HR: 14:55h
AN: U33B-06 [Abstracts]
TI: Noble Gases in the Earth's Core?
AU: * Jephcoat, A P
EM: andrew@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford
Parks Road, Oxford, OX1 3PR
United Kingdom
AU: * Jephcoat, A P
EM: andrew@earth.ox.ac.uk
AF: Diamond Light Source, Rutherford Appleton Laboratory, Chilton, OX11 0QX
United Kingdom
AU: Bouhifd, M A
EM: alib@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford
Parks Road, Oxford, OX1 3PR
United Kingdom
AU: Heber, V
EM: v.heber@open.ac.uk
AF: Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA
United Kingdom
AU: Kelley, S P
EM: s.p.kelley@open.ac.uk
AF: Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA
United Kingdom
AB:
Chemical inertness, surface volatility and low abundance
have made the noble gases a unique trace elemental and isotopic system
for constraining the formation and evolution of the solid Earth and
its atmosphere. This geochemical role parallels
extensive physical-property measurements on the condensed rare gases
alone at the pressures equivalent to those of the Earth's deep mantle and core
from diamond-anvil cell (DAC) experiments.
Traditional geochemical approaches to the processes of
planetary evolution have involved crystal-melt partitioning
at low pressures relevant more to near-surface degassing.
The degree of compatibility has fluctuated among different
studies and largely rests with the conclusion that, for
common upper mantle phases, the noble gases are highly
incompatible. But the long-known high $^3$He/$^4$He ratios for some ocean-island basalts and more recent observations for
some
of the rare gases (Ne, Ar and possibly Xe) that there is a
solar component emanating from the Earth, continue to raise
questions on the source reservoir as well as on accretionary
and incorporation processes. Changes in models of mantle convection style have made it harder to rely on the deep mantle as
a reservoir, and the core
has remained a particularly unfavourable location either because of
difficulty in constructing a retention mechanism during
planetary accretion or simply because of lack of
data: Partitioning studies at pressure are rare and complicated
by the difficulty in reproducing not only absolute concentrations,
but confinement of gas in high-pressure apparatus and post-run analysis.
We have investigated noble gas solubility in silicate liquids at
high pressures in a DAC (relevant to a magma-ocean model of the early Earth)
that suggests that the detailed composition
and structure of silicate liquids may act as an important
control on the level of incompatibility. The long-held idea
of partial melting as a single-stage, efficient process
for extracting noble gases from the Earth's mantle at all
depths, may well be oversimplified. For molten metal compositions
interacting with silicate melt,
Matsuda et al. (1993) defined the near-zero limits
of noble gas solubility expected in metal with increasing
pressure. We re-visit the phenomenological aspect of (saturated) noble gas
solubility in metals with new experiments in noble gas pressure-transmitting
media in the laser-heated DAC. First results with argon
analysed with SEM methods suggest up to
an order of magnitude higher partition coefficient
(D(Ar)$_{Fe/sil}$ $\sim$ 0.1) for liquids in the DAC at 5 GPa.
We have also recovered samples for analysis with more sensitive
UV laser-ablation mass spectroscopic techniques that
provide additional, depth-resolved constraints on noble gas solubility
at moderate pressures.
DE: 8125 Evolution of the Earth
DE: 8147 Planetary interiors (5430, 5724)
DE: 3924 High-pressure behavior
DE: 1025 Composition of the mantle
DE: 1015 Composition of the core
SC: Union [U]
MN: 2004 AGU Fall Meeting