Iron isotope systematics of the Skaergaard intrusion and implications for its liquid line of descent

Abstract

The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033 to 0.151 ‰ with an abrupt step increase at the base of Lower Zone c (LZc) within the LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) – c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite - ilmenite separates straddle bulk rock compositions with fractionation factors (∆56Femt-ilm) of 0.081 to 0.239 ‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents, having unradiogenic Sr like gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (∆56Fegranophyre-ferrodiorite ≥ 0.1 ‰) reaching a maximum δ56Fe of 0.217 ‰ for UBS granophyre. A fused xenolith from UBS has δ56Fe = 0.372 ‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modelled Skaergaard liquids. Forward modelling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modelling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb – LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe-enrichment at nearly constant silica, does not lead to a rise in δ56Fe towards the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si-enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liquid phase separation of the bulk magma. Crustal contamination, fluid exsolution, hydrothermal alteration and thermal diffusion are also shown to have no resolvable effect on the Fe isotope composition of the gabbroic and granophyric rocks. We conclude that the Fe isotope systematics documented in the Skaergaard intrusion reflect the dominant role of fractionating Fe-rich minerals from gabbroic through ferrodioritic to rhyolitic liquids. The success of our model to account for the observed Fe isotope systematics for Skaergaard demonstrates the utility of Fe+2 and Fe+3 force constants determined at ambient conditions to model magmatic conditions and gives critical insights into plutonic processes fractionating Fe isotopes complementary to the volcanic record.