The fingerprint of seawater circulation in a 500-meter section of ocean crust gabbros

A novel strip-sampling technique has been applied to the 500-m gabbroic sec tion 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 mo re representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, delta(18)O) analyses w ere conducted on these strip samples; for comparison, analyses were conduct ed on a small suite of protolith samples, selected for their fresh and unal tered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a sea water component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; delta(18)O down to 2.0%0; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impr egnation by late-stage melts derived locally or from deeper levels of the c rust). The average upper 500 m 735B gabbro section is somewhat lower than a verage N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and he avy 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 comprise d of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average comp osition 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 lar ge scale removal of melts from this 500-m gabbro section. Copyright (C) 199 9 Elsevier Science Ltd.