Original paper

Melt and fluid inclusions in dunite xenoliths from La Gomera, Canary Islands: tracking the mantle metasomatic fluids

Frezzotti, Maria-Luce; Touret, Jacques L.R.; Lustenhouwer, Wim J.; Neumann, Else-Ragnild

European Journal of Mineralogy Volume 6 Number 6 (1994), p. 805 - 818

35 references

published: Nov 30, 1994
manuscript accepted: Jun 20, 1994
manuscript received: Oct 11, 1993

DOI: 10.1127/ejm/6/6/0805

BibTeX file

ArtNo. ESP147050606009, Price: 29.00 €

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Abstract

Abstract Composite xenoliths in alkali-basaltic lavas on La Gomera (Canary Islands) consist of clinopyroxenespinel- dunites cut by 3-mm to 2-cm wide clinopyroxenite veins. The xenoliths are crosscut by a complex system of mainly intracrystalline late veinlets, filled by microcrystalline aggregates and glass remnants. Two different types of fluid and melt inclusions occur. Type 1: primary glass inclusions + CO2 fluid inclusions; these contain Cr-spinel and diopside daughter minerals, compositionally similar to those in dunite. CO2 inclusions (L + V at room temperature) have glass rims (10-30 % of the volume), and are always re-equilibrated. Type 1 inclusions are remnants of the magma from which the dunites were formed. Type 2: secondary silicate glass inclusions and mixed silicate glass + carbonate inclusions, occurring together with reequilibrated CO2 inclusions along fractures originating from a complex network of late veinlets. Melt inclusions contain a silicate glass which may include a spherical carbonate droplet. The glass has an ultramafic composition (MgO: 24-38 wt.%, FeO: 5-18 wt.% and SiO2: 33-46 wt.%). A high volatile content (H2O + CO2) is suggested by very low oxide totals (≈ 85 wt. %) and by high Cl contents (up to 3900 ppm). Bulk analysis of late veinlets show similar compositions, but with higher FeO/MgO ratios. The carbonates are high-Mg calcite or dolomite; the shape of the crystal and the poor crystallinity suggest that they were derived from a carbonate melt (carbonatite). Associated CO2 inclusions, always containing some magnetite, have low densities (0.56-0.19 g/cm3). We propose that the fracture-bound ultramafic glass + carbonate inclusions and the CO2 inclusions associated with Type-2 represent a trapping episode of a homogeneous, volatile-rich, CO2-saturated melt which was present in the upper mantle during Canary Islands volcanism. This melt is responsible for mantle metasomatism, but apparently distinct from the erupted lava.

Keywords

xenolithsmelt and fluid inclusionsimmiscibilityultramafic meltcarbonatitic melt.Canary Islands.