718 



UNITED STATES MINERAL RESOURCES 



Zirconium, a fairly common element, ranking 

 twentieth in order of abundance in the earth's outer 

 crust, is determined routinely in nearly all trace- 

 element investigations. Thus the average zirconium 

 contents of the predominant rock types (background 

 levels) are reasonably well documented and provide 

 a basis for the recognition of zirconium-rich prov- 

 inces or rock units of potential economic interest. 

 The highly generalized abundance data on igneous 

 rocks (table 159) reflect the well-known tendency 

 for zirconium content to increase in the differentia- 

 tion sequence ultramafic-mafic-intermediate-felsic, 

 with a particularly marked enrichment in syenitic 

 (alkalic) rocks. 



BEHAVIOR OF ZIRCONIUM IN 

 MAGMATIC PROCESSES 



The abundance and variation of zirconium in 

 suites of igneous rocks represent different stages of 

 crystallization of magmas (Chao and Fleischer, 

 1960) . Zirconium increases in each stage as differ- 

 entiation proceeds. The zirconium contents of suites 

 derived from an alkaline base magma are signifi- 

 cantly greater than those derived from a basaltic 

 magma of the tholeiitic type. In these rock suites, 

 differentiation produces about a five-fold enrichment 

 in the final products of crystallization. An unusually 

 strong enrichment of zirconium (> 1,000 ppm Zr) 

 occurs in the late differentiates of the Skaergaard 

 intrusion in Greenland as compared with the zir- 

 conium content of its initial liquid (94 ppm (parts 

 per million)), a value similar to the average zir- 

 conium content of tholeiitic basalts (Brooks, 1969). 

 Calc-alkalic and calcic rock suites have the lowest 

 zirconium contents and do not yield differentiates 

 rich in zirconium. The alkalic suites have higher 

 zirconium contents, and zirconium enrichment is 

 most pronounced (> 1,000 ppm) in the most siliceous 

 rocks of these suites. 



The strong tendency for zirconium to concentrate 

 in rocks with strong alkaline affinities is generally 

 attributed to the greater solubility of zirconium in 

 magmas relatively high in Na, K, F, CI, and OH. 

 Because of its high ionic potential, zirconium is 

 strongly partitioned into the residual liquid (Ring- 

 wood, 1955). Strongly alkalic (pantelleritic) obsidi- 

 ans contain from 1,100-3,000 ppm Zr (Carmichael, 

 1962), and riebeckite rhyolites and granites from 

 Nigeria have as much as 2,200 ppm Zr (Butler and 

 Thompson, 1965; Bowden, 1966). The contrasting 

 behavior of zirconium in the more calcic suites, 

 which have low contents of alkalis and complexing 

 ions, may thus be attributed for the most part to 

 early crystallization of zircon, which causes only 



slight to moderate enrichment in the residual 

 magmas. 



BEHAVIOR OF HAFNIUM IN 

 MAGMATIC PROCESSES 



Data on the hafnium content of rocks are rela- 

 tively scarce because of the difficulty of determining 

 hafnium in the concentration ranges encountered in 

 rocks. The average content of hafnium in the upper 

 crust of the earth is 3 ppm, approximately one- 

 fiftieth of the abundance of zirconium. 



Because hafnium and zirconium are nearly identi- 

 cal in chemical properties, the geochemical behavior 

 of hafnium during magmatic differentiation is quite 

 similar to that of zirconium. In most suites of co- 

 magmatic or spatially associated rocks, the Zr-Hf 

 ratios of the bulk rock or zircons are generally be- 

 tween one-half and twice the average Zr-Hf ratio 

 of the earth's crust. Two diabase-granophyre suites 

 from Dillsburg, Pa., and Great Lake intrusion, Tas- 

 mania, have absolute and relative enrichment of 

 hafnium in the later-formed rocks. The average Zr- 

 Hf ratio decreased from 68 to 33 in the course of 

 differentiation (Gottfried and others, 1968). In the 

 Skaergaard intrusion, however, the ratio varied in 

 an opposite manner, the enrichment of zirconium and 

 hafnium being accompanied by an increase in the 

 Zr-Hf ratio with differentiation (Brooks, 1969). 

 Limited data on differentiation products of an alkali- 

 olivine basalt indicate an increase in the Zr-Hf ratio 

 from 59 in early formed ankaramite to 85 in more 

 silica- and alkali-rich trachyte (Brooks, 1970). 



The geochemical variation of hafnium relative to 

 zirconium in calc-alkaline rocks is based mainly on 

 data obtained on zircon. Analyses for hafnium and 

 the Hf-Zr ratios in zircon from calc-alkaline plutonic 

 rocks indicate about a twofold enrichment of haf- 

 nium in zircons from granites over that found in 

 zircon from gabbroic rocks, the Zr-Hf ratio de- 

 creasing from 50-70 in the gabbros to 30-35 in the 

 granites. Variations of the Zr-Hf ratios in oulk rock 

 samples from calc-alkaline granitic masses (Condie 

 and Lo, 1971 ; Esson and others, 1968) are similar to 

 those obtained on zircon. Zircon from under- 

 saturated rocks, such as syenites and nepheline sye- 

 nites, has low hafnium contents and significantly 

 higher Zr-Hf ratios (60-150) than zircon from ordi- 

 nary granitic rocks, as shown in table 160. In con- 

 trast, zircons with the highest hafnium contents (to 

 30 percent) and unusually low Zr-Hf ratios (<20) 

 are found in zircon derived from peralkaline gran- 

 ites, granitic pegmatites, and lithium-tantalum- 

 bearing pegmatites (Knorring and Hornung, 1961; 



