Earth's volatile contents established by melting and vaporization

Abstract

The silicate Earth is strongly depleted in moderately volatile elements (such as lead, zinc, indium and alkali elements) relative to CI chondrites, the meteorites that compositionally most closely resemble the Sun. This depletion may be explained qualitatively by accretion of 10 to 20 per cent of a volatile-rich body to a reduced volatile-free proto-Earth, followed by partial extraction of some elements to the core. However, there are several unanswered questions regarding the sources of Earth's volatiles, notably the overabundance of indium in the silicate Earth. Here we examine the melting processes that occurred during accretion on Earth and precursor bodies and report vaporization experiments under conditions of fixed temperature and oxygen fugacity. We find that the pattern of volatile element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. We argue that melting and vaporization on precursor bodies and possibly during the giant Moon-forming impact were responsible for establishing the observed abundances of moderately volatile elements in Earth.

Figure 1

Concentrations in Bulk Silicate Earth (BSE) of moderately volatile elements plotted versus their condensation temperatures from a gas of solar system composition. Concentrations are expressed relative to concentrations in CI chondrite meteorites (normalised to a concentration ratio of Mg of 1.0). Elements studied here are shown in the key. Open squares refer to elements not specifically addressed by our study. Open circles refer to all elements in CV chondrites. Error bars are 1SD.

Figure 2

Concentrations of selected elements in product silicate glasses, normalised to starting concentrations, for a time series performed at 1300°C and an oxygen fugacity of 10^-7 atm. This oxygen fugacity is about 0.3 log units below the Ni-NiO buffer. Note that the order of volatility is Cd>Ag>Cu>In>Zn, in marked contrast to the anticipated relative volatilities from the condensation temperatures of Figure 1. Error bars 1 SD.

Figure 3

This shows volatility factors as a function of oxygen fugacity for the elements we investigated at 1300°C. Volatility factors were calculated from the ratios of initial to final concentrations of the elements of interest after 60 minutes reaction in the constantly stirred melt. Indicative error bars of ±10% and ±20% are greater than 1 SD for virtually all elements and conditions (see text)

Figure 4

a,b. This shows concentrations in Bulk Silicate Earth plotted as a function of measured volatility factors at log fO₂ of -11 and -13 at 1300°C. Concentrations of elements studied here are given relative to concentrations in CI chondrite meteorites (normalised to a concentration ratio of Mg of 1.0). Vertical error bars as in Figure 1. Horizontal error bars are 2 std errors. Line labelled “late veneer” refers to region where late addition of ~0.5% of chondritic material may have raised concentrations of the most volatile elements.