Radiogenic 182W and 142Nd anomalies in a 2.7 Ga old basaltic lava flow indicate long-term preservation of early mantle heterogeneities

Abstract

Understanding the origin and preservation of early mantle heterogeneities is essential for reconstructing Earth’s accretion and early differentiation history. This can be investigated using the short-lived ¹⁴⁶Sm–¹⁴²Nd and ¹⁸²Hf–¹⁸²W isotope systems, which provide insights into large-scale differentiation processes through ¹⁴²Nd and ¹⁸²W anomalies in Earth’s oldest rocks. In this study, we present ¹⁸²W data for Theo’s Flow, a thick, differentiated, mafic–ultramafic flow of tholeiitic affinity, as well as ¹⁴⁷Sm–¹⁴³Nd, ¹⁷⁶Lu–¹⁷⁶Hf, and ¹⁸⁷Re–¹⁸⁷Os isotope data and trace element concentrations, including highly siderophile elements, for Theo’s Flow and closely spatially and temporally associated Pyke Hill komatiites, both from the ∼2.7 Ga old Kidd-Munro Assemblage of the Abitibi greenstone belt, Ontario, Canada. Theo’s Flow samples are characterized by an average µ¹⁸²W value of +6.7±2.7, which, together with the previously reported positive µ¹⁴²Nd anomaly of +6.8±2.5, indicate long-term survival of mantle domains resulting from magmatic differentiation early in Earth’s history. If it is assumed that the bulk silicate Earth evolved with a suprachondritic Sm/Nd ratio, anomalies for both elements can be explained by a single silicate differentiation event at ∼4.54 Ga. Alternatively, if the bulk silicate Earth evolved with a chondritic Sm/Nd ratio, a model fractionation age of ∼4.45 Ga is obtained. In this case, due to extinction of ¹⁸²Hf by that time, the ¹⁸²W anomaly would have to have been generated by another process, such as derivation from a mantle source characterized by a deficit in late accreted materials. Regardless of the timing of the early silicate differentiation, the preservation of both ¹⁸²W and ¹⁴²Nd anomalies in the 2.7 Ga old rocks highlights the survival of early-formed mantle domains for nearly 1.8 Gyr after Earth’s formation. Comparison of isotopic and chemical data for Theo’s Flow with those for the Pyke Hill and Boston Creek komatiites, which were argued to have all been derived from melting in a single mantle plume, provides evidence for the survival of early mantle heterogeneities on a plume scale.