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| Titel |
Tracing the metasomatic and magmatic evolution of continental mantle roots: A case study of high 238U/204Pb and 232Th/204Pb clinopyroxenes from Middle Atlas (Morocco) peridotite xenoliths |
| VerfasserIn |
Nadine Wittig, D. Graham Pearson, Joel A. Baker, Svend Duggen, Kaj Hoernle |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043010
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| Zusammenfassung |
| Although comprising only 2% of Earth’s mantle, lithospheric peridotites and pyroxenites,
especially those from beneath the continental blocks, feature strongly in the literature as
deplete analogues for the entire upper mantle. This frequent usage stems from its availability
at Earth surface as xenoliths sampled by volcanic rocks, as obducted orogenic bodies marking
(paleo-) suture zones and also as samples from the current oceanic basins (abyssal
peridotites). On the other hand lithospheric mantle records ubiquitous metasomatic
enrichment (modal and cryptic) due to interaction with various fluid phases and
both, extreme depletion and metasomatic enrichment may render Earth lithospheric
mantle an important geochemical reservoir that could be distinct from the convecting
mantle.
We wish to particularly address the potential of SCLM (sub-continental lithospheric
mantle) as potential source of the intraplate volcanism and, importantly, whether SCLM has
unique lithophile (Rb/Sr, Sm/Nd, Lu/Hf, U/Pb, Th/Pb) and siderophile (Re/Os) isotope
features that would allow the discrimination of SCLM from the convecting mantle at a given
time. We will focus on off-cratonic spinel-facies peridotite xenoliths sampled by the
Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and highlight
clinopyroxenes with unusually high 238U/204Pb and 232Th/204Pb that are coupled with
homogeneous radiogenic Pb isotopes.
The Moroccan U, Th and Pb abundances in the clinopyroxenes generally exceed
estimates of primitive mantle clinopyroxene. Pb isotope compositions of these
clinopyroxenes are unusually radiogenic relative to literature data of SCLM minerals
and vary between 206Pb/204Pb = 19.93 – 20.25, 207Pb/204Pb = 15.63 – 15.66 and
208Pb/204Pb = 39.72 – 40.23. These Pb isotope systematics are associated with
the Pb isotope signature of the convecting mantle. Nevertheless, these samples
have negative Î7/4 but positive Î8/4; setting them distinctly apart from typical
HIMU. 238U/204Pb and 232Th/204Pb of these clinopyroxenes, which range from 26 to
81 and 136 to 399, respectively, more are extreme than estimates of MORB- and
HIMU-source mantle. This high-quality U-Pb and Th-Pb data and Pb isotopes allows us to
determine that the age of the metasomatic enrichment is very young (> 20 Ma)
and associated with the Quaternary intra-plate volcanism in this region although,
importantly, Pb isotopes of host volcanic rocks and SCLM clinopyroxenes do not
overlap.
When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope
range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in
that they are similar to the convecting mantle. This observation does not support
the existence of SCLM with high 238U/204Pb and 232Th/204Pb, which has been
recorded from numerous localities, for long periods of time. Consequently, the
narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest
metasomatic events are recorded by incompatible elements such as U, Th and Pb and
that such metasomatism frequently effects continental SCLM throughout Earth’s
evolution.
Numerical modelling of putative magmas from the Middle Atlas SCLM generated by
fractional, non-modal melting calculations yield extremely high 238U/204Pb and 232Th/204Pb
ratios that do not seem to be realized in terrestrial basaltic rocks. For example, the most
extreme SCLM melt generated yields 232Th/204Pb of 2250 relative to 232Th/204Pb of 200 for
the host volcanic rocks from the Middle Atlas.
Relative to the elevated 238U/204Pb and 232Th/204Pb ratios that are found in many
peridotite clinopyroxenes available from the literature, Rb/Sr, Sm/Nd and Lu/Hf data of most
of these samples are unremarkable and remain associated with values of the convecting
mantle, i.e. this data is unsuitable in tracing the age of the metasomatism, the separation of
the SCLM from the convecting mantle or the melting of SCLM. Therefore we propose
extreme U/Pb and Th/Pb could be used to tracing lithospheric melting, yet neither extreme Pb
isotope nor extreme 238U/204Pb and 232Th/204Pb have been found in terrestrial
volcanism rendering frequent SCLM melting/recycling undetectable or insignificant. |
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