A Review on Elemental and Isotopic Geochemistry

Abstract

Geochemistry is the study of the development, and distribution of chemical elements on Earth, which are found in rock-forming minerals and their byproducts, as well as in living beings, water, and the environment. The elemental geochemical variation of sediments is used to recognize the mechanisms controlling the estuarine environment and serves as a baseline for assessing the environmental effect in the future. Geochemistry is a unique field that deals with the study of mineral deposits. It also addresses the interconnections between the structures of rock, soil, water, and air, which vary according to different places. Furthermore, groundwater is the solely accessible water supply in many desert basins, particularly in developing nations. Geochemical indicators are proper instruments for addressing a diversity of hydrological issues, particularly in arid and semi-arid settings. Thermodynamically, the fugacity of oxygen (fO2) in solid earth varies by many orders of magnitude. Enstatite chondrites can have high levels of hydrogen abundance, hydrogen, and nitrogen isotope compositions like those of the earth’s mantle. The chapter deals with the basic concept of geochemistry and its types, as well as the development of geochemistry. It also explains elemental and isotopes geochemistry, human health, and medical geochemistry.

1. Introduction

Christian Friedrich Schönbein, a German-Swiss scientist, coined the word “geochemistry” in 1838 [1]. Geochemistry is the study of the chemistry of natural earth materials, and chemical processes that occur within and on the earth’s surface today and in the past. Geochemical investigations can thus cover a wide range of minerals and analyses, and they can be carried out in a variety of situations, including industrial, environmental, and educational [2]. The study of chemical processes that occur in natural sources of earth materials such as rocks, as well as the evaluation of their compositions and structures, is called geochemistry. Most geological materials are composed of inorganic minerals. The earth has hundreds of natural minerals [3]. Maybe the greatest explanation is that in geochemistry, we use chemical techniques to address geological issues; scientists use chemistry to comprehend the earth and how it operates. The basic components of the earth are the core (both inner and outer), mantle, and crust. The latter is composed of igneous, sedimentary, and metamorphic rocks. These rocks are frequently silicates rich in Mg and Fe. Scientists may find rocks with these qualities as xenoliths in lavas; they are Mg- and Fe-rich silicate rocks composed principally of olivine and pyroxenes [4]. Quartz, feldspars, amphiboles, pyroxenes, olivines, biotite, garnets, clay minerals, and calcite are among the naturally occurring minerals that make up most of the earth’s crust. Except for calcite, all these minerals are silicates. Most rock-forming minerals of the earth’s crust are made of Al, Si, and O. The fundamental primal material of soil is rock, which is broken down by weathering to generate loose debris known as soil parent material, the physical and chemical characteristics of which alter. Earlier research has discovered that the soil parent material is a significant natural source of heavy metals, determining the initial heavy metal level in the soil. Soils of various sorts are found in many geological formations. Indeed, many kinds of soils can exist within the same geological unit. Soil heavy metal contamination is distinguished by its concealment, irreversibility, and long-term nature, which complicates heavy metal pollution control [5].

Heavy metals (HMs) are naturally present in soils, and their quantity reflects the composition of the parent rocks from which they are created. Excessive or high-level concentrations of certain of these metals in soil offer major environmental concerns, including health threats to plant, animal, aquatic, and human life. Therefore, understanding the geochemical and mineralogical composition of sediments may be required to predict the fate of discharged contaminants [6]. Owing to their typical coherent behavior and sensitivity to changes in pH, redox potential, and adsorption/desorption processes, rare earth elements (REEs) compositions are often used as a proxy in groundwater geochemical studies [7]. Geochemical technologies are used in the process of mass prospecting for mineral resources. Their participation significantly improves the efficacy of geological exploration activity and helps geological scientific-technical advancement. Finding previously unknown mineral deposits needs a geochemical investigation. To emphasize the geographical relationship of geochemical patterns, create elemental correlations, or extract geochemical anomalies induced by mineralization, several approaches have been used to analyze geochemical exploration data. In recent decades, there has been a lot of interest in the normal, lognormal, power-law, multimodal, and sophisticated distribution laws of geochemical element concentrations. Methods including classic statistics, multivariate statistics, geostatistics, fractal/multifractal, machine learning, and deep learning algorithms that follow distribution laws have been used to identify geochemical anomalies associated with targeted mineralization [8].

Elemental geochemical characteristics and their ratios are extensively used to evaluate paleoenvironmental conditions, ancient water compositions, paleotectonic settings, and sedimentary rock origin.

This chapter describes the geochemistry of elemental and isotopes and their impact on the environment and humans.

2. Analytical geochemistry

Analytical geochemistry generally focuses on 5 approaches: (1) major element geochemistry; (2) trace element geochemistry; (3) mass fractionation determination; (4) age dating; and (5) radionuclides isotopes for geochemical probes. When geochemistry first began, the idea was to use conventional wet chemistry to identify the bulk main elements. One of the most prevalent methods for classifying igneous rocks was to use a total alkali versus silica) diagram. The classification of rocks is straightforward yet crucial, although an exact determination of the primary constituents is necessary. X-ray fluorescence spectroscopy was created to replace the time-consuming and difficult conventional wet chemistry, and it has been widely utilized since then.

Geochemical analyses are performed for industrial, environmental, or academic reasons; thus, involve a diverse range of materials and analytes of interest such as air, volcanic gas, water, dust, soil, rock, or biological hard tissues (particularly ancient biological tissues), as well as anthropogenic materials, such as industrial effluent and sewage sludge [9]. The analytical approaches used in geochemistry are categorized based on the physical principles into eight methods, as illustrated in Figure 1. Methods like mass spectroscopy dominate fields like isotope geochemistry, despite their wide range of sample types and machine preparation [11]. Whether the composition of rocks, glass chemistry or minerals is important in areas like sedimentary, igneous rocks, metamorphic geochemistry, as well as other domains of geochemistry directly related to petroleum research, a variety of analytical methods are frequently used. The process of figuring out the chemical interactions that exist between mineral grains is known as mineral analysis. To analyze major and trace elements, analytical techniques with high precision such as X-ray spectroscopy, ionic radiation techniques, and laser ablation processes are frequently used [12].

Figure 1.

Geochemical analyses modified from [10].

Aside from minerals, the study of fluid inclusions (small drops of fluid trapped inside crystals during the initial growth of the solution or later) has aided in the development of modern theories of ore origin, reproduction, formation, oil migration, accumulation, as well as our understanding of the importance of the liquid stage in a variety of geological processes [13]. Fission path analysis is a specialized method for extracting information from minerals that analyses the signs of physical damage caused by the spontaneous fission of the original nucleus to the crystal network. When the rate of spontaneous fission is known, the accumulation of these fission pathways can be used as a dating tool in other geological studies [14].

X-ray Fluorescence Spectrometer (XRF) is mostly used to analyze solid materials. A 100 mg sample is diluted 10 times with a flux made of pure LiBO2 and Li2B4O7 and melted into a glass bead in a Pt crucible [15]. Because the absorption of X-rays rises with mass number, lithium borate is a good material for making the glass bead for measuring the emitted X-rays [15]. The chamber is kept under a vacuum to reduce the loss of secondary X-rays. The mass numbers of N2 and O2 in the air are greater than those of Li or B. As a result, X-rays were absorbed or dispersed by these gases. As a result, X-rays were absorbed or dispersed by these gases. A vacuum is also wanted to prevent the diffracting crystals from deliquescence. To correctly find the principal elements, measurements of H₂O (−), H2O (+), LOI (loss on ignition), and the ferric/ferrous ratio (Fe(II)/Fe(III)) are necessary. The weight of absorbed water is denoted by H2O (). The weight difference between the sample at room temperature and after heating it to 105°C was measured. This is insignificant for hard silicate rock, but it becomes significant for new deep-sea sediments. The weight difference between the raw sample and that heated at 1000°C in a porcelain crucible is referred to as the LOI.

3. The evolution of geochemistry

Geochemistry developed from the same metallurgy and alchemy roots as chemistry and emerged as a distinct subfield of chemistry and geology in the nineteenth century [16]. Clarke Goldschmidt and Vernadsky’s works laid the groundwork for the field’s subsequent development and progress in the 20th and 21st centuries [17]. Geochemistry has become an integral part of all aspects of earth and planetary sciences over that time, greatly enriching our understanding of the nature and evolution of our planet [17]. Geochemistry is a modern science and belongs to the group of earth sciences that studies the composition and geological history of the earth. Geochemistry arose because of the accumulation of large numbers of geological and chemical studies, and the urgent need to solve many of the issues associated with the exploration of useful ores reservoirs [18]. The field of geochemistry has expanded, and branches such as organic geochemistry have emerged. Organic materials are the cornerstone of this branch of geochemistry means organic geochemistry in two main divisions: a) oil chemistry, asphalt, coal, humic acids, and other organic components that are found in sedimentary rocks; b) the role of organic processes and living organisms in fixing inorganic elements. This type of study is sometimes called biogeochemistry. Geochemistry also blends with other branches of science such as mathematics, physics, and statistics and has become a quantitative trend in recent years. Thermodynamics is considered the backbone of experimental geochemistry to predict the chemical reactions between minerals [19]. As for statistics, it is possible to describe the chemical composition of large groups of land units of the crust, even the interactions of igneous rocks, at statistical rates. Geochemistry has also benefited from contemporary methods in determining the properties of solid materials. These methods can be divided into chemical analysis and structural analysis techniques. The latter includes the technical method that extends to understanding the nature of chemical bonds. The ancient chemical methods of analyzing minerals and rocks depended on using acids and other chemicals to determine the proportion of major elements. There has been a tremendous development in this field, where the use of emission X-ray spectroscopy is an alternative method for the analysis of major and trace elements [9].

Elemental geochemistry is concerned with studying the distribution of chemical elements, their forms of existence, and their concentration rates in metals, rocks, and earth’s mantle [20]. This branch has made it possible to reveal many facts about the distribution of chemical elements.

Biogeochemistry studies the distribution of chemical elements and their concentrations in organic matter, the role of living organisms in the movement of these elements, and their distribution in the earth’s various atmospheres [21]. Although the weight of living organisms is estimated at 0.01% of the weight of earth’s crust, they have a significant role in geological processes, which are called biogeochemical processes. We cite as an example that iodine is obtained mainly from some types of marine plants that extract it from seawater and concentrate it in their tissues in proportions hundreds of times higher than its concentration in water. These processes also include photosynthesis and the liberation of oxygen under the influence of solar energy, which is stored in the form of fossil fuels (coal, oil, and gas). One of the crucial topics in this science is the study of the biogeochemical provinces, which are areas in which the concentrations of chemical elements change from their general rates, increasing or decreasing [21]. It was found that there are at least thirty chemical elements involved in the formation of these zones. The iodine-poor provinces, which spread in high areas, the interior parts of the continents and others, the regions poor in fluorine, which causes tooth decay, and the provinces rich in fluorine and volcanic areas and the areas of the spread of apatite rocks, and usually, cause osteoporosis.

Isotope geochemistry studies the distribution of isotopes in the chemical elements that make up natural bodies (rocks, metals, water, and organic matter) [22]. Another aspect of this science is finding the absolute ages of bodies and natural phenomena using radioactive elements such as uranium, thorium, and potassium (K40). Cosmochemistry is concerned with studying the distribution of chemical elements and their concentrations in celestial bodies, especially meteorites. It also interests the effect of cosmic rays on the components of the earth’s atmosphere, especially the gaseous atmosphere, which causes the formation of some elements such as carbon-14 [23]. Cosmological studies are helpful in verifying information about the origin and development of the earth and the evolution of geological processes. Isotopes are atoms of the same element with differing numbers of neutrons. Differences in the number of neutrons among distinct isotopes of an element indicate that the isotopes have different masses. Isotopes of the same element have the same number of protons. All isotopes of oxygen contain 8 protons, but an oxygen atom with a mass of 18 (denoted ¹⁸O) has two more neutrons than oxygen with a mass of 16 (¹⁶O).

Nuclear reactions in stars determine the primordial isotopic compositions of planetary systems. Radioactive decay, cosmic ray interactions, mass-dependent fractionations that go with inorganic and biological reactions, and anthropogenic activities such as nuclear fuel processing, reactor accidents, and nuclear-weapons testing can change isotopic compositions in terrestrial environments over time. Nuclides (isotope-specific atoms) that spontaneously break down over time to generate other isotopes are known as radioactive (unstable) isotopes. Radioactive isotopes release alpha or beta particles, as well as gamma rays, during disintegration. On geologic time scales, stable isotopes do not appear to decay to other isotopes, yet they can be generated by the decay of radioactive isotopes. Therefore, the main target of this chapter is the geochemistry of elemental and isotopes and their impact on the environment and humans. Fractionating processes cause changes in the stable isotope compositions of metals and metalloids. Radiogenic processes can cause metal stable isotope variations in some elements, such as the radioactive decay of unstable to stable isotopes [24]. Strontium (Sr) and lead (Pb), both of which have four stable isotopes, are the most important examples of environmental studies. For Sr., one isotope is influenced by radiogenic processes (⁸⁷Rb, ⁸⁷Sr decay), whereas the other three are only influenced by stable isotope fractionation (e.g., ⁸⁸Sr/⁸⁶Sr), allowing Sr. isotope signatures to be used as two-dimensional tracers by investigating radiogenic processes and stable isotope fractionation in parallel [25]. While radiogenic Sr. isotope variations have been studied for decades and applied in many fields, such as ecosystem research, natural water studies, and archaeology, stable Sr. isotope fractionation research is still in its infancy. Three of the four stable isotopes of Pb are byproducts of radioactive U-Th decay chains. As a result, stable Pb isotope fractionation in natural samples cannot be detected because there is no ratio between two isotopes that is unaffected by radiogenic processes [25]. Depending on the geochemical composition and age of the source materials, relatively large Pb isotope variations between environmental samples can be observed, and these are generally thought to outnumber potentially occurring mass-dependent and nuclear-volume effects of Pb isotopes by a factor of up to 200 [26]. Natural (e.g., ²¹⁰Pb₂₅) and anthropogenic nuclear processes produce some radioactive metal isotopes (e.g., ¹³⁷Cs). Finally, metal stable isotopes used as enriched tracers (or “spikes”) have a wide range of applications in environmental research [27].

Groundwater geochemistry studies the groundwater, which is a vital water resource that must be protected to preserve its long-term viability. Aquifer resource evaluation needs a right report on the origin, age, source, and migratory passages of groundwater. To determine provincial impacts in an aquifer using just hydrogeologic procedures, data from several wells, time-series analysis of water levels, discharge monitoring, and permeability and aquifer boundary measurements are frequently required. Deep under the Earth, water reduces the melting temperature of rock, resulting in magma that forms the continents. Deep crustal fluids, such as chloride-rich brine, transport gold, and other metals to create ore deposits. The water of the ancient oceans served as a crucible for the evolution of early life, and freshwater is the foundation of life on earth. Homogeneous hydrogeochemical reactions include only one phase, whereas heterogeneous reactions involve two or more phases, such as gas and water, water and solids, or gas and solids. In contrast to open systems, closed systems can only interchange energy with the environment, not components. Multi-tracer studies are often used to estimate hydrodynamic characteristics such as groundwater flow, mixing patterns between different groundwater sources, and recharge rate and such data are required to better water-resource management. However, in arid areas, estimation of groundwater recharge remains complicated because of spatial variability of rainfall and a low amount of recharge.

4. An insight into isotope geochemistry: history and importance

Because of their suitable geochemical and nuclear properties, isotopes are used as tracers and chronometers to study a wide range of topics, including rock and mineral chronology, reconstruction of sea-level changes, rock water interactions, and magmatic processes [28]. Isotopic observations have revealed time scales of mixing processes in the seas and atmosphere and the residence durations of marine components and gases in the atmosphere. Isotope geochemistry is a branch of geology that studies the natural fluctuations in the relative abundance of isotopes of certain elements [29]. Isotope ratio mass spectrometry measures variations in isotope abundance and can give information about the ages and origins of rocks, air, and water bodies, as well as their mixing processes. The geochemistry of radioactive isotopes is concerned with Nucleosynthesis, Radiogenic, and Stable Isotopes (Figure 2). Nucleosynthesis (i.e., the origin of elements) in stars can generate “new” heavy isotopes like Proton-proton (H to He), Helium burning (He to C), s-process (neutron capture), and p-process (proton capture). Plutonium isotopes, 239-Pu, 240-Pu, and 241-Pu are produced in nearly all nuclear reactors by neutron capture on naturally occurring uranium 238-U [30].

Figure 2.

Types of isotope geochemistry.

The measurement of the age of the earth and our solar system is arguably the most significant milestone in the use of isotopes in earth science. Isotope-based dating methods are the gold standard and are utilized to confirm non-isotope-based dating methods. Stable isotope tests of organic materials, and phosphate in bones and teeth in recovered fossils, give evidence for dietary sources ingested by humans and other animals.

The isotope geochemistry domain began after Henri Becquerel discovered a radioactivity item in 1896 [31]. Within a few years, following this astounding finding, Rutherford reported an exponential decline in radioactive material activity with time and created the notion of half-lives, paving the way for the age determination of natural compounds containing radioactive elements (Rutherford 1900) [31]. Soddy coined the term “isotope” in 1913 (Soddy 1913). Soddy (1913) and Fajans (1913) developed the principles regulating the transformation of elements during radioactive decay at the same time (1913). Rutherford determined the first radiometric age of a geologic sample on a pitchblende sample in 1905, and the ages of the number of other minerals were later determined by Strutt (1905) and Boltwood (1907) using the U-He and U-Pb techniques. The full ²³⁸U and ²³²Th chain, which is still in use today, was formed in 1913 [28]. The disequilibria between members of the U-Th series caused by differences in the geochemical properties of different elements within the chain opened a new field of study to investigate different geochemical processes like rock-water interaction, dating of inorganic precipitates, detrital and biogenic sediments, and archeological objects. One of the most important findings in this field was the discovery of substantial fractionations of ²³⁸U and ²³⁴U in rocks, rock leachates, and natural waters [32]. Protons, alpha particles, electrons, helium, nuclei of other elements, and subatomic particles are charged particles (including high-energy charged particles) that make up cosmic rays. The high-energy charged particles that reach the atmosphere interact with atmospheric components (e.g., N, O, Ar, and so on) to form a variety of cosmogenic radioactive isotopes with half-lives ranging from a few minutes to millions of years [33]. Some cosmogenic isotopes (e.g., ¹⁴C, ¹⁰Be, ⁷Be) have been used to quantify processes in earth surface reservoirs such as air-sea exchange, atmospheric mixing, ocean circulation and mixing, scavenging, sediment accumulation and mixing rates in aqueous systems, erosion rates, exposure ages, changes in cosmic ray production rates, and human civilization history. Murphy and Urey (1932), Nier and Gulbransen (1939), Dole and Slobod (1940), and Urey (1948) were among the first to quantify stable isotopes in terrestrial materials, long before the discovery of cosmogenic isotopes like ⁷Be, ¹⁰Be, and ¹⁴C. Urey (1947) and Bigeleisen and Mayer (1947) used statistical quantum mechanics and statistical thermodynamics to give the theoretical underpinning for isotopic fractionation. Stable isotope geothermometry is used at low temperatures, that is, non-magmatic temperatures, due to the dependency of the equilibrium constant on the inverse square of temperature. The fractionations are often minor at temperatures over 800°C or thereabouts, making precise temperature determination problematic [34]. However, even at temperatures in the upper mantle (1000°C or higher), fractionations remain considerable, albeit minor. Fractionation factors for minerals in the temperature range of 600° to 1300°C were obtained experimentally and matched with theoretical estimates.

Radioactive and stable isotopes have consequences on humans, both directly and as tracers of processes, that affect their well-being. Human activities such as nuclear weapons, testing, and accidents have released certain radioactive isotopes into the environment [35]. The majority are naturally formed, both from the earth’s original composition and through the ongoing synthesis of U isotopes and Th by cosmic rays, which produce a variety of radioactive daughters and ⁴⁰K in biological systems and the environment.

5. The elemental geochemistry

The elemental geochemistry of clastic sedimentary rocks offers critical information for recognizing sedimentary histories, such as chemical weathering history, nature and composition of sediment origin, sediment transit, and diagenetic history. Geochemical apprehension of these challenges constrains many basic geological topics, including paleoclimates, tectonic linkages, basin formation, diagenetic fluid movement, and crust-mantle evolution. Geochemical apprehension of these concerns constrains many basic geological topics, including paleoclimates, tectonic linkages, basin formation, diagenetic fluid movement, and crust-mantle evolution. Experience from the Mars Exploration Rovers Spirit and Opportunity, orbital spectroscopy, and experiments with Mars-like compositions, demonstrate that such approaches apply to sedimentary environments on Mars, where primary igneous compositions and aqueous conditions may differ from our terrestrial experience. The elemental geochemistry index is currently a reliable predictor of marine and lake habitats. The variation in elemental geochemical characteristics indicates the lithofacies of the shale and the sedimentary environment. Wang et al. [36] discovered that lake-water depth fluctuates from deep to shallow in a high-frequency sequence unit, and element ratios in shales shift regularly, particularly near the high-frequency cycle interface [36]. Chemostratigraphy is the study of inorganic/organic chemical changes within sedimentary sequences, depending on the rock’s elemental or isotopic composition [37]. It is a significant device for the discovery and exploitation of unconventional resources. A rock’s chemistry changes depending on its mineral makeup. Because rock chemistry is so easily measurable, the rocks may be put in a chronostratigraphic sequence framework [38]. Because the Al₂O₃/TiO₂ ratio grows from 3 to 8 for mafic igneous rocks, 8 to 21 for intermediate rocks, and 21 to 70 for felsic igneous rocks, the Al₂O₃/TiO₂ ratio of most clastic rocks is used to infer the source rock compositions [39]. Source rock composition, length of weathering, climatic conditions, and rates of tectonic uplift of the source location mainly controlled the chemical weathering of source rocks. About 75% of the upper crust’s labile material comprises feldspars and volcanic glass, and the chemical weathering of these components eventually culminates in the production of clay minerals [40, 41]. Ca, Na, and K are mainly eliminated from source rocks during chemical weathering. The quantity of these elements that survive in soil profiles and sediments produced from them is a sensitive indicator of chemical weathering severity. If the alkali contents (K₂O + Na₂O) and K₂O/Na₂O ratios in silica clastic sedimentary rocks are devoid of alkali-related post-depositional changes, then they are regarded credible indicators of the source material’s weathering severity [42].

Recently, the sequential stratigraphy of shale has been studied in many regions around the world. Those investigations revealed that, even in an environment conducive to organic matter accumulation, the characteristics and distribution of source rocks changed systemically both vertically and laterally. This alteration can vary from a few centimeters to hundreds of meters and can be laminae to super-sequence [43]. The sedimentary environment and stratigraphic stacking patterns affect the transition, which occurs at both sequence and para-sequence scales. The primary unit of characterizing organic matter enrichment is the para-sequence, and the interior rock characteristics (or litho-faces/facies) of the para-sequence also underwent systematic modifications. Previous research has found a unique association between shale formation division and its sequence hierarchy [43]. Previous studies found that the high-frequency sequence separation of lake-facies sediments occurred mostly during the deposition of coarse debris sediments [44]. Coarse-grained silt shows evident lithologic alterations, making detection of progradation and retrogradation as well as high-frequency sequence division simple. However, dividing shale sequence stratigraphy (particularly high-frequency sequence) is a tough task and a difficult point in the study of high-frequency sequence. Previous studies found that the high-frequency sequence separation of lake-facies sediments occurred mostly during the deposition of coarse debris sediments [45]. Coarse-grained silt shows evident lithologic alterations, making detection of progradation and retrogradation as well as high-frequency sequence division simple. However, dividing shale sequence stratigraphy (particularly high-frequency sequence) is a tough task and a difficult point in the study of high-frequency sequence. The elemental geochemistry index is now a reliable indicator for marine and lake ecosystems. The variation in elemental geochemical parameters reflects the lithofacies of shale and the sedimentary environment [46]. Wang et al. discovered that lake-water depth fluctuates from deep to shallow in a high-frequency sequence unit, and element ratios in shales shift regularly, particularly near the high-frequency cycle interface [36]. The periodic vertical change of organic geochemical parameters in the lacustrine basin serves as a foundation for sequence stratigraphic unit categorization and linkage. Jin et al. [47] discovered strong correlations between the vertical change of geochemical elements and para-sequence cycle changes in lake-facies shales, and the cyclic change of geochemical elements can also be used to classify the stratigraphy division of high-frequency sequences in shales. Their research established the importance of employing geochemical components in high-frequency shale sequence stratigraphic division studies, which has significant implications for shale gas and oil exploration and production.

6. Light elements in the earth

The light elements found in the earth’s core are yet unknown; however, they might include silicon, oxygen, sulfur, carbon, and hydrogen [48]. Knowledge of the earth’s building blocks, planetary accretion and core formation processes, and subsequent chemical and thermal development of the core requires an understanding of the nature of the light elements in the core. Hydrogen, carbon, oxygen, silicon, and sulfur are possibly the primary light elements [49]. The earth’s core is mostly made up of iron and nickel alloyed with lighter elements including oxygen, silicon, Sulfur, carbon, and hydrogen in amounts of 5–10 wt. %. Because oxygen is the most representative element on earth, it has been postulated as a possible main light element [50].

The density deficit of elements, such as silicon, sulfur, or oxygen was assigned. Over the years, several researchers have investigated several elements lighter than iron, such as silicon, sulfur, oxygen, hydrogen, and carbon, either alone or in combination. The existence and significance of a freezing point depression at the inner core boundary (ICB), on which major questions in geophysics and geochemistry hinge, such as to what extent are light elements released at the ICB, thus inducing compositional convection thought to power the geodynamic the temperature at the ICB lower than the melting temperature [51]. When compared to pure iron, the presence of significant amounts of low atomic number ‘light’ elements like sulfur, silicon, oxygen, carbon, and hydrogen in the earth’s outer and inner cores result in a density deficit. Core composition estimates may only be obtained indirectly due to their inaccessibility, by combining results from high-pressure tests and theoretical calculations with seismic data [51]. Restrictions on individual light element concentrations in the core might provide insights into the nature of earth’s building blocks and the path of accretion. More oxidized material from the outer Solar System is thought to have been added near the end of accumulation. Although the amounts of Si, O, and H in the core are still constrained, they would be significant inputs into such a scenario. Although core amounts of Si, O, and H are still constrained, they would be important inputs in such a scenario [52]. The behavior of core-mantle (metal–silicate) element partitioning is affected by metal composition, specifically S, Si, and O concentrations. Recognizing the core, light element concentrations would lead to a better understanding of earth’s core-mantle element partitioning and core formation conditions. The presence of H in the core suggests that H₂O was not only delivered in the final stage of accretion but was also incorporated into core-forming metals via a hydrous magma ocean. The upper bound of 900 ppm H in the bulk earth corresponds to 8100 ppm H₂O, implying that up to 35 times the current ocean mass of water could have been brought to our planet. This high-water content agrees with upper estimates for the number of oceans on earth derived from ab initio calculations of hydrogen partitioning between silicate melt and liquid iron at core-forming conditions.

Metal–silicate partitioning data should assist limit each light element’s concentrations in core-forming metals and abundances in the bulk silicate earth (albeit uncertainties remain due to unknowns regarding the degree of chemical equilibrium20) (BSE) [52].

Recognizing earth accretion, core formation, and the maintenance of the earth’s magnetic field requires constraining the core’s composition. The presence of significant amounts of low atomic number “light” elements such as sulfur, silicon, oxygen, carbon, and hydrogen in the earth’s outer and inner cores results in a density deficit relative to pure iron. The following is a display of these elements:

1. Oxygen

The oxygen fugacity of gas of solar composition is calculated using the procedure H₂ + 12O₂ H₂O after accounting for the oxygen connected to carbon in CO and other less prevalent oxides. The bulk of the Solar System’s rocky planets, including earth, developed with DIW five orders of magnitude bigger than that of solar gas, according to meteorite research. Oxygen fugacity was likely strong during the early phases of rock formation in the Solar System, as evidenced by significant levels of iron linked to oxygen in silicates in chondrite meteorites. Sublimation of water-rich and/or rock-rich dust at high dust/gas ratios may cause an increase in oxygen fugacity during rocky body formation. In this regard, we’d like to know if the mechanisms that cause rock oxidation in the Solar System are replicated in other planetary systems, and if rocky exoplanets exist as a result geophysical and geochemical characteristics of the earth are shared [53]. Geophysical and geochemical characteristics of the earth are shared. As a result, whether rocky exoplanets reflect or not, the earth’s geophysical and geological characteristic is an open question. Carbonate oxygen and carbon isotopes are often utilized as tracers of mineral precipitation temperatures and fluid sources, and bulk rock techniques are typically used to study them. The oxygen and carbon isotopic compositions of Wade Dima carbonates were examined utilizing two complementary techniques, ranging from mm-scale micro-bulk-rock investigations to m-scale in situ isotopic analyses [54].

In solid earth, the thermodynamic property of oxygen fugacity (fO₂) varies by many orders of magnitude. The capacity of fayalite-magnetite-quartz to link to equilibrium, depending on the relative concentrations of ferric and ferrous iron, is commonly portrayed. Arc magmas contain more Fe³⁺ (and hence greater estimated oxygen fugacity) than mid-ocean ridge basalts, as well as a lot of water and other volatile elements (e.g., S, C, F). They vary from MORB in that they have greater chalcophile (sulphur-loving) element concentrations and are typically found in Cu-Au-Ag ore deposits [55].

Because large-scale oxygen isotope fractionation occurs only near the earth’s surface, deviations from the average δ¹⁸O of mantle-derived rocks, whether positive or negative, are usually attributed to the presence of recycled crustal material in the mantle source or the interaction of mantle-derived melts with crustal material during ascent. Valley et al. [56] find that the δ¹⁸O of magmatic zircons have remained relatively constant (and similar to modern mantle) throughout time, back to 4.4 billion years ago (4.4 Ga). This suggests that the δ¹⁸O content of the mantle has stayed relatively stable throughout the earth’s history. Upper mantle and crust melting produce Hadean and Archaean zircons [57]. As a result, this composition of carbonate isotope ratios may be a proxy for limnological conditions. Furthermore, the differential accumulation of certain trace elements during source rock deposition may preserve information about paleoclimate conditions during source rock deposition, which can be used to categorize sedimentary environments [58].

1. Hydrogen

The core may contain the world’s largest H reservoir. The H partition coefficient between iron and silicate, which melts at high pressures and temperatures, was recently calculated by combining ab initio molecular dynamics with thermodynamic integration, and it was proven that under core-formation conditions, H prefers to partition into the iron liquid. The amount of water dissolved into the silicate mantle during core-mantle differentiation determines the optimal quantity of H in the core, which is linked to water in the materials accreting to build earth and its accretion processes. New research suggests that enstatite chondrites (EC), which are assumed to be representative of the materials that formed the earth, may contain high levels of hydrogen and have hydrogen and nitrogen isotope compositions similar to the earth’s mantle [59].

Kerogen is described as the fraction of sedimentary organic constituents of sedimentary rocks that are insoluble in common organic solvents. Kerogens are derived from organic materials such as algae, pollen, wood, vitrinite, and structureless material. Different kerogens have varying amounts of hydrogen concerning carbon and oxygen. The hydrogen content of kerogen determines the oil vs. gas yields from primary hydrocarbon-generating reactions [60].

The first hydrogen index (HI) of hydrogen-rich kerogen is higher, while the initial hydrogen index of hydrogen-poor kerogen is lower. As a result, the early HI levels of marine shales are linked to paleo productivity, terrestrial input, and preservation. The photosynthetic intensity of phytoplankton in the photic zone influences paleo productivity, which can be estimated using a range of geochemical proxies such as organic P (Porg), and biogenic Ba (Babio), excess Si (EXSiO₂), and trace elements. In theory, inorganic geochemical techniques can be used to approximate the original TOC contents and recover the original HI values of high to over-mature marine shales. Because shale formation is so complicated, it’s crucial to consider all of the influencing variables, such as the shale’s formation mechanism and depositional or diagenetic settings, as these have varying effects on geochemical parameters. Examine the inorganic proxies and use a preservation or redox perspective to compute the original hydrogen index [60].

Much experimental research has been undertaken to examine the mechanisms of hydrogen incorporation in olivine under various thermodynamic settings, because the viscosity of olivine-rich rock may decrease as impurities such as H, Fe, or Ti are integrated (e.g., pressure, temperature, fO₂, fH₂O, a SiO₂, and chemical composition). A new experimental approach involves doping olivine with a high proportion of atomic impurity before or after hydration. While these studies reveal a distinct method of H incorporation, the findings aren’t always applicable to the earth’s upper mantle [61].

1. Silicon

Silicon is the third most abundant element on the planet (atomic weight: 28.08553). (16.1% Si; since it is lithophile and reacts incompatibly with other elements during mantle melting, the oceanic and continental crusts are richer in Si than the mantle). In terrestrial conditions, silicon has a single valence state (Si⁴⁺) and does not produce volatile compounds. On the silicate earth, native Si is rare; instead, Si is almost usually bonded to O as the SiO tetrahedron. Si is a crucial cation in the silicate earth and is essential in the biosphere, with diatoms accounting for more than half of ocean primary production. It is also a result of successful continental erosion, which feeds the marine ecosystem. As a result, it’s not surprising that the early uses of Si isotopes, which were within the scope of low-precision technology, were centered on the earth’s supergene semiconductor environment [62].

At first, silicon was assumed to be a light component in the core. Since then, silicon has advanced to become one of the most promising materials. The earth’s mantle has a higher Mg/Si ratio than chondrites (“missing Si”), which can be explained by a large amount of silicon in the core. The absorption of Si into the core also explains why bulk earth has a higher Si isotopic composition than chondrites, but the explanation for such a disparity in Si isotope composition and Si isotopic fractionation factor remains a mystery. Furthermore, because silicon and iron are known to form a wide solid solution, a Fe–Si alloy could approximate the density and compressional-wave velocity of the inner core. Previous theoretical investigations and observations on the Fe–Si binary system at 21 GPa, on the other hand, have shown that the variation in Si concentration between coexisting liquid and solid is far too small to account for the observed density rise at this temperature [63].

The first technique used to precisely identify silicon isotope compositions of interest in geochemistry was gas source mass spectrometry. This method entails breaking down the sample with HF, followed by a series of chemical conversion procedures which vary across authors to generate SiF₄, it can then be injected into a mass spectrometer with a gas source for isotopic analysis. Because sample breakdown and subsequent purification usually result in isotopic fractionation, it is critical to develop a silicon purification yield that is as near to 100% feasible. Following that, the Apollo lunar sample return mission generated d30Si values that were repeated or quadrupled and claimed to fall within an “average deviation” of 0.11 or better if sample fluorination with the F₂ gas was utilized [64].

Silicon could be a light element component for geophysical and geochemical reasons. Lin et al. [65] used the Fe0.85Si0.15 Fe-rich Fe-Si alloy as an example to investigate the effect of a light element on the physical properties of Fe. This is done for several reasons, including the ones listed below: (1) in the core, the abundance of light element(s) is only 10% by weight. (2) Most light elements, such as oxygen and sulfur, have limited solubility in Fe and form intermediate compounds with Fe (such as FeO, FeS, and Fe₃S₂) over a narrow pressure-temperature range; (3) silicon readily forms alloys with Fe under ambient conditions; and (4) Fe-rich Fe-Ni and Fe-Si alloys adopt the hexagonal close-packed (hcp) structure at high pressures, imposing a significant restriction on the crystal structure of the inner core [66].

Figure 3 depicts the general pattern of 30Si fluctuations in numerous geological processes in terrestrial reservoirs. The fractionation of the silicon thermodynamic isotope induced by low-temperature geological processes such as chemical weathering, biogenic/non-biogenic precipitation, adsorption, and biological absorption is smaller at elevated temperatures. Silicon isotope geochemistry provides critical geochemical constraints for tracing bio-physicochemical processes in terrestrial environments, mineral deposit formation, hydrothermal fluid activities, meteorite and planetary evolutions, and a better understanding of the mechanisms underlying silicon isotope fractionation in common geological processes [68].

Figure 3.

Variations in silicon isotopes in nature as a consequence of host rocks, animals, and associated processes of geology [67].

Silicon (Si) is the second most prevalent element in earth’s lithosphere crust, and it is used in a variety of geochemical and biochemical processes. This “Si biosphere” is fed by chemical weathering, which produces secondary minerals with considerable negative Si isotope fractionation and a heavy Si fluid phase. The exact Si isotope content of the protolith, i.e., the continental crust, is unknown; however, it can currently be determined using inductively coupled plasma mass spectrometry with many detectors (MC-ICPMS). The exact and accurate analysis of all three stable isotopes (28Si, 29Si, and 30Si) at high mass resolution has been made possible by these methods. Igneous processes cause minor isotope differences [69].

Silicon (Si) is a crucial nutrient for photosynthetic marine diatoms and, as a result, indirectly influences the oceanic CO₂ storage capacity. Rivers transmit 85% of the silicon dissolved (DSi) that enters the ocean (Figures 3 and 4). Chemical weathering of continental silicate provides for 45% of the riverine dissolved load. Importantly, the Ca and Mg silicate rocks undergo chemical weathering. Affects atmospheric CO₂ concentrations, thereby controlling climate throughout geological periods [70].

1. Aluminum

Figure 4.

The modern-day global Si cycle is represented as just a cartoon schematic [68].

The modern-day global Si cycle is represented as just a cartoon schematic (Figure 3). The fluxes’ magnitudes (in 1012 mol yr1) and corresponding 30Si values (in percent) are displayed as the most common fractionations (ε, ‰). Inset panels show the mechanisms involved in the production of biogenic silica (BSi) and clay minerals. Lines represent particle fluxes, while solid lines indicate solute fluxes or transformations. Details can be regarded as primary text [68].

Although the role of acidity in the behavior of Aluminum in the podsolization hypothesis is not novel, it was not highlighted in the TACAD investigation. Soils containing HBEF were assessed using the podsolization theory. The organic carbon to organic A1 ratios (C/A1) decreased as soil depth increased. The O₂-horizon has a C/A1 of 228 while the Bhs3-horizon has a C/A1 of 14. Because the solubility of organic A1 complexes decreases as the C/A1 decreases, the lower C/A1 in B-horizon soils was interpreted as evidence of A1 retention, as predicted by the podsolization theory. In the Bhs 1- horizon, however, there was a small but significant loss of inorganic A1. This mobilized A1 was exported from the watershed rather than being kept in the lower B-horizons. Significant deviations from the podsolization theory were found in alkaline European locales. Dissolved A1 concentrations increased with soil type in the Netherlands. The depth (upper 40 cm) and inorganic A1 species accounted for more than 80% of dissolved Al in B-horizon soil solutions [71].

A common mineralogical characteristic of massif-type anorthosite complexes is high-aluminum orthopyroxene megacrysts (HAOMs). Their primitive geochemical characteristics (high Mg and juvenile isotope signature), high aluminum content (up to 13 wt.% Al₂O₃), and plagioclase (ilmenite, rutile, and garnet) resolution lamellae due to low-pressure re-equilibration have all been interpreted as evidence of polybasic crystallization history of their parent magmas. They also invented a widely used Alin-Opx geobarometer to analyze the weather [72]. Despite the increasingly technical and commercial importance, understanding of Ga′s geochemical behavior in the environment has still been limited. As a result, there is an urgent need to research identified Ga′s geochemical cycle, especially in comparison to its geochemical companion aluminum (Al). Gallium and aluminum are “geochemical partners” or “pseudo isotopes” because they are both elements in Periodic Table Group 13. In this case, the term “geochemical twins” is inappropriate because it relates to the geochemical interaction of two trace elements (whereas Al, unlike Ga, is frequently a substantial element). Due to their equal charge and ionic radius, Ga and Al demonstrate related geochemical activity in most natural systems. Surprisingly, there have been few comprehensive investigations of the Ga-Al combination’s geochemical behavior. The Ga/Al mass ratios (measured in g Gaper kilogram Al; g/kg) derived from published concentration data appear to be comparable for igneous rocks, sediments, and sedimentary rocks (basalts, granites, shales, loess, deep-sea clay, typical upper continental crust) and cover a small range of 0.19 g/kg to 0.41 g/kg [73].

7. Human health and medical geochemistry

Medicinal chemistry is the study of the dialectical interaction between chemical components, human health, and the biosphere (geochemistry). In the important tissues and bodily fluids, trace elements are found in trace levels [74]. The gut absorbs only extremely little amounts of vanadium that enter the body [75]. As a result, a liter of water is insufficient to fulfill vanadium’s daily requirements. Vanadium is used in the digestion and breakdown of carbohydrates, as well as the proper functioning of the circulatory system. It controls how potassium, sodium triphosphate, and other enzymes act [22]. Furthermore, receiving oxygen stimulates the hepatocytes. It also has activities in the eyes, liver, kidneys, heart, neurological system, and blood insulin management. As a result, vanadium is compared to insulin in terms of its ability to digest blood sugar. Low amounts of this element in the body cause arteriosclerosis, diabetes, sagging muscles, stunted growth, and a range of other pathological signs [76].

In a variety of ways, cobalt is needed for the health and safety of the human body’s effective functioning. It is found in vitamin B12, and it’s considered a catalyst in this vitamin because it’s linked to proteins in the body. This vitamin is involved in several processes, including ribose to deoxyribose conversion and methyl group translocation. Cobalt assists in carbohydrate and protein digestion [77]. The need for cobalt in the diet is about 0.11-ppm (mg/kg) of dry matter, while recent recommendations advocate boosting the diet up to 0.20 mg of Cobalt/kg dry matter, which appears to improve animal productivity, particularly in dairy cattle [78]. It is also responsible for monitoring the work of cells, growth, and the activity of red blood cells and bone marrow, as well as the generation of amino acids, the formation of DNA, and the important activities of the immune system, and neurological system, and digestive system. Cobalt is largely found in bones and muscles, and vitamin B12 is a rich supplier of it. Because 3 mg of this element is consumed daily and 85% of it is excreted in the urine, the heart stores the majority of it [78]. As a result, it has become clear that the chemical balance of living beings’ health is exceedingly sensitive, as minor alterations in trace components can have huge implications for public health and safety [79].

8. Conclusion

Metal biogeochemical cycling is frequently accompanied by stable isotope fractionation in natural systems, which can now be quantified thanks to recent analytical breakthroughs. As a result, over the last two decades, a new research field called environmental isotope geochemistry has emerged, complementing traditional stable isotope systems (H, C, O, N, and S) with many more elements from across the periodic table.

The most significant principles of mass-dependent and mass-independent metal stable isotope fractionation are discussed, as well as the magnitude of natural isotopic variations for various elements. Redox transformations, complexation, sorption, precipitation, dissolution, evaporation, diffusion, and biological cycling are all discussed as mechanisms that might cause metal stable isotope fractionation in environmental systems. The utility and limitations of metal stable isotope signatures as tracers in environmental geochemistry are also examined, as well as future directions. As Supporting Information, a summary of analytical methodologies for metal stable isotopes is provided, as well as an overview of the present state of research on various elements. Incorporating the results of all experimental, theoretical, and field study methods will enable the environmental geochemistry community to develop a new tool for mineral isotope analysis to address important scientific questions.