- Samarium-neodymium dating
Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on decay of a long-lived Sm isotope to a
radiogenicNd isotope. Nd isotope ratios are used to provide information on the source of igneousmelts 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
felsicminerals (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.
m-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-decayof 147Sm to 143Nd with a half lifeof 1.06 x 1011 years.
146Sm is an extinct nuclide which decays via alpha emission to produce 142Nd, with a half-life of 1.08 x 108 years.
146Sm is itself produced by the decay of 150Gd via alpha-decay with a half-life of 1.06 x 1011 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.
m and Nd geochemistry
The concentration of Sm and Nd in
silicateminerals increase with the order in which they crystallise from a magma according to Bowen's reaction series. Samarium is accommodated more easily into maficminerals, 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.
ultramaficrocks have low Sm and Nd and "high" Sm/Nd ratios. Felsicrocks have high concentrations of Sm and Nd but "low" Sm/Nd ratios ( komatiitehas 1.14ppm Sm and 3.59ppm Nd versus 4.65ppm Sm and 21.6ppm Nd in rhyolite).
The importance of this process is apparent in modeling the age of
The CHUR ModelThrough the analysis of isotopic compositions of neodymium, DePaolo and Wasserburg [ DePaolo, D. J. and Wasserburg, G. J. (1976). Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249)52. ] first discovered that terrestrial igneous rocks closely followed the Chondritic Uniform Reservoir (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 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. Dickin, A.P., 2005. Radiogenic Isotope Geology 2nd ed. Cambridge: Cambridge University Press. pp. 76-77 ] . 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’. [McCulloch, M.T. and Wasserburg G.J. 1978. Sm- Nd and Rb-Sr chronology of crustal formation. Science 200, 1003-11 ] 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.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Dating methodologies in archaeology — Dating material drawn from the archaeological record can be made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence… … Wikipedia
dating — I In geology and archaeology, the process of determining an object s or event s place within a chronological scheme. Scientists may use either relative dating, in which items are sequenced on the basis of stratigraphic clues (see stratigraphy) or … Universalium
Samarium — promethium ← samarium → europium ↑ Sm ↓ … Wikipedia
Neodymium — praseodymium ← neodymium → promethium ↑ Nd ↓ U … Wikipedia
Radiometric dating — (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.… … Wikipedia
Isotopes of samarium — Naturally occurring samarium (Sm) is composed of 4 stable isotopes, 144Sm, 150Sm, 152Sm and 154Sm, and 3 extremely long lived radioisotopes, 147Sm (1.06e|11y), 148Sm (7e|15y) and 149Sm (>2e|15y), with 152Sm being the most abundant (26.75% natural … Wikipedia
Isotopes of neodymium — Naturally occurring neodymium (Nd) is composed of 5 stable isotopes, 142Nd, 143Nd, 145Nd, 146Nd and 148Nd, with 142Nd being the most abundant (27.2% natural abundance), and 2 radioisotopes, 144Nd and 150Nd. In all, 33 radioisotopes of Neodymium… … Wikipedia
Radiocarbon dating — (sometimes simply known as carbon dating) is a radiometric dating method that uses the naturally occurring radioisotope carbon 14 (14C) to estimate the age of carbon bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated … Wikipedia
Optical dating — is a method of determining how long ago minerals were last exposed to daylight. It is useful to geologists and archaeologists who want to know when such an event occurred. Alternate names sometimes used are optically stimulated luminescence… … Wikipedia
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. Isochron dating can be further separated into … Wikipedia