Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

Abstract

[1] We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of relatively recent thermal perturbation and compositional modification associated with emplacement of kimberlitic fluids into the deep tectospheric root. Seismic velocities and densities for cratonic xenoliths differ significantly from those predicted for both primitive mantle peridotite and mantle eclogite. A model primitive mantle under cratonic P-T conditions exhibits velocities about 1% lower for P and about 1.5% lower for S, a consequence of a more fertile composition and different modal composition. Primitive mantle is also about 2% more dense at 150-km depth than low-T garnet lherzolite at cratonic P-T conditions. Similar calculations based on an oceanic geotherm are consistent with the isopycnic hypothesis of comparable density columns beneath oceanic and cratonic regions. Calculations for a hypothetical ''cratonic'' eclogite (50:50 garnet/omphacite) with an assumed cratonic geotherm produce extremely high V P and V S (8.68 km/s and 4.84 km/s, respectively, at 150 km depth) as well as high density ($3.54 gm/cc). The very high velocity of eclogite sh...

Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds

“…Density increase with depth is exhibited in the data of Kelly et al (2003), from a suite of xenoliths from the Kaapvaal craton discussed in Boyd et al (1999), going from low-T spinel (<100 km) to low-T garnet (100-180 km) to high-T garnet (>180 km) peridotites. The formulae of James et al (2004), which are very close to those of Li and Liebermann (2007), are also questionable as they are not pressure dependent.…”

“…The assumptions implicit in our implementation of these formulae are discussed in James et al (2004), and are:…”

“…Some authors, (e.g., James et al, 2004), specify their results in terms of the Fe number, which is (1. -Mg#/100).…”

“…Goes et al's (2000) formulations yield virtually the same moduli as those in Isaak et al (2006) for an Mg# of around 90, but include the pressure dependence for the bulk modulus derived by Nestola et al (2005). The formulae we use for deriving the moduli and density are those in James et al (2004), viz.,…”

“…There is clearly a wide range of reported values for the moduli, and their dependence on Mg#, temperature and pressure. For the purposes of our study, we take the formulae of James et al (2004) for olivine, orthopyroxene and garnet, and the formulae of Goes et al (2000) for clinopyroxene. Goes et al's (2000) formulations yield virtually the same moduli as those in Isaak et al (2006) for an Mg# of around 90, but include the pressure dependence for the bulk modulus derived by Nestola et al (2005).…”