Mineralogical and geochemical signatures of Quaternary pyroclasts alterations at the volcanic Trindade Island, South Atlantic.
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2020
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Abstract
This paper reports the composition and alteration products of pyroclasts in the Holocene Paredão volcano
(pyroclast 1) and Late Quaternary Morro Vermelho Formation (pyroclasts 2 and 3) of Trindade Island, South
Atlantic, Brazil using combined macromorphological, micromorphological, mineralogical and geochemical
techniques. The pyroclasts 1 and 2 are interpreted as volcanic tuff breccia deposits, whereas pyroclast 3 is a
lapilli deposit. They are dark gray in color with some altered reddish regions and show vesicles and amygdales
structures with small greenish crystals of 2.0 mm scattered throughout the matrix. The eruptions can be regarded
as Strombolian-type by producing pyroclastic deposits with coarse fragments with high vesicularity and fluidal
shape that indicate magmatic degassing and fragmentation. Petrologic and XRD data revealed a mixture of
biotite, goethite, ilmenite, anatase, magnetite, hematite, pyroxene, zeolites, and olivine as their main mineral
components. Optical microscopy analysis confirms the vesicular and amygdaloid structures, with a hypocrys-
talline texture and a pale brown stained vitreous mass classified as sideromelane, due to its basaltic composition.
The sideromelane changes to a reddish brown and yellowish-brown staining material identified as palagonite,
clearly indicating a hydrovolcanic eruption that occurs when the ascending magma comes into contact with
water. Infrared analyses in the palagonitized regions revealed the presence of halloysite, suggesting alteration of
sideromelane to tubular clay minerals. Amygdales and microfractures are partially or totally filled with zeolites,
which are formed by the percolation of water that reacts with the palagonite and precipitation of chemical elements of hydrothermal fluid. Reddish dark brown iddingsite and anhedral crystals of titaniferous magnetites occur in the fractures and edges of the olivine. These crystals are also dispersed in the matrix while some of them are zoned with Cr-rich core and Cr-poor edge, suggesting a deep mantle origin of the magma. The high trace elements content can be related to clinopyroxene (diopside) that include these elements. Geochemical data show that the pyroclasts are undersaturated in silica, plotting in the ultrabasic and foidites fields on the TAS classification diagram.
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Keywords
Palagonite, Halloysite, Zeolites, Infrared
Citation
MATEUS, A. C. C. et al. Mineralogical and geochemical signatures of Quaternary pyroclasts alterations at the volcanic Trindade Island, South Atlantic. Journal of South American Earth Sciences, v. 102, artigo 102674, 2020. Disponível em: <https://www.sciencedirect.com/science/article/pii/S0895981120302170?via%3Dihub>. Acesso em: 29 abr. 2022.