The differentiation trend of the Skaergaard intrusion and the timing of magnetite crystallization: iron enrichment revisited

Abstract

Initial studies of the Skaergaard intrusion [L.R. Wager, J. Petrol. 1 (1960) 364–398] and much of the subsequent work [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247; C. Tegner, Contrib. Mineral. Petrol. 128 (1997) 45–51; A.R. McBirney, Contrib. Mineral. Petrol. 132 (1998) 103–105] concluded that the Skaergaard magma followed an iron-enrichment trend with little or no silica enrichment until the final stages of crystallization. Several recent reports [R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 95 (1987) 451–461; R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 104 (1990) 248–254], however, have suggested that the Skaergaard magma began to follow a silica-enrichment trend in Lower Zone c (LZc) of the Layered Series where magnetite first became an abundant mineral. Magnetite in LZc, however, generally occurs in aggregates of magnetite–ulvöspinel and ilmenite–hematite that have undergone extensive subsolidus reequilibration and exsolution [E.A. Vincent, Neues Jahrb. Mineral. Abh. 94 (1960) 993–1016; E.A. Vincent, Geochim. Cosmochim. Acta 6 (1954) 1–26; A.F. Buddington, D.H. Lindsley, J. Petrol. 5 (1964) 310C357; H.R. Naslund, J. Petrol. 25 (1984) 185–212; A.R. McBirney, J. Petrol. 30 (1989) 363–397; Y.D. Jang, Petrological, Geochemical, and Mineralogical Variations in the Skaergaard Intrusion, East Greenland (Ph.D. Dissertation), State University of New York, Binghamton, NY, 1999, 219 pp.]. As a result, it is not clear if magnetite in these samples was an equilibrium, liquidus mineral fractionated from the main magma reservoir, or if magnetite crystallized as a later, interstitial mineral and did not directly affect the differentiation trend of the main Skaergaard magma. The timing of the initial crystallization of abundant magnetite and ilmenite is a key factor in understanding the trend of Skaergaard differentiation. Because V is a strongly included element in oxides, and is not strongly included in silicate minerals, the V content of an evolving magma is generally controlled by the fractionation of oxide minerals, in particular magnetite. The initial crystallization of magnetite should, therefore, be accompanied by a sudden decrease in the V content of the evolving magma, and in all of the coexisting mafic phases in equilibrium with that magma as well. The V content in Skaergaard pyroxene does not decrease significantly until the upper part of the Middle Zone (MZ), suggesting that the onset of extensive magnetite fractionation is much later than has previously been thought, and that the magnetite in LZc and the lower part of the MZ might not have been a liquidus phase at that level. The observed V trend in Skaergaard pyroxene can be modeled almost perfectly using published partition coefficients for the coexisting minerals in the Skaergaard intrusion, assuming that no magnetite fractionation occurred until the upper part of the MZ. Independently calculated trends for fO2 in the Skaergaard magma [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247] change in the upper part of the MZ to more reducing conditions. The onset of magnetite fractionation would remove Fe2O3 from the magma and could initiate such a change. The timing of magnetite fractionation will have a strong effect on whether magma evolves towards iron enrichment or silica enrichment.