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A unified continental thickness from seismology and diamonds suggests a melt-defined plate

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Science  11 Aug 2017:
Vol. 357, Issue 6351, pp. 580-583
DOI: 10.1126/science.aan0741

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A coherent depth for continental plates

The thickness of the continental portion of Earth's cold and rigid surface plates is a source of debate. Tharimena et al. analyzed a specific type of seismic signal called SS precursors to provide a robust estimate of plate thickness under the continents (see the Perspective by Savage). The values range from 130 to 190 km, which lines up well with the depth where diamonds are stable—an independent line of evidence for the depth of continents.

Science, this issue p. 580; see also p. 549

Abstract

Thick, rigid continents move over the weaker underlying mantle, although geophysical and geochemical constraints on the exact thickness and defining mechanism of the continental plates are widely discrepant. Xenoliths suggest a chemical continental lithosphere ~175 kilometers thick, whereas seismic tomography supports a much thicker root (>250 kilometers) and a gradual lithosphere-asthenosphere transition, consistent with a thermal definition. We modeled SS precursor waveforms from continental interiors and found a 7 to 9% velocity drop at depths of 130 to 190 kilometers. The discontinuity depth is well correlated with the origin depths of diamond-bearing xenoliths and corresponds to the transition from coarse to deformed xenoliths. At this depth, the xenolith-derived geotherm also intersects the carbonate-silicate solidus, suggesting that partial melt defines the plate boundaries beneath the continental interior.

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