Archaean mantle potential temperature

Calculations based on radioactive heat production show that the Archaean mantle was hotter than the modern mantle. High mantle potential temperatures calculated from the ultramafic lavas - komatiites, common in the Archaean - led to the assumption that the Archaean mantle was substantially hotter than the modern mantle. However, the recent proposal that komatiites are the product of cooler, wet mantle melting weakens the argument for a very hot Archaean mantle, and there are now good grounds for arguing that the temperature of the Earth s mantle has declined by only 100-200°C since the mid-late Archaean. 

Melt thickness therefore can be expressed as a function of the mantle potential temperature. A potential temperature of 1,280°C equates to a melt thickness of about 7 km (normal ocean floor), whereas a mantle potential temperature of 1,480°C equates to a melt thickness of about 27 km (see Fig. 3.14). Clearly, these principles are important when considering melting processes in the early Earth, since many geoscientists believe that mantle temperatures were hotter in the Archaean (Section 3.2.3). 

Archaean mantle drawn from a knowledge of the Earth s radioactive heat content and mantle potential temperatures calculated for komatiites and basalts. 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

However, as was shown in Chapter 3, the evidence of higher mantle potential temperatures in the Archaean is the subject of much debate. On the one hand, there is no doubt that the decay of radioactive elements in the mantle during early in Earth history made for a hot early mantle. What is less certain is whether or not this period extended into the mid- and late-Archaean, for the previously convincing evidence from komatiites is now looking more equivocal. 

Nisbet, E. G., Cheadle, M. J., Arndt, N. T. Bickle, M. j. 1993. Constraining the potential temperature of the Archaean mantle a review of the evidence from komatiites. Lithos, 30,291-307. 

The coohng history of the mantle Figure 3.25 shows the secular cooling curve for the mantle from Richter (1988), the calculated potential temperatures for Archaean komatiites and basalts, and an estimated temperature for the Archaean subcontinental mantle. Two important conclusions follow. First, it is clear that the Archaean mantle had both hot and cool regions. Potential temperatures calculated from dry komatiite melting temperatures imply an anomalously hot mantle source,... 

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