Volatiles in a peralkaline system: Abiogenic hydrocarbons and F-Cl-Br systematics in the naujaite of the Ilimaussaq intrusion, South Greenland
Lithos 95(3-4): 298-314
Agpaitic rocks comprise most of the exposed part of the 1.16 Ga old, 8 x 17 km large and about 1700 m thick Ilimaussaq intrusion in South Greenland. Within these, more than 600 m thick sequence of sodalite-rich "naujaites" (mainly sodalite + arfvedsonite +alkali feldspar+ nepheline + eudialyte + aenigmatite) are interpreted as a socialite flotation cumulate. Socialites show two to three different zones in cathodoluminescence (CL) and at least two zones in thin sections. The CL zones can be related to chemical differences detectable by electron microprobe, whereas relations with optical zonations are less obvious. Compositional trends in socialite reflect trends in the evolution of volatile contents in the melt. The sodalite at Ilimaussaq is almost free of Ca and closely corresponds to the pure Na-Cl socialite endmember with about 7 wt.% of Cl; S contents reach up to 0.9 wt.%. Cl/Br ratios range from 500 to 1700. Raman spectroscopy shows that S is present as [SO4](2-) in socialite, although sphalerite (ZnS) is a stable phase in naujaites. Peralkalinity and fO(2) conditions allow S2- and [SO4](2-) to be present contemporaneously.|The whole naujaite sequence is divided into two parts, an upper part with low, homogeneous S contents and Cl/Br ratios in the socialite cores, and a lower part with strongly variable and higher S contents and with Cl/Br ratios, which are decreasing downwards. The details of the S content and the Cl/Br ratio evolution show that sodalite strongly influences the halogen contents of the melt by scavenging Cl and Br.|The naujaites were formed from a highly reduced, halogen-rich magma in equilibrium with magmatic methane at about 800 degrees C, which, upon ascent, cooling and fractionation, exsolved an aqueous fluid phase. Both fluids were trapped in separate inclusions indicating their immiscibility.|Micrometer-sized aegirine crystals and primary hydrocarbon-bearing inclusions are abundant in the crystal cores. The inclusions were trapped at pressures up to 4 kbar, although the emplacement pressure of the intrusion is about I kbar. This indicates growth of the socialite during melt ascent and a very effective mechanism of trace element scavenging during socialite growth. Socialite rims are devoid of aegirine or primary hydrocarbon inclusions and probably reflect the emplacement stage. (c) 2006 Elsevier B.V. All rights reserved.