Bioalteration of basaltic glass in the oceanic crust

[1] Bioalteration of Quaternary to Early Cretaceous basaltic glass from pil low lavas of the upper oceanic crust can be documented in Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) samples from shallow to deep dri ll holes from the north to central Atlantic Ocean, Lau Basin, and Costa Ric a Rift, a wide range of marine settings. Biogenerated textures are rooted i n fractures and occur as two main types, a granular type and a tubular type . The granular type, common at all depths within the volcanic pile, appears as solid bands, semicircles or irregular patches of individual and/or coal esced spherical bodies, mostly 0.2-0.6 mum in diameter, with irregular prot rusions into the fresh glass. The tubular type is more common at deeper lev els in the crust and consists of thin tubes, sometime branching bodies, mos tly 20-30 mum long and are more common at deeper levels. The upper crust di splays a large variability in the relative importance of biotic to abiotic alteration, and the degree of bioalteration appears to decrease with depth. Thus the fraction of bioalteration of the total alteration of the glass ra nges from 20-90% in the upper 300 m down to a maximum of 10% at about 500 m depth. This might be due to a natural variability in the abundance of bioa ltered glass or to biased sampling from low drilling recovery of relatively young crust. The proportion of bioaltered to abiotically altered glass doe s not show any systematic variations with age of the crust. Thus bioalterat ion lasts as long as abiotic alteration, i.e., for as long as water is avai lable to the hydration of the oceanic crust. Evidence from heat flow measur ements suggests that hydrothermal circulation lasts until at least similar to 70 Ma, and thus the deep biosphere is likely to expand at least into cru st of this age.