Niobium 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... 

Amphibole HFSE characteristics are sensitive to the presence of Umenite and rutile. In MARID samples where rutile dominates over Umenite, Gregoire et al. (2002) note that K-richterites display large negative niobium and tantalum anomalies, whereas when Umenite dominates, these anomalies are positive. Nb/Ta ratios of amphibole vary from close to the PUM value (17.6) up to —25 (Figirre 23). Zr/Hf values are generally lower than the PUM value of 37 (Figure 23). Differences in HFSE contents between vein and disseminated amphibole (and mica) in spinel peridotites may be explained by a model in which Zr-Nb rich amphibole and mica crystallize close to, or within a melt vein in the mantle. The fractionated, chlorine-rich aqueous residual fluids from the evolved melt then crystallize low Zr-Nb, LREE-depleted amphibole or... 

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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