Serpentine: the Vaseline of subduction

Although they are seismically precarious, the major coastal cities of the Americas and East Asia that lie close to destructive plate margins probably owe their survival to a greasy assemblage of hydrated ultramafic minerals – serpentine, talc and magnesium hydroxide (brucite).  Detailed tomographic images using the records of natural earthquakes along the subduction zone beneath western North America show a zone of exceptionally reduced S-wave speeds at the “corner” formed by the subducted slab and the base of the crust (Bostock, M.G. et al. 2002.  Inverted continental Moho and serpentinization of the forearc mantle.  Nature, v. 417, p. 536-538).  This low-speed zone coincides with the fore-arc region of the destructive margin, roughly along the coast.  Normally the Moho marks a sudden increase in wave speed in the mantle underlying the crust, but here the situation is reversed (inverted).  The best explanation is that S-wave speed slows because of an abundance of weak rock, between 35 and 60 km down.  The likely candidate is mantle peridotite that has become hydrated by fluids seeping upwards from cold, wet oceanic lithosphere as it begins to be subducted.  Low-temperature, high-pressure metamorphism of hydrothermally altered basaltic crust begins to transform it to anhydrous eclogite, so releasing masses of water vapour.  It is this fluid release that is implicated in the generation of magmas beneath volcanic arcs, because it reduces the beginning-of-melting temperature in the overriding mantle wedge.  However, such partial melting is possible only when temperature is high.  In the cooler, shallow regions of the fore arc rising watery fluids serve to convert peridotite to hydrous minerals, especially serpentine.  One outcome is the creation of anomalously low-density mantle, which bulges upwards to create fore-arc ridges at some destructive margins, even squirting serpentinite upwards in bizarre mud volcanoes.  Yet all hydrated, ultramafic minerals are natural lubricants, and would act to ease sudden rupture along the subduction zone, thereby preventing extremely high-magnitude earthquakes whose surface effects would be devastating.

See also: Zandt, G. 2002.  The slippery slope.  Nature, v. 417, p. 497-498

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