Basalts mantle mixing

In order to achieve these aims, and to limit the length of the chapter, we have not provided a detailed review of theories regarding the origin of primitive arc basalts, the mixing of magmatic components derived from the upper mantle, aqueous fluids, and sediment melts, or open-system processes in the cmst including mixing and assimilation. For a more complete view of these theories, we refer the reader to the many excellent review papers and individual smdies that are, all too briefly, cited below. 

Chesley, j., Ruiz, J. Hooper, P. 1996. Crust-mantle mixing implications based on the Re-Os isotope systematics of the Columbia River basalt group. EOS Trans, American Geophysical Union, 77, 832. 

Fig. 10.5. Models showing Sr-0 isotope variation during mixing between upper crustal rocks (full triangle), mantle peridotite (star) and mantle-derived basaltic magma (full square). Solid line represents the trend of magma contamination by upper crust. Dotted line is mantle contamination trend. Dashed line represents contamination trend of Roman Province magmas by sedimentary carbonates. Numbers along the lines indicate amounts of crustal end-member. For discussion, see text.

Two extreme notions about the meaning of these components or end members (sometimes also called flavors ) can be found in the hterature. One holds that the extreme isotopic end members of these exist as identifiable species, which may occupy separate volumes or reservoirs in the mantle. In this view, the intermediate compositions found in most oceanic basalts are generated by instantaneous mixing of these species during melting and emplacement of OIBs. The other notion considers them to be merely extremes of a... 

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). 

In summary, we have recently witnessed a shift away from the classically layered mantle model in favor of whole mantle convection models, where the buoyancy of sinking slabs is the dominant driving force. Slabs can penetrate deep into the lower mantle and with the induced return flow we would expect the mantle to mix efficiently. This leaves us with an interesting dilemma. If the mantle convects as a whole, how can it preserve the large-scale and long-hved heterogeneity seen in the geochemistry of oceanic basalts ... 

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