Chondritic Earth: comparisons, guidelines and status

Tuesday, 16 December 2014
William F McDonough, Univ Maryland, College Park, MD, United States
The chemical and isotopic composition of the Earth is rationally understood within the context of the chondritic reference frame, without recourse to hidden reservoirs, collision erosion, or strict interpretation of an enstatite chondrite model. Challenges to interpreting the array of recent and disparate chemical and isotopic observations from meteorites need to be understood as rich data harvests that inform us of the compositional heterogeneity in the early solar system. Our ability to resolve small, significant compositional differences between chondrite families provide critical insights into integrated compositional signatures at differing annuli distances from the Sun (i.e., 1-6 AU). Rigorous evaluation of chondritic models for planets requires treatment of both statistical and systematic uncertainties – to date these efforts are uncommonly practiced.

Planetary olivine to pyroxene ratio reflects fO2 and temperature potentials in the nebular, given possible ISM compositional conditions; thus this ratio is a non-unique parameter of terrestrial bodies. Consequently the Mg/Si value of a planet (ie., olivine to pyroxene ratio) is a free variable; there is no singular chondritic Mg/Si value. For the Earth, there is an absence of physical and chemical evidence requiring a major element, chemical distinction between the upper and lower mantle, within uncertainties. Early Earth differentiation likely occurred, but there is an absence of chemical and isotopic evidence of its imprint. Chondrites, peridotites, komatiites, and basalts (ancient and modern) reveal a coherent picture of a chondritic compositional Earth, with compositionally affinities to enstatite chondrites. At present results from geoneutrino studies non-uniquely support these conclusions. Future experiments can provide true transformative insights into the Earth’s thermal budget, define compositional BSE models, and will restrict discussions on Earth dynamics and its thermal evolution.