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| Titel |
Recycle and fractionation of U and K in the mantle via slab subduction; noble gas isotopic evidence from Polynesian HIMU |
| VerfasserIn |
Takeshi Hanyu, Yoshiyuki Tatsumi, Jun-Ichi Kimura |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078432
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| Zusammenfassung |
| The abundance and distribution of U and K in the Earth are critical not only for isotope and
noble gas geochemistry but also for internal heat production in the mantle. While the
concentration of U in bulk silicate Earth (BSE) has been estimated from the chondritic value,
K concentration in BSE is poorly constrained. K concentration in BSE has been estimated
using U concentration in BSE multiplied by the canonical K/U ratio (13000) on the ground
that crustal and mantle-derived rocks show uniform K/U. However, such theory might be
uncertain if the subducted slab had fractionated K/U and it remained isolated as a hidden
reservoir.
We present He–Ne–Ar isotopic compositions for Polynesian HIMU lavas with radiogenic
Pb isotopic compositions. It has been widely accepted that the HIMU lavas are sourced
from subducted ancient oceanic crust. K/U of the HIMU reservoir is constrained
using the relative abundances of radiogenic and nucleogenic noble gases, because
40Ar/36Ar evolves by decay of 40K while production of 4He and 21Ne is related
with U and Th decay. In 4He/40Ar*–4He/21Ne* space (asterisks denote radiogenic
component), the HIMU lavas define a trend that is parallel to, but offset from the trend
previously observed for other ocean island basalts. Using 4He/21Ne* as a monitor of
elemental fractionation of noble gasses, fractionation-corrected 4He/40Ar* is higher
than that expected for the mantle with the canonical K/U of 13000. K/U of the
HIMU reservoir converted from 4He/40Ar* is approximately 3000. Low K/U of the
HIMU reservoir is best explained by a model where this reservoir originates from
subducted oceanic crust that preferentially lost K relative to U via dehydration during its
subduction.
Since the HIMU reservoir, involving subducted oceanic crust, is enriched in U, but not in
K, previous estimates of K/U and K concentrations for BSE, that did not take this reservoir
into consideration, will be too high. The mass balance calculation, considering continental
crust, depleted mantle, primitive mantle, and subducted crust, demonstrates that
the estimated K/U of BSE may be modified from 13000 down to 8000–10000.
Moreover, this calculation indicates that the subducted oceanic crust may make a
significant contribution to the U mantle budget, requiring there to be less in the
primitive mantle. However, a contribution from the primitive mantle is not ruled out
entirely, unless the subducted oceanic crust had a relatively high U concentration
and low K/U over geologic time and was totally preserved in a convecting mantle. |
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