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| Titel |
Anomalously high retention of radiogenic helium in native platinum |
| VerfasserIn |
O. V. Yakubovich, Yu. A. Shukolyukov, A. G. Mochalov, A. B. Kotov, S. I. Korneev |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064282
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| Zusammenfassung |
| Relatively quick migration of helium from crystal structures has been known for a long time.
However there is a group of minerals – native metals - where stability of radiogenic helium is
essentially high [1]. Helium, due to its very low solubility in metals, assembles in atomic
clusters - “bubbles” of nanometer size. Migration of helium “bubbles” as a whole from
the crystal structures needs relatively high temperature near the melting point of
metals.
On that ground of special interest are platinoids with melting points (and, consequently,
temperatures of “explosion-like” release of radiogenic helium) of more than 1550 oC In this
respect we believe that the method based on natural radioactivity of platinum is
promising.
To verify the idea of anomalously high retention of radiogenic helium in native platinum
and to check the efficiency of the proposed 190Pt-4He method of isotope geochronology, we
studied independent mineral aggregates of native platinum from chromite-bearing dunites of
Galmoenan plutonic complex (10 individual samples) (Koryak-Kamchatka belt, Russia) and
Konder massif (5 individual samples) (Khabarovsk district,Russia). Because native platinum
always has admixture of Fe, Cu etc. for reliable determination of concentration of platinum in
the samples in our study we used electron microscope JSM-6510LA with JED 2200 add-on.
Amount of 4He in native platinum was determinate on mass-spectrometer complex
MSU-G-01-M.
Native platinum consists of 6 isotopes. Among them two isotopes are α-radioactive and
decay according to following schemes:
190Pt -4He+186Os-4He+182W
192Pt -4He+188Os
Presumably, in native platinum there is always a certain amount of uranium and thorium,
absorbed in the process of crystallization. However influence of helium generation from
uranium becomes more marked at growing of 238U/Pt ratio and beginning with 238U/Pt -
10-5 should be taken into consideration. For the same reason helium produced by the decay
of 192Pt and 186Os can be neglected [2].
Obtained data was used to build a 4Herad -190Pt isochrone. The age calculated by the
tangent of the isochrone slope angle for Galmoenan massif and for neighbor placer is: 72.3 ±
3.4 mln years. The obtained age value is comparable with the results of isotope datings,
which were made previously by different methods of isotope geochronology. Rb-Sr, U-Pb,
K-Ar, 39Ar-40Ar, Sm-Nd isotope systems shown formation of the complex from 80 to 50
mln.years ago.
For the samples from Konder massif was also obtained a good isochrone. The age
calculated by the tangent of the isochrone slope angle for Konder “alluvial” placer is
112±7 mln years. That data is also in a good correspondence with other geological
data.
Thus the data provide the ability of direct 190Pt-4He dating of native platinum. And
additionally reaffirm the evidence of anomalously high retention of radiogenic helium in
native metals up to the temperatures near their melting points.
This study was supported by the RFFI, projects no.07-05-00634 and 11-05-12046-ofi-m-2011,
the program of the Department of Earth Sciences of the RAS ”Natural Isotope Systems:
Methods of Investigations.”
References:
[1]Shukolyokov et al., Petrology, 2012, v 20, p 1, p 1-21Â
[2] Shukolyukov et al., Dokl.Earth Science, 2011 v. 441 p 1 p 1579-1582 |
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