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| Titel |
Lower mantle contribution to the genesis of carbonatites: the noble gases and carbon isotopic evidence |
| VerfasserIn |
J. Mata, M. Moreira, C. Mourão, M. Ader, R. Doucelance |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028031
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| Zusammenfassung |
| It has been demonstrated that only 20 to 40% of the subducted CO2 is extracted by
decarbonatation of descending slab. This, and the fact that some carbonatites exhibit
HIMU-like Pb, Nd and Sr isotopic signatures, led to models invoking a carbonatite origin by
partial melting of ancient subducted carbonated oceanic crust. A recycled carbon origin for
diamonds has also been suggested. However the mantle is the largest carbon reservoir of the
Earth and a non-recycled (i.e. primordial) origin for the carbon in carbonatites cannot be
discarded.
Some of the Cape Verde oceanic carbonatites present low 4He/3He ratios (down to
46,700; R/Ra up to 15.5) demonstrating that they sample a reservoir characterized by low
time-integrated (U+Th)/3He. Such a reservoir, being clearly distinct from the crust or
from the upper mantle sampled by the 60 000 km long oceanic ridge system, is
thought to be localized in the lower mantle. For continental carbonatites several
authors has also interpreted noble gases isotopic compositions as reflecting the
contribution of a reservoir with time-integrated (U+Th)/(3He, 22Ne) and 40K/36Ar
lower than the upper mantle, thus endorsing the contribution of the lower mantle
[1;2;3].
Some carbonatites are also characterized by 129Xe anomalies relatively to the air
(129Xe/130Xe up to 6.94 in Cape Verde). Considering that the recycling of carbonates,
eventually characterized by high Te and Ba contents, would with time increase
simultaneously the 129Xe and 130Xe, the observed 129Xe anomalies cannot be explained by
models calling upon crustal carbonate recycling. We interpret them in terms of an ancient
mantle origin by decay of the now extinct 129I. Moreover, experimental work has
demonstrated that crustal carbonates are unlikely to be transported to lower mantle depth
levels as a consequence of its removal by melting reactions. Thus, all the above described
lower mantle signals are indicative of a non-recycled, lower mantle, origin for carbon, unless
we admit that, during ascent, deep-seated mantle plumes entrain recycled carbon from
the upper mantle. Nevertheless many carbonatites are also characterized by δ13C
values (-8.0 to -4.25 per mil in Cape Verde) lighter than those characterizing crustal
inorganic carbonates, endorsing the role of primordial carbon to the genesis of
carbonatites.
Taking into account that a recycled origin for some carbonatites is inescapable from
carbon and noble gases signatures [e.g. 4; 5] we conclude that multiple origins (recycled vs.
primordial) are possible for the carbon involved in the generation of carbonatitic
magmas.
Sasada et al. (1997) – Geochim. Cosmochim. Acta, 61: 4219-4228
Marty et al. (1998)- Earth Planet. Sci. Lett., 164: 179-192
Tolstikhin et al. (2002) - Geochim. Cosmochim. Acta, 66: 881-901
Ray et al. (1999) – Earth Planet Sci. Lett., 170; 205-214
Basu & Murty (2006) – Chem. Geol., 234: 236-250
This is a contribution from the FCT/FEDER project PLINT (POCTI/CTA/45802/2002) |
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