The Low-δ¹â¸O Late-Stage Ferrodiorite Magmas in the Skaergaard Intrusion: Result of Liquid Immiscibility, Thermal Metamorphism, or Meteoric Water Incorporation into Magma?

• Published in: The Journal of Geology
• Main Authors: Bindeman, I.N., Brooks, C. K., McBirney, A. R., Taylor, H. P.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: University of Chicago Press 2008
• Subjects: Geology; Greenland; East Greenland
• Online Access: https://doi.org/10.1086/591992

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 ¹â¸O 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-δ¹â¸O ferrodiorite magmas (δ18O = ∼ 3‰–4‰) because after the intrusion solidified, much of the intrusion and its overlying roof rocks were heavily overprinted by low-δ¹â¸O meteoric-hydrothermal fluids. We consider three possible ways of producing these low-δ¹â¸O ferrodiorite magmas. (1) At isotopic equilibrium, liquid immiscibility may cause separation of a higher-δ¹â¸O, higher-SiOâ‚‚ 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-δ¹â¸O 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-δ¹â¸O stoped blocks of the upper border series. Such reactive exchange between residual melt and adjacent hydrothermally altered, water-saturated rocks contributed low-δ¹â¸O crystalline components and low-δ¹â¸O pore water to the residual melt. The low-δ¹â¸O zircon and sphene may have crystallized directly from this ...