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Melting of Iron at Earth's Inner Core Boundary Based on Fast X-ray Diffraction
Earth's core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at 330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.
Journal Article
An early geodynamo driven by exsolution of mantle components from Earth’s core
Experiments show that magnesium oxide can dissolve in core-forming metallic melts at very high temperatures; core formation models suggest that a giant impact during Earth’s accretion could have contributed large amounts of magnesium to the early core, the subsequent exsolution of which would have generated enough gravitational energy to power an early geodynamo and produce an ancient magnetic field.
The formation of Earth's early magnetic field
James Badro
et al
. present experiments showing that, at sufficiently high temperatures, magnesium oxide can dissolve in core-forming metallic melts. They then use core-formation models to demonstrate that a high-temperature event during Earth's accretion, such as the Moon-forming giant impact, could have contributed large amounts of magnesium to the early Earth's core. As the core subsequently cooled, the ensuing exsolution of buoyant magnesium oxide would have generated enough gravitational energy to power an early geodynamo and produce Earth's ancient magnetic field.
Recent palaeomagnetic observations
1
report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth
2
is the primary driver of Earth’s present-day geodynamo
3
,
4
,
5
, but the inner core is too young
6
to explain the existence of a magnetic field before about one billion years ago. Theoretical models
7
propose that the exsolution of magnesium oxide—the major constituent of Earth’s mantle—from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core
8
,
9
,
10
and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models
11
, we show that extreme events during Earth’s accretion (such as the Moon-forming giant impact
12
) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution
7
of buoyant magnesium oxide would have taken place at the core–mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.
Journal Article
Low Core-Mantle Boundary Temperature Inferred from the Solidus of Pyrolite
The melting temperature of Earth's mantle provides key constraints on the thermal structures of both the mantle and the core. Through high-pressure experiments and three-dimensional x-ray microtomographic imaging, we showed that the solidus temperature of a primitive (pyrolitic) mantle is as low as 3570 ± 200 kelvin at pressures expected near the boundary between the mantle and the outer core. Because the lowermost mantle is not globally molten, this provides an upper bound of the temperature at the core-mantle boundary (TCMB). Such remarkably low TCMB implies that the post-perovskite phase is present in wide areas of the lowermost mantle. The low TCMB also requires that the melting temperature of the outer core is depressed largely by impurities such as hydrogen.
Journal Article
Crystallization of silicon dioxide and compositional evolution of the Earth’s core
Melting experiments with liquid Fe–Si–O alloy at the pressure of the Earth’s core reveal that the crystallization of silicon dioxide leads to core convection and a dynamo.
Crystallization in the Earth's core
The Earth's core contains large amounts of iron (Fe), but its density, about ten per cent less than that of pure iron, indicates the presence of lighter elements in the outer core, potentially including silicon (Si) and oxygen (O). To simulate the early Earth, Kei Hirose and co-authors present melting experiments on liquid Fe–Si–O alloy at the pressures of the Earth's core in a laser-heated diamond-anvil cell. They find that an initial Fe–Si–O core would be able to crystallize silicon dioxide (SiO
2
) as it cools. The authors conclude that if crystallization proceeds from the top of the core, the sinking of SiO
2
-depleted Fe–Si–O liquid would have been more than enough to power core convection and a dynamo in the early Earth.
The Earth’s core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40–60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean
1
,
2
,
3
,
4
,
5
predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core
6
,
7
,
8
,
9
. However, only the binary systems Fe–Si and Fe–O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe–Si–O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe–Si–O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO
2
) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe–Si–O core crystallizes SiO
2
as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity
10
,
11
, from as early on as the Hadean eon
12
. SiO
2
saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.
Journal Article
Hottest places on the planet
\"Simple text and full-color photographs describe the hottest places on the planet\"-- Provided by publisher.
Book
N6-methyladenosine modification of the 5′ epsilon structure of the HBV pregenome RNA regulates its encapsidation by the viral core protein
Hepatitis B virus (HBV) contains a partially double-stranded DNA genome. During infection, its replication is mediated by reverse transcription (RT) of an RNA intermediate termed pregenomic RNA (pgRNA) within core particles in the cytoplasm. An epsilon structural element located in the 5′ end of the pgRNA primes the RT activity. We have previously identified the N6-methyladenosine (m⁶A)–modified DRACH motif at 1905 to 1909 nucleotides in the epsilon structure that affects myriad functions of the viral life cycle. In this study, we investigated the functional role of m⁶A modification of the 5′ ε (epsilon) structural element of the HBV pgRNA in the nucleocapsid assembly. Using the m⁶A sitemutant in the HBV 5′ epsilon, we present evidence that m⁶A methylation of 5′ epsilon is necessary for its encapsidation. The m⁶A modification of 5′ epsilon increased the efficiency of viral RNA packaging, whereas the m⁶A of 3′ epsilon is dispensable for encapsidation. Similarly, depletion of methyltransferases (METTL3/14) decreased pgRNA and viral DNA levels within the core particles. Furthermore, the m⁶A modification at 5′ epsilon of HBV pgRNA promoted the interaction with core proteins, whereas the 5′ epsilon m⁶A site–mutated pgRNA failed to interact. HBV polymerase interaction with 5′ epsilon was independent of m⁶A modification of 5′ epsilon. This study highlights yet another pivotal role of m⁶A modification in dictating the key events of the HBV life cycle and provides avenues for investigating RNA–protein interactions in various biological processes, including viral RNA genome encapsidation in the context of m⁶A modification.
Journal Article