Joint seismic and mineral physics analysis of upper mantle temperature and composition

Jeroen RitsemaUniversity of Michigan

Wenbo Xu and Xin Wan, University of MichiganPaul Cupillard, IPG Paris & UC BerkeleyBenoit Tauzin, IPG Strasbourg Lars Stixrude and Carolina Lithgow-Bertelloni, University College London

Heterogeneity in the deep mantle

Gibbs or blobs?

Interpreting the interpretations

REVELATION  20:10“NOBODY PARTIES IN HELL”

From the Garnero Galleries

HellUnited Christian Bikers .com

(websitepointedout byStephenStackhouse)

K.E. Bullen

§6.1.2 Inferences on the Earth’s interiorThe nether regions of the Earth are inaccessible in the ordinary sense. Before the time of Newton, when evidence about them was nearly totally lacking, it was not necessarily unreasonable to describe the Earth in terms of models invoking say a Hell, or a subterranean monster shaking itself to cause earthquakes. The subsequent growth of evidence has lowered the plausibility of such models.

LLSVP

S wave data and large-scale mantle heterogeneity

Path segments and reflection points of S and SS waves,coloured red when S or SS is slow and blue when S or SS is fast;

Seismic tomography: blurred and limited vision

Simons et al. EOS, 2006 Thurber and Ritsema, Treatise in Geophysics, 2007

The number 1 problem in tomography: incomplete data coverage,and, hence, heterogeneous model resolution.

Tomographic “image” of geodynamic models

McNamara & Zhong, Nature, 2005Ritsema, McNamara, & Bull JGR, 2007

Red, white and blue

The question has been raised: what are the reds and blues?May I add: what is the white?• PREM includes layers with an uncertain geophysical origin• PREM is already a product of the modeling of seismic data• In tomography, PREM is modified

The PHysical Reference Earth Model (P-REM)

Cammarano et al., EPSL 2003:Physical Reference Earth Models(P-REF or P-REM):seismic velocity models based on physical/chemical constraints.

A P-REM is not necessarily the best solution to explain (global) seismological data, but it has a clear geophysical/geochemicalbasis that may simplify seismic data interpretation.

Simplicity:Homogeneous compositionAdiabatic geotherms

A model for mantle chemistry

• 6 oxides for bulk chemistry;• Depleted Mid-Ocean-Ridge-Basalt-Mantle (DMM) for mantle bulk chemistry• MORB model of WH [2005]• 18% of basalt fraction produces the DMM composition

Mixing in the mantle

Xu et al., 2008 Brandenburg & van Keken, 2007

• Consider the basaltic and harburgitic components in disequilibrium• Determine the effects of “mechanical mixing” on phase equilibria• Determine the effects of “mechanical mixing” on seismic velocities• Determine the implications for constraining upper mantle structure (i.e., temperature)

Hofman & White, 1982; Allègre & Turcotte, 1986; Christensen & Hoffman, 1994; Xie & Tackley, 2004

Phase equilibria

(f XB + [1-f] XH)Pyrolite

f (XB) + [1-f] (XH)Basalt + Harzburgite

• Calculations following Stixrude and Lithgow Bertelloni, JGR [2005] and updated parameter sets from Xu et al., EPSL, [2008]• Basalt fraction f can be used as a compositional metric

Ita & Stixrude, 1992; Bina & Hellfrich, 1992; Collony & Petrini, 2002; Cammarano et al, 2003Mattern et al., 2005; Stixrude & Lithgow-Bertelloni, 2005

Shear velocity profiles for adiabatic temperature profiles

All the effects of composition, phase equilibria and temperature on seismic velocity(and waveforms and traveltimes) are incorporated in a self-consistent manner.

Traveltime modeling of the transition zone

We shall not make tomographic corrections for the uppermost mantle(they may do more harm than good) but model data directly usingpredictions with P-REM. So,

+ geotherms =

Seismic velocity profiles waveforms traveltime predictions

Here, we only vary the potential temperature of an adiabatic temperature profiles

Data analysis

• Application I: SS precursors• Application II: P410s and P660s

SS precursors

Shearer, 1991; Flanagan & Shearer, 1999; Deuss & Woodhouse, 2001; Gu & Dziewonski, 2002;Schmerr & Garnero, 2006; Houser et al., 2008

SS precursors data

SS precursors synthetics

S410S–S660S (i.e., T410-660) variations are robust indicators oftransition zone temperatures. The traveltimes of S660S and S410Swith respect to SS are also affected by uppermost mantle structure.

Temperature in the transition zone

• Measure S660S-S410S in stacks of recorded and computed waveforms, using the same source-receiver geometry• Determine the potential temperature for each stack that best explains S660S-S410S• On average, the average temperature in the Mechanical Mixture is 1620 K. For the EA it is 100 K higher.

Ritsema et al., EPSL, 2009

Overview

• Application II: P410s and P660s

Receiver functions: P-S converted waves

Vinnik, 1977; Shen et al., 1998; Chevrot et al., 1999; Li et al., 2003; Gilbert et al., 2003;Lawrence & Shearer, 2006

Receiver functions: observations

Tauzin et al., 2008

Receiver functions: modeling

Ritsema et al., Geology, 2009

Summary and Conclusions

• For the same bulk chemistry, the Mechanical Mixture (MM) and Equilibrium Assemblage (EA) have different phase equilibria and, hence, different seismic structures.• Seismic velocities in the MM are higher than in the EA, and transition zone gradients are slightly steeper.• Since MM velocities are higher than EA velocities, best fitting geotherms for the MM are 100 K lower than for the EA.

Summary and Conclusions• Traveltimes of Ps converted waves and SS precursors can be explained by the MM for a potential temperature of 1620 K, consistent with analyses of magma compositions [Herzberg et al., 2007; Putirka, 2008].• Traveltime variations imply large-scale (1000+ km) temperature variations of 200 K• Thus, the MM in combination with the 1620 K adiabatic geotherm is a reasonable physical reference model for explaining seismic traveltimes through the transition zone.• Unexplained are the steep velocity gradients in the mantle, which may require sub-adiabatic temperatures or compositional gradients.

Seismic heterogeneity and physical reference models

anomalies from PREM anomalies from PhREM

Seismic tomography

ANALYSIS OF WAVEFORM ANOMALIES SEISMIC IMAGE