Exogenic processes and the origin of the sialic crust |
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| Authors: | C T Walker J G Dennis |
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| Institution: | 1. Department of Geological Sciences, California State University, 90840, Long Beach, California
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| Abstract: | Large scale segregation of sialic crust from mantle material may have taken place by magmatic differentiation, partial melting or by exogenic processes. Weathering is a little considered, but verifiable process by which free silica can be generated in quantity from mafic material. In proto-Archean time acid ocean water must have been highly reactive. Even submarine weathering by such acid water could lead to large scale generation of free silica. Evidence from soil formation supports this view. Recrystallization of weathering products would take place by anatexis in the course of relatively intense recycling processes in pre-Archean and early Archean times. Much Fe and Mg in shales and carbonates would segregate downward as mafic xenoliths. The sialic fraction would be returned to shallow crustal levels by diapiric ascent and upward melting. This model is valid only if the sialic crust has grown little since Archean time. We show that “primitive” isotope ratios in later sialic rocks which may suggest gradual accretion of sialic crust throughout geologic time can also be explained by recycling mechanisms. We conclude that exogenic processes in pre-Archean times could have generated most of the sialic crust. |
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