Reconciling mantle attenuation-temperature relationships from seismology, petrology, and laboratory measurements

Abstract

Seismic attenuation measurements provide a powerful tool for sampling mantle properties.Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry meltfree rocks, but require $10 2 210 3 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q 21 S ), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q S $ 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 C. The Q S measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q S too high by a factor of 1.5-5. Modifying models to include effects of H 2 O and rheologydependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H 2 O effects are modest. Therefore, high H 2 O in the arc source region cannot explain the low Q S , nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q S can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

“…The discrepancy may partly arise from differences in lateral resolution between our body wave data and laterally averaged surface wave attenuation, but the faster-spreading EPR may also be less attenuating overall than the JdF. The lowest previously observed values of mantle Q S < 25 were imaged at the Lau back-arc ( 26 ). …”

“…Incorporating mineral physics arguments that scale diffusion creep rates to attenuation ( 17 , 26 ), the values of Q S in our model correspond to at least a ~150 × viscosity reduction between depths of 50 and 150 km (Table 1). Although this quantitative estimate is linked to several assumptions in our model, a qualitative weakening effect of partial melt at the grain boundaries is well established for crystalline aggregates ( 40 ).…”

“…( 26 ). Assuming all viscous and viscoelastic processes scale similarly ( 26 , 51 ), the effects of melt, water, and grain size were incorporated by scaling relaxation time scales for all processes (τ) involved in the anelasticity calculation$\mathrm{normal\tau }={\mathit{x}}_{\mathrm{\phi }}{true(\frac{{\mathit{C}}_{\mathrm{italicO}\mathrm{italicH}}}{{\mathit{C}}_{\mathrm{italicO}{\mathit{H}}_{\mathit{R}}}}true)}^{\mathit{r}}{\mathit{d}}^{\mathit{m}}{\mathrm{\tau }}_{0}false(\mathrm{italicP},\mathrm{italicT}false)$where r = 1.0, m = 3, and τ 0 ( P , T ) is a simple Arrhenius function ( 16 ). The fits of Faul and Jackson ( 30 ) have been superseded in the mineral physics literature by Jackson and Faul ( 16 ), but the later calibrations provided markedly worse agreement to seismological observations, when extrapolated to upper mantle conditions.…”

“…Grain size, d , was computed using the paleowattmeter ( 53 ), augmenting grain growth parameters to account for water concentration ( 26 , 54 ). We computed strain rates assuming that a half-spreading rate of 30 mm/year is accommodated over a 100-km-thick layer, giving $\stackrel{.}{\mathrm{ϵ}}\approx {10}^{-14}{\mathrm{s}}^{-1}$.…”

Mafic High‐Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

“…These attenuation predictions are based on the calibrations ofJackson and Faul (2010) modified to include the effects of H 2 O as defects in olivine and of variable grain size, followingAbers et al (2014). These attenuation predictions are based on the calibrations ofJackson and Faul (2010) modified to include the effects of H 2 O as defects in olivine and of variable grain size, followingAbers et al (2014).…”