2002 Fall AGU Abstract

Bina, C. R., Phase transition complexity and multiple seismic reflectors in subduction zones, Eos, Transactions of the American Geophysical Union, 83, Fall Meeting Supplement, S52C-05, 2002.

S52C-05 (Invited)

Patterns of seismic wavespeeds within subduction zones are significantly affected by the laterally varying phase relations associated with slab thermal structures, especially in the upper mantle where equilibrium phase changes contribute fine structure to the overall fast anomalies expected from thermal effects alone. Such lateral variations in the properties of associated seismic reflectors contribute to non-uniformity in the visibility of and detectable topography on seismic ``discontinuities'' in the transition zone.

The 410-km seismic discontinuity is usually attributed largely to the alpha -> beta phase transition in (Mg,Fe)2SiO4 olivine, more accurately written as the alpha -> alpha + beta -> beta reaction series. The apparent sharpness of and apparent topography on the associated seismic wavespeed feature have been subjects of debate, especially in regions near subduction zones.

I argue that some of the apparent discrepancies arise from the common oversimplification of representing the relevant phase relations in the vicinity of subducting slabs simply as an alpha -> beta transition. The low temperatures of subduction zones cause the equilibrium olivine phase relations within slabs to shift from the alpha -> alpha + beta -> beta reaction series to the more complex alpha -> alpha + gamma -> alpha + beta -> beta reaction series. Even lower temperatures in slab interiors cause the entire alpha -> alpha + beta -> beta -> beta + gamma -> gamma reaction series to give way to an alpha -> alpha + gamma -> beta + gamma -> gamma reaction series.

The more complex reaction series within cold subduction zones yields bifurcations in seismic wavespeed contrasts. The alpha -> alpha + gamma onset of the reaction series is strongly uplifted to shallower depths, but this transition is relatively broad. The alpha + gamma -> alpha + beta and colder alpha + gamma -> beta + gamma reactions, on the other hand, are univariant and therefore very sharp, but these transitions are not as strongly uplifted. The resulting seismic wavespeed signature to be expected above 410 km in a subduction zone consists of a strongly uplifted diffuse contrast overlying a weakly uplifted sharp contrast. Indeed, the univariant portions of the olivine reaction series in a slab may potentially give rise to the brightest seismic reflectors in the upper mantle.

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