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Published November 2008 | public
Journal Article

The Low-δ¹⁸O Late-Stage Ferrodiorite Magmas in the Skaergaard Intrusion: Result of Liquid Immiscibility, Thermal Metamorphism, or Meteoric Water Incorporation into Magma?

Abstract

We report new laser fluorination oxygen isotope analyses of selected samples throughout the Skaergaard intrusion in East Greenland, particularly relying on ∼1-mg separates of the refractory, alteration-resistant minerals zircon, sphene, olivine, and ferroamphibole. We also reexamine published oxygen isotope data on bulk mineral separates of plagioclase and clinopyroxene. Our results show that the latest-stage, strongly differentiated magmas represented by ∼3 to 6 km³ of ferrodiorites around the Sandwich Horizon (SH), where the upper and lower solidification fronts met, became depleted in ¹⁸O by about 1.5‰–2‰ relative to the original Skaergaard magma and the normal mantle-derived mid-ocean ridge basalt. Earlier studies did not recognize these low-δ¹⁸O ferrodiorite magmas (δ18O = ∼ 3‰–4‰) because after the intrusion solidified, much of the intrusion and its overlying roof rocks were heavily overprinted by low-δ¹⁸O meteoric-hydrothermal fluids. We consider three possible ways of producing these low-δ¹⁸O ferrodiorite magmas. (1) At isotopic equilibrium, liquid immiscibility may cause separation of a higher-δ¹⁸O, higher-SiO₂ granophyric melt, thereby depleting the residual Fe-rich ferrodiorite magma in 18O. However, such a model would require removal of many cubic kilometers of coeval granophyre, a greater proportion than is observed anywhere in the intrusion; there is no evidence that any such magmas erupted to the surface and were eroded. (2) While direct migration of low-δ¹⁸O water seems implausible, we consider a model of "self-fertilization," whereby oxygen from meteoric waters entered the SH magma by devolatilization and exchange with hydrated, low-δ¹⁸O stoped blocks of the upper border series. Such reactive exchange between residual melt and adjacent hydrothermally altered, water-saturated rocks contributed low-δ¹⁸O crystalline components and low-δ¹⁸O pore water to the residual melt. The low-δ¹⁸O zircon and sphene may have crystallized directly from this contaminated low-δ¹⁸O melt, even though the entire mineral assemblage did not, simplifying the mass balance problem. (3) Finally, after SH crystallization, fracturing, and subsequent subsolidus meteoric-hydrothermal alteration depleted these rocks in ¹⁸O, intrusion of the 660-m-thick Basistoppen sill, emplaced 150–200 m above the still hot SH, may have reheated and partially melted these late-stage differentiates. In this scenario, zircon and sphene could have crystallized from a low-δ¹⁸O partial melt, while other minerals may have simply reequilibrated. We favor models 2 and 3 and discuss their strengths and weaknesses.

Additional Information

We thank R. Larsen (Technical University, Trondheim, Norway) for the sample of pegmatite from the lower zone and T. Nielsen (Geological Survey of Denmark and Greenland) for help in extracting the picrite from the Danish Lithosphere Center collections in Copenhagen. M. Hamilton (University of Toronto) is thanked for supplying mineral separate from the Sandwich Horizon. D. Naslund is thanked for fruitful discussions on Skaergaard geology and sample donation. I. Veksler and an anonymous reviewer are thanked for their fruitful reviews. Analyses were supported by National Science Foundation grant EAR-0537872 to I. N. Bindeman.

Additional details

Created:
August 22, 2023
Modified:
October 24, 2023