Basalts from ODP Site 648, recovered from Serocki Volcano, a near-axis volcano on the Mid-Atlantic Ridge, are normal mid-ocean ridge basalts, depleted in K, Rb, and LREE. They have homogeneous source regions with 87Sr/88Sr = 0.70234 - 0.70245 (N = 6); 143Nd/,44Nd = 0.51318 - 0.51322 (N = 8); 208Pb/204Pb = 18.53 - 18.74; 207Pb/205Pb = 15.40 – 15.45; 208Pb/204Pb = 37.83 – 36.14 (n= 3). The recovered materials are generally fresh with minor initial alteration that can be identified petrographically as the minor occurrence of groundmass clays and/or black halos. We have analyzed four separates of black halos, two of them with adjoining fresh bulk rock samples for a comparison of their chemical and isotopic composition. All black halos analyzed occur on or very close to the surfaces of rock fragments, and appear to be of finer groundmass grain size and/or contain less groundmass plagioclase, suggesting a different early cooling history. Two of the black halo samples analyzed were indistinguishable from fresh rock, and two displayed higher K, Rb, loss-on-ignition, and 87Sr/86Sr ratios, and lower K/Rb, chemical variations that suggest higher degree of ocean floor alteration.

Supplement to: Park, Kye-Hun; Staudigel, Hubert (1990): Radiogenic isotope ratios and initial seafloor alteration in submarine Serocki Volcano basalts. In: Detrick, R; Honnorez, J; Bryan, WB; Juteau, T; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 106/109, 117-121