Temporal and spatial trends in mantle metasomatism and redox state beneath the Kimberley Craton (Northwest Australia)

Abstract

The Kimberley Craton of northwest Australia hosts a suite of kimberlites, lamproites and ultramafic lamprophyres that range in age from ∼1257 (Argyle) to ∼20 Ma (Ellendale). Mantle xenocrysts and xenoliths entrained in these rocks provide the opportunity to examine how the composition and structure of the cratonic lithospheric mantle has evolved through time. In this study we present the first oxygen fugacity ( f O2) profile through the cratonic lithospheric mantle beneath the Ellendale and Argyle lamproite pipes. The f O2 estimates are based on the atomic Fe3+/ΣFe in garnet xenocrysts determined by the flank method accompanied by geothermobarometric and trace element data. The f O2 values indicate a reduced lithosphere beneath the Argyle and Ellendale pipes, within the range of values found for other diamondiferous cratonic lithosphere, but with a systematic trend to more oxidized conditions (by up to 5 log units in f O2) toward the base of the lithosphere. This trend is accompanied by increases in TiO2, FeOT, LREE (light rare-earth elements) and in V/Sc, Sm/Er and Zr/Y values with increasing temperature (T). Garnet xenocrysts with higher Fe3+/ΣFe from the base of the lithosphere have sinusoidal REE patterns in contrast to the mostly LREE-depleted patterns of garnets at shallower levels in the lithosphere. The oxidation and enrichment of the deeper lithosphere is the result of metasomatism by infiltrating hydrous, possibly carbonate-bearing, silicate melts. At Argyle, the base of the lithosphere was also infiltrated by hydrous fluids that formed abundant Proterozoic eclogitic diamonds with subduction signatures. The composition, thickness and redox state of the lithosphere beneath the Argyle and Ellendale lamproites is similar apart from the base of the Argyle lithosphere being more oxidized and minor differences in the exact values for some trace element ratios (i.e., Ti/Eu, Zr/Y, and Sm/Yb). Existing clinopyroxene paleogeotherms indicate cool conductive paleogeothermal gradients with lithospheric thicknesses similar to those of seismic tomographic models. These results suggest that the Kimberley Craton has remained relatively stable since at least the Mesoproterozoic, despite the emplacement of lamproites, kimberlites and ultramafic lamprophyres at ∼840–800 Ma and eruption of continental flood basalts of the Kalkarindji large igneous province (LIP) at 511 Ma. The long-term stability of the lithosphere of the Kimberley Craton has allowed the survival of diamonds of differing ages and the preservation of ancient, enriched mantle domains that gave rise to the Miocene lamproites with extremely enriched Sr-Nd-Hf isotopes that formed by prolonged unperturbed radiogenic ingrowth since the Paleoproterozoic. Show more