Iron-Titanium oxide minerals in the Bjerkrem-Sogndal massif, south-western Norway

Ilmenite and magnetite are investigated from the point of view of their distribution, micro-texture, and chemical composition (major and minor elements) in the Bjerkreim-Sokndal massif (Egersund area, South-Rogaland, SW. Norway). This massif is an igneous layered lopolith made up of cumulates of the anorthosite-mangerite suite. The lower part of the massif presents a rhytmic structure.
The microtextures of ilmenite result from simple exsolution of ilmenite-hematite solid solution. Magnetite contains intergrowths of ilmenite formed by oxidation-exsolution of ulvöspinel-magnetite solid-solution.
In the stratigraphic sequence, on a large scale, ilmenite appears first alone, and is then accompanied by magnetite; its hematite content decreases towards the top of the massif, while the titanium content of the magnetite increases. On the scale of the rhythms, similar trends but of a lesser amplitude are also observed.
Evidence of deuteric readjustment of the orthomagmatic composition of the two oxides is provided (1) by the observation of microtextures at the contact betwen grains (zoning of primary ilmenite and rim of secondary ilmenite) (2) by the existence of differences in chemical composition between isolated grains and grains in contact, and (3) bythe determination of the equilibrium temperature by means of the Buddington and Lindsley geothermonmeter.
Reconstruction of the T-fO2 orthomagmatic conditions is provided in two particular levels of the massif shows that the reducing character of the magma increases during differentiation. The sudden changes in the oxide assembalge at the base of the rythmic units reflect a sudden increase in the fO2 of the magma. These increases, as shown by variation in Cr, Ni and Co, are due to recurrences of the basic character of the magma.
The variations of the minor elements Mn, V, Ga, and Zn are interpreted in terms of the influence of the deuteric readjustment. It follows that the ratios Mn/Fe2+ ; Ga/Fe3+, and Zn/Fe2+ increase and that the ratio V/Fe3+ decreases in the magma in the course of differentiation. The distribution of Mn between ilmenite and magnetite is discussed.
Intermittent supplies of undifferentiated magma are proposed as the geological mechanism controlling the chemical recurrencess associated with the rhythmic structure.