ND ISOTOPE EVIDENCE FOR A LREE DEPLETED 3.4 GA ARCHEAN MANTLE AND IMPLICATIONS FOR EARLY FRACTIONATION OF EARTH RESERVOIRS

The origin and destruction of the Earths primary crust following planetary differentiation remains one of the most elusive chapters in Earth history. The original chemical characteristics for most ancient rocks have all been destroyed or disturbed, preventing an accurate assessment for the timing and extent of crustal growth, mantle depletion, and surface recycling. A new Sm-Nd isochron for the Lower Lava greenstones of the Western-Iron Ore Group (W-IOG) greenstone belt, Singhbhum craton, Eastern India, demonstrates that 3.43 ± 0.07-billion-year-old mantle-derived Lower Lava greenstones had an initial εNd = +5.7 ± 0.3 (initial ¹⁴³Nd/¹⁴⁴Nd ratio of 0.508465 ± 67; MSWD = 1.1; n= 9), consistent with extraction from a mantle with an evolution similar to that of the Lunar mantle. This age agrees with a concordant 3,392 ± 29 Ma U-Pb zircon age (MSWD=10.4) measured on a tuffaceous unit intermediate within the stratigraphic succession. Additionally, the Lower Lava isochron age is also consistent with the younger 3.37 Ga Bonai granite, which has an intrusive relationship with the W-IOG. The inferred long-lived greater than chondritic Sm/Nd reservoir may be analogous to the missing complementary e_Hf > 0 reservoir to that of Singhbhum Hadean-Eoarchean zircons with e_Hf < 0. The presence of a Hadean LREE enriched crust, as implied by the initial zircon Hf-isotopic data from Singhbhum, complements the existence of a LREE depleted mantle reservoir as measured by our Nd results, thus arguing that the antiquity of the depleted mantle source was not an artifact. If the Lower Lavas depleted mantle reservoir were preserved today, assuming a linear growth separating from a chondritic Earth at 4.56 Ga, its present Nd isotopic composition would have evolved along an ε_Nd trajectory steeper, e_Nd = +23, than that of the modern depleted mantle, e_Nd = +10, suggesting that crustal recycling has suppressed Earth’s mantle by a minimum of 13 ε_Nd units over 3.4 Ga geologic time. Unlike the Moon, a direct consequence of Earth developing an early active tectonic regime led to the wholesale recycling of the LREE enriched crust which mixed back into its complementary depleted mantle reservoir, in effect fertilizing the mantle and suppressing the ε_Nd growth.