A part of 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. In geologic time, it behaves as a viscous fluid.

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 clues which tell us the real age of the ocean island basalts. High ³He/⁴He ratios means it comes from a deep mantle source. The extreme in He-C-Pb-Sr is due to the recycling of material within the transition zone. The interaction of the deep mantle with the transition zone is what creates the variability in ocean island basalts.

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. As U and Th are the parent isotopes to helium-4, if there is no parent isotope at the start of the radioactive decay, there will be little daughter isotope produced.

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

Negative Nb anomalies are characteristic of subducted material. Subducted material includes continental-sources sediments from the ocean floor that gets subducted into the mantle. The trace element patterns look more like the crust than the mantle, which means that the fluid inclusions' trace element record is not recording any mantle processes.

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, the elevated levels of helium-3 isotope indicates that these fluid inclusions are coming from a very ancient reservoir located deep inside the Earth.

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.

The outer rim may be contaminated by helium-4 (4He) from the surrounding material. 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. This removal is an experimental design to attempt to get a pure analysis of the helium only from the diamond.

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 that belong to the Earth's transition zone (410 to 660 km depth). Kimberlite pipes are an explosive and geologically "quick" process that brings diamonds up to the Earth's surface with little to no interaction with the surrounding rocks, making them ideal for this study. The sizes of these diamonds ranged between 1.3 mm to 6 mm. Despite the size limitation, the research team could successfully detect the helium gases trapped in these diamonds.

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 (usually the denser one) protrudes into the Earth’s mantle.

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.

Mid-ocean ridges occur where two tectonic plates pull apart and the mantle below wells up and then rapidly cools due to the interaction with the ocean's cold water. This is what makes up oceanic crust.

This occurs when magma inside the Earth's mantle gets polluted by small amounts of crustal rocks—that is, rocks from Earth's crust. Shallower crustal rocks are more felsic, which means they contain more feldspar and quartz.

While ⁴He is formed as a result of radioactive decay, the origin of ³He has been traced to collision with meteorites. Thus a high ³He/⁴He 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 ³He. 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).

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.

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

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.

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.

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

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.

The term ‘δ¹³C-δ¹⁸O’ 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.

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.

Picogram is a unit of measurement of weight and it is equivalent to one-trillionth (10^-12) 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.

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.

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.

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 ³He/⁴He 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.

Conventionally, a high ³He/⁴He 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 ³He/⁴He content may also arise from the upper mantle region.

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

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.

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.

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

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).

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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