eochemical insights into formation of enigmatic ironstones from Rio Grande rise, South Atlantic Ocea

Autor(es)

Benites, Mariana ^[1] ; Hein, James R. ^[2] ; Mizell, Kira ^[2] ; Farley, Kenneth A. ^[3] ; Treffkorn, Jonathan ^[3] ; Jovane, Luigi ^[1]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Inst Oceanog, Praca Oceanog 191, BR-05508120 Sao Paulo, SP - Brazil

^[2] US Geol Survey, PCMSC, 2885 Mission St, Santa Cruz, CA 95060 - USA

^[3] CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 - USA

• Tipo de documento: Artigo Científico
• Fonte: MARINE GEOLOGY; v. 444, FEB 2022.
• Citações Web of Science: 0

Resumo

Rio Grande Rise (RGR) is an intraplate oceanic elevation in the South Atlantic Ocean that formed at a hotspot on the Mid-Atlantic Ridge during the Cretaceous. In spreading center and hotspot environments, ironstones form mainly by biomineralization of reduced Fe from hydrothermal fluids or oxidation of sulfide deposits. However, RGR has been considered aseismic and volcanically inactive for the past 46 Ma. Here, we investigate the origin of ironstones collected from the summit of RGR using multiple techniques: petrographic observations, X-ray diffraction, U-Th/He geochronology, and chemical composition. The ironstones from RGR consist of finely laminated goethite containing igneous rock fragments, carbonate fluorapatite, and calcite. Our results suggest that Fe oxyhydroxides were precipitated by Fe-oxidizing bacteria forming bacterial mats. The bacterial Fe mats underwent compaction, dewatering, goethite crystallization, and cementation that created the ironstone deposits. U-Th/He geochronology reveals protracted goethite minimum ages extending from the late Miocene to the Quaternary, probably due to multiple generations of mats, slow mineralization rates, and Fe-oxide dissolution-reprecipitation cycles. Flame-like goethite structures underneath FeMn crusts and a chimney-shaped goethite sample with a central channel indicate that the dewatering fluid flowed upward through the deposits, or a thermal fluid source may have been introduced from below the ironstone deposits. High Fe/Mn ratios, low trace metals contents (Ni + Co + Cu), and very low Fe/REY ratios suggest ironstone precipitation from a hydrothermal fluid; however, REYSN plots and bivariate Ce-SN /Ce-SN{*} versus Y-SN/Ho-SN and Ce-SN /Ce-SN{*} versus Nd plots are inconclusive, and a proximal source of magma was unlikely during the period of mat formation. Given this evidence, we hypothesize that a geothermal circulation system may have facilitated ironstone mineralization at RGR. (AU)

• Processo FAPESP: 19/15587-3 - Formação de crostas de ferromanganês e história de fosfatização no Rio Grande Rise
• Beneficiário: Mariana Benites
• Modalidade de apoio: Bolsas no Exterior - Estágio de Pesquisa - Doutorado
• Processo FAPESP: 14/50820-7 - Marine ferromanganese deposits: a major resource of E-tech elements
• Beneficiário: Frederico Pereira Brandini
• Modalidade de apoio: Auxílio à Pesquisa - Temático
• Processo FAPESP: 18/17061-6 - Relação entre ambiente e bactérias magnetotáticas
• Beneficiário: Luigi Jovane
• Modalidade de apoio: Auxílio à Pesquisa - Programa de Pesquisa sobre Mudanças Climáticas Globais - Regular
• Processo FAPESP: 18/05114-8 - Crostas de ferro e manganês e fosforitas da Elevação do Rio Grande
• Beneficiário: Mariana Benites
• Modalidade de apoio: Bolsas no Brasil - Doutorado