Temporal evolution of 142Nd signatures in SW Greenland – new insights from high precision MC-ICP-MS measurements

The short-lived 146Sm-142Nd decay system is a powerful tool to investigate early geodynamic processes. Here, we introduce a new analytical protocol that allows measuring 142Nd/144Nd isotope compositions at the lowest ppm-level by MC ICP-MS. This new method is applied to various Archean to Proterozoic rock types (TTGs, boninites, tholeiites, dikes) from the Isua region, SW Greenland. While our results are consistent with previously reported elevated 142Nd/144Nd compositions[1,2], our study is the first to statistically resolve small 142Nd/144Nd isotope differences between rocks from the 3.7 Ga and the 3.8 Ga units of the Isua supracrustal belt. Further, we generally observe a strong decrease of 142Nd anomalies between 3.8 Ga and 3.4 Ga, likely linked to the progressive admixture of unfractionated bulk silicate Earth material that progressively replenished the mantle beneath the Isua region. Such a geodynamic model is also in accord with observational constraints from trace elements and long- and short-lived radiogenic isotopes. The recurrence of elevated 142Nd/144Nd compositions in younger Mesoarchean rocks (~2.97-3.08 Ga) and one Proterozoic dike (~2.0 Ga) can be explained by mantle sources that were influenced by older Eoarchean recycled components or crustal contamination, respectively. This is also consistent with unradiogenic initial ɛ176Hf and ɛ143Nd values and enriched incompatible trace element patterns that have been reported for some of these rocks[3].

[1] Caro et al. (2006), GCA, 70, 164-191. [2] Saji et al. (2018), GPL, 7, 43-48. [3] Szilas et al. (2015), MM, 79, 857-876.