Opaque mineral alteration states correlated with temperature in an active geothermal system

Abstract

ALTERATION of opaque minerals, especially the iron–titanium oxides, has become a very widely used technique in rock and palaeomagnetism and in geothermometry¹. The high (≥ 600 °C) temperature oxidation of titanomagnetite and ilmenite is a well-known process in nature^2,3 and in experimental petrology^4,5. In contrast, the alteration of these oxides at low (< 300 °C) temperatures is relatively poorly known. This is partly due to the lack of data on the stability of various alteration products in the 0–300 °C temperature range. Another difficulty is the absence of a characteristic sequence of textural features that can be used in a classification of progressive alteration. Here we describe how investigations on the Azores drill hole of 1973, located in the lower flank of the active volcano Agua de Pau, San Miguel, Azores⁶ have filled this gap.