Mineral physics-based Interpretation of the LAB:
partial melting or sub-solidus processes?
T51C-2603
1. Introduction
•Recent seismological studies using high-frequency body
waves show relatively sharp and large velocity reduction at
the oceanic lithosphere-asthenosphere boundary (LAB).
•The key features that need to be explained by any viable
model include
(1)a sharp (<20 km width) and a large (5-10%) velocity drop,
(2) at 70 km depth in the old oceanic upper mantle and
(3) the age-dependent LAB depth in the young oceanic
upper mantle.
Tolulope Olugboji*, Shun-ichiro Karato, Jeffrey Park
Department of Geology and Geophysics, Yale University
3.2 RF Synthetics: Sensitivity Tests
Sensitivity tests indicate that a melt layer should be easily
resolvable.
Model parameter grid-search using data places constraints
on the relevant grain-boundary-sliding parameters, with the
associated uncertainty: peak intensity, activation energy,
characteristic period (frequency), and peak broadness
parameter.
The main controls are the temperature gradient, the rapid
transition from dry to wet conditions, and the peak
broadening parameters.
Figure 6. Shear modulus and attenuation data with the model fits from optimal
parameters in the grid search.
Figure 8. A comparison of attenuation and relative velocity with depth, for the
absorption band model (ABM), the grain boundary sliding model (GBS), and both
contributions (ABM+GBS).
Figure 1. Seismic measurements of the LAB overlaid on temperature contours using
three thermal models. The measurements show that partial melting is difficult
2. Objectives
1. We show, using synthetic Receiver Functions (RF), that
the model of melt accumulation at the LAB are difficult to
reconcile with seismological observations.
2. We present a detailed analysis of a new version of subsolidus model where the role of grain-boundary sliding is
included.
Figure 3. A layer of ~20 km should be detectable at modest frequencies ~0.2 Hz.
4. Description of New Subsolidus Model
Incorporating grain-boundary sliding (gbs), theoretically, can
produce a large velocity reduction
T51C-2603
6.1. Model Predictions: Depth of Relaxation
The characteristic frequency varies with temperature, and
water content as described below, We describe the
predicted relaxation depth, using the parameters estimated
from the grid-search.
6.2 Model Predictions: Sharpness
7. Model Predictions (Summary):
A. Age-Dependence, B. Sharpness, C. Uncertainty
The model predictions on the age-dependence of the LAB
depth, and sharpness, with its associated uncertainty, are
broadly consistent with recent seismic observables.
3.1 RF Synthetics: Missing Double Polarity
RF synthetics through a melt layer should have a a doublepolarity signature. This diagnostic feature is not observed in
published data.
Figure 4. Schematic diagram showing how shifts in the characteristic frequency of the
grain-boundary sliding peak leads to the observed LAB (adapted from [Karato,
2012]).
5.1 Grid-Search for GBS Parameters
Figure 9. The predicted LAB depth (dark line) and sharpness (circles) is age-dependent for
young oceans, with a transition to a constant depth for old oceans.
Selected References
1. Olugboji,T.M., Karato, S., Park, J. (2012). Structures of the lithosphere-asthenosphere
boundary: mineral physics modeling and seismological signatures. Geochemistry,
Geophysics, Geosystems, submitted.
2. Karato, S. (2012). On the origin of the asthenosphere. Earth and Planetary Sci. Lett., 321-
322, 95-03, doi:10.1016/j.epsl.2012.01.001.
3. Jackson, I., and U.H. Faul (2010). Grainsize-sensitive visco-elastic relaxation in olivine:
Towards a robust laboratory-based model for seismological application, Phys. Earth Planet.
Inter., 183(1-2), 151
Figure 7. The depth of relaxation is age-dependent
Figure 2. Synthetic receiver functions (P and S) computed using a model given in (D).
For a melt layer, a double-polarity signal is expected (Psd1/SPd1 and Psd2/Spd2).
RESEARCH POSTER PRESENTATION DESIGN © 2012
www.PosterPresentations.com
Figure 5.. Bi-variate probability density functions, PDFs, are calculated, using a grid search.
Maximum likelihood is indicated by warmer colors (chi-squared values ~ 1)
4. Levin, V., and J. Park (1998). P-SH Conversion oin Layered Media with Hexagonally
Symmetric Anisotropy: A CookBook, Pure and Applied Geophysics, 151(2-4), 669-697
Contacts
*[email protected]