Chondritic Earth Model

Abstract

GEOCHEMISTS have frequently used the abundances of elements in meteorites as a guide to the overall composition of the Earth. The abundances in the chondritic meteorites have been commonly used on the assumption that they represent the closest approach to the composition, for the non-volatile elements, of the primitive unfractionated material in the solar system. Recent work on trace element abundances in chondrites has shown many complications in this picture, and fractionation of the elements, by processes not yet understood, appears to have occurred among the various classes of chondrites. Thus the ordinary chondrites are depleted in the chalco-phile elements such as lead, thallium and bismuth¹, compared with the carbonaceous chondrites, which are accordingly thought to be more primitive. Attempts to use the composition of the carbonaceous chondrites are hampered by a lack of data for some trace elements, but the overall similarity in composition of the lithophile elements with that of the high iron group of chondrites enables use to be made of the larger quantity of analytical data existing for that group. The abundances and general geochemistry of the chalcophile elements are too imperfectly known to enable useful calculations to be made at present. In this communication, attention is directed to some problems and consequences of the chondritic Earth model. A fuller discussion, for a total of forty-four elements, will be presented elsewhere².