Abstract
This paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical ‘anomaly.’ Typical observed anomalies are high electrical conductivity of the order of 0.1 S/m or greater, velocity decreases of 7–10%, seismic Q values less than 100, and a frequency band for seismic effects in the region near 1 Hz. Existing theories of electrical conduction in partial melts and of frequency‐dependent seismic properties together with recent measurements of melt electrical conductivity, viscosity, and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods. From electrical anomalies, mainly sensitive to melt volume and its interconnection, one can require a minimum melt fraction of several percent at temperatures close to the solidus (1150°–1300°C). However, seismic models demand only a small volume in very flattened shapes (aspect ratio ⋍0.001, melt fraction ∼0.1%). Further, if melt configuration permits seismic dissipation in bulk, that is, there exist flattened voids intersecting more or less equant voids, then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies. Observed equilibrium textures of partly melted peridotite together with inferred melt‐solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern. Thus if partial melt causes mantle geophysical anomalies, it should exist in a variety of void shapes and probably of sizes. While the association of electrical and elastic anomalies with indications of reduced density, volcanism, and high heat flow makes the hypothesis of partial melting an attractive explanation, the minimum physical requirement is for existence of relatively high temperature.
Published Version
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