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Zircon chronochemistry by excimer laser ablation ICP-MS
Michael Palin, Ian Campbell and Charlotte Allen
The accessory mineral zircon (ZrSiO4) contains a wealth of petrogenetic information for igneous rocks of intermediate to felsic composition and provenance constraints for siliciclastic sediments. Excimer laser ablation (ELA)-ICP-MS permits in situ (³15 micrometer spot) measurement of U-Th-Pb isotopes and selected major and trace elements in zircon. Instrumental mass bias in the quadrupole ICP-MS and elemental fractionation induced during ablation are corrected for by reference to zircon and glass standards, a robust procedure because of reproducible ablation by the ArF excimer laser.
The capability of ELA-ICP-MS for simultaneous collection of U-Th-Pb isotope and major and trace element data has been used to examine evolution of Cenozoic igneous suites associated with major Cu±Au deposits in Chile, the Philippines, and Indonesia. Using a lattice-strain model of mineral-melt trace element partitioning, the REE contents of zircon can provide estimates of relative magmatic oxidation state. Shallow intrusions spatially associated with ore display exhibit anomalously high oxidation states and in some cases are preceded by oxidation trends in older units. These patterns can be explained by the interdependent relations of oxidation state, sulfur speciation and solubility, and metal partitioning in silicate magmas.
Rapid analysis by ELA-ICP-MS (2 min/spot) enables studies of detrital zircons on scales previously unrealized. These capabilities have been used in studies of modern and ancient detrital zircons from a variety of geologic settings including: large rivers in North and South America, India and China; Neogene volcanics in the Asia-Pacific region; Neoproterozoic through Paleozoic sandstones of the Appalachians; and Proterozoic conglomerate and Archean quartzite in Brazil. By analyzing large numbers of grains (minimum n = 60, typical n = 100) for both their age and chemical composition, tighter constraints can be placed on the provenance of these siliciclastic materials.