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| Titel |
Noble gas isotope signals of mid-ocean ridge basalts and their implication
for upper mantle structure |
| VerfasserIn |
Nicole A. Stroncik, Samuel Niedermann |
| 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 |
250126433
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| Publikation (Nr.) |
 EGU/EGU2016-6152.pdf |
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| Zusammenfassung |
| The geochemical structure of the upper mantle in general and its noble gas isotopic structure
in particular have been the subject of countless studies, as both provide important insights
into mantle dynamic processes and are essential for the formulation of mantle geodynamic
models. This contribution presents a noble gas study of basaltic glasses derived
from the Mid-Atlantic-Ridge (MAR) between 4 and 12˚ S, an area well known
for its high degree of lithophile isotope heterogeneity and exhibiting anomalous
crustal thickness. The Sr, Nd, Pb and Hf isotopies along this segment of the MAR
range from ultra-depleted (more than NMORB) to highly enriched, and different
concepts have been proposed to explain the observed isotopic signatures. Here
we show that the high degree of heterogeneity is not confined to the isotopes of
the lithophile elements, but is also shown by the noble gas isotopes and noble gas
interelement ratios, such as e.g. 3He/22NeM or 4He/40Ar*. 3He/4He, 21Ne/22Neextra and
40Ar/36Ar range from 7.3 to 9.3 RA, from 0.05 to 0.08, and from 346 to 37,400,
respectively. Nevertheless, the majority of the Ne isotope data are clearly aligned along a
single mixing line in the Ne-three-isotope diagram, represented by the equation
20Ne/22Ne=70.5 x 21Ne/22Ne + 7.782, with a slope distinctly different from that of the
MORB line, indicating that the ultra-depleted material is characterised by a significantly
more nucleogenic 21Ne/22Ne isotopy than the normal depleted mantle. We show,
based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and
178Hf/177Hf, that the ultra-depleted material erupted at this MAR segment was most likely
produced by an ancient, deep melting event. This implies that isotopic heterogeneities
in the upper mantle are not solely caused by the injection of enriched materials
from deep-seated mantle plumes or by crustal recycling but may also be due to
differences in the depth and degree of melting of upper mantle material within the
lifetime of our planet. In addition, the observed along-ridge and off-axis isotope
patterns in conjunction with spatial distribution patterns of off-axis seamounts in the
area show that the upper mantle can be extremely heterogeneous in space and time
even at small scale and that the mixing efficiency of either large or small scale
convection processes in the upper mantle is thus lower than commonly assumed. |
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