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

• Published in: Chemical Geology
• Main Authors: Hasenstab-Dübeler, Eric, Tusch, Jonas, Hoffmann, J. Elis, Fischer-Gödde, Mario, Szilas, Kristoffer, Münker, Carsten
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Archean; Hadean differentiation; Isua supracrustal belt: 142Nd; Mantle heterogeneity; MC ICP-MS; Ameralik; Greenland
• Online Access: https://curis.ku.dk/portal/da/publications/temporal-evolution-of-142nd-signatures-in-sw-greenland-from-high-precision-mcicpms-measurements(7d630ece-8793-434b-996f-b1b01f3a967b).html; https://doi.org/10.1016/j.chemgeo.2022.121141; https://curis.ku.dk/ws/files/332118363/Temporal_evolution.pdf

Measurements of 142 Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143 Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ 142 Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143 Nd/ 144 Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390 −0.060 +0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142 Nd/ 144 Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ 142 Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ 142 Nd = +9.8 ± 1.0). Even lower but homogeneous μ 142 Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε 143 Nd (i) and εHf (i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ 142 Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147 Sm[sbnd] 143 Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules ...