Archean deep melting and post-Archean metasomatism of the cratonic mantle: A garnet Lu-Hf isotope record

Abstract

Mantle xenoliths provide a direct window into the subcontinental lithospheric mantle (SCLM) beneath Archean cratons. Age constraints on the melting, metasomatism and deformation that have affected the SCLM are crucial for developing insight into continental dynamics and craton development. Obtaining constraints is nevertheless difficult. Garnet geochronology – and Lu-Hf geochronology in particular – has good prospects in this regard, because it uniquely allows dating of a rock forming mineral and petrogenetic indicator in the mantle. However, the behavior of the Lu-Hf system at mantle conditions is not yet well-understood, and stringent sample size requirements and issues relating to melt infiltration typically impede obtaining reliable garnet ages for single xenoliths. In this study, we tested the capabilities of modern Lu-Hf methods to date garnet in kimberlite-hosted mantle xenoliths that record mantle melting, metasomatism and shearing. The samples are from the Siberian, Kaapvaal and Slave cratons, and include garnet-bearing olivine pyroxenites, lherzolites and websterites, as well as orthopyroxenite with exsolved garnet, and include both sheared and granular lithologies. The analyses yielded low-dispersion Lu-Hf isochrons for single xenoliths with age uncertainties down to 0.3 % and ages up to 3.0 Ga. Garnet in equilibrium with clinopyroxene in coarse lherzolite and websterite samples records Proterozoic ages, whereas high-Cr garnet from coarse harzburgite and orthopyroxenite samples lacking clinopyroxene yields Archean ages. The age data do not reflect ‘normal’ volume diffusion-controlled chronometric closure but instead indicate strong age retention barring events of fluid or melt infiltration. Only sheared samples yielded Lu-Hf ages close to the age of kimberlite eruption. These ages highlight that deformation and metasomatism are ongoing within otherwise stable cratonic roots. Initial Hf isotope compositions obtained from the Lu-Hf isochrons approximate chondritic values in the Archean and become gradually more superchondritic with time. These results indicate that the cratonic mantle, as well as the melts that metasomatized it since the Archean, derive from a moderately depleted mantle reservoir that has existed since at least 3 Ga. Together, the new Lu-Hf data provide a new temporal framework for the petrological development of the SCLM and establish Lu-Hf garnet geochronology as prime geochronometer for dating mantle processes.