Rubidium-strontium dating
Encyclopedia
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
.
The utility of the rubidium
-strontium
isotope
system results from the fact that 87Rb (one of the isotopes of rubidium
) decays to 87Sr with a halflife of 49 billion years. Different mineral
s in a given geologic setting can have a distinctly different ratio of strontium-87 to strontium-86 (87Sr/86Sr) as a consequence of different ages, original Rb/Sr values and the initial 87Sr/86Sr.
If these minerals crystallized
from the same silicic melt, each mineral had the same initial 87Sr/86Sr as the parent melt. However, because Rb substitutes for K in minerals and these minerals have different K
/Ca
ratios, the minerals will have had different Rb/Sr ratios.
During fractional crystallization
, Sr tends to become concentrated in plagioclase
, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma
may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation.
Highest ratios (10 or higher) occur in pegmatite
s.
Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.
such as a granite
that contains several major Sr-bearing minerals including plagioclase feldspar
, K-feldspar
, hornblende
, biotite
, and muscovite
. Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
The ideal scenario according to Bowen's reaction series
would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende (i.e.; tonalite
or diorite
), which is low in K (and hence Rb) but high in Sr (as this substitutes for Ca), which proportionally enriches the melt in K and Rb. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock.
One of the major drawbacks (and, conversely, the most important use) of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids. Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonate
d hydrothermal fluids present during metamorphism or magmatism.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock (generally during potassic alteration or calcic (albitisation
) alteration. Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed.
Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock.
The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same 87Sr/86Sr as the mineral.
, forensics and palaeontology because the 87Sr/86Sr of a skeleton, sea shell or indeed a clay artefact is directly comparable to the source rocks upon which it was formed or upon which the organism lived. Thus, by measuring the current-day 87Sr/86Sr ratio (and often the Nd-Nd ratios as well) the geological fingerprint of an object or skeleton can be measured, allowing migration patterns to be determined.
In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers (for example U-Th
) leads to greater uncertainties in the exact shape of the Sr isotope seawater curve.
Radiometric dating
Radiometric 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...
technique that geologists
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
use to determine the age of rocks.
Development of this process was aided by Fritz Strassmann
Fritz Strassmann
Friedrich Wilhelm "Fritz" Strassmann was a German chemist who, with Otto Hahn in 1938, identified barium in the residue after bombarding uranium with neutrons, which led to the interpretation of their results as being from nuclear fission...
, who later went on to discover nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
with Otto Hahn
Otto Hahn
Otto Hahn FRS was a German chemist and Nobel laureate, a pioneer in the fields of radioactivity and radiochemistry. He is regarded as "the father of nuclear chemistry". Hahn was a courageous opposer of Jewish persecution by the Nazis and after World War II he became a passionate campaigner...
and Lise Meitner
Lise Meitner
Lise Meitner FRS was an Austrian-born, later Swedish, physicist who worked on radioactivity and nuclear physics. Meitner was part of the team that discovered nuclear fission, an achievement for which her colleague Otto Hahn was awarded the Nobel Prize...
.
The utility of the rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
-strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
system results from the fact that 87Rb (one of the isotopes of rubidium
Isotopes of rubidium
Rubidium has 32 isotopes, with naturally occurring rubidium being composed of just two isotopes; 85Rb and the radioactive 87Rb . Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 to 60 days. Standard atomic mass is 85.4678 u.87Rb has a half-life of...
) decays to 87Sr with a halflife of 49 billion years. Different mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s in a given geologic setting can have a distinctly different ratio of strontium-87 to strontium-86 (87Sr/86Sr) as a consequence of different ages, original Rb/Sr values and the initial 87Sr/86Sr.
If these minerals crystallized
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...
from the same silicic melt, each mineral had the same initial 87Sr/86Sr as the parent melt. However, because Rb substitutes for K in minerals and these minerals have different K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
/Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
ratios, the minerals will have had different Rb/Sr ratios.
During fractional crystallization
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...
, Sr tends to become concentrated in plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation.
Highest ratios (10 or higher) occur in pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s.
Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.
Example
For example, consider the case of an igneous rockIgneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
such as a granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
that contains several major Sr-bearing minerals including plagioclase feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, K-feldspar
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, and muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
. Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
The ideal scenario 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...
would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende (i.e.; tonalite
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
or diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
), which is low in K (and hence Rb) but high in Sr (as this substitutes for Ca), which proportionally enriches the melt in K and Rb. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
Calculating the age
The age of a sample is determined by analysing several minerals within the sample. The 87Sr/86Sr ratio for each sample is plotted against its 87Rb/86Sr ratio on a graph called an isochron. If these form a straight line then the samples are consistent, and the age probably reliable. The slope of the line dictates the age of the sample.Sources of error
Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus deriving an accurate 87Sr/86Sr ratio for the mineral or whole rock sample.Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock.
- The system must have remained closed to Rb and Sr diffusion from the time at which the rock formed or fell below the closure temperatureClosure temperatureIn radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature at which a system has cooled so that there is no longer any exchange of parent or...
(generally considered to be 650 °C); - The minerals which are taken from a rock to construct an isochronIsochron datingIsochron 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...
must have formed in chemical equilibrium with one another or in the case of sediments, be deposited at the same time; - The rock must not have undergone any metasomatismMetasomatismMetasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
which could have disturbed the Rb-Sr system either thermally or chemically
One of the major drawbacks (and, conversely, the most important use) of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids. Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
d hydrothermal fluids present during metamorphism or magmatism.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock (generally during potassic alteration or calcic (albitisation
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
) alteration. Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed.
Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock.
Geochronology
The Rb-Sr dating method has been used extensively in dating rocks. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the 87Sr/86Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same 87Sr/86Sr as the mineral.
Isotope geochemistry
Initial 87Sr/86Sr ratios are a useful tool in archaeologyArchaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
, forensics and palaeontology because the 87Sr/86Sr of a skeleton, sea shell or indeed a clay artefact is directly comparable to the source rocks upon which it was formed or upon which the organism lived. Thus, by measuring the current-day 87Sr/86Sr ratio (and often the Nd-Nd ratios as well) the geological fingerprint of an object or skeleton can be measured, allowing migration patterns to be determined.
Strontium isotope stratigraphy
Strontium isotope stratigraphy relies on recognised variations in the 87Sr/86Sr ratio of seawater over time. The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined. This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not completely understood for older sequences.In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers (for example U-Th
Uranium-thorium dating
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral...
) leads to greater uncertainties in the exact shape of the Sr isotope seawater curve.