Boninite als Fenster für Spurenelementtransport in Subduktionszonen
Zusammenfassung der Projektergebnisse
The aim of this research project was to examine the trace element inventory of boninites and associated arc tholeiites, with an emphasis on the extended high field strength element group (HFSE: W, Sb, Nb, Ta, Mo, Zr, Hf). Boninites are an important endmember of subduction zone magmas, as their budget of incompatible elements is mostly derived from subduction components that overprinted the mantle sources of these magmas. Two classic boninitetholeiite assemblages were initially chosen for this project, high-Ca boninites from Cyprus and low Ca boninites from Cape Vogel, Papua New Guinea. Based on our newly developed analytical protocols, we measured critical HFSE ratios such as Ta/W, Nb/Ta or Zr/Hf by isotope dilution to ensure the greatest accuracy possible. Melt modelling indicates that the low-Ca boninites originate from much more refractory mantle sources than the high-Ca boninites from Cyprus. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. In contrast, the HFSE ratios in the PNG boninites are best explained by the addition of melt-like slab components in equilibrium with garnet–amphibolitic mafic oceanic crust. Our results suggest the subarc mobility of HFSE decreases in the order Sb > W–Mo > Nb–Ta > Zr–Hf. Systematics of Mo–W may serve as a novel tracer for the amount, composition and redox state of subducted pelagic sediments. The observed selective enrichment of W relative to other highly incompatible elements like Th, U, and Nb-Ta in subduction rocks spurred further investigations, and high-precision HFSE measurements were also performed on rocks from other tectonic settings (MORBs and OIBs). These data reveal a selective depletion of W in OIBs and MORBs relative to immobile HFSE that is complementary to the selective W enrichment observed in arc rocks and the continental crust. Therefore we re-assessed the W mass balance in the silicate Earth with a now lower W abundance and higher Hf/W ratio in the primitive mantle. The model age for the formation of the Earth’s core that is inferred from Hf-W systematics is therefore slightly younger (up to 38 Myrs after solar system formation) than previously calculated (ca. 33 Myrs).
Projektbezogene Publikationen (Auswahl)
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(2008): Mobility of W in subduction zones. Earth and Planetary Science Letters 274 (1-2), 82-92
König, S., Münker, C., Schuth, S., Garbe Schöberg, D.
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(2010): Boninites as windows into variable trace element mobility in subduction zones. Geochimica et Cosmochimica Acta 74 (2), 684-704
König, S., Münker, C., Schuth, S., Luguet, A., Hoffmann, J.E., Kuduon, J.
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(2010): Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochimica et Cosmochimica Acta 74 (24): 7236-7260
Hoffmann, J.E., Münker, C., Polat, A., König, S., Mezger, K., Rosing, M.T.
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(2011) Deep melting of old subducted oceanic crust recorded by superchondritic Nb/Ta in modern island arc lavas. Earth and Planetary Science Letters 301, 265–274
König S. & Schuth S.
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(2011): The Earth’s tungsten budget during mantle melting and crust formation. Geochimica et Cosmochimica Acta 75 (8), 2119-2136
König, S., Münker, C., Hohl, S., Paulick, H., Barth, A.R., Lagos, M., Pfänder, J., Büchl, A.
