Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland

Abstract

Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ~3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB.Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism.Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ~3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ~3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ~3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.