N, Ar and Pb isotopic co-variations in magmatic minerals: Discriminating fractionation processes from magmatic sources in Monteregian Hills, Québec, Canada

Abstract

Nitrogen, argon and helium isotopic compositions were measured by vacuum crushing in pyroxenes and amphiboles from mafic rocks belonging to the Monteregian Hills (Québec, Canada). For comparison, lead isotopic compositions were measured in plagioclase in the same samples. The goal of this study was to constrain the mantle sources of this igneous province, resolving a controversial issue that has persisted for more than 30years. The measured He and Ar show solubility-controlled elemental fractionation while N is affected by kinetic isotopic fractionation during melt degassing. In contrast, the 40Ar/36Ar ratio seems not to be affected by kinetic fractionation, but likely reflects mantle sources. The 40Ar/36Ar and 208Pb/206Pb ratios are meaningfully correlated, suggesting the mixing between a plume source showing high 208Pb/206Pb (≤2.06) and low 40Ar/36Ar ratios (~1200) and a recycled source such as HIMU showing low 208Pb/206Pb (≤1.95) and near-atmospheric 40Ar/36Ar ratios (~300). The N2/36Ar and 206Pb/204Pb ratios are inversely correlated. Although the N2/36Ar ratios are diluted by an atmospheric-like component, this relationship can be interpreted as the mixing between a recycled component (HIMU) and a mantle source that ambiguously could be either a plume source or the depleted mantle. The samples showing less radiogenic 206Pb/204Pb ratios show δ15N values of −7.6 to −7.4‰, which points to a MORB-type mantle source (δ15N=−5±2‰) rather than a plume source (δ15N=+3‰). Thus, there is contrasting evidence arguing for either a plume or a depleted mantle component. This ambiguity could be resolved by assuming that the plume source provided heat to melt a sub-continental mantle source. Thus, part of the volatile budget might be issued from this distinct mantle source. This study demonstrates the great potential of coupling radiogenic isotopes together with Ar and N isotopes. Nitrogen and argon are efficiently recycled in the mantle and thus can be helpful in tracing the crustal sources that affect the Sr–Nd–Pb isotopic systems in oceanic and continental volcanism.