Constraining Upper Mantle Thermal Anomalies Using Seismic Methods

Andrew Nyblade
Department of Geosciences
Penn State University
University Park, PA  16802

	Much can be learned about geodynamic processes associated 
with major tectonic features on Earth by mapping temperature 
variations within the upper mantle.  In many of the continents, it 
is possible to combine a number of observables to constrain, with 
some degree of certainty, thermal anomalies within the lithosphere 
and sublithospheric mantle.  Complementary data often used to do this 
include heat flow, gravity, magnetic and seismic data, and 
pressure-temperature (P-T) estimates from xenoliths.  In Antarctica, 
the availability of direct measures of subsurface temperatures from 
heat flow and P-T estimates are limited, and consequently one must 
place greater emphasis on proxy measures of Earth's temperature field 
provided by gravity, magnetic and seismic data.  Seismic data can be 
used in a variety of ways to constrain temperatures within the upper 
mantle, provided that the thermoelastic properties of mantle minerals 
are known.  Using laboratory measurements of these properties, in 
combination with models for the average mineralogical composition 
of mantle rocks, it is possible to make inferences from seismic 
wave speeds about temperature variations in the mantle.  However, caution 
must be exercised in making such inferences because the uncertainties 
associated with the thermoelastic properties and mantle composition 
can lead, in some instances, to errors in temperature estimates 
that are as large as the inferred temperature variations themselves.  
In  addition, effects of anelasticity and partial melts on seismic 
velocities are poorly known and can add to even greater uncertainty 
in temperature estimates.  It is also common to use topography on 
the 410 and 660 km discontinuities to place constraints on 
temperature variations in the upper mantle.  These discontinuities 
are largely the result of phase transformations in olivine that are
both temperature and pressure dependent.  The Clapeyron slopes of
 these phase transformations have been experimentally determined, 
thus enabling seismologists to use variations in transition zone 
thickness into constrain temperature variations at depths greater 
than 400 km in the mantle.  Seismic methods to estimate variations 
in upper mantle temperatures have not yet been applied extensively 
in Antarctica, but they have been elsewhere, and therefore examples 
will be provided from other continents.