1965 Volume 53 Issue 6 Pages 215-227
Some petrochemical features of ultramafic rocks are firstly discussed by taking account of mantle norms and M values. The existence of iron-rich ultramafic intrusives, e.g., the Union Bay complex, is pointed out and world-wide occurrences of these complexes are suggested. Commonly the rocks constituting ultramafic intrusives contain less aluminium than mantle peridotite, as represented by peridotite inclusions in basaltic rocks, and have a considerable wide range in their M values, whereas peridotite inclusions show relatively uniform M values. Secondly, the distribution relation of aluminium with four-fold co-ordination and silicon between co-existing clinopyroxenes and orthopyroxenes is discussed. Although tschermakitic molecule may be more soluble in clinopyroxenes than in orthopyroxenes at a given pressure, AlIV contents in orthopyroxenes are greater than those in clinopyroxenes from ordinary ultramafic intrusives. The reverse relation is observed in pyroxenes from peridotite inclusions and the Lizard primary peridotites which were formed at higher pressure. This is interpreted on the basis of physico-chemical considerations.