Adbanced mineralogy, vol. 2: Methods and instrumentations: Results and recent developments

Abstract

We present the first combined 146,147Sm–142,143Nd and 176Lu–176Hf 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. 142Nd measurements confirm and extend the database of 142Nd excesses in Isua samples (μ142Nd = 7–16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined 146,147Sm–142,143Nd systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of ~ 4.53, ~ 4.47, and ~ 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 142Nd results and consistent with a superchondritic initial 143Nd/144Nd ratio (ε143Nd3.7Ga = + 1.41 ± 0.98). In contrast, the corresponding initial ε176Hf3.7Ga = − 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 147Sm–143Nd, 176Lu–176Hf, and 142Nd 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 ~ 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic 176Hf/177Hf (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.