Basalts niobium/thorium

Niobium and tantalum are roughly interpolated between thorium and lanthanum in the Iherzolites, but the Nb/Ta values are variable. Lenoir et al. (2001) have revealed an abrupt change in the Nb/Ta ratio across the Ronda recrystallization front (Van der Wal and Vissers, 1993, 1996), from subchondritic values (—10) in the spinel tectonite domain ( ahead of the front) to near-chondritic (—16) in the granular domain ( behind the front). They tentatively ascribed this observation to a change from lithospheric conditions, under which Nb-Ta would be dominantly controlled by very small amounts of titanium oxides precipitated from volatile-rich small-volume melts (Bodinier et al., 1996), to more astheno-spheric conditions, under which the titanium oxides would be dissolved into basaltic partial melt and Nb-Ta redistributed between silicate phases. The harzburgites are systematically enriched in niobium and tantalum relative to... 

Tantalum and niobium in arc magmas are depleted relative to REEs, so that Nb/La and Ta/La are lower than in the primitive mantle and in MORBs. Depletion of primitive arc magmas in tantalum and niobium relative to lanthanum and thorium (Figure 14) is ubiquitous (a few niobium-enriched lavas—e.g., Kepezhinskas et al. (1997)—form a distinct anomaly which will not be discussed in this chapter). For this reason, and because thorium, tantalum, and niobium are relatively immobile in low temperature alteration of basalts, low Nb/Th and Ta/Th have been used as discriminants between arc magmas and both... 

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