1. RESULTS
CONCLUSION
CONVECTIVE INSTABILITY OF STAGNANT SLABS AT THE BASE OF THE MANTLE TRANSITION ZONE
MATTHEW H. MOTOKI1 MAXIM D. BALLMER2,3
1School of Ocean & Earth Sciences & Technology, University of Hawaii at Manoa, mmotoki@hawaii.edu
2Earth-Life Science Institute, Tokyo Institute of Technology, ballmer@elsi.jp 3Institute of Geophysics, ETH Zurich, maxim.ballmer@erdw.ethz.ch
Figure 3. Intraplate volcanism in Europe may be related to upwellings
rising near the tip of a retreating slab.
• The onset age of convective instability rising out
of a slab that stagnates at the base of the upper
mantle increases with increasing plate age and
decreasing viscosity or thermal expansivity.
• Onset ages are on the order of tens of Myrs.
The slab fully disintegrates after >100 Myrs.
• Instabilities are triggered sooner on the young
side of a fracture zone (or most likely near any
other heterogeneity).
• The instabilities separate the slab’s harzburgite
and eclogite; a small fraction of eclogite is
transported to the base of the lithosphere
• Mostly harzburgitic plumes rising out of the slab
entrain some eclogite (and hydrated mantle), an
alternative explanation for intraplate volcanism.
Figure 2. Intraplate volcanism is typically associated with slab stagnation in the MTZ.
Figure 1. Slabs commonly stagnate at the base of the upper mantle.
MOTIVATION
43°N
39°N
30°N
a) 43°N
b) 39°N
c) 30°N
a
b
c
Huang & Zhao (2006);
Fukao et al. (2009)
Modifiedwithpermissionfrom
Conradetal.(2011)
Conradetal.(2011)
Zhao&Ohtani(2009)
Faccenna et al. (2010)
Figure 4. Vertical root-mean square velocities at the bottom of the box as a
function of age for models with (a,b) age of the plate at the time of subduction
slab, (c) variable mantle viscosity η0, and (d) variable thermal expansivity α;
black crosses show onset ages.
Figure 7. Evolution of the average volume
fraction of (a) harzburgite and (b) eclogite
with depth. The cutoff at the lower end of
the colorscales (blue vs. white) is at 0.02%.
Figure 6. Convective instability rising out of a heterogeneous slab; (a) vertical root-mean square velocites for a slab with a fracture zone (FZ). Plate ages at the trench
on both sides of the FZ are 25 Myr and slab (see legend), respectively. (b-d) Snapshots of composition for a slab of ages 25 and 35 Myrs on both sides of the FZ.
DISCUSSION
Figure 8. Global-scale thermochemical mantle convection model
with persistent compositional layers. (A) radial compositional profile
and (B) snapshot of composition after 4.6 Myrs model time.
Figure 5. Two-dimensional numerical model predictions. Snapshots of (a-d) composition and (e-h) temperature (oC) for the reference case.
Faccenna et al. (2010)
Ballmer et al.,
(Sci.Adv., 2015)
CITATION: Motoki and Ballmer, G-cubed, 2015