Editing Age of Earth (section)

===Modern radiometric dating===
Radiometric dating continues to be the predominant way scientists date geologic time scales. Techniques for radioactive dating have been tested and fine-tuned on an ongoing basis since the 1960s. Forty or so different dating techniques have been utilized to date, working on a wide variety of materials. Dates for the same sample using these different techniques are in very close agreement on the age of the material.{{citation needed|date=March 2015}} Possible [[radioactive contamination|contamination]] problems do exist, but they have been studied and dealt with by careful investigation, leading to sample preparation procedures being minimized to limit the chance of contamination.{{citation needed|date=March 2015}}

====Use of meteorites====
An age of 4.55 ± 0.07 billion years, very close to today's accepted age, was determined by [[Clair Patterson|Clair Cameron Patterson]] using uranium–lead isotope dating (specifically [[lead–lead dating]]) on several meteorites including the [[Canyon Diablo (meteorite)|Canyon Diablo meteorite]] and published in 1956.<ref name="Patterson">{{cite journal | last=Patterson | first=Claire | title=Age of meteorites and the earth | journal=Geochimica et Cosmochimica Acta | url=http://es.ucsc.edu/~rcoe/eart206/Patterson_AgeEarth_GeoCosmoActa56.pdf | date=1956 | volume=10 | issue=4 | pages=230–237 | access-date=2009-07-07 | doi=10.1016/0016-7037(56)90036-9 | bibcode=1956GeCoA..10..230P | url-status=live | archive-url=https://web.archive.org/web/20100621045217/http://es.ucsc.edu/%7Ercoe/eart206/Patterson_AgeEarth_GeoCosmoActa56.pdf | archive-date=2010-06-21 }}</ref> The quoted age of Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research.
[[File:Paterson isochron animation.gif|thumb|left|upright=1.6|Lead isotope isochron diagram showing data used by Patterson to determine the age of Earth in 1956.]]Most geological samples from Earth are unable to give a direct date of the formation of Earth from the solar nebula because Earth has undergone differentiation into the core, mantle, and crust, and this has then undergone a long history of mixing and unmixing of these sample reservoirs by [[plate tectonics]], [[weathering]] and [[hydrothermal circulation]].

All of these processes may adversely affect isotopic dating mechanisms because the sample cannot always be assumed to have remained as a closed system, by which it is meant that either the parent or daughter [[nuclide]] (a species of atom characterised by the number of neutrons and protons an atom contains) or an intermediate daughter nuclide may have been partially removed from the sample, which will skew the resulting isotopic date. To mitigate this effect it is usual to date several minerals in the same sample, to provide an [[isochron]]. Alternatively, more than one dating system may be used on a sample to check the date.

Some meteorites are furthermore considered to represent the primitive material from which the accreting solar disk was formed.<ref>{{cite conference | author=Carlson, R. W.| author2=Tera, F.
 | title=Lead–Lead Constraints on the Timescale of Early Planetary Differentiation
 | book-title=Conference Proceedings, Origin of the Earth and Moon | page=6
 | publisher=Lunar and Planetary Institute
 | date=December 1–3, 1998
 | location=Houston, Texas | url=http://www.lpi.usra.edu/meetings/origin98/pdf/4066.pdf |access-date=2008-12-22 | archive-url= https://web.archive.org/web/20081216214311/http://www.lpi.usra.edu/meetings/origin98/pdf/4066.pdf| archive-date= 16 December 2008 | url-status= live}}</ref> Some have behaved as closed systems (for some isotopic systems) soon after the solar disk and the planets formed.{{citation needed|date=March 2015}} To date, these assumptions are supported by much scientific observation and repeated isotopic dates, and it is certainly a more robust hypothesis than that which assumes a terrestrial rock has retained its original composition.

Nevertheless, ancient [[Archean|Archaean]] lead [[ores]] of [[galena]] have been used to date the formation of Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead–lead isotope systems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1% margin for error.<ref>Dalrymple (1994) pp. 310–341</ref>

Statistics for several meteorites that have undergone isochron dating are as follows:<ref name="BGDarymple">{{cite book
 | author=Dalrymple, Brent G.
 | title=Ancient Earth, Ancient Skies: The Age of the Earth and Its Cosmic Surroundings
 | url=https://archive.org/details/ancientearthanci0000dalr
 | url-access=registration
 | date=2004
 | publisher=[[Stanford University Press]]
 | isbn = 978-0-8047-4933-6
 | pages = [https://archive.org/details/ancientearthanci0000dalr/page/147 147], 169
}}
</ref>

{| <!-- is [[Figure space]], the width of one digit. -->
!colspan=4 align=left| 1. St. Severin (ordinary chondrite)
|-
|width=1em| || 1. || Pb-Pb isochron || 4.543 ± 0.019 billion years
|-
| || 2. || Sm-Nd isochron || 4.55  ± 0.33 billion years
|-
| || 3. || Rb-Sr isochron || 4.51  ± 0.15 billion years
|-
| || 4. ||| Re-Os isochron || 4.68  ± 0.15 billion years
|-
!colspan=4 align=left| 2. Juvinas (basaltic achondrite)
|-
| || 1. || Pb-Pb isochron || 4.556 ± 0.012 billion years
|-
| || 2. || Pb-Pb isochron || 4.540 ± 0.001 billion years
|-
| || 3. || Sm-Nd isochron || 4.56  ± 0.08 billion years
|-
| || 4. || Rb-Sr isochron || 4.50  ± 0.07 billion years
|-
!colspan=4 align=left| 3. Allende (carbonaceous chondrite)
|-
| || 1. || Pb-Pb isochron || 4.553 ± 0.004 billion years
|-
| || 2. || Ar-Ar age spectrum || 4.52  ± 0.02 billion years
|-
| || 3. || Ar-Ar age spectrum || 4.55  ± 0.03 billion years
|-
| || 4. || Ar-Ar age spectrum&nbsp; || 4.56  ± 0.05 billion years
|}

====Canyon Diablo meteorite====
{{Further|Age of the Solar System|Canyon Diablo (meteorite)}}[[File:Barringer Crater aerial photo by USGS.jpg|thumb|upright|[[Meteor Crater|Barringer Crater]], Arizona, where the Canyon Diablo meteorite was found.]]

The [[Canyon Diablo (canyon)|Canyon Diablo]] meteorite was used because it is both large and representative of a particularly rare type of meteorite that contains [[sulfide]] minerals (particularly [[troilite]], FeS), metallic [[nickel]]-[[iron]] alloys, plus silicate minerals. This is important because the presence of the three mineral phases allows investigation of isotopic dates using samples that provide a great separation in concentrations between parent and daughter nuclides. This is particularly true of uranium and lead. Lead is strongly [[chalcophile|chalcophilic]] and is found in the sulfide at a much greater concentration than in the silicate, versus uranium. Because of this segregation in the parent and daughter nuclides during the formation of the meteorite, this allowed a much more precise date of the formation of the solar disk and hence the planets than ever before.

[[File:Canyon-diablo-meteorite.jpg|thumb|upright|left|Fragment of the Canyon Diablo iron meteorite.]]

The age determined from the Canyon Diablo meteorite has been confirmed by hundreds of other age determinations, from both terrestrial samples and other meteorites.<ref>{{cite conference
 | author=Terada, K.| author2=Sano, Y.
 | title=In-situ ion microprobe U-Pb dating of phosphates in H-chondrites | book-title=Proceedings, Eleventh Annual V. M. Goldschmidt Conference
 | publisher=Lunar and Planetary Institute
 | date=May 20–24, 2001
 | location=Hot Springs, Virginia | url=http://www.lpi.usra.edu/meetings/gold2001/pdf/3306.pdf | access-date=2008-12-22
 | bibcode=2001eag..conf.3306T | archive-url= https://web.archive.org/web/20081216214310/http://www.lpi.usra.edu/meetings/gold2001/pdf/3306.pdf| archive-date= 16 December 2008 | url-status= live}}</ref> The meteorite samples, however, show a spread from 4.53 to 4.58 billion years ago. This is interpreted as the duration of formation of the solar nebula and its collapse into the solar disk to form the Sun and the planets. This 50 million year time span allows for accretion of the planets from the original solar dust and meteorites.

The Moon, as another extraterrestrial body that has not undergone plate tectonics and that has no atmosphere, provides quite precise age dates from the samples returned from the Apollo missions. Rocks returned from the Moon have been dated at a maximum of 4.51 billion years old. [[Martian meteorites]] that have landed upon Earth have also been dated to around 4.5 billion years old by [[lead–lead dating]]. Lunar samples, since they have not been disturbed by weathering, plate tectonics or material moved by organisms, can also provide dating by direct [[electron microscope]] examination of [[cosmic ray]] tracks. The accumulation of dislocations generated by high energy cosmic ray particle impacts provides another confirmation of the isotopic dates. [[Environmental radioactivity#Activation products from cosmic rays|Cosmic ray dating]] is only useful on material that has not been melted, since melting erases the crystalline structure of the material, and wipes away the tracks left by the particles.