Highly permeable and layered Jurassic oceanic crust in the western Pacific

Bulk permeability was determined from drillstring packer measurements in ODP Hole 801C in Jurassic oceanic crust in the western Pacific. The values average 8 x 10 -14 m 2 over 93 m of open hole, or 4 X 10 -13 m 2 if the permeable interval is confined to an 18 m thick hydrothermal zone within oceanic basement. These values are about 1-10 times higher than those reported for the upper sections of Holes 395A, 504B and 735B in 3.7, 5.9 and 11 Ma old oceanic crust respectively. The discovery that 160 Ma old oceanic crust contains a zone with extremely high permeability is not predicted by any model of ocean crustal evolution. The high permeability interval appears to be associated with the burial by off-ridge volcanism of a zone of hydrothermal precipitates and altered basalts. The generality of this result may depend on the generality of off-ridge volcanism necessary to create and preserve such zones of high permeability within the ocean crustal section. Although the crustal and permeability structures detected at Hole 801C may not fit easily into currently accepted models of crustal accretion and aging, the generality of off-ridge volcanism suggests that these structures could actually be typical of large portions of the world's oceanic crust. 1. Geological background O c e a n Dri l l ing P rog ram ( O D P ) H o l e 801C was dri l led th rough sed iments and into oceanic crust of Midd le Jurassic age (160 Ma) at 18°38.5'N, 156°21.6 'E in the Pigafe t ta Basin of the wes te rn Pacific O c e a n (Fig. 1) dur ing O D P Leg 129 [1]. This is a dr i l lhole into the wor ld ' s o ldest in-situ oceanic crust [2-4]. Because of its ex t reme age, all mode ls for the physical evolut ion of oceanic crust predic t that it would have an ex t remely low 1 A. Arnaud-Vanneau, D.D. Bergersen, Y. Bogdanov, H.W. Bohrmann, B. Buchardt, G. Camoin, D.M. Christie, J.J. Dieu, P. Enos, E. Erba, J. Fenner, J.S. Gee, J.A. Haggerty, MJ. Head, P.R.N. Hobbs, H. Ito, L. Jansa, J.W. Ladd, J.M. Lincoln, M. Nakanishi, J.G. Ogg, B.N. Opdyke, P.N. Pearson, I. Premoli Silva, T.M. Quinn, F.R. Rack, D.K. Watkins and P.A. Wilson porosi ty (and by inference, low bulk permeabil i ty) , a l though no m e a s u r e m e n t s of bulk permeabi l i ty had been m a d e in in-situ oceanic crust o lder than 11 Ma. We des igned a comprehens ive set of logging and permeabi l i ty m easu rem en t s for Ho le 801C in o rde r to provide an e n d m e m b e r character iza t ion of very old oceanic crust fo rmed by rapid seaf loor spreading. These m easu rem en t s were conduc ted af ter reent ry into Ho le 801C dur ing Leg 144 [5]. We successfully logged most of the exposed basemen t at the bo t tom of the hole below the caving pipe with geophysical , geochemical , fo rmat ion microscanner , t e m p e r a t u r e and m a g n e t o m e t e r tools, and then conduc ted permeabi l i ty exper iments . Our most surprising result was the permeabi l i ty de te rmina t ion , which we repor t he re in detai l using some of the geophysical logs and the t e m p e r a t u r e log as ancillary informat ion. 0012-821X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved 72 R.L. LARSON ET AL. 2. Geophysical logging results Hole 801C was first logged with a geophysical combination string, consisting of natural gamma, lithodensity, neutron porosity, long-spaced sonic, and temperature tools (Fig. 2). Temperature data from this first logging run indicate that the thermal regime in Hole 801C is dominantly conductive. The water in the borehole appears to have thermally equilibrated with the surrounding formation in the 2.5 yr since the hole was drilled. The geophysical logs of Leg 144 confirmed the general three-layer stratigraphy determined by coring on Leg 129 (Fig. 2). The open portion of the hole below 480 m below seafloor (mbsf) consists of alkali basalts from 480 to 510 mbsf, a hydrothermal zone of chemical precipitate and altered alkali basalts from 510 to 528 mbsf, and tholeiitic basalts from 528 mbsf to the bottom of the hole at 594 mbsf. The alkali basalts are 4-14 Ma younger than the tholeiitic basalts, and thus 130°E 150°E 170°E 170°W