Re–Os, Sm–Nd isotope- and REE systematics on ultramafic rocks and pillow basalts from the Earth's oldest oceanic crustal fragments (Isua Supracrustal Belt and Ujaragssuit nunât area, W Greenland)

Abstract

Ultramafic rocks and associated pillow basalts from the western–northwestern sector of the Isua Supracrustal Belt (ISB, West Greenland) reveal a complex pattern of trace elements, mainly with respect to their rare earth elements (REE). The heterogeneity of LREE patterns, in combination with widely scattering εNd[ T=3.81 Ga] values between −3.7 and +12.4, indicate that the REE systematics of the samples were disturbed by one (or more) metamorphic events which took place some considerable time after the original crystallization of the Isua belt. Furthermore, LREE-rich metasomatic fluids associated with the emplacement of tonalites into the supracrustals may have significantly altered the primary trace element budget. Extreme variations in whole rock γOs[ T=3.81 Ga] values of these rocks ranging from −8500 to +9 are favorably interpreted to derive from Re-additions during fluid hydrothermal alterations accompanying the early and late Archean metamorphic events. In contrast, the Re–Os isotope system remained intact in ultramafic-hosted chromites from Isua and from the Ujaragssuit nunât area about 20 km SE of the ISB. An average γOs[ T=3.81 Ga] value of +1.3 ±2.5 (2 σ) for the chromite population covers the entire range defined by the chondritic average, the convecting primitive upper mantle (PUM) [Nature 383 (1996) 517; Geochim. Cosmochim. Acta 65 (2001) 1311] and models involving outer core entrainment at this time. Given the uncertainty in the origin of the ultramafic rocks and, thus, the chromites, and the very high degree of alteration of these rocks within the ISB, our data do not conclusively allow to further impinge on the petrogenesis of the ISB mantle-derived rocks. Our data are the first though to vaguely hint at an early Archean mantle with a possible slightly suprachondritic initial 187Os/ 188Os isotopic composition. Similarities of unaltered pillow basalts with Phanerozoic boninites [Chem. Geol. 184 (2002) 231] may indicate a formation in an intra-oceanic subduction zone like geodynamic process, in which extreme early (Hadean?) depletion of a mantle source by melt extraction was followed by a second stage of melting induced by enriched subduction components. Alternatively, core–mantle interaction as a plausible mechanism for possible weak 187Os enrichments in these rocks remains a valid hypothesis.