Sidney L. M. Mello

Geophysical and geochemical evidence for cold upper mantle beneath the Equatorial Atlantic Ocean

This paper presents geophysical and geochemical evidence for the possible existence of cold, subducted lithosphere beneath the Saint Paul Fracture Zone of the Equatorial Atlantic Ocean. The ocean floor along the fracture zone is characterized by a high percentage of abyssal peridotites. The abyssal peridotites were emplaced by tectonic uplift of the oceanic lithosphere. The top of the ridge is exposed at Saint Peter and Saint Paul islets. The Os isotopic composition of these abyssal peridotites indicate the presence of old depleted mantle material in this region with Re-depletion model ages (TRD) from 0.32 to 1.1Ga. Melt inclusions in plagioclase phenocrysts of the MORB close to this location have boninitic composition, suggesting that some basalts originated from low-degree mantle melting. The global tomography models show fast seismic velocities in the upper and lower mantle of the Equatorial Atlantic Ocean, consistent with the presence of cold subducted lithosphere. The range of Re-depletion model ages are consistent with paleo-reconstructions of plate motion, suggesting that the fossil subducted slab was formed during the closure of both the Iapetus and the Rheic oceans.

Morfoestrutura da Cordilheira Mesoceânica no Atlantico Sul entre 0ºS e 50ºS

A variacao morfologica da Cordilheira Mesoatlântica Sul (CMAS) e analisada com foco na segmentacao estrutural, gravimetria residual e a geoquimica de basaltos. O estudo estabelece uma analise morfoestrutural da CMAS, inferindo sobre os processos tectonicos e magmaticos em subsuperficie que originam e influenciam a topografia observada. A analise morfologica da CMAS, em escala da ordem de 1.000 km, revelou a existencia de domos topograficos, ao norte e ao sul da CMAS, separados por uma regiao central deprimida. A partir desta analise foram definidos tres dominios topograficos, nomeados aqui Norte, Central e Sul, centrados em torno de 10oS, 25oS e 40oS. Estes resultados sugerem um magmatismo mais intenso sob o eixo da cordilheira nos Dominio Norte e, principalmente no Dominio Sul, enquanto na regiao do Dominio Central predominam os processos tectonicos, com um magmatismo esparso. Tal segmentacao de longo comprimento de onda da CMAS esta relacionada a processos profundos no manto astenosferico, sugerindo uma estrutura de densidade do manto regionalmente diferenciada em todo Atlântico Sul (AS). Ao norte e ao sul, o eixo da cordilheira parece ser alimentado por fontes profundas no manto, coerente com os domos topograficos observados, enquanto a topografia menos elevada observada na regiao central provavelmente esta relacionada a um adensamento e/ou resfriamento mantelico. A analise morfologica da CMAS em escala de maior detalhe revelou a existencia de 20 segmentos tectonicos, cuja morfoestrutura apresenta uma notavel variacao ao longo da CMAS, intra e intersegmentos. Um vale axial, com profundidade media da ordem de 3.200 m, e em geral observado no eixo da CMAS, mas em certas regioes sua ausencia e marcante. A largura do vale central, bem como a elevacao das montanhas do rifte tambem e variavel, sugerindo assim forte controle de processos de acrescao locais variaveis no tempo e no espaco.

Analysis of seafloor depth anomalies between the ascension and St. Helena Islands, South Atlantic

Observed seafloor depths from 12 kHz bathymetry coupled with seafloor spreading magnetic anomalies were used to obtain depth anomalies across the Mid-Atlantic ridge axis between 2oS and 18oS. The theoretical seafloor depths were calculated from the plate cooling model and crustal ages depicted from seafloor spreading magnetic anomalies between 0 to 25 Ma. We noticed that the seafloor spreading magnetic anomalies are symmetric with respect to the ridge axis between Ascension and St. Helena Islands, but the depth anomalies are quite asymmetric. The east side of the ridge axis is shallower (~ 1,000 meters) than the west side, probably as a result of a thermal upwelling of the lithosphere. We suggest that a mantle plume located on the African Plate, which could represent a larger swell linked to Ascension, St. Helena and Fernando Po-Annobon Volcanic Lineament, might cause this type of anomaly.

Metallic sulfide deposits in deep sea

The geological processes that lead to the formation of a new oceanic crust may cause intense hydrothermal activity. This activity supports the existence of an exotic fauna in deep ocean and also forms mineral deposits. Since the discovery of the first hydrothermal field on the East Pacific Rise, there has already been found about 100 hydrothermal sites associated with important metallic sulfide deposits. These sites occur along the oceanic spreading centers in different tectonic settings, but are not continuous globally. They are localized at specific sites where hot rocks are connected to faults and fractures, creating a permeable pathway for the hydrothermal circulation. Seawater penetrates into the oceanic crust and is modified to hydrothermal fluid with high temperature (up to 400°C), low pH and Eh. This fluid interacts with the rocks leaching and transporting metals, which precipitate as massive sulfides at the seafloor or as stockwork in the subseafloor. Metallic sulfide deposits can reach considerable size ranging from 1 to 100 million tones with high concentrations of copper, zinc, lead and precious metals such as gold and silver. Some of these deposits have recently attracted the interest of the scientific community and the international mining industry. They are good examples of modern analogues for understanding the genesis of volcanic-hosted massive sulfides on land and show strong potential mining in the near future. Here we provide a review on the deep-sea sulfide deposits, focusing on its formation, methods of exploration, location and characteristics (mineralogy/metal contents) of the main sites, and also the potential mining impacts.