ALBERT

Description: 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.

Description: Basalts in Hole 648B, located in the rift valley of the Mid-Atlantic Ridge at 23°N in crust estimated to be less than 100,000 years old, are mainly fresh, but small amounts of secondary phases are found on fracture surfaces and in alteration halos within the rocks. The halos are defined by dark bands 1-4 mm thick that have developed parallel to fracture surfaces or pillow margins and which in some cases have migrated some centimeters into the rock. The dark bands are the principal locus of secondary phases. The secondary phases are olive-green and yellow protoceladonites, of composition and structure intermediate between celadonite and iron-rich saponite, red (Mn-poor) to opaque (Mn-rich) iron oxyhydroxides, mixtures of protoceladonite and iron oxyhydroxide, and rare manganese oxides. These phases occur mainly as linings or fillings of open spaces in the basalt within the dark bands. Sulfides and intersertal glass are the only primary phases that can be seen to have been altered. Where dark bands have migrated into the rock, the rock behind the advancing band is almost devoid of secondary phases, implying redissolution. The potassium and magnesium in the secondary phases could have been supplied from ambient seawater. The aluminum in the protoceladonites must have been derived from local reaction of intergranular glass. The source of iron and silica could have been intergranular glass or low temperature mineralizing solutions of the type responsible for the formation of deposits of manganese oxides and iron oxyhydroxides and silicates on the seafloor.

Description: ODP Legs 106 and 109 drilled zero-age crust at Site 648 on the axis of the Mid-Atlantic Ridge and recovered sparsely to moderately phyric but chemically homogeneous basalt. Plagioclase phenocrysts in Site 648 basalts exhibit a variety of zoning patterns and compositions including: (1) tabular (An84) cores mantled by more sodic zones (An80 and An75), (2) tabular (An80) cores mantled by narrow (An84 and An75) zones, (3) skeletal cores (An80-78) enclosing sodic (An75) patches, and (4) rounded cores (An75) surrounded by skeletal (An80-78) mantles. Backscattered electron images have been used to define zone morphologies in 28 crystals; major element compositions of the zones have been determined by electron microprobe; and Mg, K, Ti, Fe, and Sr concentrations have been determined by ion microprobe. Equilibrium partitioning of minor and trace elements between crystals and liquids occurred during stable growth of tabular cores and inner rims. Disequilibrium partitioning occurred during rapid growth of crystal rims, resulting in enrichment of incompatible trace and minor elements, and during growth of calcic zones with skeletal and acicular morphology, resulting in enrichment of Fe and Sr and depletion of Mg, K, and Ti. Crystal/liquid partition coefficients estimated from the composition of inner rims and glass are DMg = 0.03, DK = 0.15, DTi = 0.028, DFe = 0.05, and DSr = 1.82. Using these partition coefficients, we have calculated the composition of primitive liquid in equilibrium with phenocryst cores. The estimated liquid composition is similar to primitive basalt compositions from the Kane Fracture Zone except for a much higher Ca/(Ca + Na) ratio which resembles values for MAR plume basalts. The diversity in plagioclase morphologies and compositions is explained by mixing of melts at different stages of evolution and stored in separated but interconnected reservoirs, as previously suggested by Kuo and Kirkpatrick (1982, doi:10.1007/BF00376957).