IODP Hole U1473A at Atlantis Bank (Southwest Indian Ridge; Expedition 360): Formation & metamorphism of felsic veins in gabbros and their role for fluid and mass transfer: first results

Abstract:

Hole U1473, located on the summit of Atlantis Bank at the ultra-slow spreading Southwest Indian Ridge was drilled to 789.7 m below seafloor (mbsf) during IODP Expedition 360. It consists of massive gabbros cut by nearly 400 felsic veins, which are evolved, SiO₂-enriched lithologies comprising ~1.5 vol% of the drill core. For their formation 3 endmember models are discussed: (1) fractional crystallization as consequence of oxide saturation at a late stage of tholeiitic magma evolution; (2) anatexis of mafic rocks in the deep oceanic crust, triggered by hydrothermal fluids; (3) liquid immiscibility in an evolved MORB system.

Mineral assemblages in the felsic veins include mainly plagioclase, amphibole, Fe-Ti oxides ± quartz and minor zircon, apatite, ± titanite, ± biotite, ± K-feldspar. Vein minerals often show strong zoning, which is especially expressed in amphiboles easily visible by their color ranging from brown to green (Fig. 1a) corresponding to compositions from pargasite via pargasitic amphiboles, magnesiohornblendes to tremolite/actinolite. Moreover, zoning patterns can be observed in plagioclases from the veins, in which their An contents vary from 33.55 down to 4.75. This is distinctly lower than in the plagioclases of the host gabbros, which show no zoning at all. Clinopyroxenes on the contact between felsic veins and host gabbros show reactions towards amphibole (Fig. 1d). Early results display that TiO₂ in amphiboles is strongly enriched in the host rock (up to ~4.5 wt%) and confined to brown colored amphiboles, whereas the brown counterpart from the vein is lower in TiO₂ (~1.2 wt%), sometimes changing from brown to green and therefore decreasing gradually to very low contents (~0.2 wt% TiO₂) (Fig. 1b).

F and Cl contents help to distinguish between magmatic and seawater derived fluids controlling the amphibole formation. In profiles, Cl sometimes parallels the zoning pattern for TiO₂. F seems to be unaffected (Fig. 1c).

Characteristic features observed in ~1/3 of all samples are granophyric textures exposed as intergrowth of quartz and (albitic) plagioclase (Fig. 2a). Merely in one sample we observed a special variety of this intergrowth. Here, both domains of the texture are plagioclase slightly varying in average composition, instead of plagioclase and quartz (Fig. 2b, 2c).