PICRITE EVIDENCE FOR WIDESPREAD FE-RICH HETEROGENEITIES IN THE ARCHEAN MANTLE

The high iron content (FeO^TOT>14 wt. %) of the global suite of Neoarchean ferropicritic lavas and plutonic rocks cannot be produced by the melting of the “pyrolitic” (Mg # =0.88-0.92) peridotite that is thought to make up the bulk of the Earth’s mantle. Although most would agree that the Mg #’s of their mantle sources are low (< 0.85), possibly approaching those of the Martian mantle or ordinary chondrites (>0.75), the composition and mineralogy of ferropicrite sources remains poorly constrained. Olivine and garnet do not coexist on the liquidii of ferropicrite liquids under any pressure, fuelling a debate as to whether the mantle sources of ferropicrite are garnet-free peridotite or olivine-free garnet-pyroxenite.

Two types of Neoarchean ferropicrites are recognized on the basis of trace element profiles, Ni contents, and Sc/Fe ratios on six Archean cratons (Superior, Slave, W. Churchill, Yilgarn, Kaapvaal, Baltic). Alkaline ferropicrites have fractionated trace element profiles (Nb/Y_MORB ≥3), resembling those of modern ocean island basalts (OIB), and have low Sc/Fe ratios (<3 x 10^-4). Their Ni contents are higher than those of the comparably magnesian primary melts of pyrolitic mantle and are similar to the Ni-rich tholeiitic basalts from Hawaii. In contrast, subalkaline ferropicrites have less fractionated trace element profiles, and are characterized by low Nb/Y_MORB ratios (<3), and relatively high Sc/Fe ratios (>3 x 10^-4). The Ni contents of the subalkaline ferropicrites are low relative to the melts of normal terrestrial mantle at equivalent MgO concentrations.

The Sc content of melts is strongly controlled by the residual mineralogy of the source (D^Sc_ga~1-6; D^Sc_ol <0.5), and the Sc/Fe ratio is an effective discriminant between olivine and garnet-dominated sources. The low Sc/Fe ratios and Ni contents of the alkaline ferropicrites overlap those of the Hawaiian tholeiitic basalts, and suggest derivation from secondary garnet-pyroxenite sources. In contrast, the high Sc/Fe ratios and low Ni contents of the subalkaline ferropicrites are more consistent with the melting of garnet-free peridotite. The widespread association of ferropicrites with more ubiquitous Fe-poor (ultra)mafic rocks indicates a heterogeneous Neoarchean mantle, containing domains of Fe-rich peridotite and garnet-pyroxenite.