Radiometric Dating and 4.4 GY Jack Hills Zircon Crystals

Radiometric Dating

Knowing the rate of decay for various elements allows scientists to determine the age of Earth’s geological material. This technique, known as radiometric dating, radioactive dating, or radioisotope dating, relies on known half-lives of radioactive isotopes. Harnessing this physical process as a scientific tool, the oldest materials on Earth may be accurately dated. Such materials are rocks and minerals from the Hadean Eon, spanning ~500 Myr from planetary formation to the Archaean Eon, (4.54 - 4 GYA). The Hadean Eon has widely been regarded as an extremely unstable time, with molten, magmatic surfaces and near-constant meteor impacts, rendering the early Earth uninhabitable (8.3, 2019). However, recent and ongoing research based on radiometric dating continues to contradict this belief, providing insight into the conditions of the early Earth.

Isotopes and Radioactive Decay

Isotopes are variations of the same chemical element with different numbers of neutrons in their nuclei. As protons are what determine the atomic number of an element, isotopes of an element remain in the same place on the periodic table and hold (mostly) the same chemical properties. Differing numbers of nucleons mean that isotopes have different masses, and hence varying physical properties. Some isotopes are stable, while some are unstable, and will ultimately undergo radioactive decay.

The occurrence of radioactive decay is impossible to calculate for a single atom due to its nature as a stochastic process. However, a large amount of the same radioactive atoms has a constant rate of radioactive decay. The rate of radioactive decay is measured through half-lives, with one half-life being the duration of time it takes for half of a given amount of a sample to decay. A decaying nucleus will ultimately transform into a different chemical element, a result of a change in its number of protons. By knowing the constant rate of decay for radioactive atoms and by calculating the abundance of decay products in a given sample, the age of fossils, geological materials, and a geological timeline for Earth may be calculated (How Did Scientists Calculate the Age of Earth?, n.d.).

Prior to radiometric dating, scientists could only estimate the relative age of Earth. Up until the mid-19th century, it was believed the Earth was significantly younger than we know it to be today. In 1862, William Thompson (also known as Lord Kelvin), a British physicist and mathematician, published a calculation positing the Earth was no older than 400 million years, with conservative estimates of only 20 million years (The Age of the Earth - Lord Kelvin’s Heat Loss Model as a Failed Scientific Clock: Matthew Rognstad, n.d.). His work was based upon the time it would’ve taken for Earth to cool and solidify from its molten origins. Kelvin did not take into account heat released from radioactive decay (observed in 1896 by French physicist Henri Becquerel), the liquid state of Earth’s outer core, or the plasticity of the mantle. Kelvin’s calculations were based on the assumption that the Earth was solid and that conduction was the source of heat transfer, ultimately rendering his work inaccurate.

Radiometric dating was introduced in the early 20th century, by pioneers such as Ernest Rutherford, Bertram Boltwood, and Arthur Holmes. This relatively modern technique has provided unparalleled insight into the evolution of our planet and has been vital in establishing the geological timescale for Earth. This technique can also be used to date fossilized life forms or man-made artifacts. There are various types of radioactive decay by which atomic nuclei can transform to reach a more stable state. This transformation occurs when “parent” isotopes release particles via alpha or beta decay (among others).

Alpha Decay

In alpha decay, an alpha particle is emitted -- two protons and two neutrons -- from the nucleus of an unstable isotope. This decay is mediated by the nuclear force and the electromagnetic force. The nuclear force, which holds protons and neutrons together in a nucleus, is identical to the strong force when binding quarks inside a nucleon. The electromagnetic force works in opposition by repelling nucleons of like charges. Alpha decay occurs in heavy nuclei, where the number of nucleons is so great that it threatens to disequilibriate the forces holding the nucleus together. This type of decay offers stabilization by decreasing nuclei size.

Through this decay process, an alpha particle is ejected and the mass number of the parent isotope is decreased by four and the atomic number is decreased by two. Alpha particles are identical to a helium nucleus. For example, an atom of uranium, U-238, undergoes alpha decay to become an atom of Thorium, Th-234. This transmutation results in a decrease of four by the atomic number (238 to 234) and a decrease of two by the atomic mass (92 to 90). Alpha decay balances the nuclear force and the electromagnetic force in atomic nuclei (Alpha Decay - an Overview | ScienceDirect Topics, n.d.).

Beta Decay

Beta decay processes are governed by the weak force -- another of the four known fundamental forces. The flavor-change of a quark by the emission of a W-boson (a force-carrier particle) is what transforms nucleons in atomic nuclei: proton to neutron or vice versa. Beta-plus decay (β+) occurs when a proton is transformed into a neutron. During beta-plus decay, a positron (antielectron) and neutrino are emitted. Conversely, beta-minus decay (β– ) occurs when a neutron is transformed into a proton and an electron and antineutrino are emitted. This transformation of nucleons is an attempt to stabilize the ratio of protons to neutrons in an atomic nucleus. The particle pairs emitted in the process: electron/positron and (anti)neutrino are produced in the decay reaction. They cannot be found in the nucleus prior to the decay.

For instance, a proton is composed of two up quarks and one down quark. Through beta-plus decay, one of the proton’s up quarks will transform into a down quark. This turns the proton into a neutron, composed of two down quarks and one up quark. This interaction is mediated by the emission of a W boson accompanied by the creation of a positron/neutrino pair.

In beta decay, the total number of nucleons remains the same, meaning the total mass remains the same. However, the number of protons changes, consequently changing the atomic number. During beta-minus decay (neutron transforms into a proton), the atomic number increases. During beta-plus decay (proton transforms into a neutron), the atomic number decreases. For example, thorium-234 will become protactinium-234 through beta-minus decay. During this reaction, the atomic mass stays the same, yet the atomic number increases by one due to the transformation of a neutron to a proton. Th-234 has an atomic number of 90, which is increased to 91 when transmuted to pa-234.

There are other types of radioactive decay, including gamma decay, cluster decay, nuclear fission, and electron capture, which can also be considered a type of beta decay because it is similarly a result of the weak force. Through a decay chain, parent isotopes will continuously decay into more stable “daughter” isotopes. Decay chains can have a series of both alpha and beta decay. By calculating the ratio of parent-daughter isotopes in a sample, the decay chain may be extrapolated backward to determine the age of the sample (17.3: Types of Radioactivity: Alpha, Beta, and Gamma Decay - Chemistry LibreTexts, n.d.).

Types of Radiometric Dating

The most common methods for radiometric dating include radiocarbon dating, potassium-argon dating, and uranium-lead dating. Radiocarbon dating, also known as carbon-14 dating, can be used for organic materials. In this reaction, 14C undergoes beta-minus decay, transmuting into nitrogen-14. 14N is a stable isotope of nitrogen. However, due to its relatively short half-life of 5,730 years, 14C can only be used to accurately date samples less than 60,000 years old. Potassium-argon dating (K-Ar dating), in which potassium-40 undergoes a decay chain into argon-40 with a half-life of 1.25 Gyr. Uranium-lead dating occurs with uranium-238 decaying into lead-206 with a half-life of 4.5 Gyr, or uranium-235 decaying into lead-207 with a half-life of 704 Myr (Radiometric Age Dating - Geology (U.S. National Park Service), n.d.).

The Oldest Rocks

O’Neil et al. published a study in 2008 that explored the oldest known whole rocks, located in the Nuvvuagittuq greenstone belt, a site of exposed bedrock in northern Quebec, Canada. By analyzing the abundance of various isotopes of neodymium and samarium in rock samples, geologists Jonathan O’Neil of McGill University and Richard Carlson of the Carnegie Institution of Washington established a range of age between 3.8 and 4.28 Gyr (O’Neil et al., 2008). The oldest rocks found -- referred to as faux amphibolite -- are believed to be from Earth’s primordial volcanic deposits. This discovery pushed back the oldest rocks by 250 Myr. Prior to this, the oldest rocks were 4.03 Gyr old, found in the Acasta Gneiss rock-body in the Northwest Territories in 1999. (Bowring & Williams, 1999).

The Oldest Material on Earth

While the ancient bedrock in Quebec harbors the oldest whole rocks found on Earth, the Canadian site does not contain the oldest material to have been found on Earth. Detrital zircon grains crystals, found in Western Australia, have been of scientific interest for decades (Compston & Pidgeon, 1986). Wilde et al. published a study in 2001 that reevaluated the age of zircon mineral grains found in Jack Hills, Australia, dating them as far back as 4.4 Gyr with the U-Th-Pb geochronometer (Wilde et al., 2001). U-Pb dating is especially effective for zircon crystals because no lead is included in the formation of zircon, meaning that any lead found in the sample is radiogenic. U-Pb dating is also effective because it has two “parallel” decay chains: 238U to 206Pb and 235U to 207Pb, increasing the total accuracy. The existence of two dating techniques within the overall U–Pb system functions as a built-in method of verification (Schoene, n.d.).

Zircon

Zircon crystals are nesosilicate minerals. A silicate is a type of polyatomic anion -- a so-called “molecular ion” composed of two or more covalently-bonded atoms (as opposed to a monatomic ion containing only one atom). Ions hold a net charge due to the gain or loss of an electron. Positive ions are referred to as cations, whereas negative ions are referred to as anions. Polyatomic ions act as a single unit, and just as regular ions do, they possess a net charge (7.9, 2016).

Zircon crystals are categorized as nesosilicates, a group within the larger branch of silicates. Various types of silicates are differentiated based on topological structure. In general, silicates are composed of tetrahedral units of silicon and oxygen, with one central, positively charged silicon cation (Si+4) bonded to four negatively charged oxygen anions (O-2), resulting in SiO4 with a net charge of (-4). (Silicate Mineral | Definition & Types | Britannica, n.d.). These tetrahedral units can be linked together in distinctive patterns, resulting in distinctive structures, such as pyrosilicates, cyclicsilicates, or tectosilicates. The group nesosilicates (also called orthosilicates) is just one among the many classifications. Nesosilicates are composed of isolated silicate tetrahedrons, meaning none of the oxygens are shared between tetrahedra. Instead, the structure is held together by interstitial cations. Interstitial atoms fill cavities in crystal structures that are unoccupied by the atoms of the structure itself (Nesosilicate - an Overview | ScienceDirect Topics, n.d.).

As a member of the nesosilicate family, zircon crystals hold the chemical formula: ZrSiO4. Zircon is extremely abundant in Earth’s crust, existing in all three main rock types. Zircon has been found in igneous rocks as accessory minerals (meaning its presence does not alter the root name of a rock as an essential mineral would). Igneous rocks, also known as magmatic rocks, are formed through the solidification of magma. Zircon has also been found in metamorphic rocks and in sedimentary rocks as detrital grains. Such detrital grains were the subject of the Jack Hills study. In granite rocks, zircon crystals are typically in the range of 0.1 - 0.3 mm. The mineral is also relatively hard, holding a 7.5 on the Mohs hardness scale (the range of the scale is 1-10), is highly resistant to heat and weathering, and can survive multiple cycles of transport and re-deposition. Zircon’s heat-resistance preserves the mineral across geological eons, providing one of the most reliable materials to use for radiometric dating.

Zircon Crystals in Jack Hills

Findings published in the 2001 study by Wilde et al. reevaluated the ages of Jack Hills zircon crystals. Using Uranium-Thorium-Lead radioactive decay processes for radiometric dating, the isotopic composition of zircon samples was analyzed with the sensitive high-resolution ion microprobe (SHRIMP). Samples were additionally imaged through cathodoluminescence. This method of imaging directs an electron beam at a sample to discern how the sample emits and scatters light, as well as the general structure of the sample.

Ion Microprobe

The SHRIMP instrument emits an ion beam at a given sample in a vacuum. These highly energetic “primary ions” bombard the surface of the sample, causing secondary ions to be ejected from the sample’s surface through a physical process called sputtering. This technique enables researchers to extract and analyze the sputtered particles and determine their elemental and isotopic composition. It also allows the spatial resolution of the sample to be observed on the scale of micrometers (Ion Microprobe - an Overview | ScienceDirect Topics, n.d.).

Results of Reexamining Jack Hills Zircon Crystals

Through the analysis of 100 zircon grains, the oldest material ever to be found on Earth was discovered. A broken fragment of a larger zircon crystal only 220 by 160 µm, titled (W74/2-36), had multiple sites analyzed. Five of the six initial analyses of grain W74/2-36 yielded 207Pb/206Pb ages above 4.3 Gyr. Four further analyses of W74/2-36 were conducted. The sites for these additional analyses were intentionally chosen to be located away from identifiable cracks in the crystal. Three sites gave 207Pb/206Pb ages of 4,355 ± 4 Myr, 4,341 ± 6 Myr, and 4,364 ± 6 Myr, with 100, 98, and 95% concordancy, respectively. The fourth site was of the largest interest, located at the pointed, broken termination of the crystal. This site gave a 207Pb/206Pb age of 4,404 Gyr ± 8 Myr and was 99% concordant. For perspective, a zircon grain aged at 4.404 Gyr existed only ~150 Myr after the maximum age for the Earth.

The Implication of a Hadean Hydrosphere

Measurements of oxygen isotope ratios in the zircon grains indicate an interaction with a hydrosphere. This measurement is taken by calculating the ratio between oxygen-16 and oxygen-18 in a given sample. Both isotopes of oxygen contain eight and ten neutrons respectively. The ratio in the sample is then compared to a standard 16O to 18O ratio to reach a value known as delta-O-18 (δ18O). Due to the difference in the two isotopes’ masses, the δ18O value can reveal much about the temperature and climate of the sample’s environment – higher values indicate cooler temperatures. For example, because 16O is lighter, it will evaporate first, while water droplets with high concentrations of O18 will precipitate first (Isotope Analysis, n.d.). Using the Edinburgh Cameca ion microprobe, Wilde et al. analyzed the oxygen isotope ratios of two sites, both close to two of the SHRIMP analysis sites. These analyses yielded higher than expected δ18O values of 5 - 7.4%. This directly contradicts the conventional understanding of the Hadean Eon which posits that the early Earth was a violent and chaotic environment ruled by magmatic oceans and ceaseless meteor impacts. This understanding does not allow for the existence of a hydrosphere as far back in Earth’s history, which newer research suggests. A hydrosphere at this time also suggests the earlier formation of continental crusts (Sleep, 2010) and implies that life could have originated on an earlier Earth (Ancient Crystals Suggest Earlier Ocean, 2006). Once thought to be entirely inaccessible, an understanding of the Hadean Eon can now be based on empirical evidence. Improved techniques of radiometric dating will continue to further our understanding of the Hadean Eon and guide us in piecing together our planetary past.