How irreversible heat transport processes drive Earth's interdependent thermal, structural, and chemical evolution

• Published in: Gondwana research 2013-09, Vol.24 (2), p.490-500
• Main Authors: Hofmeister, A.M., Criss, R.E.
• Format: Article
• Language: English
• Subjects: Core formation; Earth's bulk composition; Layered convection; Missing carbon; Secular cooling
• ISSN: 1342-937X; 1878-0571
• DOI: 10.1016/j.gr.2013.02.009

Because magmatism conveys radioactive isotopes plus latent heat rapidly upwards while advecting heat, this process, not convection, links and controls Earth's thermal and chemical evolution. On this basis, we present an alternative view of Earth's internal workings. Earth's beginning involved cooling via explosive outgassing of substantial ice (mainly CO) buried with dust during accretion. High carbon content is expected from Solar abundances and ice in comets. Reaction of CO with metal provided a carbide-rich core while converting MgSiO3 to olivine via oxidizing reactions. Because thermodynamic law indicates that primordial heat from gravitational segregation is neither large, nor carried downwards, whereas differentiation forced radioactive elements upwards, formation of the core and lower mantle greatly cooled the Earth. Reference conductive geotherms, calculated by using accurate thermal diffusivity data, require that heat-producing elements are sequestered above 670km which limits convection to the upper mantle. These irreversible beginnings limit secular cooling to radioactive wind-down, permitting deduction of Earth's inventory of heat-producing elements from today's heat flux. Coupling this estimate with meteoritic data indicates that Earth's oxide content has been underestimated. Density sorting segregated a Si-rich, peridotitic upper mantle from a refractory lower mantle with high Ca, Al and Ti contents, consistent with diamond inclusion mineralogy. Early and rapid differentiation means that internal temperatures have long been buffered by freezing of the inner core, allowing survival of crust as old as ~4Ga. Magmatism remains important. Melt escaping through stress-induced fractures in the rigid lithosphere imparts a lateral component and preferred direction to upper mantle circulation. Mid-ocean magma production over 4.5Ga has deposited a slab volume at 670km that is equivalent to the transition zone, thereby continuing differentiation by creating a late-stage chemical discontinuity near 400km. [Display omitted] •Earth's thermal, chemical, and physical evolution are inseparable.•Formation of core and lower mantle layers substantially cooled the Earth.•Earth is mostly refractory, containing large amounts of Ca–Al–Ti oxides.•CO ice incorporated in the protoearth led to a carbide core and olivine.•Secular cooling is derived solely from long-lived isotopes.