Strontium and Neodymium Isotope Systematics of Plutonic Rocks From the Atlantis Massif, Mid-Atlantic Ridge

Gabbroic rocks and associated basaltic dikes from Atlantis Massif, Mid-Atlantic Ridge at 30°N (IODP Exp. 304 & 305, Hole U1309D), have been examined in order to understand magmatic processes, water rock interactions and source components at the slow-spreading Mid-Atlantic Ridge. The recovered rocks range from plagioclase-bearing peridotite, troctolite, gabbro and gabbronorite to evolved oxide gabbro, and diabase. One sample about every 20-30 m from 43 to 1410 mbsf of different lithologies, plus discrete sampling of intrusive doleritic and basaltic dikes and a closer sampling (about every half a meter) through an ultramafic section, has been done. 88 samples have been analysed for Nd and Sr isotope compositions. The 143Nd/144Nd ratio of the plutonic rocks range from 0.513141 to 0.513215 with an average of 0.513187. There seems to be no systematic variation in the Nd-isotope compositions through the core. However, repeated analyses of single samples are needed to decide if the observed variations are larger than can be explained by analytical errors alone.The 87Sr/86Sr ratios vary from 0.70250 to 0.70351. The Sr-isotopic composition has been shifted slightly more towards to seawater values in the upper 800m than in the lower 600m of the cored section, showing that fluid-rock interactions decrease downward. The water-rock induced shift in Sr-isotope compositions in these layer 3 rocks is however significantly less then the shift reported from the sheeted dike complex sampled by ODP Hole 504B. The associated doleritic and basaltic dykes show on average 87Sr/86Sr ratios that are slightly higher and 143Nd/144Nd ratios that slightly lower than the plutonic rocks. This indicates that the plutonic rocks and the associated dikes were derived from slightly different mantle sources.