Primitive earth melting

The primitive earth long remained covered in darkness, wrapped in dense burning clouds into which water vapor poured continuously from volcanic emissions. When temperatures finally cooled sufficiently, the clouds began to melt into rain. At first, falling on incandescent rock, the rain evaporated, but the evaporation... 

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of... 

Laboratory study of the first lunar samples brought to Earth by Apollo 11 ruled out all ideas that the Moon might be a primitive object, i.e. an object that had remained rather cool after its accumulation with only minor melting processes induced by large impacts on the surface. On the contrary, it became evident from the chemical analysis that the Moon, like the Earth, is a highly differentiated object. On the Moon many chemical elements are strongly enriched or depleted as compared with their abundances in carbonaceous chondrites which apart from the most volatile elements, are believed to be most representative of solar matter. Thus it became clear that, at least in the upper 200 km, extensive melting processes must have occurred. 

Except for the small amount of evidence for early mantle melting we are in the dark about how and when Earth s continents first formed (Figure 13). We already have pointed out that in its early stages Earth may have had a magma ocean, sustained by heat from accretion and the blanketing effects of a dense early atmosphere. With the loss of the early atmosphere during planetary collisions, the Earth would have cooled quickly, the outer portions would have solidified and it would thereby have developed its first primitive crust. 

The least fractionated rocks of the Earth are those that have only suffered core formation but have not been affected by the extraction of partial melts during crust formation. These rocks should have the composition of the PM, i.e., the mantle before the onset of crust formation. Such rocks are typically high in MgO and low in AI2O3, CaO, Ti02, and other elements incompatible with mantle minerals. Fortunately, it is possible to collect samples on the surface of the Earth with compositions that closely resemble the composition of the primitive mantle. Such samples are not known from the surfaces of Moon, Mars, and the asteroid Vesta. It is, therefore, much more difficult to reconstruct the bulk composition of Moon, Mars, and Vesta based on the analyses of samples available from these bodies. 

Asahara Y. and Ohtani E. (2001) Melting relations of the hydrous primitive mantle in the CMAS-H2O system at high pressures and temperatures, and imphcations for generation of komatiites. Pkys. Earth Planet. Inter. 125, 31-44. 

Kushiro 1. (1998) Compositions of partial melts formed in mantle peridotites at high pressure and their relation to those of primitive MORE. Phys. Earth Planet. Inter. 107, 103-110. 

Shimizu K., Komiya T., Hirose K., Shimizu N., and Maruyama S. (2001) Cr-spinel, an excellent micro-container for retaining primitive melts-impUcations for a hydrous plume origin for komatiites. Earth Planet. Sci. Lett. 189, 177-188. 

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., Eaure 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 cmst. 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 cmst 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., 1979). 

Yogodzinski G. M. and Kelemen P. B. (1998) Slab melting in the Aleutians implications of an ion probe study of clinopyroxene in primitive adakite and basalt. Earth Planet. Sci. Lett. 158, 53-65. 

The primitive carbonaceous meteorites, which include the hydrated CI and CM meteorites and mostly-anhydrous meteorites such as the Allende CV meteorite [69], reach Earth from the asteroid belt between Mars and Jupiter. Asteroid reflectance properties display a remarkably systematic distribution as a function of heliocentric distance for asteroids in this belt, and hence meteorite types, with the most primitive ones located farthest from the sun. Asteroid hydration occurred when internal heating melted (water) ice that had co-accreted with dust, chondrules and refractory inclusions in the solar nebula. These asteroids form the IR spectroscopic C-class with clays, carbon and organics at the surface similar to CI and CM meteorite parent bodies [70]. They and the Allende CV parent body, which apparently did not accrete (much) ice, are from the same zone of the asteroid belt. Even more primitive asteroids closer to Jupiter still contain co-accreted ices, organic materials and silicate dust. They define the IR spectroscopic primitive (P)-and dark (D)-class [70] bodies that include comet nuclei and many near-Earth asteroids [10]. 

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