Combined Nd and Hf isotope evidence for deep-seated source of Isua lavas
Résumé
We present the first combined Sm-146.147-Nd-142,Nd-143 and Lu-176-Hf-176 study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm-Nd and Lu-Hf whole-rock isochrons yield identical ages within error: 3.72 +/- 0.08 and 3.67 +/- 0.07 Ga, respectively. Nd-142 measurements confirm and extend the database of Nd-142 excesses in Isua samples (mu Nd-142 = 7-16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined Sm-146,Sm-147-Nd-142,Nd-143 systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of similar to 4.53, similar to 4.47, and similar to 4.32 Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm-Nd whole-rock isochron is in accordance with Nd-142 results and consistent with a superchondritic initial Nd-143/Nd-144 ratio (epsilon Nd-143(3.7Ga) = +1.41 +/- 0.98). In contrast, the corresponding initial epsilon Hf-176(3.7Ga) = -1.41 +/- 0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm-Nd and Lu-Hf isotope systems display incongruent relative parent-daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies Sm-147-Nd-143, Lu-176-Hf-176, and Nd-142 results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53-4.32 Ga) deep mantle cumulate. Since the similar to 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic Hf-176/Hf-177 (Scherer et al., 2010), dissociation of the Lu-Hf and Sm-Nd isotope systems may not be a ubiquitous feature of the Hadean mantle, but rather reflects that a deep-seated source was involved in the formation of ISB lavas