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Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz–muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β -quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β -quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in particular regions of elevated temperature that are only weakly deforming, with coupled anomalously low bulk velocities and seismic anisotropy. [ABSTRACT FROM AUTHOR]

We present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the Cretaceous deep lower crust (∼40–65 km) of Fiordland, New Zealand. Both rock types show a distinct foliation defined by cm-scale compositional banding. Seismic properties are estimated using the Asymptotic Expansion Homogenisation – Finite Element (AEH-FE) method that, unlike the commonly used Voigt–Reuss–Hill homogenisation, incorporates the phase boundary network into calculations. The predicted mean P- and S-wave velocities are consistent with previously published data for similar lithologies from other locations (e.g., Kohistan Arc), although we find higher than expected anisotropies ( A V P ∼ 5.0–8.0%, A V S ∼ 3.0–6.5%) and substantial S-wave splitting along foliation planes in granulites. This seismic signature of granulites results from a density and elasticity contrast between cm-scale pyroxene ± garnet stringers and plagioclase matrix rather than from crystallographic orientations alone. Banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small. The origin of compositional banding in Fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs. Hence, we identify a new potential source of anisotropy within this geotectonic setting. [ABSTRACT FROM AUTHOR]

Abstract: The Namib Sand Sea on the west coast of Namibia is one of the world's oldest desert region and based on cosmogenic dating it has likely existed since the earlier Pleistocene. Among the possible sand sources, geomorphological and petrographic evidence points towards the Orange River catchment as the most prominent one. Little is known about the dynamics of transport and mixing of the sand during the desert formation and this is because the information about the Namib Sand Sea generally rests upon study sites at its edges. Here, we present a statistical analysis of magnetic components in sand samples collected along a south to north transect through the desert and at two inland sites. The magnetic components are rock fragments mainly of basaltic origin. Their statistically uniform distribution in the Namib Sand Sea indicates no significant sand source other than the Orange River and thus a predominant northward direction of the sand transport. A northward transport and the absence of a magnetic trend along the transect suggests mixing of the sand prior to its deposition in the Namib Sand Sea, most likely during river transport and under high current conditions along the shoreline. Finally, the uniform magnetic pattern provides compelling evidence for a stable erosion regime in the Orange River catchment with a steady release of magnetic components at least since the Pleistocene. [Copyright &y& Elsevier]