Fe-Ti oxides

Figure 10,20 Major element concentrations in effusive products of Boina series plotted as functions of degree of fractional crystallization, based on equation 10.132. I = first discontinuity (transition from olivine-dominated to plagioclase-dominated fractionation) II = second discontinuity (appearance of Fe-Ti oxides) III = third discontinuity (field of Si02 oversaturated trachytes apatite and Mn-oxides precipitate) IV = beginning of peralkalinity field. From Barberi et al. (1975), Journal of Petrology, 16, 22-56. Reproduced with modifications by permission of Oxford University Press.

The Boina emission products show a more or less continuous differentiation trend from transitional basalt to pantellerite. In the nonperalkaline field, the transition from basalt to ferro-basalt is dominated first by olivine F > 0.65) and then by plagioclase F > 0.45), with minor clinopyroxene. A second differentiation step with the appearance of Fe-Ti oxide crystals begins at F = 0.45 Fe and Ti decrease abruptly. Less marked discontinuities are also observed at F = 0.3 (silica-oversaturated trachytes) and F = 0.15 (peralkalinity field). 

In summary, the composition of Fe-Ti oxides in magmatic rocks provides the pet-rologist with important information about the oxygen fugacity and temperature and also the silicon activity of the magma. It also has a strong effect on the magnetic properties of these phases (see Chap. 6 7). 

Most lava flows exhibit poorly porphyritic textures. Main phenocryst phases include leucite and clinopyroxene olivine and Fe-Ti oxides occur in minor amounts. Groundmass contains the same phases plus nepheline,... 

Pyroclastic rocks include scoria and pumices, which show variably por-phyritic textures with dominant leucite phenociysts and minor clinopyrox-ene, some mica, and rare olivine, set in a microcrystalline to glassy matrix that has been generally affected by strong secondaiy processes, with abundant zeolites and clay minerals (Fomaseri et al. 1963 Trigila et al. 1995). Accessory phases include Fe-Ti oxides, apatite and rare garnet. 

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