A Raman spectroscopic study of the igneous rocks on Marion Island: a possible terrestrial analogue for the geology on Mars
The geology of Marion Island is typical of the oceanic volcanoes forming the Hawaiian archipelago, which is generally seen as an analogue for the geology on Mars. Because of the cold climate, the relatively young igneous rocks on Marion Island exhibit no visible alteration from the original rocks, and we used Raman spectroscopy to study examples of the igneous rocks on the island as potential model substances for the parent material on Mars. Three types of volcanic material were studied: namely, a grey lava (270 000-70 000 BP) as well as black lava and red scoriae cones, both formed at a later date (12 000 BP to present). A few Raman spectra of volcanic ash from the island are included for comparison purposes. The basic elemental content of the three different lavas are the same by X-ray fluorescence (XRF), but there is a variation in the relative amounts of crystalline and amorphous phases present in X-ray diffraction (XRD) data and Raman measurements, which can be related to cooling rate and oxidising conditions during the formation process. Fosterite, sanidine, labrodorite, anorthite and diopside were identified in the grey lava. In the black lava, broad bands in the spectra indicated the presence of many amorphous phases, but fosterite, anorthite, diopside, magnetite and ulvöspinel could be identified through their Raman spectra. In the red scoriae, fosterite, anorthite, diopside, hematite and pseudobrookite were identified, the last two minerals acting as colouring agent. Haematite was also identified as a thin layer covering feldspar glass. The spectra of the volcanic ash vary much and represent a variety of amorphous/glassy phases. This study emphasises the strength of Raman spectroscopy to identify a whole range of disordered phases. The igneous rock formations on Marion Island, consisting of older grey lava, black lava and red scoriae cones, were studied with Raman spectroscopy, as possible model substances for the geology on Mars.