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| Titel |
Genesis of Ultra-High Pressure Garnet Pyroxenite in Orogenic Peridotites and its bearing on the Isotopic Chemical Heterogeneity in the Mantle Source of Oceanic Basalts |
| VerfasserIn |
María Isabel Varas Reus, Carlos J. Garrido, Claudio Marchesi, Delphine Bosch, Károly Hidas |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143805
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| Publikation (Nr.) |
 EGU/EGU2017-7559.pdf |
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| Zusammenfassung |
| The genesis of ultra-high pressure (UHP) garnet pyroxenites in orogenic peridotite massifs
and its implications on the formation of chemical heterogeneities in the mantle and on basalt
petrogenesis are still not fully understood. Some UHP (diamond-bearing) garnet pyroxenites
have isotopic, and major and trace element compositions similar to the recycled oceanic
crustal component observed in oceanic basalts [1–6]. These pyroxenites hence provide an
exceptional opportunity to investigate in situ the nature and scale of the Earth’s mantle
chemical heterogeneities.
Here, we present an integrated geochemical study of UHP garnet pyroxenites
from the Ronda (Betic Belt, S. Spain) and Beni Bousera (Rif Belt, N. Morocco)
peridotite massifs. This investigation encompasses, in the same sample, bulk rock major
and trace elements, as well as Sr-Nd-Pb-Hf isotopic analyses. According to their
Al2O3 content, we classify UHP garnet pyroxenites into three groups that have
distinct trace elements and Sr-Nd-Pb-Hf isotopic signatures. Group A pyroxenites
(Al2O3: 15 – 17.5 wt. %) are characterized by low initial 87Sr/86Sr, relatively high
143Nd/144Nd, 206Pb/204Pb and 176Hf/177Hf ratios, and highly variable 207Pb/204Pb
and 208Pb/204Pb ratios. Group B pyroxenites (Al2O3 < 14 wt. %) have isotopic
signatures characterized by relatively high initial 87Sr/86Sr and low 143Nd/144Nd,
206Pb/204Pb and 176Hf/177Hf ratios. Group C pyroxenites (Al2O3 ∼ 15 wt. %)
display relatively low initial 87Sr/86Sr and 206Pb/204Pb ratios, high 143Nd/144Nd and
176Hf/177Hf ratios, and 207Pb/204Pb and 208Pb/204Pb ratios similar to Group B
pyroxenites.
The major and trace element, and isotopic compositions of the studied Ronda and Beni
Bousera UHP garnet pyroxenites lend support to the “Marble Cake Mantle” model [7] for the
genesis of these pyroxenites. This model envisions the mantle source of oceanic basalts as a
mélange of subducted, ancient oceanic crust —-represented by garnet pyroxenites in orogenic
peridotites—- intimately mixed with peridotites by mantle convection. The present study
reveals, however, that besides this exotic component of ancient recycled oceanic crust, the
genesis of these pyroxenites requires a previously unnoticed component of recycled lower
continental crust akin to the lower crustal section of the lithosphere where these UHP
garnet pyroxenites now reside in. The results of this study provide a new recipe for
the marble cake hypothesis for the genesis of UHP garnet pyroxenites in orogenic
peridotites. Furthermore, it establishes a connection between the genesis of UHP
pyroxenites, the composition of the continental crust and the generation of Earth’s mantle
heterogeneities.
References:
[1] Pearson, D. G., Davies, G. R. & Nixon, P. H. (1993). Geochemical constraints on the
petrogenesis of diamond facies pyroxenites from the Beni Bousera peridotite massif, North
Morocco. Journal of Petrology 34, 125-172.
[2] Blichert-Toft, J., Albarède, F. & Kornprobst, J. (1999). Lu-Hf Isotope systematics of
garnet pyroxenites from Beni Bousera, Morocco: implications for basalt origin. Science 283,
1303-1306.
[3] Garrido, C. J. & Bodinier, J. L. (1999). Diversity of mafic rocks in the Ronda
peridotite: Evidence for pervasive melt-rock reaction during heating of subcontinental
lithosphere by upwelling asthenosphere. Journal of Petrology 40, 729-754.
[4] Marchesi, C., Garrido, C.J., Bosch, D., Bodinier, J.-L., Gervilla, F., Hidas, K., 2013.
Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda
peridotite massif, southern Spain. Earth and Planetary Science Letters 362, 66-75. doi:
10.1016/j.epsl.2012.11.047.
[5] Marchesi, C., Dale, C.W., Garrido, C.J., Pearson, D.G., Bosch, D., Bodinier, J.-L.,
Gervilla, F., Hidas, K., 2014. Fractionation of highly siderophile elements in refertilized
mantle: Implications for the Os isotope composition of basalts. Earth and Planetary Science
Letters 400, 33-44. doi: 10.1016/j.epsl.2014.05.025
[6] Montanini, A. & Tribuzio, R. (2015). Evolution of recycled crust within the mantle:
Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern
Apennines, Italy). Geology 43, 911-914.
[7] Allègre, C. J. & Turcotte, D. L. (1986). Implications of a two-component marble-cake
mantle. Nature 323, 123-127. |
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