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George Helffrich

Bio: George Helffrich is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Mantle (geology) & Transition zone. The author has an hindex of 40, co-authored 116 publications receiving 4912 citations. Previous affiliations of George Helffrich include University of Tokyo & Carnegie Institution for Science.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors examined the Clapeyron slopes of the perovskite-forming reactions of the spinel-forming reaction and concluded that the bulk of the evidence indicates a greater Clapeyreron slope magnitude for the 410 than for the 660.
Abstract: The depths, widths, and magnitudes of the 410-km and 660-km seismic discontinuities are largely consistent with an isochemical phase change origin, as is the observation that the topography on these discontinuities is negatively correlated and significantly smaller than predicted for chemical changes. While most thermodynanlic studies of the relevant phase changes predict greater topography on the 410 than the 660, recent seismic studies suggest greater topography on the 660. The seismic results are consistent with some recent thermochemical studies which suggest that the Clapeyron slopes of the perovskite-forming reactions exceed in magnitude those of the spinel-forming reactions; however, we have reexamined the relevant Clapeyron slopes in light of other, more recent, experimental studies as well as the requirements of internal thermodynamic consistency. We conclude that the bulk of the evidence indicates a greater Clapeyron slope magnitude for the 410 than for the 660. Thus the recent seismic results are unexpected. One explanation might be that lateral temperature variations near 660 km depth exceed those near 410, consistent with a model of the 660 as a thermal boundary layer. An alternate interpretation, which requires neither a thermal boundary nor metastable olivine, is that the 410 does possess greater topography but is simply less visible seismically than the 660. This latter idea, and recent short-period observations of P'410P' seismic phases in conjunction with an elevated 660, is consistent with thermodynamic modeling of subduction zones illustrating the extreme broadening of the olivine n+P transition in cold slab interiors and, conversely, its sharpening in regions of high temperature.

512 citations

Journal ArticleDOI
02 Aug 2001-Nature
TL;DR: It is shown that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume.
Abstract: Seismological images of the Earth’s mantle reveal three distinct changes in velocity structure, at depths of 410, 660 and 2,700 km. The first two are best explained by mineral phase transformations, whereas the third—the D0 layer—probably reflects a change in chemical composition and thermal structure. Tomographic images of cold slabs in the lower mantle, the displacements of the 410-km and 660-km discontinuities around subduction zones, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. We show that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume. T he Earth’s mantle comprises 82% of its volume and 65% of its mass. It constitutes virtually all of the silicate part of the Earth, extending from the base of the crust (0.6% of Earth’s silicate mass) to the top of the metallic core (Fig. 1). When the core segregated from the silicate and gas of the proto-Earth, it incorporated high concentrations of the siderophile elements, leaving lithophile elements in the silicate mantle. (Words in bold are explained in the glossary; see Box 1.) Thus, the current composition of the mantle has core formation imprinted on it—as pronounced depletions in, for example, Fe, Ni, S, W, Pt, Au and Pb relative to the chondritic meteorites 1,2 , which are used to constrain the composition of the whole Earth. Owing to the fractionation of lithophile radioactive parent isotopes such as 238 U and 182 Hf from their siderophile daughters 206 Pb and 182 W, core formation can be dated as the time at which the evolution of the isotopic compositions of Pb and W diverged from the meteorite trend. The result (about 4.5 Gyr ago) corresponds to 50‐100 Myr after the formation of the oldest meteorite bodies in the Solar System 3,4 .

404 citations

Journal ArticleDOI
TL;DR: In this article, the authors review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities, and conclude that it is viable and describe how discontinuity have been and can be used to probe the physical and chemical state of the mantle.
Abstract: Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. Thermodynamic principles govern phase transformations, making them sensitive to changes in the mantle's ambient conditions through the thermodynamic response: Changes in temperature or composition shift the transformation to a different pressure, creating topography on a level discontinuity. With this use as an exploratory tool for the mantle in mind, we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities. Reviewing the evidence for and against the phase transition model, we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone.

260 citations

Journal ArticleDOI
02 Mar 2017-Nature
TL;DR: The results demonstrate that the liquidus field of silicon dioxide (SiO2) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe-Si-O core crystallizes SiO2 as it cools, setting limits on silicon and oxygen concentrations in the present-day outer core.
Abstract: 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. 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 (SiO2) as it cools. The authors conclude that if crystallization proceeds from the top of the core, the sinking of SiO2-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 ocean1,2,3,4,5 predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core6,7,8,9. 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 (SiO2) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe–Si–O core crystallizes SiO2 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 conductivity10,11, from as early on as the Hadean eon12. SiO2 saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.

183 citations

Journal ArticleDOI
09 Dec 2010-Nature
TL;DR: In this paper, the authors present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300 km of the outer core.
Abstract: Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores. Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core, and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300 km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe-O-S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.

165 citations


Cited by
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TL;DR: In this paper, the authors introduced the importance of subducted oceanic crustal age on arc petrogenesis and demonstrated that Archean TTD crustal generation processes are also present in selected high-Al Phanerozoic TTD terranes.
Abstract: The petrogenesis of trondhjemite-tonalite-dacite (TTD) involves all major petrologic models in various tectonic settings. A specific subtype of TTD, high-Al type, is the one most commonly associated with Archean gneiss terranes. During the Archean, continental crust formation was operating at an elevated rate relative to the Phanerozoic, and the generation of high-Al TTD played an integral role in its nucleation and growth. High heat flow, rapid convection, and subduction of hotter, smaller plates were unique tectonic elements to the Archean which optimized conditions required for transformation of subducted oceanic crust into sial via partial melting. Anatexis of Archean mid-ocean ridge basalt (MORB) under eclogitic to garnet amphibolitic conditions produced weakly peraluminous to metaluminous high-Al TTD with low heavy rare earth elements (HREE), Y, Nb, K/Rb, and Rb/Sr and high La/Yb and Sr/Y. This study demonstrates that Archean TTD crustal generation processes are also present in selected high-Al Phanerozoic TTD terranes. The Cenozpic high-Al TTD suites are commonly found in tectonic settings which are thought to recreate the elevated Archean thermal gradients, i.e., at sites of young, hot oceanic plate subduction. These relationships imply a petrologic continuity of TTD generation through time. A fertile zone of melting is envisioned at 23–26 kbar (75–85 km) and 700–775°C, where wet partial melting of the subducting slab occurs concurrently with dehydration reactions. At this depth, the converting mantle wedge continuously feeds hot mantle material to the wedge-slab interface, creating strong temperature gradients, intraslab fluid migration, and slab melting. In summary, in modern arc terranes where young ( 30 Ma) oceanic crust is subducted, mantle-derived magmas are dominant, giving rise to basaltandesite-dacite-rhyolite (BADR) fractionation suites. This study introduces the importance of subducted oceanic crustal age on arc petrogenesis.

1,351 citations

Journal ArticleDOI
TL;DR: In this article, anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity, implying that the mantle plays a major, if not dominant, role in orogenies.
Abstract: Seismic anisotropy beneath continents is analyzed from shear-wave splitting recorded at more than 300 continental seismic stations. Anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity. The observed coherence with crustal deformation implies that the mantle plays a major, if not dominant, role in orogenies. No evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are coupled to general mantle circulation.

1,091 citations

Journal ArticleDOI
05 Feb 2004-Nature
TL;DR: It is shown that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish in reef fish population dynamics.
Abstract: Mangrove forests are one of the world's most threatened tropical ecosystems with global loss exceeding 35% (ref 1) Juvenile coral reef fish often inhabit mangroves, but the importance of these nurseries to reef fish population dynamics has not been quantified Indeed, mangroves might be expected to have negligible influence on reef fish communities: juvenile fish can inhabit alternative habitats and fish populations may be regulated by other limiting factors such as larval supply or fishing Here we show that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish Mangroves in the Caribbean strongly influence the community structure of fish on neighbouring coral reefs In addition, the biomass of several commercially important species is more than doubled when adult habitat is connected to mangroves The largest herbivorous fish in the Atlantic, Scarus guacamaia, has a functional dependency on mangroves and has suffered local extinction after mangrove removal Current rates of mangrove deforestation are likely to have severe deleterious consequences for the ecosystem function, fisheries productivity and resilience of reefs Conservation efforts should protect connected corridors of mangroves, seagrass beds and coral reefs

1,086 citations

Journal ArticleDOI

919 citations