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| Titel |
Terrestrial cosmogenic 3He: where are we 30 years after its discovery? |
| VerfasserIn |
Pierre-Henri Blard, Raphaël Pik, Kenneth A. Farley, Jérôme Lavé, Yves Marrocchi |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134954
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| Publikation (Nr.) |
 EGU/EGU2016-15735.pdf |
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| Zusammenfassung |
| It is now 30 years since cosmogenic 3He has been detected for the first time in
a terrestrial sample (Kurz, 1986). 3He is now a widely used geochemical tool in
many fields of Earth sciences: volcanology, tectonics, paleoclimatology. 3He has the
advantage to have a high "production rate" to "detection limit" ratio, allowing surfaces
as young as hundred of years to be dated. Although its nuclear stability implies
several limitations, it moreover represents a useful alternative to 10Be in mafic
environments.
This contribution is a review of the progresses that have been accomplished since this
discovery, and discuss strategies to improve both the accuracy and the precision of this
geochronometer.
1) Measurement of cosmogenic 3He
Correction of magmatic 3He. To estimate the non-cosmogenic magmatic 3He,
Kurz (1986) invented a two steps method involving crushing of phenocrysts (to
analyze the isotopic ratio of the magmatic component), followed by a subsequent
melting of the sample, to extract the remaining components, including the cosmogenic
3He:
3Hec = 3Hemelt −4Hemelt x (3He/4He)magmatic (1)
Several studies suggested that the preliminary crushing may induce a loss of cosmogenic
3He (Hilton et al., 1993; Yokochi et al., 2005; Blard et al., 2006), implying an underestimate
of the cosmogenic 3He measurement. However, subsequent work did not replicate these
observations (Blard et al., 2008; Goerhing et al., 2010), suggesting an influence of
the used apparatus. An isochron method (by directly melting several phenocrysts
aliquots) is an alternative to avoid the preliminary crushing step (Blard and Pik,
2008).
Atmospheric contamination. Protin et al. (in press) provides robust evidences for a large
and irreversible contamination of atmospheric helium on silicate surfaces. This unexpected
behavior may reconcile the contrasted observations about the amplitude of crushing loss. This
undesirable atmospheric contamination is negligible if grain fractions smaller than 150 mm
are removed before melting.
Correction of radiogenic 4He and nucleogenic 3He. Equation 1 is valid only if the 4He
extracted by melting is entirely magmatic. To account for a possible radiogenic 4He
component, it is crucial to properly estimate the radiogenic 4He production rate, by
measuring the U, Th and Sm concentrations of both phenocryst and host, and the
phenocryst size. Estimating the nucleogenic 3He also requires measuring Li in the
phenocryst.
Accuracy of analytical systems. A recent inter-laboratory comparison involving 6
different groups indicated systematic offsets between labs (up to 7%) (Blard et al., 2015).
Efforts must be pursued to remove these inaccuracies.
2) Production rates
Absolute calibration. There are 25 3He calibration sites among the world, from
-47˚ S to 64˚ N in latitude, and from 35 to 3800 m in elevation. After scaling
these production rates to sea level high latitude, this dataset reveals a significant
statistical dispersion (ca. 13%). Efforts should be focused on regions that are free
of data and others, such as the Eastern Atlantic that yields values systematically
off.
3He/10Be cross calibrations. Some studies (Gayer et al., 2004 ; Amidon et al., 2009)
identified an altitude dependence of the 3He/10Be production ratio in the Himalayas, while
other data from the Andes and Africa did not (Blard et al., 2013b ; Schimmelpfennig et al.,
2011). There is thus a crucial need for new data at high and low elevation, with and without
snow, to precisely quantify the cosmogenic thermal neutron production. Artificial target
experiments may also be useful. |
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