Barium peridotites

The distribution of lithophile trace elements (REE + mbidium, caesium, strontium, barium, yttrium, zirconium, hafnium, niobium, tantalum, thorium, and uranium) normalized to primitive mantle (PM) values are illustrated in Figure 16 for a range of peridotite lithologies from the Ronda orogenic Iherzolite massif, and in Figure 17 for ophiolitic and abyssal refractory peridotites. 

The abyssal peridotites from the Izu-Bonin-Mariana Forearc resemble the ophiolitic peridotites. They are, nevertheless, distinguished by more prominent enrichments in alkaline and alkaline-earth elements (rubidium, caesium, strontium, and barium), possibly reflecting a supra-subduction imprint (Parkinson and Pearce, 1998). Except for one sample, they lack the niobium enrichment relative to FREE that is observed in the Ronda and... 

Ionov et ai, 1997). Both strontium and barium are higher in amphibole from xenohths than those from massif peridotites (e.g., Vannucci et ai, 1995). Numerous ion-probe and laser/solution ICP-MS studies have found wide variation in amphibole REE patterns depending on textural context and tectonic setting (Witt- Eickschen and Harte, 1994 Vannucci et al, 1995 Ionov and Hofmann, 1995 Johnson et al, 1996 Chazot etai, 1996 Vasellieta/., 1995 Ionov eta/., 1997 Pearson and Nowell, 2002 Gregoire et ai, 2002). 

In summary, amphibole, along with mica, is the dominant silicate host for niobium in peridotitic xenoliths. In mica-absent assemblages amphibole also dominates the barium and tantalum budgets (Ionov et al, 1997 Eggins et al, 1998) and its presence strongly alfects the bulk rock Zr/Nb ratio. 

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