Neutron-activation determinations of Ir in rocks of the Southern California and Boulder (Montana) batholiths show that the Ir content is low and variable in both batholiths, extending over 1 to 2 orders of magnitude and having a combined median of 0.006 ppb Ir. The data suggest that the calc-alkalic Boulder batholith (0.002-0.5 ppb Ir, median 0.007) may be slightly enriched in Ir relative to the calcic Southern California batholith (< 0.001-0.12 ppb Ir, median 0.005). No conspicuous relationship between Ir content and differentiation parameters is evident; however, the Boulder batholith rocks do show a slight tendency to concentrate Ir in the more mafic rocks. Our results in conjunction with previous analyses of mafic rocks leads to a crustal-abundance estimate of 0.03 ppb Ir. ?? 1974.
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Iridium in some calcic and calc-alkalic batholithic rocks of the western United States