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  • richardmitnick 4:54 pm on April 25, 2022 Permalink | Reply
    Tags: "Is Earth’s Core Rusting?", , , Core rust could slow seismic wave velocities by as much as 44% for Vs and 23% for Vp, , , , How do we know whether rusting has been happening at the Core-Mantle Boundary [CMB]?, If rust is actually present where the outer core meets the mantle (the core-mantle boundary or CMB) scientists may need to update their view of Earth’s interior and its history., If subduction carries hydrous minerals deep into Earth’s mantle they may “rust” the iron outer core forming vast sinks of oxygen that can later be returned to the atmosphere., Iron on Earth’s surface—whether in simple nails or mighty girders—reacts gradually when exposed to moist air or oxygenated water through a chemical reaction known as oxidation., Laboratory studies indicate that iron oxide-hydroxide core rust may cause significant reductions in the velocities of seismic shear waves (Vs) and compressional waves (Vp) that pass through it., Scientists should be able to use seismic tomograms to differentiate between core rust and partial melting at the CMB., This work could also help answer questions about the Great Oxidation Event (GOE) which marked the beginning of Earth’s oxygen-rich atmosphere some 2.5 billion to 2.3 billion years ago., ULVZs-ultra-low velocity zones   

    From Eos: “Is Earth’s Core Rusting?” 

    From AGU
    Eos news bloc

    From Eos

    25 April 2022
    Jiuhua Chen
    Shanece S. Esdaille

    If subduction carries hydrous minerals deep into Earth’s mantle, they may “rust” the iron outer core, forming vast sinks of oxygen that can later be returned to the atmosphere.

    1
    Owl Rock in Arizona’s Monument Valley gets its red color from iron oxide minerals. Recent experiments suggest that iron oxides may also be forming far below Earth’s surface, at the interface between the core and the lower mantle. Credit: G. Lamar/Flickr, CC BY 2.0.

    Iron on Earth’s surface—whether in simple nails or mighty girders—reacts gradually when exposed to moist air or oxygenated water through a chemical reaction known as oxidation. The reddish-brown product of this reaction, rust, can consist of various forms of hydrous (water-bearing) iron oxides and iron oxide-hydroxide materials. In nature, the red rocks found in the arid climes of the southwestern United States and elsewhere similarly owe their color to the iron oxide mineral hematite, whereas in wetter environments, iron ore minerals like hematite weather to form the iron oxide-hydroxide mineral goethite (FeOOH).

    Deep below Earth’s surface—2,900 kilometers deep, to be precise—is a mass of mostly molten iron forming the planet’s outer core. Could it rust as well?

    In experiments, scientists have recently shown that when iron meets moisture—as water or in the form of hydroxyl-bearing minerals—at pressures close to 1 million atmospheres, similar to pressures in the deep lower mantle, it forms iron peroxide or a high-pressure form of iron oxide-hydroxide with the same structure as pyrite (i.e., pyrite-type FeOOH) [Hu et al., 2016*, Mao et al., 2017]. In other words, the oxidation reactions in these experiments do, indeed, form high-pressure rust.

    *See References with links below.

    If rust is actually present where the outer core meets the mantle (the core-mantle boundary, or CMB), scientists may need to update their view of Earth’s interior and its history. This rust could shed light on the deep-water cycle in the lower mantle and the enigmatic origins of ultralow-velocity zones (ULVZs)—small, thin regions atop Earth’s fluid core that slow seismic waves significantly (Figure 1). It could also help answer questions about the Great Oxidation Event (GOE), which marked the beginning of Earth’s oxygen-rich atmosphere some 2.5 billion to 2.3 billion years ago, and the Neoproterozoic Oxygenation Event (NOE) 1 billion to 540 million years ago, which brought atmospheric free oxygen to its present levels.

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    Fig. 1. The coloration of red rocks on Earth’s surface—as seen here near West Mitten Butte, East Mitten Butte, and Merrick Butte in Arizona—mainly results from the oxidized iron minerals hematite and goethite (top). Possible core rust deposits at the core-mantle boundary (CMB), 2,900 kilometers below Earth’s surface, could be made of iron oxide-hydroxide minerals with a pyrite-like structure. This rust material could explain the detections of ultralow-velocity zones (ULVZ) in seismic data. The ULVZ detection threshold indicates the resolution of current seismic tomography. Credit: Top: Ken Cheung/Pexels; Bottom: Mary Heinrichs/AGU.

    But how do we know whether rusting has been happening at the Core-Mantle Boundary [CMB]?

    Seismic Signatures at the Core-Mantle Boundary

    Although we can’t mine the minerals at the CMB, we can examine them in other ways. If the core rusts over time, a layer of rust may have accumulated at the CMB, exhibiting certain seismic signatures.

    Laboratory studies indicate that iron oxide-hydroxide core rust (i.e., FeOOHx, where x is 0–1) may cause significant reductions in the velocities of seismic shear waves (Vs) and compressional waves (Vp) that pass through it, much like the rocks (or partial melts, if present) in ULVZs do [Liu et al., 2017]. In fact, core rust could slow seismic wave velocities by as much as 44% for Vs and 23% for Vp, compared with the average seismic velocities as a function of depth represented in the Preliminary Reference Earth Model. These large velocity reductions would make the core rust recognizable in seismic tomography if it accumulates into piles thicker than 3–5 kilometers.

    The difficulty lies in distinguishing whether seismic anomalies in ULVZs are caused by core rust or whether they have other origins. For example, partial melting, which is commonly believed to occur at the base of the lower mantle and to be responsible for ULVZs [Williams and Garnero, 1996], could give rise to seismic velocity reductions similar to those caused by core rust.

    Scientists should be able to use seismic tomograms to differentiate between core rust and partial melting at the CMB. A seismic tomogram is normally produced through a mathematical inversion process that matches calculated and observed seismic waveforms. The inversion process requires determining possible mathematical solutions that fit the data and then choosing a “best” solution from among these on the basis of additional considerations.

    Each possible mathematical solution corresponds to a distinct set of model parameters related to the physical properties of the materials involved—for example, the relative differences in Vs, Vp, and density between a material of interest and the average of the surrounding mantle around that material.

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    Fig. 2. The ranges (orange and red) of seismic velocity ratios (δlnVs:δlnVp) for different materials proposed as sources of ultralow-velocity zones are shown here: iron-rich oxide, (Fe0.84Mg0.16)O; pyrite-type FeOOH0.7 (possible core rust composition); carbon-iron melts, (Fe-C) melts; silicate perovskite and mantle partial melts, (Mg,Fe)SiO3+partial melts; and postperovskite solid silicate, PPv (Fe0.4Mg0.6)SiO3.

    These differences can vary with the amount of the material in the mantle, but each material usually exhibits a characteristic range of values for the differential logarithmic ratio of Vs to Vp (δlnVs:δlnVp) [Chen, 2021], which can be used to distinguish materials in seismic tomograms (Figure 2). It’s known from mineral physics experiments that this ratio ranges from a lower limit of 1.2 to 1 to an upper limit of 4.5 to 1 for all possible materials explaining the origin of ULVZs. Within this broader range, ratios for core rust (pyrite-type FeOOHx) fall between 1.6 to 1 and 2 to 1 and are distinct from the other materials.

    Evidence of Core Rust Origins

    So far, seismologists have sampled about 60% of the CMB in their search for ULVZs, and they have identified nearly 50 locations of seismic anomalies, accounting for as much as 20% of the CMB area, that could represent ULVZs. Most of these areas are coupled with large low shear velocity provinces (LLSVPs) in the lowermost mantle and display a δlnVs:δlnVp of around 3 to 1, which suggests partial melting (Figure 2).

    However, some of them, located at the margins of or outside the LLSVP beneath the Pacific, display a best fit ratio of about 2:1 [Chen, 2021]. For example, a ULVZ at the northern border of the Pacific LLSVP (about 9°N, 151°W) [Hutko et al., 2009] and a cluster of ULVZs beneath northern Mexico (about 24°N, 104°W) [Havens and Revenaugh, 2001] each have δlnVs:δlnVp ratios that suggest the presence of pyrite-type FeOOHx.

    A common feature of these ULVZs is that they are located in a region of the CMB where temperatures are relatively low—a few hundred kelvins lower than average temperatures within the LLSVP. The low temperatures suggest these zones were produced by a mechanism other than melting. Notably, the region beneath northern Mexico has been identified as comprising the remnants of deep subduction deposited roughly 200 million years ago to the west of North and Central America, which supports the notion that water released from the subducting slab could have rusted the outer core at the CMB.

    The Consequences of a Rusted Core

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    Fig. 3. Core rust (FeOOH0.7) could form when a relatively cold subducting slab carrying hydrous minerals meets the outer core. Driven by mantle convection, core rust deposits from this cold region could then migrate along the core-mantle boundary to a hotter region at the root of a mantle plume, where it could become unstable and decompose into hematite (Fe₂O₃), water (H₂O), and oxygen (O₂). Credit: Mary Heinrichs/AGU.

    It is thought that the dominant mineral in Earth’s lower mantle, bridgmanite, has little ability to host water. However, rusting of the core could produce a high-capacity water reservoir at the CMB—the FeOOHx rust may contain about 7% water by weight [Tang et al., 2021]. Because core rust is heavier than the average mantle, this water reservoir would tend to stay at the CMB. Thus, water can theoretically be transported and stored just outside the core, at least until mantle convection carries it away from the cooler regions near the remnants of subducted slabs and makes it thermally unstable (Figure 3).

    Whether and when this deep water cycles back to the surface would depend largely on the thermal stability of the core rust. Some scientists, on the basis of experimental work, have claimed that FeOOHx can survive only up to 2,400 K under the pressure at the CMB [Nishi et al., 2017], whereas others have observed the presence of FeOOHx at 3,100–3,300 K at similar pressure [Liu et al., 2017]. But whatever the maximum temperature that FeOOHx can withstand, it’s likely that when core rust migrates to hotter regions of the CMB, following the flow of mantle convection, it would decompose into hematite, water, and oxygen. This process offers a possible alternative explanation for the oxygenation history of Earth’s atmosphere.

    Geological, isotopic, and chemical evidence suggests that Earth’s atmosphere was mostly or entirely anoxic during the Archean eon. Following the Archean, the first introduction of molecular oxygen into the atmosphere began about 2.4 billion years ago in the GOE. The second major rise in atmospheric oxygen, the NOE, then occurred about 750 million years ago, bringing concentrations close to today’s level.

    The causes of these oxygenation events remain uncertain. One possible explanation of the GOE is the emergence of cyanobacteria, the early photosynthesizing precursor to plants. The NOE, occurring almost 2 billion years later, has been attributed to a rapid increase in marine photosynthesis and to an increased photoperiod (i.e., longer daylight hours) [Klatt et al., 2021].

    But these explanations are far from impeccable. For example, besides a large mismatch in timing between the appearance of cyanobacteria on Earth and the GOE, several studies have indicated the possibility that a large increase in atmospheric oxygen at the beginning of the GOE was followed by a deep plunge to lower levels that extended over a few hundred million years. So far, there is no convincing explanation for this rise and fall based on cyanobacterial photosynthesis.

    Furthermore, although it is widely accepted that the GOE raised atmospheric oxygen concentrations only modestly compared with the rise during the NOE, laboratory experiments investigating the influence of photoperiod on the net oxygen export from microbial mats that host competitive photosynthetic and chemosynthetic communities suggest a contradictory result [Klatt et al., 2021]. Instead of more oxygen emerging from such mats as a result of longer daylight in the NOE, the experiments indicated that the increase in daylength, from 21 to 24 hours, during the NOE may have led to only about half the rise in oxygen seen when the daylength increased to 21 hours during the GOE.

    Changes attributed to cyanobacteria and the length of the photoperiod thus do not provide a complete or consistent explanation for the atmospheric oxygen increases during the GOE or NOE, and alternative mechanisms for the origins of these events cannot be ruled out.

    Subduction, Migration, Convection, Eruption

    Decades of research have not produced conclusive evidence about when plate tectonics began on Earth. However, some recent studies indicate that subduction began bringing hydrous minerals down to the deep mantle before 3.3 billion years ago. And experimental studies have shown that hydrous minerals in subducting slabs are capable of relaying water all the way to the CMB [Ohtani, 2019]. If so, rusting might have happened as soon as the first ancient slab met the core. The core rust could have piled up gradually at the CMB, giving rise to ULVZs. As the pile migrated away from the cooler subduction region atop the molten outer core, driven by mantle convection, it would have heated up and likely become unstable when it reached a hotter region where a mantle plume was rooted (Figure 3).

    Just as typical volcanic eruptions occur intermittently, the temperature-driven decomposition of core rust could result in fitful bursts of oxygen at the surface. In contrast to the gradual increase in oxygen from cyanobacterial photosynthesis, such a burst might have released oxygen faster than the surface environment could respond and consume it, causing a rapid initial rise and a subsequent fall of atmospheric oxygen levels.

    The accumulation of a large core rust pile and its migration to the site of thermal decomposition could take a much longer time compared with the duration of eruptions of magma at the surface. Indeed, some piles that were formed may not have reached a region hot enough to cause decomposition, and their negative buoyancy amid the surrounding deep mantle would have kept them at the CMB. The geologic record suggests that Earth’s surface was entirely covered by ocean until about 3.2 billion years ago. Net removal of water from the surface and storage in the deep mantle in core rust could have contributed to the emergence of continents in the Archean, although changes in surface topography driven by plate tectonics and the growth of buoyant continents also contributed to this emergence.

    A Potential Paradigm Shift

    Although everyone can see that iron rusts at Earth’s surface, unfortunately, no one can directly prove that Earth’s liquid iron core 2,900 kilometers below the surface is similarly rusting. However, continuing studies will help scrape away layers of uncertainty and answer major questions, such as whether core rusting is responsible for the GOE and the NOE.

    In particular, more laboratory experiments are needed to precisely determine the limits of the thermal and compositional stability of core rust in equilibrium with molten iron at the conditions of the CMB. For example, we need to investigate the equilibrium between core rust and liquid iron at high pressure and high temperature. Other studies could examine core rust thermal stability at high pressures. These experiments are challenging but doable with the current experimental capabilities of laser-heated diamond anvil cells.

    Furthermore, additional work is needed to resolve when subduction began and, specifically, when “wet subduction,” which takes hydrous minerals into the deep interior, started. Geochemical evidence suggests that wet subduction did not start until 2.25 billion years ago, instead of 3.3 billion. This late a start of wet subduction may challenge the hypothesis that core rusting was the origin of the GOE.

    Moreover, whether mantle convection involves layered circulations (i.e., separate convection cells in the lower and upper mantle), whole-mantle circulation, or some hybrid of these scenarios still requires clarification. If layered circulation prevails in the mantle, then subducting slabs would be prevented from entering the lower mantle. Thus, either whole-mantle or hybrid convection [Chen, 2016] must exist for slabs—and the hydrous minerals they carry—to reach the CMB and potentially cause rusting.

    If the pieces of the puzzle all fall into place, then rusting of the core may, indeed, be a massive internal oxygen generator on Earth—and the next great atmospheric oxygenation event could be on its way. The possibility of such an event would raise all sorts of questions about the effects it could have on environments, climate, and habitability in the future. In the near term, confirming that Earth’s core rusts would cause a paradigm shift in our understanding of the planet’s deep interior and how it has fundamentally influenced conditions and life at the surface.

    References

    Chen, J. (2016), Lower-mantle materials under pressure, Science, 351(6269), 122–123, https://doi.org/10.1126/science.aad7813.

    Chen, J. (2021), Tracking the origin of ultralow velocity zones at the base of Earth’s mantle, Natl. Sci. Rev., 8(4), nwaa308, https://doi.org/10.1093/nsr/nwaa308.

    Havens, E., and J. Revenaugh (2001), A broadband seismic study of the lowermost mantle beneath Mexico: Constraints on ultralow velocity zone elasticity and density, J. Geophys. Res., 106(B12), 30,809–30,820, https://doi.org/10.1029/2000JB000072.

    Hu, Q., et al. (2016), FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen-hydrogen cycles, Nature, 534(7606), 241–244, https://doi.org/10.1038/nature18018.

    Hutko, A. R., T. Lay, and J. Revenaugh (2009), Localized double-array stacking analysis of PcP: D″ and ULVZ structure beneath the Cocos plate, Mexico, central Pacific, and north Pacific, Phys. Earth Planet. Inter., 173(1), 60–74, https://doi.org/10.1016/j.pepi.2008.11.003.

    Klatt, J. M., et al. (2021), Possible link between Earth’s rotation rate and oxygenation, Nat. Geosci., 14(8), 564–570, https://doi.org/10.1038/s41561-021-00784-3.

    Liu, J., et al. (2017), Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones, Nature, 551, 494–497, https://doi.org/10.1038/nature24461.

    Mao, H.-K., et al. (2017), When water meets iron at Earth’s core–mantle boundary, Natl. Sci. Rev., 4(6), 870–878, https://doi.org/10.1093/nsr/nwx109.

    Nishi, M., et al. (2017), The pyrite-type high-pressure form of FeOOH, Nature, 547(7662), 205–208, https://doi.org/10.1038/nature22823.

    Ohtani, E. (2019), The role of water in Earth’s mantle, Natl. Sci. Rev., 7(1), 224–232, https://doi.org/10.1093/nsr/nwz071.

    Tang, R., et al. (2021), Chemistry and P–V–T equation of state of FeO2Hx at the base of Earth’s lower mantle and their geophysical implications, Sci. Bull., 66(19), 1,954–1,958, https://doi.org/10.1016/j.scib.2021.05.010.

    Williams, Q., and E. J. Garnero (1996), Seismic evidence for partial melt at the base of Earth’s mantle, Science, 273(5281), 1,528–1,530, https://doi.org/10.1126/science.273.5281.1528.

    See the full article here .

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  • richardmitnick 4:52 pm on January 5, 2022 Permalink | Reply
    Tags: "Scientists discover leftovers of Earth’s dramatic formation", , , , , , The chemicals; rocks and layers that make up ULVZs have largely been sitting unchanged for billions of years and the early days of the planet's formation., ULVZs-ultra-low velocity zones   

    From The Australian National University (AU) : “Scientists discover leftovers of Earth’s dramatic formation” 

    ANU Australian National University Bloc

    From The Australian National University (AU)

    1
    Researchers have uncovered the most detail ever of the mysterious structures laying between the Earth’s mantle and core, also providing the strongest evidence yet they started life as an ocean of molten magma that eventually sunk.

    A team of international researchers, including scientists from The Australian National University (ANU), used thousands of computer-modelled seismic waves to examine Ultra-Low Velocity Zones (ULVZs) beneath the Coral Sea between Australia and New Zealand. The area was selected because of the high frequency of earthquakes and the seismic waves these events unleash.

    ULVZs sit at the bottom of the planet’s mantle and on top of its liquid metal outer core, and are so thin that they are normally invisible to tomographic imaging. For decades, scientists have speculated they are leftovers of the violent processes that shaped the early Earth.

    Study co-author, Professor Hrvoje Tkalčić from ANU, said the team’s findings confirm the chemicals, rocks and layers that make up ULVZs have largely been sitting unchanged for billions of years and the early days of the planet’s formation.

    “For a long time no-one really knew for sure what these mysterious ULVZs were made up of. Now, we’ve developed the clearest picture yet. Using advances in seismology and mathematical geophysics made at ANU we’ve shown that ULVZs are made up of layers,” Professor Tkalčić said.

    “Over billions years of the Earth’s shaping and reshaping, these zones have churned close to the planet’s core but largely remained intact.

    “It’s like an egg in a cake that doesn’t get mixed in with the rest of the ingredients but stays as yoke and egg white, despite the constant mixing all around it.

    “This is a really significant breakthrough as we have unlocked not only a clue as to how the early Earth formed but confirmed ULVZs are clumps of leftovers from this process that are pretty much the same as they were billions of years ago.”

    The study, published in Nature Geoscience, was led by Dr Surya Pachhai from The University of Utah (US), with much of the research completed as part of his PhD at ANU.

    According to Dr Pachhai the most surprising finding in the study is that ULVZs are made up of a lot more diverse materials than first thought.

    “ULVZs are not homogenous but contain strong structural and compositional variations within them,” he said.

    “We found that this type of ULVZs can be explained by chemical heterogeneities created at the very beginning of the Earth’s history and that they are still not well mixed after 4.5 billion years of mantle convection.”

    See the full article here .

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  • richardmitnick 3:31 pm on December 30, 2021 Permalink | Reply
    Tags: "Bayesian inversion", , "Possible chemical leftovers from early Earth sit near the core", A planetary object about the size of Mars may have slammed into the infant planet. As a result a large body of molten material known as a magma ocean formed., An alternate hypothesis: that the ultra-low velocity zones may be regions made of different rocks than the rest of the mantle—and that their composition may hearken back to the early Earth., , , Between the crust and the iron-nickel core at the center of the planet is the mantle., , How can we have any idea what's going on in the mantle and the core? Seismic waves., It's not an ocean of lava—instead it's more like solid rock-but hot and with an ability to move that drives plate tectonics at the surface., Modeling suggests that it's possible some of these zones are leftovers from the processes that shaped the early Earth., Over the following billions of years as the mantle churned and convected the dense layer would have been pushed into small patches showing up as the layered ultra-low velocity zones we see today., , , , Scientific discovery provides tools to understand the initial thermal and chemical status of Earth's mantle., Scientists on the surface can measure how and when the waves arrive at monitoring stations around the world., The ocean would have sorted itself out as it cooled with dense materials sinking and layering on to the bottom of the mantle., The team used a reverse-engineering approach., , They can back-calculate how the waves were reflected and deflected by structures within the Earth., Ultra-low velocity zones sit at the bottom of the mantle atop the liquid metal outer core., ULVZs-ultra-low velocity zones, What does it mean that there are likely layers?   

    From The University of Utah (US) via phys.org : “Possible chemical leftovers from early Earth sit near the core” 

    From The University of Utah (US)

    via

    phys.org

    December 30, 2021

    1
    Credit: Pixabay/CC0 Public Domain.

    Let’s take a journey into the depths of the Earth, down through the crust and mantle nearly to the core. We’ll use seismic waves to show the way, since they echo through the planet following an earthquake and reveal its internal structure like radar waves.

    Down near the core, there are zones where seismic waves slow to a crawl. New research from the University of Utah finds that these enigmatic and descriptively-named ultra-low velocity zones are surprisingly layered. Modeling suggests that it’s possible some of these zones are leftovers from the processes that shaped the early Earth—remnants of incomplete mixing like clumps of flour in the bottom of a bowl of batter.

    “Of all of the features we know about in the deep mantle, ultra-low velocity zones represent what are probably the most extreme,” says Michael S. Thorne, associate professor in the Department of Geology and Geophysics. “Indeed, these are some of the most extreme features found anywhere in the planet.”

    The study is published in Nature Geoscience and is funded by The National Science Foundation (US).

    Into the mantle

    Let’s review how the interior of the Earth is structured. We live on the crust, a thin layer of solid rock. Between the crust and the iron-nickel core at the center of the planet is the mantle. It’s not an ocean of lava—instead it’s more like solid rock-but hot and with an ability to move that drives plate tectonics at the surface.

    How can we have any idea what’s going on in the mantle and the core? Seismic waves. As they ripple through the Earth after an earthquake, scientists on the surface can measure how and when the waves arrive at monitoring stations around the world. From those measurements, they can back-calculate how the waves were reflected and deflected by structures within the Earth, including layers of different densities. That’s how we know where the boundaries are between the crust, mantle and core—and partially how we know what they’re made of.

    Ultra-low velocity zones sit at the bottom of the mantle atop the liquid metal outer core. In these areas, seismic waves slow by as much as half, and density goes up by a third.

    Scientists initially thought that these zones were areas where the mantle was partially melted, and might be the source of magma for so-called “hot spot” volcanic regions like Iceland.

    “But most of the things we call ultra-low velocity zones don’t appear to be located beneath hot spot volcanoes,” Thorne says, “so that cannot be the whole story.”

    So Thorne, postdoctoral scholar Surya Pachhai and colleagues from The Australian National University (AU), The Arizona State University (US) and The University of Calgary (CA) set out to explore an alternate hypothesis: that the ultra-low velocity zones may be regions made of different rocks than the rest of the mantle—and that their composition may hearken back to the early Earth.

    Perhaps, Thorne says, ultra-low velocity zones could be collections of iron oxide, which we see as rust at the surface but which can behave as a metal in the deep mantle. If that’s the case, pockets of iron oxide just outside the core might influence the Earth’s magnetic field which is generated just below.

    “The physical properties of ultra-low velocity zones are linked to their origin,” Pachhai says, “which in turn provides important information about the thermal and chemical status, evolution and dynamics of Earth’s lowermost mantle—an essential part of mantle convection that drives plate tectonics.”

    The Tectonic Plates of the world were mapped in 1996, Geological Survey (US).

    Reverse-engineering seismic waves

    To get a clear picture, the researchers studied ultra-low velocity zones beneath the Coral Sea, between Australia and New Zealand. It’s an ideal location because of an abundance of earthquakes in the area, which provide a high-resolution seismic picture of the core-mantle boundary. The hope was that high-resolution observations could reveal more about how ultra-low velocity zones are put together.

    But getting a seismic image of something through nearly 1800 miles of crust and mantle isn’t easy. It’s also not always conclusive—a thick layer of low-velocity material might reflect seismic waves the same way as a thin layer of even lower-velocity material.

    So the team used a reverse-engineering approach.

    “We can create a model of the Earth that includes ultra-low wave speed reductions,” Pachhai says, “and then run a computer simulation that tells us what the seismic waveforms would look like if that is what the Earth actually looked like. Our next step is to compare those predicted recordings with the recordings that we actually have.”

    Over hundreds of thousands of model runs, the method, called “Bayesian inversion,” yields a mathematically robust model of the interior with a good understanding of the uncertainties and trade-offs of different assumptions in the model.

    One particular question the researchers wanted to answer is whether there are internal structures, such as layers, within ultra-low velocity zones. The answer, according to the models, is that layers are highly likely. This is a big deal, because it shows the way to understanding how these zones came to be.

    “To our knowledge this is the first study using such a Bayesian approach at this level of detail to investigate ultra-low velocity zones,” Pachhai says, “and it is also the first study to demonstrate strong layering within an ultra-low velocity zone.”

    Looking back at the origins of the planet

    What does it mean that there are likely layers?

    More than four billion years ago, while dense iron was sinking to the core of the early Earth and lighter minerals were floating up into the mantle, a planetary object about the size of Mars may have slammed into the infant planet. The collision may have thrown debris into Earth’s orbit that could have later formed the Moon. It also raised the temperature of the Earth significantly—as you might expect from two planets smashing into each other.

    “As a result, a large body of molten material, known as a magma ocean, formed,” Pachhai says. The “ocean” would have consisted of rock, gases and crystals suspended in the magma.

    The ocean would have sorted itself out as it cooled, with dense materials sinking and layering on to the bottom of the mantle.

    Over the following billions of years, as the mantle churned and convected, the dense layer would have been pushed into small patches, showing up as the layered ultra-low velocity zones we see today.

    “So the primary and most surprising finding is that the ultra-low velocity zones are not homogenous but contain strong heterogeneities (structural and compositional variations) within them,” Pachhai says. “This finding changes our view on the origin and dynamics of ultra-low velocity zones. We found that this type of ultra-low velocity zone can be explained by chemical heterogeneities created at the very beginning of the Earth’s history and that they are still not well mixed after 4.5 billion years of mantle convection.”

    Not the final word

    The study provides some evidence of the origins of some ultra-low velocity zones, although there’s also evidence to suggest different origins for others, such as melting of ocean crust that’s sinking back into the mantle. But if at least some ultra-low velocity zones are leftovers from the early Earth, they preserve some of the history of the planet that otherwise has been lost.

    “Therefore, our discovery provides a tool to understand the initial thermal and chemical status of Earth’s mantle,” Pachhai says, “and their long-term evolution.”

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    The University of Utah (US) is a public coeducational space-grant research university in Salt Lake City, Utah, United States. As the state’s flagship university, the university offers more than 100 undergraduate majors and more than 92 graduate degree programs. The university is classified in the highest ranking: “R-1: Doctoral Universities – Highest Research Activity” by the Carnegie Classification of Institutions of Higher Education. The Carnegie Classification also considers the university as “selective”, which is its second most selective admissions category. Graduate studies include the S.J. Quinney College of Law and the School of Medicine, Utah’s only medical school. As of Fall 2015, there are 23,909 undergraduate students and 7,764 graduate students, for an enrollment total of 31,673.

    The university was established in 1850 as the University of Deseret by the General Assembly of the provisional State of Deseret, making it Utah’s oldest institution of higher education. It received its current name in 1892, four years before Utah attained statehood, and moved to its current location in 1900.

    The university ranks among the top 50 U.S. universities by total research expenditures with over $486 million spent in 2014. 22 Rhodes Scholars, three Nobel Prize winners, two Turing Award winners, three MacArthur Fellows, various Pulitzer Prize winners, two astronauts, Gates Cambridge Scholars, and Churchill Scholars have been affiliated with the university as students, researchers, or faculty members in its history. In addition, the university’s Honors College has been reviewed among 50 leading national Honors Colleges in the U.S. The university has also been ranked the 12th most ideologically diverse university in the country.

     
  • richardmitnick 4:24 pm on September 20, 2018 Permalink | Reply
    Tags: , Evidence suggests that subducting slabs of the earth's crust may generate unusual features spotted near the core, Experiment used a diamond anvil cell which is essentially a tiny chamber located between two diamonds, ULVZs consist of chunks of a magnesium/iron oxide mineral called magnesiowüstite, ULVZs-ultra-low velocity zones   

    From Caltech: “Experiments using Diamond Anvils Yield New Insight into the Deep Earth” 

    Caltech Logo

    From Caltech

    09/20/2018
    Robert Perkins
    (626) 395-1862
    rperkins@caltech.edu

    1
    The diamond anvil in which samples of magnesiowüstite were placed under extreme pressure and studied. Credit: Jennifer Jackson/Caltech

    2
    Cross-section illustration shows slabs of the earth’s crust descending through the mantle and aligning magnesiowüstite in ultra-low velocity zones.

    Evidence suggests that subducting slabs of the earth’s crust may generate unusual features spotted near the core.

    Nearly 1,800 miles below the earth’s surface, there are large odd structures lurking at the base of the mantle, sitting just above the core. The mantle is a thick layer of hot, mostly plastic rock that surrounds the core; atop the mantle is the thin shell of the earth’s crust. On geologic time scales, the mantle behaves like a viscous liquid, with solid elements sinking and rising through its depths.

    The aforementioned odd structures, known as ultra-low velocity zones (ULVZs), were first discovered in 1995 by Caltech’s Don Helmberger. ULVZs can be studied by measuring how they alter the seismic waves that pass through them. But observing is not necessarily understanding. Indeed, no one is really sure what these structures are.

    ULVZs are so-named because they significantly slow down the speeds of seismic waves; for example, they slow down shear waves (oscillating seismic waves capable of moving through solid bodies) by as much as 30 percent. ULVZs are several miles thick and can be hundreds of miles across. Several are scattered near the earth’s core roughly beneath the Pacific Rim. Others are clustered underneath North America, Europe, and Africa.

    “ULVZs exist so deep in the inner earth that they are impossible to study directly, which poses a significant challenge when trying to determine what exactly they are,” says Helmberger, Smits Family Professor of Geophysics, Emeritus.

    Earth scientists at Caltech now say they know not just what ULVZs are made of, but where they come from. Using experimental methods at high pressures, the researchers, led by Professor of Mineral Physics Jennifer Jackson, have found that ULVZs consist of chunks of a magnesium/iron oxide mineral called magnesiowüstite that could have precipitated out of a magma ocean that is thought to have existed at the base of the mantle millions of years ago.

    The other leading theory for ULVZs formation had suggested that they consist of melted material, some of it possibly leaking up from the core.

    Jackson and her colleagues, who reported on their work in a recent paper in the Journal of Geophysical Research: Solid Earth, found evidence supporting the magnesiowüstite theory by studying the mineral’s elastic (or seismic) anisotropy; elastic anisotropy is a variation in the speed at which seismic waves pass through a mineral depending on their direction of travel.

    One particularly unusual characteristic of the region where ULVZs exist—the core-mantle boundary (CMB)—is that it is highly heterogenous (nonuniform in character) as well as anisotropic. As a result, the speed at which seismic waves travel through the CMB varies based not only on the region that the waves are passing through but on the direction in which those waves are moving. The propagation direction, in fact, can alter the speed of the waves by a factor of three.

    “Previously, scientists explained the anisotropy as the result of seismic waves passing through a dense silicate material. What we’re suggesting is that in some regions, it is largely due to the alignment of magnesiowüstite within ULVZs,” says Jackson.

    At the pressures and temperatures experienced at the earth’s surface, magnesiowüstite exhibits little anisotropy. However, Jackson and her team found that the mineral becomes strongly anisotropic when subjected to pressures comparable to those found in the lower mantle.

    Jackson and her colleagues discovered this by placing a single crystal of magnesiowüstite in a diamond anvil cell, which is essentially a tiny chamber located between two diamonds. When the rigid diamonds are compressed against one another, the pressure inside the chamber rises. Jackson and her colleagues then bombarded the sample with x-rays. The interaction of the x-rays with the sample acts as a proxy for how seismic waves will travel through the material. At a pressure of 40 gigapascals—equivalent to the pressure at the lower mantle—magnesiowüstite was significantly more anisotropic than seismic observations of ULVZs.

    In order to create objects as large and strongly anisotropic as ULVZs, only a small amount of magnesiowüstite crystals need to be aligned in one specific direction, probably due to the application of pressure from a strong outside force. This could be explained by a subducting slab of the earth’s crust pushing its way to the CMB, Jackson says. (Subduction occurs at certain boundaries between earth’s tectonic plates, where one plate dives below another, triggering volcanism and earthquakes.)

    “Scientists are still in the process of discovering what happens to the crust when it’s subducted into the mantle,” Jackson says. “One possibility, which our research now seems to support, is that these slabs push all the way down to the core-mantle boundary and help to shape ULVZs.”

    Next, Jackson plans to explore the interaction of subducting slabs, ULVZs, and their seismic signatures. Interpreting these features will help place constraints on processes that happened early in Earth’s history, she says.

    The study is titled “Strongly Anisotropic Magnesiowüstite in Earth’s Lower Mantle.” Jackson collaborated with former Caltech postdoctoral researcher Gregory Finkelstein, now at the University of Hawai’i, who was the lead author of this study. Other colleagues include Wolfgang Sturhahn, visitor in geophysics at Caltech; as well as Ayman Said, Ahmet Alatas, Bogdan Leu, and Thomas Toellner of the Argonne National Laboratory in Illinois. This research was funded by the National Science Foundation and the W. M. Keck Institute for Space Studies.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.


    Stem Education Coalition

    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

    Caltech campus

     
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