Models mantle geochemistry

The chemical and isotopic compositions of various earth materials now make up the data used to build models to explains the formation of the Earth, its evolution the genesis of the different terrestrial units continents, mantle, core, ocean etc.. .. From a descriptive and qualitative early stage, geochemistry has become explanatory and quantitative. In this new context modeling is a key method. 

McDonough, W.F. (2004) Composition model for the Earth s core, in Treatise on Geochemistry Volume 2 The Mantle and Core (eds R.W. Carlson, HD. Holland and K.K. Turekian Editors-in-Chief), Elsevier Science, pp. 547-68. 

Crystal field theory is one of several chemical bonding models and one that is applicable solely to the transition metal and lanthanide elements. The theory, which utilizes thermodynamic data obtained from absorption bands in the visible and near-infrared regions of the electromagnetic spectrum, has met with widespread applications and successful interpretations of diverse physical and chemical properties of elements of the first transition series. These elements comprise scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. The position of the first transition series in the periodic table is shown in fig. 1.1. Transition elements constitute almost forty weight per cent, or eighteen atom per cent, of the Earth (Appendix 1) and occur in most minerals in the Crust, Mantle and Core. As a result, there are many aspects of transition metal geochemistry that are amenable to interpretation by crystal field theory. 

An important development stemming from heterogeneous accretion models is that they introduced the concept that the Earth was built from more than one component and that these may have been accreted in separate stages. This provided an apparent answer to the problem of how to build a planet with a reduced metallic core and an oxidized sihcate mantle. However, heterogeneous accretion is hard to reconcile with modem models for the protracted dynamics of terrestrial planet accretion compared with the shortness of nebular timescales. Therefore, they have been abandoned by most scientists and are barely mentioned in modem geochemistry literature any more. 

Davies G. F. (2002) Stirring geochemistry in mantle convection models with stiff plates and slabs. Geochim. Cosmochim. Acta 66, 3125-3142. 

Hofmann A. W. (1989a) Geochemistry and models of mantle circulation. Phil. Trans. Roy. Soc. London A 328, 425-439. 

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 ... 

One of the fundamental difficulties with the whole mantle convection model is that geochemical heterogeneities cannot be sustained over geological timescales. And yet, observations from trace element and isotope geochemistry show that the mantle is and has... 

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