Thorium peridotites

Attree RW, Cabell MJ, Cushing RL, Pieroni JJ (1962) A calorimetric determination of the half-life of thorium-230 and a consequent revision to its neutron capture cross section. Can J Phys 40 194-201 Bateman H (1910) Solution of a system of differential equations occurring in the theory of radioactive transformations. Proc Cambridge Phil Soc 15 423-427 Beattie PD (1993) The generation of uranium series disequilibria by partial melting of spinel peridotite ... 

Beattie P (1993b) Uranium-thorium disequihbria and partitioning on melting of garnet peridotite. Nature 363 63-65... 

Blatter DL, Carmichael ISE (1998) Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26 1035-1038 Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

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. 

Most Ronda peridotites show thorium variations consistent with LREE variations. In other words, thorium generally plots on extrapolations of the LREE segments on the PM-normalized diagrams and Th/La is roughly correlated with La/Sm. Both ratios are generally lower than PM values in the Iherzolites, close to PM values in the harzburgites and variable in the dunites. 

LREE enrichment relative to PUM in whole-rock peridotites is more likely to be a product of grain-boundary enrichment or clinopyroxenes becoming LREE enriched. Micas are poor in thorium and uranium but high in lead compared to clinopyroxene so that U/Pb is low. Some micas can have up to 20 ppm lead and Rosenbaum (1993) proposes that they are the main mantle lead repository. More recent data have shown that amp-hiboles can have comparably high lead (Table 9). 

A key feature is that all the nuclides of interest are highly incompatible in common mantle mineral phases (Table 2). Clinopyroxene and garnet (present in normal peridotitic mantle at depths greater than —80 km) are the principal host minerals for uranium and thorium in the solid phase, although even in these phases partition coefficients do not exceed 0.05. An important consideration is the sense of uranium and thorium fractionation imparted by the presence of different minerals. This can be conveniently expressed in terms of Du/ >Th- Minerals with Du/ >Th > 1 retain uranium over thorium and contribute to 23 Th-excesses in a coexisting melt, whereas those with Du/ >Th < 1 help to create 23 3xh-deficits. The effects of different minerals are then weighted by their absolute partition coefficients and modal abundance to control the bulk partition coefficient of the mantle, and thus determine the sense of fractionation of thorium from uranium in a coexisting melt. 

A more minor consideration is the role of orthopyroxene. At 1.5 GPa orthopyroxene has Du an order of magnitude less than clinopyroxene, and the small lattice site strongly favors uranium over thorium (Blundy and Wood, 2003a Wood et al., 1999). The Z>u/Dxh for orthopyroxene is consequently high ( 2.5, Table 2) and may influence the bulk partition coefficient despite its low absolute partition coefficients, especially in highly depleted peridotites. 

MORE melting and dehydration in serpentinized peridotite was illustrated in Section 3.17.5.4. Here, an example of the effects on trace-element transfer as a function of fluid rock ratios is illustrated. The layered structure of the oceanic lithosphere may cause the fluid rock ratio in the sedimentary layer to be greater than one (as fluids from the underlying mafic and peridotitic layers must rise through the sediments). Thus, some of the trace elements (e.g., beryllium, thorium) commonly considered to be only efficiently mobilized by melts could also be quite effectively mobilized by fluids, if the entire subducted lithosphere is considered. 

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