Constraints on continental growth models from Nb/U ratios in the 3.5 Ga Barberton and other Archaean basalt-komatiite suites

Abstract

The average Nb/U ratio of modern mid-ocean ridge and oceanic island basalts (MORBs and OIBs) is 47 compared with 34 for the primitive mantle and this difference is attributed to the separation of the continental crust from the mantle. Variation in the Nb/U ratio in basalts of different ages can therefore be used to identify the time at which the bulk of the continental crust separated from the mantle. This study reports Nb/U ratios for a suite of 3.5 Ga basalts and komatiites from the Barberton Mountain Land in South Africa, the oldest well-preserved, basalt-komatiite suite, and compares the results with published Nb/U ratios for other Archaean basalt-komatiite suites. The average Nb/U of the Barberton basalts and komatiites is 43, which is essentially the same as the average of all of the other Archaean basalt-komatiite suites (42). These values can be used to calculate the total amount of continental crust extracted from the mantle source region of these magmas as a percentage of the amount extracted from the modern mantle. The values obtained are 75 percent for the source region to the Barberton basalts and 70 percent for the average source region of all of the Archaean basalt-komatiite suites considered in this study. The lack of systematic variation in Nb/U through the Archaean precludes the possibility that the late Archaean was a period of rapid growth of continental crust. The rate of growth of the continental crust can be further constrained by the observation that Nb/U in the modern MORB and OIB sources are the same. This requires that the net rate of increase in the mass of the continental crust was negligible over the last 1.5 billion years. Finally, Nd and Hf model ages of Earth's oldest sediments, and the rarity of zircons older than 4.0 Ga in these rocks, indicate that little continental crust formed between 4.5 to 4.0 Ga. Taken together these constraints suggest that 4.5 to 4.0 Ga was a period of slow growth for the continental crust and that it was followed by a period of rapid growth between 4.0 and 3.0 Ga, with approximately 70 percent of the continental crust having formed by the end of that period. Crustal growth was again slow between 2.5 and 0 Ga and may have reached a steady state, with no net increase in the mass of continental crust during that period. If the average age of the preserved continental crust of 1.4 to 1.8 Ga, based on Nd model ages of modern sediments, is correct, much of the 70 percent of the continental crust that had formed by 3.0 Ga must have been destroyed by recycling through the mantle. The rate of recycling required is substantial but less than that envisaged by Armstrong (1968, 1991), especially between 4.5 and 4.0 Ga.