Samarium-neodymium dating
Encyclopedia
Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on decay of a long-lived samarium
(Sm) isotope to a radiogenic neodymium
(Nd) isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data. The various reservoirs within the solid earth will have different values of initial 143Nd/144Nd ratios, especially with reference to the mantle
.
The usefulness of Sm-Nd dating is the fact that these two elements are rare earths
. They are thus, theoretically, not particularly susceptible to partitioning during melting of silicate rocks
. The fractionation
effects of crystallisation of felsic
minerals (see above) changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the 143Nd/144Nd ratios with ingrowth of radiogenic 143Nd.
The mantle is assumed to have undergone chondritic evolution, and thus deviations in initial 143Nd/144Nd ratios can provide information as to when a particular rock or reservoir was separated from the mantle within the Earth's past.
In many cases, Sm-Nd and Rb-Sr
isotope data are used together.
The two elements are joined in a parent-daughter relationship by the alpha-decay of 147Sm to 143Nd with a half life of 1.06 years.
146Sm is an almost-extinct nuclide which decays via alpha emission to produce 142Nd, with a half-life of 1.08 years.
146Sm is itself produced by the decay of 150Gd
via alpha-decay with a half-life of 1.79 years.
An isochron
is calculated normally. As with Rb-Sr and Pb-Pb isotope geochemistry
, the initial 143Nd/144Nd ratio of the isotope system provides important information on crustal
formation and the isotopic evolution of the solar system.
minerals increase with the order in which they crystallise from a magma according to Bowen's reaction series
. Samarium is accommodated more easily into mafic
minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase faster relative to samarium. Thus, as a rock undergoes fractional crystallization from a mafic to a more felsic composition, the abundance of Sm and Nd changes, as does the ratio between Sm and Nd.
Thus, ultramafic rocks have low Sm and Nd and high Sm/Nd ratios. Felsic
rocks have high concentrations of Sm and Nd but low Sm/Nd ratios (komatiite
has 1.14 parts per million (ppm) Sm and 3.59 ppm Nd versus 4.65 ppm Sm and 21.6 ppm Nd in rhyolite
).
The importance of this process is apparent in modeling the age of continental crust
formation.
(CHUR) line.
Chondritic meteorites are thought to represent the earliest (unsorted) material that formed in the solar system before planets formed. They have relatively homogeneous trace element signatures and therefore their isotopic evolution can model the evolution of the whole solar system and of the ‘Bulk Earth’.
After plotting the ages and initial 143Nd/144Nd ratios of terrestrial igneous rocks on a Nd evolution vs. time diagram¸ DePaolo and Wasserburg determined that Archean rocks had initial Nd isotope ratios very similar to that defined by the CHUR evolution line.
Since epsilon units are larger and therefore a more tangible representation of the initial Nd isotope ratio, by using these instead of the initial isotopic ratios, it is easier to comprehend and therefore compare initial ratios of crust with different ages. In addition, epsilon units will normalize the initial ratios to CHUR, thus eliminating any effects caused by various analytical mass fractionation correction methods applied.
In order for a TCHUR age to be calculated, fractionation between Nd/Sm would have to have occurred during magma extraction from the mantle to produce a continental rock. This fractionation would then cause a deviation between the crustal and mantle isotopic evolution lines. The intersection between these two evolution lines then indicates the crustal formation age. The TCHUR age is defined by the following equation:
The TCHUR age of a rock, can yield a formation age for the crust as a whole if the sample has not suffered disturbance after its formation. Since Sm/Nd are rare-earth elements (REE), their characteristic immobility enables their ratios to resist partitioning during metamorphism and melting of silicate rocks. This therefore allows for crustal formation ages to be calculated, despite any metamorphism the sample has undergone.
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
(Sm) isotope to a radiogenic neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
(Nd) isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data. The various reservoirs within the solid earth will have different values of initial 143Nd/144Nd ratios, especially with reference to the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
.
The usefulness of Sm-Nd dating is the fact that these two elements are rare earths
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
. They are thus, theoretically, not particularly susceptible to partitioning during melting of silicate rocks
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
. The fractionation
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
effects of crystallisation of felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
minerals (see above) changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the 143Nd/144Nd ratios with ingrowth of radiogenic 143Nd.
The mantle is assumed to have undergone chondritic evolution, and thus deviations in initial 143Nd/144Nd ratios can provide information as to when a particular rock or reservoir was separated from the mantle within the Earth's past.
In many cases, Sm-Nd and Rb-Sr
Rubidium-strontium dating
The rubidium-strontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.Development of this process was aided by Fritz Strassmann, who later went on to discover nuclear fission with Otto Hahn and Lise Meitner....
isotope data are used together.
Sm-Nd radiometric dating
Samarium has five naturally occurring isotopes and neodymium has seven.The two elements are joined in a parent-daughter relationship by the alpha-decay of 147Sm to 143Nd with a half life of 1.06 years.
146Sm is an almost-extinct nuclide which decays via alpha emission to produce 142Nd, with a half-life of 1.08 years.
146Sm is itself produced by the decay of 150Gd
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
via alpha-decay with a half-life of 1.79 years.
An isochron
Isochron dating
Isochron dating is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks...
is calculated normally. As with Rb-Sr and Pb-Pb isotope geochemistry
Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
, the initial 143Nd/144Nd ratio of the isotope system provides important information on crustal
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
formation and the isotopic evolution of the solar system.
Sm and Nd geochemistry
The concentration of Sm and Nd in silicateSilicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
minerals increase with the order in which they crystallise from a magma according to Bowen's reaction series
Bowen's reaction series
Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be found together while others are almost never associated with one another...
. Samarium is accommodated more easily into mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase faster relative to samarium. Thus, as a rock undergoes fractional crystallization from a mafic to a more felsic composition, the abundance of Sm and Nd changes, as does the ratio between Sm and Nd.
Thus, ultramafic rocks have low Sm and Nd and high Sm/Nd ratios. Felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
rocks have high concentrations of Sm and Nd but low Sm/Nd ratios (komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
has 1.14 parts per million (ppm) Sm and 3.59 ppm Nd versus 4.65 ppm Sm and 21.6 ppm Nd in rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
).
The importance of this process is apparent in modeling the age of continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
formation.
The CHUR model
Through the analysis of isotopic compositions of neodymium, DePaolo and Wasserburg discovered that terrestrial igneous rocks closely followed the Chondritic Uniform ReservoirChondritic unfractionated reservoir
The CHondritic Unfractionated Reservoir or CHUR is a scientific model in astrophysics and geochemistry for the mean chemical composition of the part of the Solar Nebula from which, during the formation of the Solar System, chondrites formed...
(CHUR) line.
Chondritic meteorites are thought to represent the earliest (unsorted) material that formed in the solar system before planets formed. They have relatively homogeneous trace element signatures and therefore their isotopic evolution can model the evolution of the whole solar system and of the ‘Bulk Earth’.
After plotting the ages and initial 143Nd/144Nd ratios of terrestrial igneous rocks on a Nd evolution vs. time diagram¸ DePaolo and Wasserburg determined that Archean rocks had initial Nd isotope ratios very similar to that defined by the CHUR evolution line.
Epsilon notation
Since 143Nd/144Nd departures from the CHUR evolution line are very small, DePaolo and Wasserburg argued that it would be useful to create a form of notation that described 143Nd/144Nd in terms of their deviations from the CHUR evolution line. This is called the epsilon notation whereby one epsilon unit represents a one part per 10,000 deviation from the CHUR composition. Algebraically, epsilon units can be defined by the equation:
Since epsilon units are larger and therefore a more tangible representation of the initial Nd isotope ratio, by using these instead of the initial isotopic ratios, it is easier to comprehend and therefore compare initial ratios of crust with different ages. In addition, epsilon units will normalize the initial ratios to CHUR, thus eliminating any effects caused by various analytical mass fractionation correction methods applied.
Nd Model Ages
Since CHUR defines initial ratios of continental rocks through time, it was deduced that measurements of 143Nd/144Nd and 147Sm/144Nd, with the use of CHUR, could produce model ages for the segregation from the mantle of the melt which formed any crustal rock. This has been termed ‘t-CHUR’.In order for a TCHUR age to be calculated, fractionation between Nd/Sm would have to have occurred during magma extraction from the mantle to produce a continental rock. This fractionation would then cause a deviation between the crustal and mantle isotopic evolution lines. The intersection between these two evolution lines then indicates the crustal formation age. The TCHUR age is defined by the following equation:
The TCHUR age of a rock, can yield a formation age for the crust as a whole if the sample has not suffered disturbance after its formation. Since Sm/Nd are rare-earth elements (REE), their characteristic immobility enables their ratios to resist partitioning during metamorphism and melting of silicate rocks. This therefore allows for crustal formation ages to be calculated, despite any metamorphism the sample has undergone.