Early development of black halos during low-temperature alteration of young oceanic basalts

The most visible effect of low-temperature alteration during seawater - basalt interaction is the formation of variously colored zones adjacent to exposed surfaces, cracks, and veins along which aqueous solutions once circulated. Young oceanic basalts commonly exhibit a single black zone contiguous with exposed surfaces that surrounds apparently unaltered dark gray cores. These "black halos" are the earliest evidence of oceanic crust alteration since they are observed alone in zero age oceanic basalts and are overprinted by later alteration processes in older samples. Black halos are ubiquitous in basalts from the upper oceanic crust of all ages. In order to constrain the mechanisms and processes responsible for the formation of black halos we have undertaken a detailed petrographical, mineralogical, and geochemical study of a suite of basalts that range in age from 0.54 to 2.73 My recovered during DSDP Legs 54 and 70 from the Galapagos Spreading Center (GSC). Black halo widths range from a few millimeters up to 4 - 5 cm. The black halos are characterized by the precipitation of secondary minerals in vesicles and void spaces along with common replacement of olivine when present. Primary pyroxene is rarely altered and plagioclase is always fresh. Secondary minerals consist mainly of clay minerals (celadonitic phyllosilicates, smectites, and mixtures), iron oxyhydroxides, carbonates, and rare quartz. Geochemical analyses and oxygen isotope ratios clearly demonstrate that precipitation of secondary phases control the chemical changes resulting from the formation of black halos. Elements such as K, Rb, Cs, and B as well as H₂O⁺ content are all increased in black halos relative to least-altered dark gray basalts and unaltered glass. Black halos also have elevated δ ¹⁸O ratios due to the precipitation of ¹⁸O enriched secondary minerals. We are also able to demonstrate that the mechanism of black halo formation is at least a two stage process and is not simply related to ageing.