Cosmochemical Fractionation Trend and Terrestrial Array
Channel: Dominik Hezel
Date: 2023-04-12

An intriguing combination of two trends is seen when Al/Si is plotted vs. Mg/Si. Terrestrial rocks mainly fall along a trend with a negative slope. This is explained by fractional crystallisation: melts are enriched in Si and Al and plot at low Mg/Si and high Al/Si, while residual rocks have complementary ratios. This negative correlation is called the ›terrestrial array‹. Bulk chondrites, on the contrary, fall on a straight positive correlation in this plot, i.e., Mg and Al de/increase together. This is called the ›cosmochemical fractionation trend‹. CI chondrites and the Sun plot close together on the cosmochemical fractionation trend. Curiously, the bulk silicate Earth (BSE) plots almost exactly on the intersection of the terrestrial array and the cosmochemical fractionation trend. The BSE composition is, however, deficient in Si when compared to the Sun. One explanation might be a significant fraction of Si (up to several wt%) in the Earth’s core. Alternatively, the Earth was formed from a Si-poor reservoir. A further observation is that carbonaceous chondrites (CC) have similar Mg/Si ratios, but vary in Al/Si ratios, likely due to various Ca,Al-rich inclusion (CAI) abundances. These CAIs were likely added later to the CC. In general, the cosmochemical fraction trend seems to indicate variations in Si among the various planetary materials. The origin of this variation is, however, still unclear.