Thallium contents of chondrites

Abstract

The thallium contents of a number of equilibrated and “primitive” stony meteorites, i.e. carbonaceous (C) and unequilibrated ordinary chondrites (UOC), have been determined by neutron activation analysis. The technique has been proved by analysis of a synthetic sample and eight standard rocks in which T1 and Rb vary directly over concentration ranges of a factor of about 2000 for each. The chondritic analyses confirm T1 to be a “strongly-depleted” element. The mean T1 abundances in groups C1, C2 and C3 are 96, 46 and 32 atoms/10 9 Si atoms respectively, which lead to ratios (relative to C1) of C1: C2: C3 = 1.00:0.48:0.33, nearly the same as those predicted by a two-component model. The T1 contents of 11 UOC are intermediate to those of carbonaceous and equilibrated ordinary chondrites and decrease exponentially with increasing equilibration of the silicate phases. The trends exhibited by T1 and other “strongly-depleted” (i.e. Bi, C, In and primordial Ar, Kr and Xe), “normally-depleted” and “undepleted” elements are compared in ordinary chondrites. The Bi and T1 results suggest that the UOC and carbonaceous chondrites were not formed seriatim but that during formation of these chondrites, these elements were affected equally.