DISTRIBUTION OF POROSITY IN A SECTION OF UPPER OCEANIC-CRUST EXPOSED IN THE TROODOS OPHIOLITE

The porosity structure of a section of old oceanic crust was character ized by a detailed study of the volcanic sequence of the Troodos Ophio lite, The distribution and abundance of porosity was compared to the a mount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages, Eight sites with one dominant lithology and contrasting alterati on histories were studied. Two types of porosity were quantified: macr oscopic (macrofractures, vesicles/vugs, +/- interpillow zones) and tot al (macroscopic and laboratory-measured microscopic porosity). Crustal accretion produced a vertically zoned porosity structure due to diffe rences between pillows that formed during the main and waning stages o f crustal construction. Mean initial macroscopic porosity ranges from 14 to 17% in uppermost pillows and from 6 to 10% in the underlying pil lows and massive flows, Macroscopic porosity was reduced by 10-11% whe re alteration was most pervasive (i.e., less than or equal to 250 m of the paleoseafloor). Beneath this zone, pillow and flow outcrops showe d decreases of <1-3% and 2-5%, respectively, The combined effects of v olcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity (4-8%). The greatest decr ease in macroscopic porosity was localized where abyssal hill topograp hy or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone, which comprises similar to 10% of th e volcanic pile. Thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography, whe re crests of abyssal hills remain unsedimented for long periods of tim e. Porosity data derived from Deep Sea Drilling Project/Ocean Drilling Program cores and borehole logs show similar relationships, which imp lies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity ca nnot be uniquely described by seismic data.