Urey ratio and the structure and evolution of Earth's mantle

Abstract

The Urey ratio describes the contribution of internal heat production to planetary‐scale energy balance, and knowing this thermal budget for Earth is essential to understand its long‐term evolution. Internal heat production is provided by the decay of radiogenic elements, whose budget is constrained by geochemical models of Earth. Understanding the thermal budget thus requires contributions from both geophysics and geochemistry. The purpose of this review is to elucidate various geochemical and geophysical arguments and to delineate the most likely thermal budget of Earth. While the bulk Earth Urey ratio is probably ∼0.35, the convective Urey ratio is estimated to be ∼0.2. That is, only ∼20% of convective heat flux seems to originate from radiogenic elements at present, so the rest should be supported by secular cooling. A likely scenario for Earth's thermal history indicates that the peculiarity of today's significant imbalance between heat production and heat loss may result from the initiation of plate tectonics in the early Earth.

The effects of deep water cycling on planetary thermal evolution

“…As seen on Figure 15 the Urey ratio varies with the amount of water recycled through the system. Recent studies propose relatively low values for the presentday (mantle) Urey ratio in the range of about 0.21 -0.49 [Jaupart et al, 2007;Korenaga, 2008]. Our modeling shows that increased water recycling tends to lower the Urey ratio.…”

“…The decrease predicted by our simulations is from a value of 0.8 to 0.65. The lowest value is still too high to match present-day Urey ratio estimates [Jaupart et al, 2007;Korenaga, 2008] but the trend, in terms of water cycling effects, is in the correct direction.…”

The geochemistry and petrogenesis of the Paleoproterozoic du Chef dyke swarm, Québec, Canada

“…This evidence, along with consistent trace element geochemistry suggests that the du Chef dykes are be the product of mantle plume-driven magmatism as proposed by Buchan (2004) andSöderlund et al (2010). However, alternate models of the cooling of the mantle [e.g., Korenaga (2008) and Abbot et al (1994)] indicate that the du Chef dykes are not the product of an anomalously hot mantle plume. Continued research into the thermal evolution of the mantle and derivation of robust models which estimate the temperature of the upper mantle at ~2.4…”

“…10 along with the three secular cooling models described above. Sample DC011 records a T p 87°C and 179°C higher than is predicted for the ~2.4 Ga mantle by the models of Richter (1988) and Davies (2009) respectively, but 138°C lower than is predicted by the model of Korenaga (2008). Determining the veracity of the disparate models presented in Fig.…”

“…This initial temperature increase is followed by an increasingly rapid drop in T P to a present day values of 1350°C. Davies (2009) suggests that the low urey ratio (heat produced by radioactive decay/heat loss) used by Korenaga (2008) is extreme and instead favours a model of constantly decreasing temperature from an initial upper mantle temperature of 1800°C at 4.5 Ga to reach a modern day temperature of 1300°C.…”