Mineralogical and geochemical signatures of Quaternary pyroclasts alterations at the volcanic Trindade Island, South Atlantic.

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.