[Baps] 3 talks next week: Ahrens, Showman, Aurnou

[Sarah Stewart-Mukhopadhyay]

We have 3 planetary talks at Harvard next week:

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MONDAY APRIL 13: EPS COLLOQUIUM, 4 PM
http://www.eps.harvard.edu/seminars/epscolloque.php

Shock-Induced Transformation and Melting of Lower Mantle Minerals:
Implications for Earth Evolution
Thomas J. Ahrens
Caltech

Shock wave techniques can define high-pressure melting relations for
deep Earth minerals by several methods. Pressure-volume-energy
equations of state and spectral radiation shock temperature
measurements are sensitive to the conditions where Hugoniots of lower
mantle mineral compositions cross phase boundaries including both
polymorphic phase transitions and partial to complete melting. Method
1 uses the velocity of isentropic rarefaction waves to observe the
loss of shear modulus upon melting. Method 2 use radiative temperature
along the Hugoniot to seek a deflected in temperature upon
intersection of Hugoniot and melting curve. Method 3 applies in the
case when the liquid phase is denser than the coexisting solid
phase(s). In the absence of any other known polymorphic phase change a
sudden density increase is attributed to melting. For SiO2, all three
of these methods define the shock pressure and temperature where the
Hugoniot of fused silica passes from stishovite to partial melt (73
GPa, 4600 K) and where the Hugoniot of crystal quartz passes from the
CaCl2 structure to partial melt (116 GPa, 4900 K). In the case of
Mg2SiO4, the forsterite Hugoniot passes from periclase+perovskite
phases to melt before 152 GPa and 4300 K, whereas initial wadsleyite
material follows a colder path, transforming from
periclase+post¨Cperovskite to melt before 151 GPa and 4160 K. Recently,
we extended the range of the MgSiO3 glass Hugoniot and demonstrated
that this glass transforms into the perovskite structure from 80 to
100 GPa. Above 100 GPa and extending to over 160 GPa, the shock state
is molten. Since shock states derived from crystal enstatite are also
molten above 160 GPa, precise determination of the high pressure
Gr¨¹neisen parameter,¦Ã , can be obtained from the finite difference
V[dP/dE]V. As previously seen in Mg2SiO4 liquid, ¦Ã for molten MgSiO3
increases markedly with compression, going from 0.5 to 1.6 over the 0
to 135 GPa range. This property gives rise to larger than expected
isentropic rises in temperature with depth in model magma oceans
encompassing the entire mantle. Taking into account our interpretation
of the shock melting data and a critically evaluated subset of the
published database of equation of state determinations from static
(multi-anvil and diamond anvil) methods, we construct a proposed deep
mantle and core geotherm. We note the similarity of our resulting
magma ocean model [Asimow, 2008] and that of Labrosse et al. [2007].
The notion put forward by Labrosse et al. that the present ULVZ is a
very thin remnant of the ancient magma ocean, which initially started
to crystallize at mid-lower mantle depths, deserves further study.
Evidently, if the ULVZ is indeed a dynamically stable, partially
molten remnant of the primordial magma ocean, it is a candidate for
hosting the deep Earth's hidden reservoir of incompatible
elements,including both a substantial portion of the global inventory
of heat¨Cproducing elements and the missing primordial noble gas
isotopes. The density of such melts contributes to this reservoir
being effectively unsampled by either solid-state mantle convection or
magmatic fluids derived from the very deep Earth. Asimow, P.D., 2007.
Magmatism and the evolution of the Earth's interior. Geochimica Et
Cosmochimica Acta, 71(15): A40. Labrosse, S., Hernlund, J.W. and
Coltice, N., 2007. A crystallizing dense magma ocean at the base of
the Earth's mantle. Nature, 450(7171): 866-869.

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THURSDAY APRIL 16, CFA TALK, 11 AM
Pratt Conference Room, CfA, 60 Garden St.

The atmospheric circulation of hot Jupiters
Adam Showman
University of Arizona

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THURSDAY APRIL 16, EPS SOLID EARTH PHYSICS SEMINAR, 2 PM
Faculty Lounge, 4th floor, Hoffman Lab, 20 Oxford St.

Boundary layer transitions in rotating convection and dynamo systems
Jon Aurnou
UCLA

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