Continental crust melting

In contrast to the southern volcanic zone, Parinacota volcano lies on very thick continental crust (> 70 km) in the central volcanic zone of Chile. Bourdon et al. (2000a) showed that young Parinacota lavas encompass a wide range of U-Th disequilibria. excesses were attributed to fluid addition to the mantle wedge but °Th-excesses in lavas from the same volcano are more difficult to explain. The lavas with °Th-excesses also have low ( °Th/ Th) (< 0.6) characteristic of lower continental crust characterized by low Th/U and in their preferred model. Bourdon et al. (2000a) attributed the °Th-excesses to contamination by partial melts, formed in the presence of residual garnet, of old lower crustal materials. 

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. 

The Earth s crust and, indeed, the crusts of all differentiated bodies, are enriched in incompatible elements relative to their mantles. This reflects the partial melting of mantle material and extraction and transport of the basaltic melt to the surface. On Earth, further partial melting of the basaltic crust in the presence of water produces magma compositions even richer in silica (andesite and granite), which form the bulk of the continental crust. Because other differentiated bodies are effectively dry, this second level of differentiation did not occur. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

It is likely that the ancient crust of Mars is more mafic than the Earth s continental crust. Pervasive andesite may signal crustal fractionation, but the identity and significance of andesitic rocks is disputed. The martian crust is relatively more voluminous than the Earth s cmst, perhaps because it is not recycled. The cmst is characterized by high concentrations of incompatible hthophile elements, but fractionations are not as extreme as in terrestrial continental cmst, which has experienced repeated partial melting events over a protracted geologic history. 

The principal division of the Earth into core, mantle, and crust is the result of two fundamental processes, (i) The formation of a metal core very early in the history of the Earth. Core formation ended at —30 million years after the beginning of the solar system (Kleine et aL, 2002). (ii) The formation of the continental crust by partial melting of the silicate mantle. This process has... 

The general, incompatible-element depleted nature of the majority of MOREs and their sources is well explained by the extraction of the continental crust. Nevertheless, the bulk continental crust and the bulk of the MORE sources are not exact chemical complements. Rather, the residual mantle has undergone additional differentiation, most likely involving the generation of OlEs and their subducted equivalents. In addition, there may be more subtle differentiation processes involving smaller-scale melt migration occurring in the upper mantle (Donnelly et al., 2003). It is these additional differentiation processes that have... 

The techniques illustrated in Figures 17 and 18 can be used to establish an approximate compatibility sequence of trace elements for mantle-derived melts. In general, this sequence corresponds to the sequence of decreasing (normalized) abundances in the continental crust shown in Figure 2, but this does not apply to niobium, tantalum, and lead for which the results discussed in the previous section demand rather different positions (see also Hofmann, 1988). Here I adopt a sequence similar to that used by Hofmann (1997), but with slightly modified positions for lead and strontium. 

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