Picrite evidence for more Fe in Archean mantle reservoirs

Abstract

Recently discovered Archean amphibolites at the Lake of the Enemy, N.T., Canada, have alkaline ferropicrite compositions (MgO ∼14 wt%) with fractionated REE profiles (La/Yb = 10–15), high Nb/La ratios (1–2), and an average ε Nd value of +1.1 at 2.66 Ga, suggestive of a mantle source that was slightly enriched relative to model `MORB' reservoirs. The Fe-rich nature of these ferropicrites, together with other Archean ferropicrites and komatiites, indicates that the Archean equivalents of `OIB' primitive magmas were up to 30% richer in Fe than today. The similar Mg content of Phanerozoic alkaline picrites argues against higher mantle potential temperatures being responsible for the higher Fe content of the Lake of the Enemy ferropicrites, and low alkalis do not support an explanation involving the effects of alkalis or oxidation state on Fe–Mg partitioning. The simplest explanation is that the mantle sources of some Archean hotspot magmas had more Fe than those of modern equivalents. The relatively Fe-rich character of Archean komatiitic lavas, on the other hand, may reflect higher temperatures, and thus depths of initiation of melting, in the Archean. This interpretation requires, however, an absence of low-pressure (<3.0 GPa) primary melts in the Archean. Alternatively, the Fe-rich character of komatiitic magmas may also reflect Archean mantle sources that contained more Fe than pyrolite models (Mg# ∼0.89). Proterozoic picritic lavas resemble their Archean equivalents, while most Mesozoic picritic lavas are indistinguishable from those of today. These data indicate that the Fe content of the Earth's mantle may have decreased over time, and requires either a sink for Fe in the core or the isolation of Fe-rich material in the lower mantle.