods suggests that the thin cracks have a preferred orientation parallel to the ridge crest on-axis, and oblique to the ridge crest off-axis which is thought to reflect the pattern of southward propagation of the ridge system inferred from regional tectonic and bathymetric studies. Modelling of P-S mode converted arrivals arising from conversion at the top of the melt lens provided additional constraints on the properties of the melt lens. In conjunction with the 3-D tomographic results, this work suggests that the southernmost ridge segment in the study area has recently become magmatically active following a period of amagmatic extension suggested by its morphology, thus providing evidence for episodic melt supply at this ridge. As part of the suggestions for further work, a theoretical investigation of survey resolution was undertaken to test commonly adopted acquisition geometries with a view to optimising the design and cost-effectiveness of future 3-D controlled-source tomographic experiments.

Orientador: Antenor Zanardo / Banca: Marcos Aurélio Farias de Oliveira / Banca: Antonio José Ranalli Nardy / Banca: Eliane Aparecida Del Lama / Banca: Gergely Andres Julio Szabó / Resumo: As rochas máficas e ultramáficas em foco estão inseridas na Faixa Itapira/Amparo. Ocorrem na forma de corpos tabulares a lenticulares e são representadas por metaperidotitos, xistos ultramáficos e anfibolitos. Rochas metassomáticas aparecem associadas às máficas e ultramáficas. A litoquímica juntamente com dados de campo, a petrografia e a geocronologia mostram que os processos tectono-metamórficos que atuaram sobre essas rochas provocaram mudanças químicas e mobilidade de elementos maiores, menores, traços e terras-raras. O contexto geológico juntamente com os dados obtidos sugere que essas rochas sejam derivadas de fragmentos de crostas oceânicas embutidas na crosta continental durante o Paleoproterozóico e não de ofiólitos brasilianos, como anteriormente aventado. Indicam, também, que não houve geração de material juvenil no Neoproterozóico, apenas retrabalhamento de rochas mais antigas. Diante da possibilidade de os litotipos atribuídos ao Grupo Itapira terem sido gerados em mais de um ciclo geotectônico optou-se pela denominação de Complexo Itapira / Abstract: The metamafic and metaultramafic rocks studied in this work are located in Itapira/Amparo belt. They occur as tabular to lenticular bodies and are represented by metaperidotites, ultramafic schists and amphibolites. Metassomatic rocks are associated with these rocks. Lithochemistry, field data, petrography and geochronology indicate that the tectonic-metamorphic processes which actuated over the studied region produced chemical changes and the mobility of major, minor, trace and rare earth elements. Geological context with such data suggest that these rocks were originated from oceanic crust pieces emplaced in continental crust during Palaeoproterozoic, not from brazilian ophiolites like avocated before. They also indicate that there were no primary material generation on the Late Proterozoic, just reworking of older rocks / Doutor

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This thesis discusses the results from the first multi-source and multi-streamer three-dimensional multichannel seismic experiment conducted over a mid-ocean ridge environment. Prestack time migration was applied to the dataset resulting in the most detailed reflection images of a spreading center and its flanks to date. The key products from this work are maps of crustal velocities, crustal thickness, and Moho transition zone (MTZ) reflection character for a section of the fast-spreading East Pacific Rise (EPR) from 9°37.5’N to 9°57’N, excluding the area from 9°40’N to 9°42’N where no data were collected. Moho reflections were imaged within ~92% of the study area. The derived average crustal thickness and average crustal velocity for the investigated ~880 km2 area are 5920±320 m and 6320±290 m/s, respectively. The average crustal thickness varies little from Pacific to Cocos plate suggesting mostly uniform crustal production in the last ~180 Ka. Detailed analysis of the crustal thickness and MTZ reflection character shows that the third-order segmentation is governed by melt extraction processes within the uppermost mantle while the fourth-order ridge segmentation arises from mid- to upper-crustal processes. This analysis also suggests that both the mechanism of lower-crustal accretion and the volume of melt delivered to the crust vary along the investigated section of the EPR. More efficient mantle melt extraction is inferred at latitudes from 9°42’N to 9°51.5’N, with greater proportion of the lower crust accreted from the AML than for the rest of the study area. Larger volume of melt is delivered to the crust from 9°37.5’N to 9°40’N than to the investigated crust further north. At some locations, the Moho reflections are for the first time unambiguously imaged below the AML away from any ridge discontinuity suggesting that the Moho is formed at zero age at least at some sections of the spreading centers. The first study of the melt content of mid-crustal off-axis magma lenses (OAML), done using amplitude variation with offset technique calibrated for a magmatic plumbing system, shows that these magma bodies contain 0 to 20% melt. This suggests that OAMLs likely contribute little to the overall crustal formation.

abstract: Seismic observations have revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Pacific and Africa. One hypothesis for the origin of LLSVPs is that they are caused by accumulation of subducted oceanic crust on the core-mantle boundary (CMB). Here, I perform high resolution geodynamical calculations to test this hypothesis. The result shows that it is difficult for a thin (~ 6 km) subducted oceanic crust to accumulate on the CMB, and the major part of it is viscously stirred into the surrounding mantle. Another hypothesis for the origin of LLSVPs is that they are caused by thermochemical piles of more-primitive material which is remnant of Earth's early differentiation. In such case, a significant part of the subducted oceanic crust would enter the more-primitive reservoir, while other parts are either directly entrained into mantle plumes forming on top of the more-primitive reservoir or stirred into the background mantle. As a result, mantle plumes entrain a variable combination of compositional components including more-primitive material, old oceanic crust which first enters the more-primitive reservoir and is later entrained into mantle plumes with the more-primitive material, young oceanic crust which is directly entrained into mantle plumes without contacting the more-primitive reservoir, and depleted background mantle material. The result reconciles geochemical observation of multiple compositional components and varying ages of oceanic crust in the source of ocean-island basalts. Seismic studies have detected ultra-low velocity zones (ULVZs) in some localized regions on the CMB. Here, I present 3D thermochemical calculations to show that the distribution of ULVZs provides important information about their origin. ULVZs with a distinct composition tend to be located at the edges of LLSVPs, while ULVZs solely caused by partial melting tend to be located inboard from the edges of LLSVPs. This indicates that ULVZs at the edges of LLSVPs are best explained by distinct compositional heterogeneity, while ULVZs located insider of LLSVPs are better explained by partial melting. The results provide additional constraints for the origin of ULVZs. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2015

This dissertation contributes to our understanding of the use of the Li-isotopic composition of seawater as a tracer of the earth system with a focus on the role of low-temperature hydrothermal systems within the lava section of the ocean crust. Experiments were conducted to study the exchange coefficient (D(Li/Ca)) and isotopic fractionation factor (α; 1000ln(α)=Δ) for lithium between inorganic calcite and an aqueous solution as a function of solution chemistry. These experiments show that, under the conditions used, D(Li/Ca) negatively correlates with solution H+/Ca2+ ratio (and the solution pH) and Δ positively correlates with solution pH. The change in D(Li/Ca) with solution chemistry is interpreted as indicating that Li is incorporated into calcite as LiHCO3, and hence depends on solution H+/Ca2+. A series of diffusion experiments were performed to test whether changes in pH led to changes in the aqueous Li speciation that would lead to changes in the relative diffusivity of the two Li-isotopes, but no such changes were observed. It is proposed that the change in Δ with changing solution pH may either reflect a kinetic or equilibrium isotope fractionation associated with changing solution chemistry. These results have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification. The Li-content and isotopic composition of rocks altered by low-temperature, off-axis hydrothermal systems in the upper oceanic crust were studied to better understand the role of these systems in controlling the Li-isotopic composition of seawater. Results of a detailed study of DSDP Holes 417A, 417D and 418A from 119 Myr Western North Atlantic Ocean basin show that the Li content of the lavas decreases with depth in the upper ~30 m below sediments while the Li-isotopic composition increases from a low value and then does not show systematic variation in deeper sections. No evidence was found to support a role for a change in mineralogy of alteration products to explain the observed variation in Li composition of lavas. There is also no evidence for the modification of the composition of hydrothermal fluid due to a mixture with sediment pore-fluid. Simple one-dimensional fluid flow and fluid-rock reaction models also cannot explain the observed variation. Instead, a model of free-circulation of seawater through the upper few tens of meters of the lavas, and leakage of modified fluid into the deeper portion of the lava pile seems to explain the observed variations best. To investigate the role of low-temperature off-axis hydrothermal systems on the Li content and isotopic composition of seawater from the Cretaceous to modern era, five DSDP/ODP holes with crustal age spanning from 13.6 to 95 Myr were studied in combination with results from previous studies. Results suggest that the average amount of Li added to the upper oceanic crust decreases, while its average Li-isotopic composition increases, from the Cretaceous to the modern. The simplest explanation for these variations may be a decrease in Li concentration and an increase in Li-isotopic composition, of seawater over this time interval. / Graduate