Ultra-refractory domains in the oceanic mantle lithosphere sampled as mantle xenoliths at ocean Islands

Nina S C Simon*, Else Ragnhild Neumann, Costanza Bonadiman, Massimo Coltorti, Guillaume Delpech, Michel Grégoire, Elisabeth Widom

*Corresponding author for this work

Research output: Contribution to journalArticle

52 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)1223-1251
Number of pages29
JournalJournal of Petrology
Volume49
Issue number6
DOIs
Publication statusPublished - Jun 2008
Externally publishedYes

Keywords

  • Mantle xenoliths
  • Ocean islands
  • Recycled oceanic mantle lithosphere
  • Ultra-depleted mantle

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