Journal of Petrology Advance Access originally published online on May 3, 2008
Journal of Petrology 2008 49(6):1223-1251; doi:10.1093/petrology/egn023
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Ultra-refractory Domains in the Oceanic Mantle Lithosphere Sampled as Mantle Xenoliths at Ocean Islands
1Physics of Geological Processes, University of Oslo, Po Box 1048, Blindern, N-0316 Oslo, Norway
2Earth Science Department, University of Ferrara, Via Saragat, 1, 44100 Ferrara, Italy
3Umr Cnrs 8148 Ides, ‘Interactions Et Dynamique Des Environnements De Surface’, Faculté Des Sciences, Université Orsay-Paris Sud, Bât. 504, 91405 Orsay Cedex, France
4Observatoire Midi-Pyrénées, Umr 5562-DTP, Omp, Université Toulouse III, Avenue E. Belin, 31400 Toulouse, France
5Department of Geology, Miami University, 114 Shideler Hall, Oxford, Oh 45056, Usa
RECEIVED JUNE 29, 2007; ACCEPTED APRIL 11, 2008
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Abstract |
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Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.
KEY WORDS: mantle xenoliths; ocean islands; recycled oceanic mantle lithosphere; ultra-depleted mantle
*Corresponding author. Telephone: +47-22856922. Fax: +47-22855101. E-mail: n.s.c.simon{at}fys.uio.no
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