Testing for Pb isotopic differences between minerals in the Kiglapait layered intrusion by LA-ICP-MS

Abstract

Mineral-scale Pb isotopic variations in cumulates of the Kiglapait intrusion, the largest troctolitic intrusion of the Proterozoic Nain Plutonic Suite in Labrador (Canada), are evaluated directly in thin section by single spot LA-ICP-MS analysis of minerals with low Pb concentrations (e.g., plagioclase = 0.31–3.3 ppm, clinopyroxene = 0.06–1.1 ppm) at high spatial resolution (<100 μm). Analysis of reference materials (BCR-2G, NIST 612, KL2-G, MASS-1) demonstrates that the results are accurate despite the very low amounts of Pb measured (0.04–18 pg) to preserve high spatial resolution. The Pb isotopic ratios of all plagioclase spot analyses overlap within uncertainty and with co-existing clinopyroxene. There are no significant differences between the cores and rims of plagioclase or clinopyroxene, between interstitial or cumulus augite, and with the analyses of interstitial sulfide, biotite, and symplectites (late-stage reactive microstructures). The LA-ICP-MS results support closed-system crystallization of the constituent phases (primocrysts, interstitial minerals) in the Kiglapait cumulates. The results contrast with published Pb isotopic ratios of mineral separates (unleached, leached, leachates) from the Kiglapait intrusion analyzed in solution by MC-ICP-MS where only the relatively unradiogenic leached plagioclase, taken as representative of the initial isotopic composition at 1307 Ma, has the same isotopic composition as that defined by the LA-ICP-MS analyses. A radiogenic Pb component, volumetrically minor and not detected by the LA-ICP-MS analyses (n = 483), appears to have been introduced by a cryptic alteration event from high-temperature fluids during the earliest stages of cooling of the Kiglapait intrusion. Determination of the Pb isotope compositions of common rock-forming minerals, characterized by low Pb concentrations (a few ppm), in mafic layered intrusions can be achieved rapidly by LA-ICP-MS and has the potential to yield significant new insights related to the petrogenesis of plutonic rocks, and phenocryst-bearing volcanic rocks, in the Earth's crust.