Planetary differentiation basalts

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. 

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. 

Many meteorites have never been subjected to processes of planetary differentiation. These imdifferentiated meteorites come fix>m planetesimals that were never molten and resemble die composition of the solar nebula at the time and place of their formation. They exhibit approximately solar system composition, are called chondritic meteorites, and represent one big group of meteorites. The second group consists of the differentiated meteorites which represent pieces of partially or totally molten parent bodies. Examples of this group are meteoritic basalts (eucrites) or iron meteorites. The latter are pieces of the segregated core of the parent body. 

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