Samarium-146
Samarium-146 (¹⁴⁶Sm) is a radioactive isotope of the rare earth element samarium (Sm), belonging to the lanthanide series of the periodic table. It is an extinct radionuclide, meaning it no longer occurs naturally on Earth in detectable quantities because it has a relatively short half-life compared to the age of the solar system. The isotope played a crucial role in the early history of the solar system, particularly in the dating of planetary differentiation and meteorites through isotopic studies.
Atomic Characteristics and Basic Data
Samarium-146 is one of several isotopes of samarium, which has the atomic number 62. Its properties are defined by its nuclear configuration and radioactive decay characteristics.
Key data:
- Chemical Symbol: Sm
- Isotope: Samarium-146 (¹⁴⁶Sm)
- Atomic Number: 62
- Mass Number: 146
- Atomic Mass: Approximately 145.917 u
- Type: Radioactive (extinct radionuclide)
- Half-Life: Around 68–103 million years (recent studies suggest a value closer to 68 million years)
- Decay Mode: Alpha decay
- Decay Product: Neodymium-142 (¹⁴²Nd)
- Energy Released: Approximately 2.5 MeV per decay
Discovery and Historical Context
Samarium itself was first discovered in 1879 by French chemist Paul Émile Lecoq de Boisbaudran, but the specific isotope samarium-146 was identified much later through advances in nuclear chemistry and mass spectrometry.
The scientific significance of ¹⁴⁶Sm emerged in the mid-20th century, when researchers studying isotopic ratios in meteorites found variations in neodymium isotopes (Nd) that could be explained by the radioactive decay of ¹⁴⁶Sm. This discovery provided a new method for determining the age of early solar system materials, known as the Sm–Nd dating system.
Radioactive Decay and Transformation
Samarium-146 undergoes alpha decay, transforming into neodymium-142 (¹⁴²Nd) through the emission of an alpha particle (two protons and two neutrons). The decay equation can be represented as:
62146Sm→60142Nd+24He{}^{146}_{62}\text{Sm} \rightarrow {}^{142}_{60}\text{Nd} + {}^{4}_{2}\text{He}62146Sm→60142Nd+24He
This process releases energy and alters the isotopic composition of neodymium found in ancient rocks and meteorites. Since ¹⁴⁶Sm has a relatively short half-life compared with the age of the Earth (~4.5 billion years), nearly all of it has decayed away, making it extinct in nature today. However, its former presence can still be inferred from excesses of its decay product, ¹⁴²Nd.
Role in Geochronology
The Samarium–Neodymium (Sm–Nd) dating system is a fundamental tool in isotope geochemistry, particularly in understanding early planetary differentiation and crust–mantle evolution.
The ¹⁴⁶Sm–¹⁴²Nd chronometer is especially valuable for dating early events in the solar system’s history (within the first few hundred million years). It is used to determine:
- The age of meteorites and planetary differentiation events.
- The time of core–mantle separation on planetary bodies such as the Earth, Moon, and Mars.
- The formation ages of achondrites (stony meteorites from differentiated parent bodies).
By measuring isotopic anomalies in neodymium and comparing them with the expected natural abundance ratios, scientists can estimate when samarium–neodymium fractionation occurred during early planetary formation.
Importance in Cosmochemistry and Planetary Science
¹⁴⁶Sm is a key isotope in cosmochemical studies because it provides a time marker for the early solar system chronology. Its relatively short half-life allows scientists to date processes that occurred within the first 500 million years after solar system formation.
Applications include:
- Meteorite Dating: Determining the crystallisation ages of meteorites and their differentiation histories.
- Lunar Studies: Used to estimate the time of the Moon’s magma ocean solidification and differentiation.
- Planetary Evolution: Helps trace the timing of core formation and crustal development on terrestrial planets.
Isotopic evidence of ¹⁴⁶Sm decay in ancient rocks also suggests that Earth’s crust–mantle system differentiated very early, possibly within the first 100 million years of planetary formation.
Modern Measurement and Research
Recent advances in mass spectrometry and isotope ratio analysis have improved the precision of ¹⁴⁶Sm–¹⁴²Nd dating. The exact half-life of ¹⁴⁶Sm has been the subject of ongoing research and debate:
- Earlier studies estimated it at 103 million years.
- More recent measurements (post-2012) suggest a shorter half-life of 68 million years.
This revision affects the calibration of the ¹⁴⁶Sm–¹⁴²Nd chronometer and the inferred timing of early planetary differentiation events. For example, a shorter half-life implies that planetary bodies formed and differentiated more rapidly than previously thought.
Production and Laboratory Use
Though extinct in nature, samarium-146 can be artificially produced in nuclear reactors or particle accelerators through neutron irradiation or specific nuclear reactions involving other samarium or neodymium isotopes.
In the laboratory, it is primarily used for:
- Nuclear physics experiments, particularly studies of alpha decay and nuclear structure.
- Calibration of mass spectrometers for isotope ratio measurements.
- Astrochronological modelling, to simulate early solar system isotopic evolution.
Relation to Other Samarium Isotopes
Natural samarium is composed of both stable and radioactive isotopes, with the stable ones being ¹⁴⁴Sm, ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵²Sm, and ¹⁵⁴Sm. Another long-lived radioactive isotope, Samarium-147 (¹⁴⁷Sm), has a half-life of about 106 billion years and decays to neodymium-143 (¹⁴³Nd).
Together, the ¹⁴⁷Sm–¹⁴³Nd and ¹⁴⁶Sm–¹⁴²Nd systems form complementary dating methods, covering vastly different timescales:
- ¹⁴⁷Sm–¹⁴³Nd system: Used for long-term geological processes.
- ¹⁴⁶Sm–¹⁴²Nd system: Used for early solar system events.
Significance in Understanding the Early Earth
Isotopic studies involving ¹⁴⁶Sm have profoundly influenced theories about the formation and evolution of Earth’s mantle and crust. The decay of this isotope contributed to isotopic anomalies in ancient terrestrial and lunar rocks, providing evidence that the Earth’s silicate differentiation occurred very early in its history.
Moreover, the presence of ¹⁴⁶Sm in the early solar system indicates that it was synthesised shortly before the Sun’s formation, possibly in a nearby supernova explosion, making it an important tracer of stellar nucleosynthesis.
Summary of Key Points
- Samarium-146 (¹⁴⁶Sm) is a now-extinct radioactive isotope of samarium.
- It decays by alpha emission to neodymium-142 (¹⁴²Nd).
- Half-life: Approximately 68–103 million years, making it useful for early solar system chronology.
- Its decay system, ¹⁴⁶Sm–¹⁴²Nd, helps date early planetary differentiation and meteorite formation.
- It has been crucial in refining the understanding of Earth’s early geochemical evolution and planetary formation processes.
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