PII S0016-7037(99)00309-9 The fingerprint of seawater circulation in a 500-meter section of ocean crust gabbros STANLEY R. HART, 1 JERZY BLUSZTAJN, 1 HENRY J. B. DICK, 1 PETER S. MEYER, 1,2 and KARLIS MUEHLENBACHS 3 1 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 2 Rhode Island College, Providence, RI 02908, USA 3 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3 (Received December 2, 1998; accepted in revised form July 2, 1999) Abstract—A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os,  18 O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3;  18 O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m gabbro section. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Newly formed oceanic crust undergoes extensive chemical exchange with seawater during hydrothermal circulation pro- cesses. This interaction exerts a major control on the chemistry of many elements in the oceans; it also profoundly overprints the chemistry of the oceanic crust, leading to characteristic chemical signatures in island arc volcanism and in the mantle during crustal recycling. A complete understanding of these chemical interactions between ocean crust and seawater de- pends on knowing the elemental fluxes between the oceans and the entire ocean crust section. Although a significant body of research has been done on the extrusive and sheeted dike sections of the crust (Alt et al., 1986; Becker et al., 1989; Hart and Staudigel, 1989; Humphris and Thompson, 1978a, 1978b; Staudigel et al., 1981), no comprehensive trace element and isotopic study of the deeper crust has been achieved. By using a unique “strip sampling” approach, we report here on the Sr, Nd, Pb, Os, and oxygen isotopic character of the first 500 m of gabbro drilled at hole 735B during leg 118; in addition, a full suite of major and trace element data is also reported. The overall alteration budget of 735B gabbros is quite min- imal, and the predicted isotopic signature of aged lower oceanic crust is not akin to signatures found in those ocean island plume basalts that are thought to contain recycled oceanic crust. 2. GEOLOGIC SETTING, SITE 735 Site 735 is situated on a shallow (700 m) wave-cut platform on the transverse ridge system east of the Atlantis II fracture zone, SE Indian ridge; 25 km 2 of gabbro are exposed on this platform. Hole 735B is on 11 to 12 m.y. crust, 95 km south of the active spreading ridge, and 18 km east of the active transform plate boundary. Hole 735B was revisited in 1997 during leg 176, and is now 1.5 km deep. The scientific party found major changes in the vertical stratigraphy, with the rocks becoming less altered and deformed, and with an increasing abundance of olivine gabbro relative to ferrogabbro with depth (Dick et al., 1998; Natland et al., 1998). Dick et al. (1991b) proposed that this crustal section was formed beneath the median valley of the SW Indian Ridge 11 m.y. ago, some 15 to 19 km inboard of the ridge–transform intersection. Unroof- ing of the crustal section occurred on detachment faults created when the upper crust was periodically welded to the cold lithospheric plate at the ridge–transform intersection. The un- roofed section was uplifted into the rift mountains along normal faults, reaching sea level within a few million years. Thus, compared to normal deep-ocean crust, the high-temperature phase of hydrothermal circulation at 735B was probably some- what abbreviated, and the section then “quenched” through the retrograde metamorphism by which normal crust is affected. The major lithology at hole 735B is olivine gabbro, with associated oxide gabbros, gabbro norites and troctolites (Rob- inson and Von Herzen et al., 1989; Robinson et al., 1991; Bloomer et al., 1991; Dick et al., 1991a; Ozawa et al., 1991). The gabbros are not layered, but appear to represent co-min- gling of numerous small intrusive bodies. The section under- went ductile shear (Cannat, 1991a), from hyper-solidus down to amphibolite conditions; in the most deformed section (upper 200 m) pyroxene is strongly altered to amphibole, and amphibole veins are ubiquitous (Stakes et al., 1991). In areas of less intense shear deformation (200 to 400 m), the dominant deformation is * Author to whom correspondence should be addressed. Dr. S. R. Hart, Woods Hole Oceanographic Chemistry & Geochemistry MS 25, 360 Woods Hole Rd., Woods Hole, MA 02543-1541 (shart@whoi.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 23/24, pp. 4059 – 4080, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 4059