A sporadic low-velocity layer atop the western U.S. mantle transition zone and short-wavelength variations in transition zone discontinuities

B Schmandt,S M Hansen,K G Dueker,Z Zhang,J J Jasbinsek

doi:10.1029/2011gc003668

Abstract

Teleseismic receiver function analysis of data from six dense arrays in the western U.S. is used to investigate mantle transition zone (MTZ) discontinuities and the prevalence of a low-velocity layer atop the 410 km discontinuity (410-LVL). Negative polarity Ps arrivals indicative of a low-velocity layer with a top 25–60 km above the 410 are identified in 8–11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors. The 410-LVL is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410. The 669 km mean depth of the 660 km discontinuity (660) and the magnitude of 660 topography suggest variable hydration, locally approaching saturation, in addition to >150 K lateral temperature variations beneath five arrays. Mean amplitudes of P410s and P660s increase monotonically with period from 2 to 10 s; however, greater variations are observed in the frequency dependence of P410s compared to P660s implying 410 thickness is more heterogeneous. Variable 410 thickness is attributed to changes in hydration modulating the width of the olivine-to-wadsleyite transition interval. Frequency dependence of P660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems. Sporadic detection of the 410-LVL, the magnitude and length scales of MTZ discontinuity topography, and inferred variations in hydration support the occurrence of vigorous small-scale convection in the western U.S. mantle. Comparison of receiver functions with body wave tomography suggests small-scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410-LVL.

Highlights

Comparison of receiver functions with body wave tomography suggests small‐scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410‐LVL
Prior results support the existence of additional smaller velocity discontinuities associated with olivine and nonolivine phase transformations [Shearer, 1990; Revenaugh and Jordan, 1991; Simmons and Gurrola, 2000; Lawrence and Shearer, 2006a], are consistent with hydration of nominally anhydrous minerals at mantle transition zone (MTZ) depths [van der Meijde et al, 2003], and sporadically detect a negative velocity gradient 20–80 km above the MTZ that is frequently attributed to partial melt [Revenaugh and Sipkin, 1994; Song et al, 2004; Jasbinsek and Dueker, 2007; Vinnik and Farra, 2007; Courtier and Revenaugh, 2006, 2007; Jasbinsek et al, 2010; Vinnik et al, 2010; Schaeffer and Bostock, 2010; Tauzin et al, 2010]
Improved resolution of these more complex velocity discontinuities and their lateral variations is potentially diagnostic of fundamental characteristics of mantle dynamics including: prevalence and length scales of mass transfer across the MTZ, mantle hydration levels and abundances of minerals other than olivine, and the physical and chemical processes stimulated by cold slab or hot plume fluxes across the MTZ

Summary

Schmandt

[1] Teleseismic receiver function analysis of data from six dense arrays in the western U.S is used to investigate mantle transition zone (MTZ) discontinuities and the prevalence of a low‐velocity layer atop the 410 km discontinuity (410‐LVL). The 669 km mean depth of the 660 km discontinuity (660) and the magnitude of 660 topography suggest variable hydration, locally approaching saturation, in addition to >150 K lateral temperature variations beneath five arrays. Sporadic detection of the 410‐LVL, the magnitude and length scales of MTZ discontinuity topography, and inferred variations in hydration support the occurrence of vigorous small‐scale convection in the western U.S mantle. Zhang (2011), A sporadic low‐velocity layer atop the western U.S mantle transition zone and short‐wavelength variations in transition zone discontinuities, Geochem.

Introduction

Data and Methods

Results

Discussion

Thickness and Heterogeneity of the 410

Conclusions