The Skaergaard liquid line of descent revisited |
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Authors: | Peter Thy Charles E Lesher Christian Tegner |
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Institution: | (1) Department of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA;(2) Department of Earth Sciences, University of Aarhus, 8000 Aarhus C, Denmark |
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Abstract: | There is a fundamental conflict between the suggestion that the iron content of Skaergaard liquids increases during Fe–Ti
oxide fractionation and the observation that at the same time oxygen fugacity () drops by two log-units below the fayalite-magnetite-quartz oxygen buffer (FMQ). A new petrographic study of average Skaergaard
gabbros shows that the total modal content of Fe–Ti oxides is about 22% in the early LZc and markedly decreases to below 5%
in the UZc. Forward modeling based on these modal constraints, as well as experimental results on Skaergaard-related dikes,
predicts that fractionation of troctolitic LZa gabbros drives the derivative liquid towards a high-iron content. Strong iron
enrichment continues, together with a small decline in silica, during LZb crystallization due to the appearance of augite
as a fractionating phase. The fractionation of Fe–Ti oxides in the LZc initially suppresses iron enrichment and reverses the
silica trend to one of slight enrichment. However, continued evolution into the UZ produces liquids with maximum UZc FeO*
content of 23–25 wt.% and SiO2 content of 53 wt.% (FeO* is total iron as FeO). The maximum in FeO* is dependent on several factors of which the Fe–Ti oxide
mode has the strongest effect. The during crystallization of the LZc is widely thought to have been at, or slightly below, the fayalite-magnetite-quartz oxygen
buffer (FMQ). Under closed system evolution, incorporation of ferric iron into augite during formation of the LZb restricts
the increase in to about 0.1 log-units above FMQ (=0.1 ΔFMQ). Likewise, crystallization of the LZc through the UZa, involving Fe–Ti oxide
minerals, leads to a decline in of less than 0.1 ΔFMQ. Crystallization of the UZb-c gabbros results in oxidation to a maximum of 0.5 ΔFMQ. This behavior
can account for the iron-rich character of the UZ gabbros, as well as, the low modal content of Fe–Ti oxides. Thus, evolved
Skaergaard liquids are high in iron and contain a modest amount of SiO2. Our modeling result do not account for a strong drop in through the layered series. Such a drop would require an unacceptably high proportion of Fe–Ti oxides and high-magnetite
content in the fractionating assemblage.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Greenland Skaergaard intrusion Petrology Liquid line of descent Layered series Iron enrichment Silica depletion Fe– Ti oxides Ilmenite Magnetite Oxygen fugacity |
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