ALBERT

Description: The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, high-frequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core–mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m b from 3.2 to 6.0 recorded at epicentral distances of 119.3° to 138.8° from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKP df arriving 3–15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F -statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.

Description: Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multianvil apparatus at 6 GPa and 1400°C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivine + clinopyroxene + garnet ± orthopyroxene). The multiply saturated hydrous melts have higher (Mg + Fe)/Si and Al/Ca compared with hydrous melts produced at lower pressures. The melt compositions are similar to those determined near the dry peridotite solidus at ~1700°C, when compared on an H 2 O-free basis. Melt H 2 O contents were determined to be ~11 wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (〉70 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H 2 O-rich melt then at the base of this zone, at ~220 km, these results imply that the melt must contain 15–16 wt % H 2 O. The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na 2 O but enriched K 2 O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K 2 O and H 2 O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K 2 O and H 2 O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of ~0·2 wt % melting of a garnet peridotite source rock enriched with 1·7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.