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Lower Continental Crust

Re–Os isotope evidence for the composition, formation and age of the lower continental crust

Knowledge of the composition of the lower continental crust is important not only for understanding the formation and evolution of the crust as a whole, but also to evaluate the petrogenesis of continental basalts and the chemical effect of the lower crust recycling into the mantle. We presented Re–Os isotope data for two well characterized suites of lower-crustal xenoliths from North Queensland, Australia, which have average major and trace element compositions similar to estimates of the bulk lower continental crust. Our data indicate that the lower crust has 1 to 2 times as much osmium, about half as much rhenium, and is less radiogenic than the upper continental crust. We interpret the Re-Os isotope systematics to indicate that assimilation and fractional crystallization of basaltic melts are important processes in the formation of the lower crust, and lead to dramatic changes in the osmium isotopic composition of mafic lavas that pond and fractionate there. A consequence of this is that the Re-Os isotopic system should not be relied on to yield accurate mantle extraction ages for continental rocks.

Related Publications:
Saal, A.E., Rudnick, R.L., Ravizza, G.E. and Hart, S.R. - (1998) - Re-Os isotope evidence for the composition, formation and age of the lower continental crust. Nature 393, 58-61. [48]

Other project collaborators:
R. Rudnick (University of Maryland); G. Ravizza (University of Hawaii); S. Hart (WHOI)