Significance of the terrestrial Mg/Si ratio

Abstract

Chondritic interplanetary dust particles and also Halley's Comet dust particles from the outer parts of the solar system are enriched in volatile elements and silicon relative to Type 1 carbonaceous chondrites (asteroid belt). Rietmeijer has suggested that these chemical differences imply that there has been a radial chemical differentiation between the two source regions in the inner and outer solar system. The existence of this radial chemical zonation raises the question of whether C1 chondrites can be regarded as having primordial compositions. It is well known that the Earth's upper mantle has an Mg/Si atomic ratio (1.27) which is greater than the C1 ratio (1.05). Petrochemical studies of Venusian basalts and meteorites representing the eucrite parent body (EPB) indicate that the mantles of both Venus and the EPB are similar to Earth in that they are rich in olivine relative to pyroxene. Moreover Venus and the EPB possess Mg/Si ratios that are also similar to Earth and higher than the C1 value. Spectral evidence shows the S-type asteroids which predominate in the inner asteroid belt possess high olivine/pyroxene and Mg/Si ratios. These characteristics seem to predominate throughout the inner solar system. During formation of the terrestrial planets, elements more volatile than silicon were depleted and may have been transported outwards to recondense in the lower-temperature environment of the outer asteroid belt. Some Si may also have been lost in this manner, although not enough to alter planetary Mg/Si ratios. However, recondensation of this Si on the relatively small mass of dust particles in the asteroid belt would have caused a substantial enrichment of Si relative to Mg, which was inherited by C1 chondrites that subsequently formed in this region. It thus seems likely that it is the Mg/Si ratio of the inner planets ( ∼ 1.27), which is more representative of the solar nebula value rather than the C1 ratio. An analogous process of radial chemical fractionation may also have occurred in the outer solar nebula, with volatile elements and silicon lost from the growing giant planets being recondensed onto cosmic interplanetary dust particles and cometary bodies further out from the Sun.