Radiogenic strontium-87 model of continent formation

TLDR: In this paper, a model is formulated in which the proportion and age of reworked material that is incorporated into new sialic basement are related to the Sr87/Sr86 development in the various materials involved.

Abstract: Rubidium and strontium are used as tracers in the history of differentiation of the continental sial, and for this purpose it is assumed that the Rb/Sr ratio follows the aggregate enrichment of sialic components sufficiently well to represent an approximately linear measure of the entire process. Measurements of the Sr87/Sr86 ratios in basalts and other igneous rocks of subsialic origin suggest that the source regions of sial have had a Sr87/Sr86 ratio in the range 0.705 to 0.710 in different areas and at different times in the span of geologic history. Measurements of the Rb/Sr ratio in sialic rocks of the continental crystalline basement have indicated that the weighted average value for this ratio is in the vicinity of 0.25. This means that the Sr87/Sr86 ratio in average sialic rocks of the continental basement would increase at a faster rate than that in the subsialic source regions, by an amount equal to approximately 0.010 per billion years. This is more than five times the standard deviation precision error in measurement. A comparison of the ratio Sr87/Sr86 with the geologic age of the rock, in samples of typical sialic basement rocks leads to the conclusion that there has been a continuous generation of primary sial from subsialic source regions that has caused the continental areas to grow roughly in proportion to the extent of the geological age provinces. In North America this areal growth is estimated to be at the average rate of about 7000 km2/m.y., and it seems to have been operative over most of geologic time. A model is formulated in which the proportion and age of reworked material that is incorporated into new sialic basement are related to the Sr87/Sr86 development in the various materials involved. The model is generalized with all the parameters left open. An example set of values for the various parameters is tested on the model to indicate where there is need for more definitive data. This example set is derived from data available at present which, although very limited, indicate clearly that if the reworked material is young enough it can constitute a major proportion of a new sialic basement area, but if it is ancient (1 to 2 b.y. in primary age prior to incorporation) it could not be present in more than a minor proportion of the typical new basement.