The evolution of strontium isotopes in the upper continental crust

Abstract

RADIOGENIC isotopes in clastic sediments record how the average age and composition of the exposed continental crust has changed with time. Most of the continental crust is> 1.5 Gyr old, and the upper crust is characterized by high Rb/Sr ratios. Critically, however, late Precambrian and younger sediments have surprisingly low depositional ⁸⁷Sr⁸⁶Sr ratios, given their old source ages and high measured Rb/Sr (refs 1, 2). This paradox has prompted suggestions that the ⁸⁷Sr⁸⁶ ratio in clastic sediments is buffered by a significant flux of new (low ⁸⁷Sr⁸⁶) material from the upper mantle, with the implication that relatively large quantities of ⁸⁷Sr were generated in the sedimentary reservoir, and subsequently recycled into the upper mantle^{1, 2}. We argue here that the strontium isotope paradox instead largely reflects the gradual increase in Rb/Sr with time in the upper continental crust. The Rb/Sr ratio of the upper crust is increased by intracrustal melting and fractional crystallization in plagioclase-bearing systems, and by weathering, erosion and sedimentation. The longer material resides in the continental crust, the more likely it is to be reworked by magmatic and sedimentary processes, and so to acquire high Rb/Sr ratios. One corollary is that ∼50% less⁸⁷Sr has been generated in clastic sediments in the past 2.0 Gyr than in models that assume constant crustal Rb/Sr (Fig. 3 of ref. 2).