Two independent processes responsible for compositional zonation in mafic dykes of the Åland-Åboland Dyke Swarm, Kestiö Island, SW Finland Sofya Yu. Chistyakova ⁎, Rais M. Latypov Department of Geosciences, University of Oulu, Oulu, Finland abstract article info Article history: Received 8 April 2008 Accepted 21 March 2009 Available online 5 April 2009 Keywords: Dolerite dykes Anomalous internal zonation Magma differentiation processes Detailed sampling across three dolerite dykes of different size (small, 7 cm; middle, 75 cm; and thick, 675 cm) of the Åland-Åboland dyke swarm has revealed an internal zonation of an anomalous nature. The small, almost glassy dyke exhibits a systematic inward decrease in whole-rock MgO and Mg# (indicating a normal fractionation trend) together with a simultaneous increase in normative An and Cpx and decrease in whole-rock Zr, Y, CaO, TiO 2 (indicating a reverse fractionation trend). The middle dyke shows similar compositional trends across its narrow margins, but in the more crystalline interior whole-rock MgO and Mg# gradually but steadily increase inwards. As a result normal and reverse fractionation trends of the margins grade to exclusively reverse fractionation trends of the interior. The thick, almost totally crystalline dyke exhibits an internal zonation similar to that of the middle dyke, with fractionation trends becoming only much more pronounced in the centre of the dyke. The almost glassy nature of small dyke suggests that its anomalous compositional zonation most likely resulted from temporal changes in the composition of magma as it formed the dyke. The mechanism(s) responsible for such systematic changes in composition of inflowing magma remains, however, unknown. The margins of middle and thick dyke form in a similar way whereas their interiors formed by in situ cumulate growth against dyke sidewalls. This process resulted in a gradual inward increase in the proportion of cumulus phases owing to magma crystallization in progressively less supercooled conditions with increasing distance from cold country rocks. The compositional zonation of these dolerite dykes is thus produced by two independently operating mechanisms: successive changes in composition of inflowing magma (an external liquid-state process) and an in situ cumulate growth on dyke sidewalls (an internal crystal-liquid process). Based on the relatively minor development of internal zonation in interiors of middle and thick dykes, the former mechanism appears to be several times more effective in causing magma differentiation than the latter. It remains to be determined whether these two processes are a general reason for the formation of marginal reversals in mafic–ultramafic dykes, sills and large layered intrusions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction One remarkable feature of mafic–ultramafic intrusions and related ore deposits is the occurrence of marginal zones in which mineral crystallization sequences and compositional trends are distinctly opposite to that predicted by phase equilibria diagrams. Such marginal zones are commonly characterized by a decrease in the number of crystallizing phases in progressively formed rocks while minerals become gradually more primitive inwards from intrusive contacts. Examples include marginal reversals in mafic sills and layered intru- sions, reverse concentric zonation in Alaska-type complexes, D-shaped compositional profiles in komatiitic lava flows, reverse internal zona- tion in massive sulphide bodies and iron oxide ores, etc. Such marginal zones are almost universally developed in magmatic bodies irrespective of their age, geographical location, size, form, and even composition of parental magmas, strongly indicating that some fundamental process or processes are involved in their genesis (Latypov et al., 2007). In attempts to resolve this puzzle, igneous petrologists have sug- gested about ten different hypotheses which we have recently reviewed (Latypov, 2003a,b; Latypov et al., 2007). Among them are magma supercooling (Miller and Ripley, 1996), contamination of magma by country rocks (e.g. Tyson and Chang, 1984), successive emplacement of magma that becomes increasingly more primitive with time (e.g. Morse, 1981), crystal settling of intratelluric phenocrysts or newly grown crystals in the chamber (e.g. Ariskin et al., 2003), intrusion of successively more primitive batches of magma carrying progressively more phenocrysts (e.g. Lightfoot and Naldrett, 1984), flow differentia- tion during intrusion of olivine or orthopyroxene phenocryst-laden magma (e.g. Marsh, 1996; Gibbs and Henderson, 2005), crystallization from compositionally stratified magma in the chamber (Wilson and Lithos 112 (2009) 382–396 ⁎ Corresponding author. Department of Geosciences, P.O. Box 3000, FIN—90014, University of Oulu, Finland. Tel.: +358 8 553 1479; fax: +358 8 5531484. E-mail address: sofya.chistyakova@oulu.fi (S.Y. Chistyakova). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.03.037 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos