Hf-Nd isotopes from ultramafic and mafic rocks in the western Dharwar Craton, India, record early Archean mantle heterogeneity

Abstract

Both primary mantle differentiation and early extraction of oceanic and continental crust can have resulted in a chemically heterogeneous Archean mantle. Evidence for heterogeneity of the Archean mantle is preserved in the correlated Hf and Nd isotopes in mafic (basaltic) magmatic rocks and their ‘decoupling’ in ultramafic rocks, especially in komatiites, from the Archean Dharwar Craton, India. The komatiites and komatiitic basalts from the western Dharwar Craton were emplaced at ~3.25 Ga and have large spread in initial radiogenic isotope compositions with initial ɛHf (ranging from +0.9 to +21.3) and initial ɛNd from +0.2 to +7.4 that do not correlate. In contrast, the ɛHfi and ɛNdi in contemporaneous mafic (ɛHfi = +5.9 ± 11.2; ɛNdi = +3.2 ± 7.6) and felsic (ɛHfi = +4.3 ± 4.6; ɛNdi = +2.7 ± 4.2) rocks in the same area correlate, as is typical of rocks that are the product of magmatic processes involving melting and magmatic differentiation. The high initial ɛHf and the lack of correlation (‘decoupling’) of ɛHf and ɛNd in the komatiites and komatiitic basalts indicate that the source had superchondritic Lu/Hf which is attributed to early mantle differentiation at great depth, possibly in the perovskite stability field. This finding indicates that the primordial mantle was mineralogically and chemically layered and this layering was the result of fractionation processes very early in Earth's history, probably during magma ocean solidification. During crystallization of a primordial magma ocean, fractionation of Mg-perovskite with minor Ca-perovskite component could have resulted in a melt with high Lu/Hf and nearly unfractionated Sm/Nd. The correlation of the initial Hf isotope composition of the komatiites and komatiitic basalts with their MgO content, and thus melting temperatures, indicates the presence of a highly fractionated component derived from the deep mantle. The coupling of ɛHf-ɛNd in mafic to felsic igneous rocks and the decoupling in contemporaneous ultramafic rocks is thus the result of mantle material originating from different depths involving different ancient fractionated reservoirs. The existence of a chemical and isotopically heterogeneous Archean mantle that supplied material to form the continental crust has to be accommodated in crustal growth and preservation models that commonly assume a nearly homogeneous and gradually depleting mantle reservoir as the source of the continental crust.