Diagenetic and postdiagenetic changes in the composition of an Archean pillow

An Archean basaltic pillow of greenschist metamorphic facies and a modern sea-floor pillow with minor diagenetic alteration are comparably zoned from rim to core as follows: a 'glassy' rim, a zone of incipient crystallization, and a crystalline interior, but the petrography of each is appropriate to its metamorphic stage. The Archean pillow varies markedly in composition inward, with the major changes taking place at the rim and at the margin of the crystalline interior; the centre is a broad zone of fairly uniform composition. Relative to the centre zone, alkalis and silica are greatly depleted and iron, lime, and magnesia greatly enriched at the rim, whereas the proportions are approximately reversed at the margin of the crystalline interior. The abundance and, in part, composition of metamorphic minerals reflect the same zoning: chlorite and epidote predominate at the rim, and feldspars concentrate at the margin of the crystalline interior. The bulk of the crystalline interior is a uniform intergrowth of actinolite–epidote–chlorite–albite–sphene–quartz–calcite. Epidote is iron–enriched and iron–depleted at the rim and the margin of the crystalline interior respectively. These variations can be attributed to a pre-existing diagenetic alteration analogous to that recognized in the modern pillow where palagonitization of the glassy rim is attended by oxidation of iron and a concentration of certain elements, notably alkalis, at the pillow margins. With metamorphism, epidote develops at the pillow rims because of oxidation induced by palagonitization and the residual glass is replaced by chlorite. Elements incompatible with these minerals migrate to the margin of the crystalline interior where they can be accommodated in the developing metamorphic assemblage. Little net change in the composition of the pillow is evident.