First Ce-Nd isotope measurements of middle and lower continental crust samples support massive lower crust recycling over Earth's history

Abstract

Long-lived radioactive isotopes provide valuable information on the evolution of Earth's geological reservoirs. Coupled measurements of the 138La-138Ce and 147Sm-143Nd systems have attracted much interest, but some critical reservoirs, such as the deep continental crust, have yet to be investigated. To address this gap, we report Ce-Nd isotope measurements of 35 crustal samples from Canada, western Europe, and Siberia, with Archean to Phanerozoic ages. Most samples from western Europe and Siberia are interpreted to represent the deepest continental crust because of low SiO2 content (< 55 wt%) and generally mafic compositions. In contrast, Precambrian composites from the Canadian shield have highly felsic compositions (61–71 wt% SiO2) and represent the old upper continental crust. Xenoliths from the Massif Central (France) and samples from uplifted massifs in the Ivrea-Verbano zone (Italy) plot on the Ce-Nd mantle array. Precambrian Canadian composites and xenoliths from Udachnaya (Siberian Craton) plot to the left of the mantle array, with the Siberian samples farthest from the array. The La-Ce system yields a ca. 1.8 Ga age for the Siberian xenoliths, distinct from the ca. 2.7 Ga Sm-Nd errorchron age. The La-Ce system was most likely reset during a large-scale episode of delamination and rejuvenation of the Archean lower lithosphere established in Siberia. A change in the La/Ce ratio during this event explains an increasing deviation with time of the Ce-Nd isotopic composition of the Siberian deep crust from the mantle array. We introduce a new parameter (θ) to quantify the likelihood of any rock evolving away from the mantle array. It represents the angle between the evolution vector of a sample and the mantle array, and it is calculated from measured La/Ce and Sm/Nd ratios. Samples that plot to the right of the mantle array have θ > 0, whereas samples that plot to the left of the mantle array have θ < 0. The angle θ was used to compare our samples to a worldwide granulite compilation (1581 samples) because it is independent of the age and the initial isotopic composition of the samples. The majority of lower crust rocks have negative θ, suggesting they are prone to evolve to the left of the mantle array, but to a lesser extent than the Siberian samples that are characterized by the most negative θ values. Thus, the Siberian samples with the most unusual Ce-Nd isotopic compositions are the only group to plot close to the deep crust end-member calculated from classical mass-balance budget of the bulk silicate Earth. These results suggest that the lower crust sampled by Siberian xenoliths is only a minor component in the terrestrial deep crust. We use this database to estimate new parent/daughter ratios for the lower continental crust: 138La/142Ce = 0.00380 ±0.00007 and 147Sm/144Nd = 0.128 ±0.007. Measurements and models are finally reconciled when considering massive recycling of lower continental crust through Earth's history, totaling 3 to 4 present-day, continental crust masses.