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Applications of LA-ICP-MS analysis to zircon : assessing downhole fractionation and pre-treatment effects for U-Pb geochronology and trace element variations in accessory minerals from the Bushveld Complex

University of British Columbia

Zircon and other U-Th-Pb-bearing minerals are now recognized as key geochemical and geochronological tracers of the evolution of late-stage fractionated interstitial melt in mafic layered intrusions. Two separate, yet complementary, applications of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on zircon from layered intrusions are presented with the goals of advancing the analytical capabilities of LA-ICP-MS and showcasing the powerful geochemical fingerprinting of zircon. The effects of downhole fractionation, the time-dependent evolution of Pb-U ratios during laser ablation, represents a significant limitation on the accuracy of U-Th-Pb zircon geochronology by LA-ICP-MS. Exponential downhole correction models developed from the analyses of three common zircon reference materials (Plešovice, Temora-2, 91500) and applied to low-U zircon from Precambrian mafic intrusions (Laramie, Bushveld, Stillwater) indicate that successful correction requires careful matrix-matching the reference zircon to the unknowns. Pre-treatment protocols, including annealing and leaching, applied to all analyzed zircon produces strong effects on downhole fractionation with correlative impact on the relative accuracy of the calculated ages as a function of the downhole behaviour in the reference material used. In the Paleoproterozoic Bushveld Complex, the world’s largest layered intrusion, the trace element systematics of zircon provide temperature-composition constraints on the near-solidus crystallization of mafic-ultramafic cumulates and overlying granitic rocks. Zircon occurs with other late-stage interstitial minerals (e.g., quartz, biotite, Na-plagioclase) and crystallized at temperatures ranging from 950°C down to 690°C based on Ti-in-zircon thermometry. Forward modeling using rhyolite-MELTS of proposed Bushveld parental magmas yields similar zircon saturation temperatures from melts of intermediate-silicic composition, representing less than ~15% remaining melt, and reproduces the observed mineral assemblages. Anomalously high and variable Th/U in zircon from the Critical Zone (e.g., UG2 chromitite, Merensky Reef) reflects U loss to late, oxidized Cl-rich fluids that exsolved from the fractionated interstitial melt, a process that may be a characteristic feature of large open-system layered intrusions (e.g., Neoarchean Stillwater Complex). The presence of late-stage interstitial zircon and other accessory minerals in layered intrusions provides new in situ geochemical and geochronological tools for evaluating the origin and evolution of mafic-ultramafic magmatism in the Earth’s crust throughout geological time.

Text

2017-01-21

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/60201

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/