Basalts crust/mantle differentiation

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. 

It is likely that there is no primitive, undifferentiated mantle still preserved within the Earth s mantle. Nd-isotopes indicate that the mantle experienced a major differentiation event, perhaps as early as 30 Ma after the formation of the solar system, in which a Fe-rich basaltic crust formed on a magma ocean and was... 

The recognition of a 142Nd anomaly within the mantle implies that the Earth experienced a major, very early differentiation event. The study by Boyet and Carlson (2005) showed that lunar basalts also have elevated 142Nd/144Nd ratios relative to primitive chondrites, implying that the Moon was formed from an Earth that had already experienced major differentiation. This means that the early differentiation of the Earth took place within 30 Ma of the formation of the solar system. Whilst the precise nature of this differentiation event is not known, a favored model is the formation of an Fe- and trace element-enriched basaltic crust, perhaps as an initial crust to a magma ocean. It is postulated that this crust is now isolated from the convecting mantle and is located deep within the lower mantle. 

Studies of the short-lived isotope 142Nd in primitive chondritic meteorites have recently shown that the Earth s mantle does not have a chondritic 142Nd-isotope ratio (Boyet Carlson, 2005). The implication of this finding is that the Earth experienced a major differentiation event in which an Fe-rich, trace element-enriched basaltic crust formed on a magma ocean. This crust has subsequently been removed and isolated from the convecting mantle and may now be represented by the D" layer (Tolstikhin Hofmann, 2005). Similar 142Nd results on lunar samples suggest that this differentiation took place before the formation of the Moon, that is, within 30 Ma of the formation of the solar system (Boyet Si Carlson, 2005). 

II- Early Archaean mantle differentiation related to the extraction of basaltic crust... 

III - Mantle differentiation related to the extraction of the continental crust The 143Nd-isotope record of the Earth s mantle provides a record of the progressive extraction of the continental crust, from about 3.0 Ga to the present-day (Fig. 3.27). Continental crust is created from the mantle in two steps. First, basaltic crust is produced by the partial... 

In this present version of the model the D" layer is thought to have originated very early in Earth history, as an early, incompatible element- and metal-rich basaltic crust, enriched during late accretion (4,540-4,000 Ma) with chondritic material. There is support from Nd-and Hf-isotopes for the existence of this very early differentiate of the mantle (see Sections 3.2.3.1 and 3.2.3.2). This crust, when subducted, had a bulk density which exceeded that of the mantle and numerical modeling experiments confirm that it would have stabilized at the core-mantle boundary (Davies, 2006). 

Mantle-derived basalts, on the other hand, have a relatively uniform composition with 8 Li values of 4 2%o (Tomaszak 2004 Elliott et al. 2004). The continental crust generally has a lighter Ei isotope composition than the upper mantle from which it was derived (Teng et al. 2004). Considering the small Li isotope fractionation at high temperature igneous differentiation processes (Tomaszak 2004), pristine... 

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. 

These granites can be generated by extreme differentiation of basaltic magmas, or by remelting mafic mantle-derived materials that have had a short residence time in the crust. Both models effectively result in the formation of new cmst and in differentiation, but as the resultant granitic magmas are isotopically primitive, it may be difficult to isolate which of the two processes was responsible for their formation. 

If the model outlined above is valid, sihcic magmatism in the LFB involved net crustal growth, as juvenile mantle-derived liquids, or their differentiates were instrumental in the formation and compositional evolution of hornblende granites, and, to a lesser extent, the cordierite granites. The amount of new crust generated is estimated by determining the overall mantle component present within both granitic types, and this is best done isotopically, since the trace-element ratios are poorly constrained for the potential basaltic end-members. 

A study of the radiogenic isotope memory of the Earth s mantle clearly shows that the mantle is neither an independent part of the Earth system nor has it been for a long time. But rather, it records a history of the extraction and recycling of both basaltic and continental crust. This raises two very important questions. First, is our isotopic record of the mantle representative of the whole mantle or only the upper mantle Second, is there any primitive, undifferentiated mantle still preserved within the large mass of the differentiated Earth s mantle ... 

Fresh, Cenozoic oceanic basalts typically span a narrow range in 5 O ( 5 to 7 %o for whole rocks), restricting the abundance of subducted crust in their mantle sources and crustal contaminants added to them during differentiation to amounts less than 10 wt %. Furthermore, these data suggest that isotopic fractionations during melting, metasomatism, and metamorphism in the upper mantle are small... 

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