Document type
Conference papers
Document subtype
Abstract
Title
Oxyhydroxide Hydrothermal Crusts from Saldanha Hydrothermal Field
Participants in the publication
Pedro Costa (Author)
Ágata Alveirinho Dias (Author)
IDL
Summary
The Saldanha hydrothermal field is located at the top of a\nmount in the Mid-Atlantic Ridge (MAR, 36º34'N, 32º26'W, ~\n2200m). The field is covered by pelagic sediment from where\ntransparent and weak fluids vent through small orifices.\nOxyhydroxides (OH) occur mainly within the top of that\nsediment cover, forming crusts containing millimetric microchimneys.\nIn this work, the mineralogy and geochemistry of\nSaldanha OH crusts were studied using X-ray diffraction\n(XRD), Electron microprobe (EPMA), Scanning Electron\nMicroscopy / Energy Dispersive X-Ray Spectroscopy\n(SEM/EDS) and geochemical (INAA, ICPMS) analysis.\nSaldanha crusts show a zoned pattern with alternate thin\nlayers of acicular oxyhydroxides with massive\ncryptocrystalline oxides. Mineralogical and textural\nobservations collectively indicate an open space setting for\nthe growth of the oxyhydroxide minerals within the sediment\ncover.\nAlthough these phases are Mn-rich in composition, large\nvariation on MnO2 (46-83wt%), MgO (5-24wt%) and FeO\n(usually less than 1%) content was noted. Todorokite and\nbirnessite form the more massive and crystalline phases and\ndisplay the higher contents in Mn. Vernadite, on the other\nhand, form elongated structures and microconcretions\n(botryoidal textures) identified in the acicular layers. This less\ncrystalline phase displays a higher content in MgO.\nGeochemical discriminant diagrams revealed a hydrothermal\norigin for these OH phases. However, Mg and part of Mn\nmight be derived from seawater due to an extensive mixing\nbetween the ascendent hydrothermal fluid and the descendent\nseawater.\nIn conclusion, Saldanha crusts were precipitated at the\nsediment cover in the upper part of the hydrothermal system,\nas consequence of more oxidizing conditions. The depletion\nin Fe content in these phases can be explained by the\nprecipitation of this metal in the form of sulphides, found in\ndeeper parts of the system.\n
Date of Publication
2020-06-23
Institution
FACULDADE DE CIÊNCIAS DA UNIVERSIDADE DE LISBOA
Event
Goldschmidt Abstracts
Publication Identifiers
Publisher
Geochemical Society
Document Identifiers
DOI -
https://doi.org/10.46427/gold2020.478
URL -
http://dx.doi.org/10.46427/gold2020.478
Keywords
Saldanha
Oxyhydroxide
Hydrothermal Crusts
mineralogy
Tags
Marine geology
mineralogy
deep-sea hydrothermal processes