Banca de DEFESA: MAISA BASTOS ABRAM
Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.DISCENTE : MAISA BASTOS ABRAM
DATA : 27/10/2020
HORA: 14:00
LOCAL: remoto
TÍTULO:
EARLY AND MIDDLE DEVONIAN PHOSPHOGENESIS IN NORTHERN GONDWANA (PARNAÍBA BASIN, BRAZIL): PETROGRAPHIC, GEOCHEMICAL AND ISOTOPIC EVIDENCE FOR PHOSPHORUS CYCLING
PALAVRAS-CHAVES:
Pimenteira Formation – Itaim Formation - Phosphorite – Ironstone - Devonian
PÁGINAS: 120
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Geociências
SUBÁREA: Geologia
ESPECIALIDADE: Estratigrafia
RESUMO:
The Devonian is a period of climate change and epeiric seas, with phosphate successions, but with little economic importance, such as the northwestern areas of Gondwana margin in Brazil. The objective of this work is to deepen the knowledge about the occurrences of Devonian phosphates, in order to approach this theme, with the establishment of phosphogenetic models and implications on the evolution of the planet and mineral potential. Pragian to Givetian successions in the Canindé group, Itaim and Pimenteira formations, Parnaíba Basin (Brazil) were investigated. Phosphorite and phosphatic ironstone rich successions and their host rocks were investigated in an integrated study based on sedimentology, high resolution sequence stratigraphy with petrographical, mineralogical and geochemical (whole rock major and trace element, δ13C and δ18O isotope, total organic carbon/total sulfur and Corg/P ratio) data to highlight paleoproductivity, redox, recycling, diagenesis and phosphogenetic processes. Lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences (B, C, D and E) and 19 parasequences. Phosphorite occurs is in the form of concretions or pristine layers in offshore environment, near SWB (outer/inner shelf) precipitated under suboxic to anoxic conditions. It also occurs as glauconite francolite nodules in hetherolitic lithofacies precipitated in oxic to suboxic zones. Ironstone and phosphatic ironstone were deposited in more proximal settings, near FWWB. Molibdenium, V/Cr and V/(V+Ni) ratios for phosphatic ironstone and ironstone intervals indicated variations from dysoxic/suboxic conditions to more anoxic and minimal dissolved H2S in phosphatic ironstone intervals. Total sulfur versus TOC indicated variations from dysoxic, anoxic and even euxinic conditions for organic lithofacies. Distinctive MREE enriched patterns are similar to other Devonian phosphorites patterns, even for samples with authigenic francolite precipitation near surface water. The variation for the siderites was of δ18O (VPDB) -12.0 to 2.5 and δ13C-21.5 to -0.8. The variation indicates early diagenesis with pore-water sensitive to organic degradation that evoluted from sulfate reduction to iron reduction zones and possible variations in salinity due to mixing freshwater. Francolite presented more clustered data points then siderites (δ18O VPDB from -14.11‰ to -4.6‰) / (δ13C from -2.9‰ to -8.1‰), although δ18O VPDB data were strongly affected by weathering. We recorded that the limit inner/outer shelf waters were more enriched in P concentrations then shorewards regions, with the development of phosphogenesis along the shelf generating an oxygen minimum zone with geochemical data like modern upwelling sites. River discharges probably affected physical characteristics of the upper water column and produced large spatial variability in the Parnaiba basin system probably influencing vertical stratification and salinity or temperature leading to gradients of surface nutrient concentrations. Not only upwelling ferruginous water but terrestrial contribution of OM contributed to enhance shallower anoxia resulting in reduced total phosphorus burial efficiencies. The phosphorus cycle was probably affected by the global low oxygen and anoxic conditions in the Devonian. This may have affected the burial of P across the entire shelf, implicating in low economic potential for Early and Middle Devonian phosphorites, although rare earth elements and yttrium concentrations represent a potentially economic byproduct.
MEMBROS DA BANCA:
Presidente - 357060 - MICHAEL HOLZ
Interno - 003.403.165-00 - AROLDO MISI - UFBA
Interno - 286023 - JOSE MARIA LANDIM DOMINGUEZ
Externo à Instituição - CLEIDE REGINA MOURA DA SILVA
Externo à Instituição - SERGIO BERGAMASCHI - UERJ