Amphiboles isotopic ratios

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

Figure 24 Chondrite-normalized abundances of REEs in a wall-rock harzburgite from Lherz (dotted lines— whole-rock analyses), compared with numerical experiments of ID porous melt flow, after Bodinier et al. (1990). The harzburgite samples were collected at 25-65 cm from an amphibole-pyroxenite dike. In contrast with the 0-25 cm wall-rock adjacent to the dike, they are devoid of amphibole but contain minute amounts of apatite (Woodland et al., 1996). The strong REE fractionation observed in these samples is explained by chromatographic fractionation due to diffusional exchange of the elements between peridotite minerals and advective interstitial melt (Navon and Stolper, 1987 Vasseur et al, 1991). The results are shown in (a) for variable t t ratio, where t is the duration of the infiltration process and t the time it takes for the melt to percolate throughout the percolation column (Navon and Stolper, 1987). This parameter is proportional to the average melt/rock ratio in the percolation column. In (b), the results are shown for constant f/fc but variable proportion of clinopyroxene at the scale of the studied peridotite slices (<5 cm). All model parameters may be found in Bodinier et al. (1990). As discussed in the text, this model was criticized by Nielson and Wilshire (1993). An improved version taking into account the gradual solidiflcation of melt down the wall-rock thermal gradient and the isotopic variations was recently proposed by Bodinier et al. (2003).

---