Partial melting lower crust

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. 

Kempton P. D., Downes H., Sharkov E. V., Vetrin V. R., Ionov D. A., Carswell D. A., and Beard A. (1995) Petrology and geochemistry of xenoliths from the northern Baltic shield evidence for partial melting and metasomatism in the lower crust beneath an Archaean terrane. Lithos 36 (3—4), 157-184. 

Bea F. and Montero P. (1999) Behavior of accessory phases and redistribution of Zr, REE, Y, Th, and U during metamorphism and partial melting of metapeUtes in the lower crust an example from the Kinzigite Fromation of Ivrea-Verbano, NW Italy. Geochim. Cosmochim. Acta 63, 1133-1153. 

Petford N. and Gallagher K. (2001) Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma. Earth Planet. Sci. Lett. 193, 483-499. 

Figure 16 Schematic illustration of mechanisms for transfer of sediments, volcanics, and/or lower erustal gabbros into the mantle wedge from the subducting plate and the base of arc crust. Dark black line indicates position of the subduction zone below this line, material subducts at the convergence velocity. Above this line, material is carried downward more slowly. Any process leading to slow transport of low-melting point metasediment, metabasalt, or metagabbro into the mantle wedge would lead to partial melting of this material beneath an arc.

Figure 1 Systematics of Nd- and Hf-isotopic evolution in the bulk Earth, continental crust, and mantle. Daughter elements Nd and Hf are more incompatible during mantle melting (more likely to go into a partial melt of mantle rock) than Sm and Lu, respectively. As a result, the continental crust has a lower Sm/Nd and Lu/Hf ratio than the mantle, and lower Nd- and Hf-isotope ratios. Young continental crust has isotope ratios similar to the mantle, and the older the continental terrain, the lower the Nd- and Hf-isotope ratios. Rb-Sr behaves in the opposite sense, such that the parent element Rb is more incompatible than the daughter element Sr. (a) Schematic example of the evolution of Nd- and Hf-isotope ratios of a melt and the melt residue from a melting event around the middle of Earth history from a source with the composition of the bulk Earth, (b) The same scenario as in (a), but with the isotope ratios plotted as e d and snf. The bulk Earth value throughout geological time is defined as e d and SHf = 0> and e-value of a sample is the parts per 10 deviation from the bulk Earth value.

Longhi et al. 1999) and amphibolite (Christensen Mooney 1995 Rudnick Fountain 1995). For any of these compositions, partial melting and extraction of the anorthosite suite had to result in a general increase of restitic minerals (pyroxene and garnet). The occurrence of this process at a large scale, as the volume of mid-Proterozoic meta-anorthosites would require, would have produced a restitic bulk composition of the lower crust and a decrease of VpjVs ratios to values inconsistent with our observations. 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Faure and Hurley, 1963 Hart et al., 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental crust. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental crust is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1919). 

Galer and O Nions (1985) were the first to recognize the K-conundrum. They proposed a stepwise model whereby, through much of Earth history, the mantle had a nearly constant k with values equivalent to that of the BSE and only within the last 600 Ma was the ratio reduced to its present value. They suggested that the mantle operated as an open system in which there was an exchange of U and Th between the upper and lower mantle. In this two-layer mantle model, the removal of U and Th from the upper mantle to the crust by partial melting is counterbalanced by the flux of high-K lower mantle into the upper mantle (Fig. 3.31). 

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