IN SITU LEAD ISOTOPE GEOCHEMISTRY BY LA-ICP-MS OF CUMULUS AND INTERSTITIAL MINERALS FROM THE KIGLAPAIT LAYERED INTRUSION

The in situ isotopic compositions of minerals can provide robust records of the sources and processes involved during crystallization of magma in the Earth’s crust. Layered intrusions represent natural laboratories for assessing differentiation processes of mantle-derived magma and they play a key role in illustrating compositional diversity in magmas. In this study, we have developed an analytical protocol for determining the Pb isotope composition of cumulus and interstitial minerals directly in thin section by LA-ICP-MS and applied it to the 1307 Ma Kiglapait intrusion of coastal Labrador, Canada. Solution analyses of mineral separates by multi-collector ICP-MS (MC-ICP-MS) document puzzling Pb isotopic differences between coexisting plagioclase (unleached, leached, leachates) and mafic minerals in the Kiglapait intrusion. In situ Pb isotopes were determined in cumulus plagioclase, interstitial and cumulus clinopyroxene, sulfide, and different types of interstitial material by LA-ICP-MS at the Pacific Centre for Isotopic and Geochemical Research. The age-corrected in situ Pb isotopic ratios of plagioclase (²⁰⁸Pb/²⁰⁶Pb_i = 2.40 ± 0.05, 2SD, n = 361) overlap those of leached plagioclase measured by solution MC-ICP-MS (²⁰⁸Pb/²⁰⁶Pb_i = 2.40 ± 0.03, n = 10). Notably, there is no difference in composition between plagioclase cores and rims, whereas solution analyses of unleached plagioclase (²⁰⁸Pb/²⁰⁶Pb_i = 2.08-2.37) and leached plagioclase yield distinct compositions. In situ Pb isotopic ratios of clinopyroxene cores and rims (²⁰⁸Pb/²⁰⁶Pb_i = 2.41 ± 0.13, n = 56) overlap with those of plagioclase and not with the initial ratios of bulk mafic and clinopyroxene separates. There is no Pb isotopic difference within analytical uncertainty between plagioclase/clinopyroxene and analyses of interstitial sulfide and biotite, and symplectites (late-stage reactive microstructures). The in situ results provide strong evidence for Pb isotopic homogeneity between all constituent phases in the Kiglapait cumulates. The source of the radiogenic Pb component, recorded by solution analyses, remains elusive. Application of in situ Pb isotope measurements by LA-ICP-MS has the potential to yield breakthroughs in our understanding of the petrogenesis of layered intrusions.