Isotope geochemistry
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added Pa:Th oceanic isotope section
'''Isotope geochemistry''' is an aspect of [[geology]] based upon study of the relative and absolute concentrations of the [[chemical element|elements]] and their [[isotopes]] in the [[Earth]]. Broadly, the field is divided into two branches: [[stable isotope|stable]] and [[radiogenic]] isotope geochemistry.
==Lead-lead isotope geochemistry==
[[Lead]] has four stable [[isotopes]] - <sup>204</sup>Pb, <sup>206</sup>Pb, <sup>207</sup>Pb, <sup>208</sup>Pb and one common radioactive isotope <sup>202</sup>Pb with a [[half-life]] of ~53,000 years.
Lead is created in the Earth via decay of [[transuranic]] [[Chemical element|elements]], primarily [[uranium]] and [[thorium]].
Lead isotope [[geochemistry]] is useful for providing [[radiometric dating|isotopic dates]] on a variety of materials. Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in [[igneous rocks]], the source of [[sediments]] and even the origin of people via [[isotopic fingerprint]]ing of their teeth, skin and bones.
It has been used to date [[ice core]]s from the Arctic shelf, and provides information on the source of atmospheric lead [[air pollution|pollution]].
Lead-lead isotopes has been successfully used in [[forensic science]] to fingerprint bullets, because each batch of ammunition has its own peculiar <sup>204</sup>Pb/<sup>206</sup>Pb vs <sup>207</sup>Pb/<sup>208</sup>Pb ratio.
==Samarium-neodymium==
{{main|Samarium-neodymium dating}}
[[Samarium]]-[[neodymium]] is an isotope system which can be utilised to provide a date as well as [[isotopic fingerprint]]s of geological materials, and various other materials including archaeological finds (pots, ceramics).
<sup>147</sup>Sm decays to produce <sup>143</sup>Nd with a half life of 1.06x10<sup>11</sup> years.
Dating is achieved usually by trying to produce an [[isochron dating|isochron]] of several minerals within a rock specimen. The initial <sup>143</sup>Nd/<sup>144</sup>Nd ratio is determined.
This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir - which is an approximation of the chondritic material which formed the solar system. CHUR was determined by analysing [[chondrite]] and [[achondrite]] meteorites.
The difference in the ratio of the sample relative to CHUR can give information on a model age of extraction from the mantle (for which an assumed evolution has been calculated relative to CHUR) and to whether this was extracted from a granitic source (depleted in radiogenic Nd), the mantle, or an enriched source.
==Rhenium-osmium==
[[Rhenium]] and [[osmium]] are chalcophile elements which are present at very low abundances in the crust. Rhenium undergoes [[radioactive decay]] to produce osmium. The ratio of non-radiogenic osmium to radiogenic osmium throughout time varies.
Rhenium prefers to enter [[sulfide]]s more readily than osmium. Hence, during melting of the mantle, rhenium is stripped out, and prevents the osmium-osmium ratio from changing appreciably. This ''locks in'' an initial osmium ratio of the sample at the time of the melting event. Osmium-osmium initial ratios are used to determine the source characteristic and age of mantle melting events.
==[[Protactinium]]:[[Thorium]] Ratios - <sup>231</sup>Pa / <sup>230</sup>Th==
[[Uranium]] is well mixed in the ocean, and its decay produces <sup>231</sup>Pa and <sup>230</sup>Th at a constant activity ratio (0.093). The decay products are rapidly removed by [[adsorption]] on settling particles, but not at equal rates. <sup>231</sup>Pa has a residence equivalent to the residence time of [[deep water]] in the [[Atlantic]] basin (around 1000 yrs) but <sup>230</sup>Th is removed more rapidly (centuries). [[Thermohaline circulation]] effectively exports <sup>231</sup>Pa from the Atlantic into the [[Southern Ocean]], while most of the <sup>230</sup>Th remains in Atlantic sediments. As a result, there is a relationship between <sup>231</sup>Pa/<sup>230</sup>Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment <sup>231</sup>Pa/<sup>230</sup>Th ratio, while slower overturning increases this ratio. The combination of [[d13C]] and <sup>231</sup>Pa/<sup>230</sup>Th can therefore provide a more complete insight into past circulation changes.
==Noble gas isotopes==
===Helium-3===
[[Helium-3]] was trapped in the planet when it was created. Some <sup>3</sup>He is being added by meteoric dust, primarily collecting on the bottom of oceans (although due to [[subduction]], all oceanic [[tectonic plates]] are younger than continental plates). However, <sup>3</sup>He will be degassed from oceanic sediment during [[subduction]], so cosmogenic <sup>3</sup>He is not affecting the concentration or [[noble gas]] ratios of the [[Mantle (geology)|mantle]].
Helium-3 is created by [[cosmic ray]] bombardment, and by [[lithium]] spallation reactions which generally occur in the crust. Lithium [[spallation]] is the process by which a [[fast neutron|high-energy neutron]] bombards a [[lithium]] atom, creating a <sup>3</sup>He and a <sup>4</sup>He ion. This requires significant lithium to adversely affect the <sup>3</sup>He/<sup>4</sup>He ratio.
All degassed helium is lost to space eventually, due to the average speed of helium exceeding the [[escape velocity]] for the Earth. Thus, it is assumed the helium content and ratios of [[Earth's atmosphere]] have remained essentially stable.
It has been observed that <sup>3</sup>He is present in [[volcano]] emissions and [[oceanic ridge]] samples. How <sup>3</sup>He is stored in the planet is under investigation, but it is associated with the [[mantle (geology)|mantle]] and is used as a marker of material of deep origin.
Due to similarities in [[helium]] and [[carbon]] in [[magma]] chemistry, outgassing of helium requires the loss of [[Volatiles|volatile components]] ([[water]], [[carbon dioxide]]) from the mantle, which happens at depths of less than 60 km. However, <sup>3</sup>He is transported to the surface primarily trapped in the [[crystal]] lattice of minerals within [[fluid inclusions]].
Helium-4 is created by [[radiogenic]] production (by decay of [[uranium]]/[[thorium]]-series [[chemical element|element]]s). The [[continental crust]] has become enriched with those elements relative to the mantle and thus more He<sup>4</sup> is produced in the crust than in the mantle.
The ratio ('''R''') of <sup>3</sup>He to <sup>4</sup>He is often used to represent <sup>3</sup>He content. '''R''' usually is given as a multiple of the present atmospheric ratio ('''Ra''').
Common values for '''R/Ra''':
* Old continental crust: less than 1
* [[oceanic ridge|mid-ocean ridge]] [[basalt]] (MORB): 7 to 9
* Spreading ridge rocks: 9.1 plus or minus 3.6
* [[Hotspot (geology)|Hotspot]] rocks: 5 to 42
* Ocean and terrestrial water: 1
* Sedimentary formation water: less than 1
* Thermal spring water: 3 to 11
<sup>3</sup>He/<sup>4</sup>He isotope chemistry is being used to date [[groundwater]]s, estimate groundwater flow rates, track water pollution, and provide insights into [[hydrothermal]] processes, [[igneous]] [[geology]] and [[ore genesis]].
* [http://www.geotrack.com.au/uthhe/u-th-he-techinfo.htm (U-Th)/He dating of apatite as a thermal history tool]
* [http://lvo.wr.usgs.gov/helium.html USGS: Helium Discharge at Mammoth Mountain Fumarole (MMF)]
==Ground water isotopes==
===Tritium/helium-3===
[[Tritium]] was released to the atmosphere during atmospheric testing of nuclear bombs. Radioactive decay of tritium produces the noble gas [[helium-3]]. Comparing the ratio of tritium to helium-3 (<sup>3</sup>H/<sup>3</sup>He) allows estimation of the age of recent [[ground water]]s.
* [http://water.usgs.gov/lab/3h3he/background/ USGS Tritium/Helium-3 Dating]
* [http://wwwrcamnl.wr.usgs.gov/isoig/period/he_iig.html Hydrologic Isotope Tracers - Helium]
==See also==
* [[Cosmogenic isotope]]s
* [[Environmental isotopes]]
* [[Geochemistry]]
* [[Isotopic signature]]
* [[Radiometric dating]]
==General online stable isotope references==
* [http://pubs.usgs.gov/info/seal2/ USGS: Stable Isotopes and Mineral Resource Investigations in the United States]
* [http://wwwrcamnl.wr.usgs.gov/isoig/res/funda.html USGS: Fundamentals of Stable Isotope Geochemistry]
* [http://www.science.uottawa.ca/~eih/ch1/ch1.htm Environmental Isotopes]
* [http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html Fundamentals of Isotope Geochemistry]
==References==
<sup>3</sup>He/<sup>4</sup>He<br>
Burnard P.G., Farley K.A., & Turner G., 1998. ''Multiple fluid pulses in a Samoan [[harzburgite]].'' Chemical Geology, 147, pp. 99-114.
Kirstein L. & Timmerman M., 2000. ''Evidence of the proto-Iceland lume in northwestern Ireland at 42Ma from helium isotopes.'' Journal of the Geophysical Society, London. Vol 157, pp. 923-927.
Porcelli D. & Halliday A.N., 2001. ''The core as a possible source of mantle helium.'' Earth and Planetary Science Letters, 192, pp. 45-56.
Re-Os<br>
Arne D., Bierlein F.P., Morgan J.W., & Stein H.J., 2001. ''Re-Os Dating of Sulfides Associated with [[gold]] mineralisation in central Victoria, Australia.'' Economic Geology, 96, pp. 1455-1459.
Martin C., 1991. ''Osmium isotopic characteristics of mantle-derived rocks.'' Geochimica et Cosmochimica Acta, 55, pp. 1421-1434.
{{Chronology}}
[[Category:Geochemistry]]
[[Category:Geophysics]]
[[Category:Geochronology]]
[[es:Geoquímica de isótopos]]
[[et:Isotoopgeoloogia]]
[[fr:Géologie isotopique]]
[[it:Geochimica isotopica]]
[[id:Geokimia isotop]]