Basaltic crust, melting

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

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. 

Petford N. and Atherton M. (1996) Na-rich partial melts from newly underplated basaltic crust the Cordillera Blanca Batholith. Peru. J. Petrol. 37, 1491-1521. 

Fluid-saturated melting of basaltic crust begins at temperatures of —650 °C at 1.5 GPa to —750 °C at 3 GPa (Figure 5). It should be noted that... 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

These geochemical arguments imply that TTGs have been derived from the mantle in two stages. First the mantle melted to form a basalt and then that basalt became hydrated and remelted to form a TTG magma. However, the fact that TTGs have mantle-like initial Sr and Nd ratios means that the basaltic TTG precursor had a short crustal residence time (i.e. there was a short time interval between the creation and destruction of the basaltic crust), the normal case for oceanic crust. 

Plume/oceanic plateau models This model is a variant of the basalt lower crust melting model described above, but in this case basalt melting takes place at the base of thick basaltic crust. Crust of this type may form through imbrication and stacking of normal-thickness oceanic crust or may form as initially thick crust in an oceanic plateau as the product of mantle plume-related magma-tism (Chapter 3, Section 3.1.5). 

Planetary crusts may be divided into three types. Primary cmsts form as a result of the initial melting of the body. The feldspathic crust of the lunar highlands forms this type of example (Fig. 11). Secondary crusts arise through later partial melting of solid planetary mantles and in the rocky inner planets of the solar system, produce basaltic melts. The lunar maria and the surfaces of Mars and Venus as well as our oceanic crust are examples (Figs. 11, 12). Remelting and reprocessing of the basaltic crust as it is returned to the mantle produces our familiar continental cmst. This is an example of a tertiary cmst, and it appears to be the sole example of this type in the solar system. 

Even though the lunar crust has been thinned dramatically beneath large impact basins, no exposures of the mantle have been recognized. It is not surprising, then, that no mantle rocks have been found among returned samples. Consequently, the composition of the lunar mantle must be determined indirectly, from the basaltic magmas that represent partial melts of mantle sources. 

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