50 Matching Annotations
  1. Mar 2020
    1. [8 ± 1 R/Ra (SD)]

      In this expression, R stands for <sup>3</sup>He/<sup>4</sup>He in an igneous rock sample. Ra symbolizes the same ratio expressed relative to the values obtained in air. Thus the value of R/Ra in MORBs is 8, and this value is within 1 standard deviation of the mean R/Ra.

    2. Although unexpected, a high-3He/4He source dominating the isotope ratio could explain why the R/Ra values are higher than those found for MORBs. This would be most visible in rocks that have low helium concentrations and low U-Th-Sm contents, such as recycled pelagic sediments strongly depleted in almost all their helium and U-Th during subduction

      A "high- 3He/4He" ratio indicates the predominance of Helium obtained from undisturbed regions of the Earth. This ratio also indicates, that chances of finding radioactive Helium (helium-4) is low. Hence, it is not a surprise, to see these kind of helium ratios in rocks with low concentrations of radioactive trace elements, such as, U and Th.

    3. H. M. Gonnermann, S. Mukhopadhyay, Nature 449, 1037–1040 (2007)

      While <sup>4</sup>He is formed as a result of radioactive decay, the origin of <sup>3</sup>He has been traced to collision with meteorites. Thus a high <sup>3</sup>He/<sup>4</sup>He ratio indicates that the rock sample has been collected from a region which has been undisturbed and very ancient. Hence it is expected that OIBs, which are conventionally identified as undegassed, primitive mantle sources, should show high concentrations of <sup>3</sup>He. However, this conventional model fails in reality. This is also known as the "Helium Concentration Paradox". This paper aims at self-consistently explain the paradox by measuring the carbon dioxide in mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs).

    4. A negative Nb anomaly characterizing the involvement of subducted material is present in all the trace-element patterns of our studied samples. This anomaly implies that a recycled crustal component, and not the ambient mantle, dominates the trace-element budget in the fluid inclusions.

      In general, the principal components of the continental crust are trace elements. These trace elements deplete with time. However, this depletion is very different for Nb compared to other trace elements, like Th and La. This is called the negative Nb anomaly.

    5. continental plume–related basalts

      When large stretches of land are covered by lava generated from a massive volcanic eruption, a type of basaltic lava is created. Sometimes, these eruptions are also caused by movement of tectonic plates in the lithosphere. Such events when combined with abnormally hot rock in the Earth's mantle (mantle plume), give rise to continental plume-related basalts.

    6. This indicates that a less degassed, high-3He/4He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts

      The composition of ocean island basalts provides vital information about how and when the different reservoirs are formed in the mantle. The high 3He/4He ratio and degassed nature of the mantle source are the clues which tell us the real age of the ocean island basalts.

    7. shallow crustal contamination

      This occurs when magma inside the Earth's mantle gets polluted by small amounts of volcanic rocks.

    8. degassing

      It is the process of removal of dissolved gases from liquids, especially aqueous solutions.

    9. T. Hanyu, I. Kaneoka, Nature 390, 273–276 (1997)

      This article reports the helium isotope data from ancient basalts located in three islands in the southern pacific ocean. The HIMU (high μ, where μ= U-238/Pb-204) sample collected from this area shows a relatively low and stable helium-3 to helium-4 ratio. While, other enriched samples show a variation in this isotope ratio.

  2. Feb 2020
    1. The He isotopic data for fluid inclusions in superdeep diamonds presented here resolve this issue by showing direct evidence that the high-3He/4He source must be present in the deep mantle, beneath a depth of 410 km.

      This is the main conclusion of this research work. The authors have presented evidence that the diamond fluid inclusions analyzed originate from the deep mantle region of the Earth, which is at least 410 kilometers underneath Earth's surface.

    2. He in particular has been used to define large-scale mantle structures

      The news article below highlights the method used to examine the Earth's interior, especially the mantle region. https://phys.org/news/2019-09-gigantic-masses-earth-mantle-untouched.html

    3. lithophile elements

      The term lithophile was coined by Goldschmidt to describe elements with affinity for silicates. The Greek word lithophile means rock-loving. These elements are primarily found in regions with higher concentrations of silicate, e.g., the mantle and crust. A few examples of lithophile elements are Li, Na, Mg, Al and Si.

    4. δ13C-δ18O

      The term ‘δ<sup>13</sup>C-δ<sup>18</sup>O’ denotes the isotopic signatures of carbon and oxygen elements. An isotopic signature is calculated from the ratio of stable isotopes (13-C/12-C or 18-O/16-O) and expressed in parts per thousand.

    5. The highest R/Ra value for our diamonds was from the Juina-5 kimberlite, and it coincides with the R/Ra of 49.8 from Baffin Island picrites (4), the highest observed in basalts.

      In the diamond fluid inclusions richest in helium-3, the <sup>3</sup>He/<sup>4</sup>He ratio is about 50 times to that of air. This considerably high isotope ratio suggests that the real age of these fluid inclusions date back to the early days of Earth's formation.

    6. kimberlite

      A kind of intrusive igneous rock that is formed deep inside the Earth’s interior. Kimberlite tends to move upwards via the upper mantle and lower and upper crusts, ultimately reaching the surface of the Earth. When the rocks move up, they carry diamonds inside them, thereby becoming an important source of diamonds.

    7. Preservation of He and its isotope signatures in diamond is supported by He heterogeneities within individual diamonds (18, 20)

      In conducting their experiments, the research team acknowledged the inherent variability in the <sup>3</sup>He/<sup>4</sup>He ratio inside each diamond. To minimize these differences, the scientists used diamonds that were free of inclusions thereby eliminating the possibility of inconsistencies in helium ratios originating from these inclusions. Then, the crushed the diamond in a vacuum to verify that 90% of the helium is present in the groundmass of the diamond. This was then followed by step-heating the crushed diamond to measure the variability in the <sup>3</sup>He/<sup>4</sup>He ratio in the diamond.

    8. trace-element

      A chemical element which constitutes less than 0.1% of a rock's composition.There is unique geochemical information stored in the variation of concentration of each trace element. Zn, Cd and Sr are a few examples of trace-elements.

    9. picogram analyses of Pb-Sr isotopes of fluid inclusions

      Picogram is a unit of measurement of weight and it is equivalent to one-trillionth (10<sup>-12</sup>) of a gram. A picogram analysis is done by weighing a sample at the picogram scale and then using other analytical techniques on this sample to get meaningful information.

    10. all diamonds show typical sublithospheric features

      In order to confirm the sublithospheric features, the authors characterized the structure of these diamonds using a technique called cathodoluminescence imaging. With this technique, light emitted by a sample when irradiated with electron radiation can be measured.

    11. Our diamonds have physical features and properties that are consistent with other diamonds from Earth’s transition zone (410 to 660 km depth), including dislocations or diffuse growth zones (database S1) and no detectable nitrogen (N) or fully aggregated N defects (database S2) (24).

      There has been an ongoing debate about the exact location of the chemical reservoirs that lie deep within the planet Earth. Timmerman and colleagues have successfully narrowed down this location to Earth's transition zone, i.e., 410 to 660 kilometers underneath Earth's surface. These conclusions are based on not only the relative abundance of helium-3 to helium-4 isotopes, but also on the physical similarities between the studied diamonds with already identified ones.

    12. Diamonds are physically and chemically robust, allowing retention of He isotope signatures that reflect their formation environment (18–20).

      Multiple investigations have been performed to study the distributions and compositions of helium in a group of well-characterized diamonds. Studies during the 1980s made the scientists aware of the unusually <sup>3</sup>He/<sup>4</sup>He ratio within individual diamonds. More recent explorations indicate that these diamonds are sources of radiogenic helium-4 and are generally found underneath the Earth’s mantle.

    13. An upper mantle location for the high-3He/4He reservoir has also been suggested on the basis of seismic anomalies, heterogeneities sampled by small degrees of melt, and modeled low–U-Th/3He domains formed through melt depletion (1, 12–17)

      Conventionally, a high <sup>3</sup>He/<sup>4</sup>He ratio is known to originate from the lower mantle region of the Earth. However, studies show that this is not always true. A new model proposed by scientists analyzed the seemingly inconsistent results and hypothesized that the high <sup>3</sup>He/<sup>4</sup>He content may also arise from the upper mantle region.

    14. The chemical evolution, nature, and scale of these different reservoirs remain problematic.

      It is said that our planet Earth is about 4.54 billion years old. This planet has undergone a series of chemical and biological evolution processes since then. However, many scientists believe that the absolute core of the Earth has remained undisturbed. There has been a lot of debate regarding the chemical composition of the different reservoirs that still exist in these primitive locations. The news article below highlights the important findings of a recent publication in PNAS. It reports the existence of a significant reservoir of methane deep under the ocean. Learn more here: https://www.sciencealert.com/scientists-identify-gigantic-reservoir-of-methane-buried-under-the-ocean

    15. transition zone

      The area that separates the Earth's upper mantle from its lower mantle. The depth of this zone is usually between 410 to 660 kilometers beneath the Earth's surface.

    16. fluid inclusions

      Small quantities of gases or liquids that remain trapped inside minerals. These inclusions provide critical insights on the geological processes in the Earth's interior.

    17. superdeep

      At depths of more than 410 kilometers underneath the Earth's surface.

    18. mantle

      A part of the Earth’s interior that lies between the dense, extremely hot core and the thin outer layer, known as the crust. It is made up of a thick rocky shell that constitutes 84% of Earth’s volume.

    19. reservoirs

      Refers to a mass of material that experiences a common set of chemical interactions. Reservoirs, in most cases, have distinct boundaries (e.g., an ocean).

    20. We studied 24 diamonds (1.3 to 6 mm in size) from the Juina-5 and Collier-4 kimberlites and São Luiz River (Juina, Brazil).

      The authors carefully inspected 24 diamonds excavated from the Juina area of Brazil. This location was chosen because the diamonds excavated from this area show characteristics similar to the ones which belong to the Earth's transition zone (410 to 660 km depth). The sizes of these diamonds ranged between 1.3 mm to 6 mm and in spite of this limitation in size, they could successfully detect the helium gases trapped in these diamonds.

    21. After establishing the sublithospheric origin for our diamonds, we measured helium isotopes of the fluid inclusions.

      The helium isotopes were measured from the fluid inclusions using mass spectrometry. Mass Spectrometry is a specialized technique, which is used to determine relative abundances of isotopes from a sample. See Essential Knowledge 1.D.2 in the AP Chemistry Course and Exam Description.

    22. We combined these data

      Here, the authors have brought together several batches of data, where each batch represents the relative abundances of isotopes present in fluid inclusions inside diamonds, like helium, Pb-Sr, trace-elements, and carbon isotopes. After collecting this data, they have plotted them in multiple graphs to highlight comparisons between them. This is very common in scientific research and requires training in data analysis and graph plotting. This is also recommended in the Science Practice 5 of AP Physics 2 Course and Exam Description.

    23. We removed the outer rim of the diamond to eliminate any 4He implantation [up to 30 μm from the diamond surface

      The primordial diamonds excavated by the scientists contained fluid inclusions consisting both helium-4 and helium-3 isotopes. The helium-4 isotopes are formed from radioactive decay of trace-elements and hence take a long time to form. While, the helium-3 isotopes exist inside Earth from the early days of formation and there has been no new generation of helium-3 isotope since then. Hence, removing the outer rim consisting of helium-4 isotopes from the diamonds narrows the search to helium-3 isotopes located in very ancient chemical reservoirs.

    24. This observation requires that the high-3He/4He source has higher He abundances than reservoirs with low 3He/4He ratios and thus supports the presence of a primordial 3He plume.

      Any increase in the helium-3 isotope abundance provides strong evidence of the location and age of the chemical reservoirs explored in this study. Helium-3 isotope formation dates back to the beginning of Earth and any recent formation of the isotope on Earth's surface has been ruled out due to its tendency to get lost in the space. Thus the the authors are confident that this spike in helium-3 is coming from very ancient reservoir deep inside the Earth.

    25. The He isotopic signatures released from the sparse fluid inclusions vary from 0.7 to 49.9 R/Ra (Figs. 1 and 2).

      This observation indicates that the R/Ra ratio measured from the sparse fluid inclusions cover a very wide range of values. Higher values indicate that the relative abundance of helium-3 to helium-4 to that in air is very high (almost 50 times). When the values are this high, there is a good chance of detection of elevated helium-3 isotope, thereby confirming the fact that these fluid inclusions are coming from a very ancient reservoir located deep inside the Earth.

    26. The carbon isotope compositions of the diamonds

      The carbon isotope compositions were measured using an instrument called Stable Isotopes mass spectrometer. Watch this video to get an idea about how this instrument works in a science laboratory: https://www.youtube.com/watch?v=SHbzEwMt-1s

    27. M. G. Jackson, J. G. Konter, T. W. Becker, Nature 542, 340–343 (2017)

      In this paper, the authors have shown that only the hottest hotspots with the slowest wave velocity draw from the ancient reservoirs formed during the early days of planet Earth.

    28. M. J. Walter et al., Science 334, 54–57 (2011)

      There has been a lot of debate regarding the exact location of the chemical reservoirs containing superdeep diamonds. In this research, the authors have done carbon isotope analysis on these diamonds and concluded that the most likely origin is in the lower mantle of Earth.

    29. D. L. Anderson, Proc. Natl. Acad. Sci. U.S.A. 95, 4822–4827 (1998)

      This research article debunks the popular outcomes of previous geological models of Earth's interior. The author shows data to disprove the previous hypothesis of correlating high R in basalts with excess of helium-3 and existence of primitive gas-rich chemical reservoirs.

    30. M. D. Kurz, J. J. Gurney, W. J. Jenkins, D. E. Lott III, Earth Planet. Sci. Lett. 86, 57–68 (1987)

      In this paper, the authors have performed a series of experiments on diamond samples in order to ascertain the chemical composition and isotope variability of diamonds from the Orapa kimberlite.

    31. F. Kaminsky, Earth Sci. Rev. 110, 127–147 (2012)

      There are several existing models of the Earth's formation. In this research, the author points out the discrepancies in the preconceived models and highlights the fact that the chemical compositions of the upper and lower mantle are different.

    32. J. C. VanDecar, D. E. James, M. Assumpção, Nature 378, 25–31 (1995)

      The author reports a seismic study of old parts of southeast Brazil and provides no concrete evidence of metamorphic rocks which are commonly found in regions occupied by subducted oceanic plates.

    33. M. Broadley et al., Geochem. Perspect. Lett. 8, 26–30 (2018)

      In this research, the authors investigate the geochemical processes involving diamond formation within the Siberian cratonic lithosphere. They particularly study the halogen and noble gas geochemistry of fluids trapped in diamonds sampled from this region.

    34. R. L. Christiansen, G. Foulger, J. R. Evans, Geol. Soc. Am. Bull. 114, 1245–1256 (2002)

      In this research paper, the author presents their data of helium isotope analysis of rock samples from the Yellowstone region. They conclude that the analysis shows no evidence of a deep mantle origin of the collected samples.

  3. Oct 2019
    1. seismic tomography

      This is an imaging technique that uses seismic waves generated by earthquakes and explosions to create computer-generated, three-dimensional images of Earth's interior. More information on how this technique works can be found here : https://www.iris.edu/hq/inclass/downloads/optional/269

    2. pelagic sediments

      These are very fine-grained particles which gradually accumulate on the ocean floor over time. These deposits comprise of both inorganic (by products of volcanic activities) and organic (marine plants and animals) matters.

    3. mid–ocean ridge basalts

      The mid-ocean ridge is one of the largest chain of volcanic mountains on Earth, with 90% of the mountains submerged underneath the ocean. A type of basaltic rock originating from volcanic eruptions in this region is known as mid-ocean ridge basalt.

    4. ocean island basalts

      Basalt is a type of igneous rocks which comprises 90% of all volcanic rocks. When these basalts are formed as a result of volcanic activities inside the ocean and away from the tectonic plate junctions, they are known as ocean island basalts.

    5. radiogenic 4He

      A radiogenic isotope is formed by the process of radioactive decay. For instance, in this case, the stable isotope helium-4 is generated from the decay of a radioactive helium-4 nucleus.

    6. slab subduction

      A slab is a part of the tectonic plate which undergoes subduction. Subduction is a geological phenomenon occurring at the junction between two tectonic plates. This involves pushing one plate below the other, so much so that the sinking plate protrudes into the Earth’s mantle.

    7. primordial undegassed reservoir

      Ancient reservoir in Earth's interior, composed of trapped gases that have not been removed.