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

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. Mc Birney, H.R. Naslund. Contrib. Mineral. Petrol. 104 (1990) 235-247, C. Tegn er. Contrib. Mineral. Petrol. 128 (1997) 45-51, A.R. McBirney, Contrib. Min eral. Petrol. 132 (1998) 103-105] concluded that the Skaergaard magma follo wed 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 suggest ed that the Skaergaard magma began to follow a silica-enrichment trend in L ower Zone c (LZc) of the Layered Series where magnetite first became an abu ndant mineral. Magnetite in LZc, however. generally occurs in aggregates of magnetite-ulvospinel and ilmenite-hematite that have undergone extensive s ubsolidus reequilibration and exsolution [E.A. Vincent. Neues Jahrb. Minera l. 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. Na slund, J. Petrol. 25 (1984) 185-212, A.R. McBirney, J. Petrol. 30 (1989) 36 3-397; Y.D. Jang, Petrological. Geochemical, and Mineralogical Variations i n the Skaergaard Intrusion, East Greenland (Ph.D. Dissertation). State Univ ersity 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 fr actionated from the main magma reservoir, or if magnetite crystallized as a later, interstitial mineral and did not directly affect the differentiatio n trend of the main Skaergaard magma. The timing of the initial crystalliza tion of abundant magnetite and ilmenite is a key factor in understanding th e trend of Skaergaard differentiation. Because V is a strongly included ele ment in oxides, and is not strongly included in silicate minerals, the V co ntent of an evolving magma is generally controlled by the fractionation of oxide minerals, in particular magnetite. The initial crystallization of mag netite should, therefore, be accompanied by a sudden decrease in the V cont ent of the evolving magma, and in all of the coexisting mafic phases in equ ilibrium with that magma as well. The V content in Skaergaard pyroxene does not decrease significantly until the upper part of the Middle Zone (MZ), s uggesting that the onset of extensive magnetite fractionation is much later than has previously been thought, and that the magnetite in LZc and the lo wer 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 usi ng published partition coefficients for the coexisting minerals in the Skae rgaard intrusion, assuming that no magnetite fractionation occurred until t he upper part of the MZ. Independently calculated trends for fO(2) in the S kaergaard 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, Contr ib. Mineral. Petrol. 104 (1990) 235-247] change in the upper part of the MZ to more reducing conditions. The onset of magnetite fractionation would re move Fe2O3 from the magma and could initiate such a change. The timing of m agnetite fractionation will have a strong effect on whether magma evolves t owards iron enrichment or silica enrichment. (C) 2001 Elsevier Science B.V. All rights reserved.