Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

We examine the modes and compositions of garnet-bearing peridotite xenoliths from the Kaapvaal Craton to quantify factors governing density and seismic velocity variations within metasomatically altered cratonic mantle. Three distinct compositional trends are resolved by principal component analysis. The first reflects differences in residue composition resulting from partial melting. The second is associated with orthopyroxene (opx) enrichment, possibly due to silica addition by subduction zone fluids in the source region of the xenoliths. The third principal component reflects garnet and clinopyroxene enrichment possibly as a consequence of melt infiltration. More than half of the mineral mode variance among Kaapvaal Craton xenoliths can be accounted for by opx enrichment. Melt depletion effects can account for as much as 30% of the variance, while less than 20% of the variance is associated garnet and clinopyroxene enrichment. Using the parameterization of Schutt and Lesher (2006) we show that at cratonic mantle temperatures and pressures, orthopyroxene enrichment results in little change in bulk density (ρbulk) and shear-wave velocity (VS), but decreases compressional wave velocities (VP) and VP/VS. In contrast, melt depletion has little effect on VP, but leads to an increase in VS and a decrease in ρbulk and VP/VS. Garnet (gt) and clinopyroxene (cpx) enrichment cause an increase in ρbulk, VP, VS, and VP/VS. The isolation of the major contributions to xenolith compositional variations among the Kaapvaal Craton suite helps to reconcile seeming discrepancies among previous studies that estimate the effects of composition on mantle density and seismic velocity based on either xenolith or laboratory data. Subtle variations in these properties with the pressure of equilibration may explain some of the fine scale structure in seismic tomograms for cratonic mantle, but overall differences among peridotite lithologies can only account for at most a 2–3% change in seismic velocities. Larger changes in lithospheric seismic properties are more likely associated with temperature differences or presence of mafic or hydrated lithologies. We also find that the density effects of melt depletion are insufficient to produce cratonic mantle that is neutrally buoyant with respect to fertile convecting mantle.

Research Highlights
► Three compositional trends are found.
► Opx enrichment reduces VP, but has little effect on density and VS.
► Melt depletion increases VS.
► Gt and cpx enrichment increases VP, VS, and density.
► Composition cannot account for more than 2–3% in velocity.