Alteration of olivine in volcanic rocks from Trindade Island, South Atlantic

The alteration of olivine (forsterite) phenocrysts in three soil profiles that were developed on similar pyroclastic rocks from Trindade Island, at different altitudes and degree of development, were investigated in this study. Optical microscopy, Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Differential Thermal and Thermogravimetric Analysis (DTA-TGA) were used to determine the mineralogical, micromorphological and geochemical transformations resulting from the alteration process. Micromorphological analyses showed fractured phenocrysts of colorless olivines with high relief at plane polarized light. These crystals are euhedral and irregularly shaped, and distributed in a dark reddish brown matrix constituted by pyroxenes, magnetites, ilmenites, kaolinite, hematite and anatase. At crossed polarized light, the olivine phenocrysts display a rim of reddish brown material in the fractures and in the borders that extinguishes parallel to their extinction direction. This reddish brown material appears to invade the crystal progressively until a complete replacement towards the top of the soil profile of higher altitude and degree of development. The olivine was identified as forsterite type and the reddish brown material has a chemical composition of Si, Fe, Mg, Al, Ca, Na, K, Mn and Ti, with predominant and variable contents of Fe, Si and Mg. The chemical composition and optical features are consistent with that found in iddingsites which may form through the incipient alteration of olivines due to the oxidation of ferrous iron to ferric iron, and with the coordination of magnesium with hydroxyl groups. The microscopic features of olivine did not allow us to identify any previous alteration by either deuteric or hydrothermal processes. However, the advance of the alteration towards the soil profile, leaving only relicts of olivine crystals or reaching their total transformation in the upper horizons, shows that weathering is the main process of iddingsite formation. This finding is corroborated by the presence of greater alteration of olivine crystals in the topmost soil at the highest altitude on the same lithology.