Chapter 4

Ridge rock chemistry

McKenzie and Mike BickleMike BickleMike Bickle (1948-present) of the University of Cambridge has combined petrological and geochemical research based in the field and laboratory with physical modelling. His goal has been to understand the processes that shape the Earth, both within the solid Earth and, latterly, in the interactions between solid Earth, water and atmosphere to determine climate cycles. set out in their paper of 1988 to calculate both how much molten rock could be produced at a mid-ocean ridge and how its composition might vary. Their approach uses clever representations of complex processes within the Earth, with data from the petrology and chemistry of rocks formed at the ridges. They find that lithospheric extension and its thermal consequences can account for the compositions of mid ocean ridge basalts produced by melting a garnet peridotiteperidotitea dense, coarse-grained plutonic igneous rock consisting mostly of the minerals olivine, clinopyroxene and orthopyroxene. mantle. It takes greater extension of the continents to generate melts, which thus have different compositions.

The McKenzie and Bickle paper represents an extension of the physical modelling of lithosphere extension and its thermal effects into the more traditional geological fields of petrology and geochemistry. In part this reflects the maturity of the plate tectonics paradigm, but it also comes about because of the development of an extensive suite of useful petrological and geochemical data. The uniform nature of tholeiitictholeiitica type of basalt oversaturated with silica. basalt from mid ocean ridges points towards some uniformity of origin. This paper successfully merges geophysical modelling of Earth processes with traditional geological means of determining the evolution of igneous and metamorphic rocks.

READ THE PAPER — McKenzie, D. and Bickle, M.J. 1988. The volume and composition of melt generated by extension of the lithosphere. Journal of Petrolology. 29, 625-79.


Locations of basaltic glass analyses, from working file “The volume and composition of melt generated by extension of the lithosphere” (1988).