Fractionation of Some Abundant Lithophile Element Ratios in Chondrites

Abstract

The abundant lithophile elements Si, Mg, Ca, A1 and Ti have been estimated (in duplicate) in 8 carbonaceous chondrites, 25 common chondrites (8 iron high and 17 iron low) and 7 enstatite chondrites. An X-ray fluorescence method was used, the precision of which, expressed as the coefficient of variation (C), is 1–2%, with the exception of Al, for which the precision appears to be a little poorer.

Because of the accuracy of the data, it has been possible to determine precisely the variation of the ratios Si/Mg, Si/Ca, Si/Al, Si/Ti and Ca/Al within each chondrite group and the fractionation, if any, of each ratio between each group.

Variation of each ratio in the 25 common chondrites is very small (C varies from 1.2 to 3.0%) but is usually larger in the other types.

With the exception of Ca/Al, which may not be fractionated at all, the degree of fractionation varies very widely. For example, Si/Mg increases by only a little more than 10% when passing from the carbonaceous to the common chondrites, whereas that of Si/Ca and of Si/Al is extreme, i.e. ~ 100% increase from the carbonaceous to the enstatite chondrites. As far as can be ascertained, fractionation of the ratios between each group is discrete.