Oxygen to the core
An
international collaboration including researchers from Lawrence Livermore
National Laboratory has discovered that Earth's core formed under more
oxidizing condition's than previously proposed. Through a series of
laser-heated diamond anvil cell experiments at high pressure (350,000 to
700,000 atmospheres of pressure) and temperatures (5,120 to 7,460 degrees
Fahrenheit), the team demonstrated that the depletion of siderophile (also
known as "iron loving") elements can be produced by core formation
under more oxidizing conditions than earlier predictions.
"We
found that planet accretion (growth) under oxidizing conditions is similar to
those of the most common meteorites," said LLNL geophysicist Rick Ryerson.
The research appears in the Jan. 10 edition of Science Express.
While
scientists know that Earth accreted from some mixture of meteoritic material,
there is no simple way to quantify precisely the proportions of these various
materials. The new research defines how various materials may have been distributed
and transported in the early solar system.
As core
formation and accretion are closely linked, constraining the process of core
formation allows researchers to place limits on the range of materials that
formed our planet, and determine whether the composition of those materials
changed with time. (Was accretion heterogeneous or homogeneous?)
"A
model in which a relatively oxidized Earth is progressively reduced by oxygen
transfer to the core-forming metal is capable of reconciling both the need for
light elements in the core and the concentration of siderophile elements in the
silicate mantle, and suggests that oxygen is an important constituent in the
core," Ryerson said.
The
experiments demonstrated that a slight reduction of such siderphile elements as
vanadium (V) and chromium (Cr) and moderate depletion of nickel (Ni) and cobalt
(Co) can be produced during core formation, allowing for oxygen to play a more
prominent role.
Planetary
core formation is one of the final stages of the dust-to-meteorite-to-planet
formation continuum. Meteorites are the raw materials for planetary formation
and core formation is a process that leads to chemical differentiation of the
planet. But meteorite formation and core formation are very different
processes, driven by different heat sources and occurring in very different
pressure and temperature ranges.
"Our
ability to match the siderophile element signature under more oxidizing
conditions allows us to accrete Earth from more common, oxidized meteoritic
materials, such as carbonaceous and ordinary chondrites," Ryerson said.
Earth's
magnetic field is generated in the core, and protects Earth from the solar wind
and associated erosion of the atmosphere. While the inner core of Earth is
solid, the outer core is still liquid. The ability to preserve a liquid outer
core and the associated magnetic field are dependent on the composition of the
core and the concentration of light elements that may reduce the melting
temperature.
"By
characterizing the chemical interactions that accompany separation of
core-forming melts from the silicate magma ocean, we can hope to provide
additional constraints on the nature of light elements in the present-day core
and its melting/freezing behavior," Ryerson said.
Other
teams members include Julien Siebert and Daniele Antonangeli (former LLNL
postdocs) from the Université Pierre et Marie Curie, and James Badro (a faculty
scholar at LLNL) from the Institut de Physique du Globe de Paris.
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Posted by Unknown
on Friday, January 11, 2013.
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