Long-term preservation of early-formed silicate reservoirs in the deep mantle: New insight from 146Sm-142Nd systematics in Hawaiian basalts

Short-lived radiogenic isotope systems provide unique constraints on the preservation of early-formed reservoirs in Earth's mantle. We present the largest high-precision μ 142 Nd dataset for a single hotspot, comprising 55 analyses of 29 shield-stage tholeiitic basalts from the Hawaiian Islands. Samples span the full geochemical range of the Loa and Kea trends and were analyzed using multiple chemical purification procedures and thermal ionization mass spectrometry to assess analytical reproducibility. New μ 142 Nd measurements are complemented by ε 138 Ce and ε 143 Nd data. The Hawaiian samples define the expected mantle array in ε 138 Ce–ε 143 Nd space and confirm the isotopic distinction between the Loa and Kea trends. Kea-trend geochemical samples with the highest ε 143 Nd values (+7.5) have the smallest recycled crust contribution into their mantle source and are therefore very good targets to search for the presence of early-formed reservoirs in Earth's mantle. Our new μ 142 Nd measurements show no significant variability (weighted mean μ 142 Nd=0.48 ± 0.27), with values indistinguishable from both the JNdi-1 reference material and the average composition of global ocean island basalts. Combined constraints from the 146,147 Sm– 142,143 Nd systems indicate that the depleted source reservoir sampled by some Kea lavas formed later than ∼400 Ma after Solar System formation. More generally, our new μ 142 Nd dataset does not provide compelling evidence for the existence of large-scale deep mantle domains that retain primordial Sm–Nd isotopic heterogeneities in this area, which is characterized by anomalous zones with low seismic velocity. This result contrasts with reported variations measured in 182 W, 40 K and 100 Ru in some lavas from the Hawaiian Islands. Without a more comprehensive dataset of measurements taken on the same samples, it is difficult to interpret these isotopic signatures and link them to specific reservoirs.