OCCURRENCE OF SIDERITE AND ANKERITE IN YOUNG BASALTS FROM THE GALAPAGOS SPREADING CENTER (DSDP HOLE-506G AND HOLE-507B)

As soon as they are emplaced on the sea floor, oceanic basalts go thro ugh a low-temperature alteration process which produces black halos co ncentrical with exposed surfaces and cracks, whereas the grey internal parts of the basaltic pieces apparently remain unaltered. This paper reports for the first time the occurrence of authigenic siderite and a nkerite in oceanic basalts and more particularly in the grey internal parts of the latter. Small (8-50 mum) crystals of zoned siderite and a nkerite have been observed in ten vesicles of two samples recovered fr om DSDP Holes 506G and 507B drilled south of the Galapagos Spreading C enter (GSC). These Fe-carbonates show a large range of chemical compos ition (FeCO3=47-88%; CaCO3=5-40%; MgCO3=1-20%; MnCO3=0-11%). Most of t hem are Ca-richer than siderite reported in the literature. The chemic al composition of the carbonate clearly reflects the fluctuation of th e fluid chemical composition during crystallization. Mn and at least p art of the Fe are thought to be hydrothermal in origin, whereas Mg and probably Ca were provided by seawater. It is proposed that siderite a nd ankerite formed at relatively low temperature (< 85-degrees-C) and is metastable. The alteration of the GSC basalts seems to have proceed ed in two stages: during the first, reducing stage, pyrite precipitate d from hydrothermal fluids. A little further in the rock, siderite pre cipitated from the fluid which had already been modified by the format ion of pyrite, and thus in a microenvironment where particular conditi ons prevailed (high P(CO2), increasing pS2- or increasing pH or increa sing or decreasing pe). During the second, oxidizing, stage of alterat ion, a seawater-dominated fluid allowed crystallization of mixtures of Fe-rich smectites and micas, and Fe-hydroxides forming the black halo s in the external portion of the basalt pieces and locally oxidizing p yrite and siderite in their innermost part. It is shown in this paper that, even at its earliest stage, and at low temperature, alteration o f the upper oceanic crust (lavas) involves fluids enriched in Fe and M n, interpreted to be of hydrothermal origin.