Basalt compositions from the Mid-Atlantic Ridge at the SMARK area (22 degrees 30'N to 22 degrees 50'N) - implications for parental liquid variabilityat isotopically homogeneous spreading centers

Major- and trace-element analyses of over 50 well-located samples from the third segment south of the Kane Transform Fault are reported. These can be grouped into four age groups based on structural relations and paleomagneti c data, Although no spatial relations are discerned and most trace-element concentrations vary with the extent of fractionation, trends of decreasing Rb and Cs and increasing Ba and Th over the past million years are revealed . The compositions of these samples are similar to basaltic rocks recovered from around the Kane Transform Fault and allow assessment of geochemical v ariability along a series of geologically diverse ridge segments both north of the transform and from greater than 150 km to the south. Most of the Va riation can be explained by polybaric fractionation with local contribution s from enriched sources. Remarkably, the extreme variations in crystal thic kness, segment morphology. depth of the median valley, and even the presenc e of the Kane Transform Fault are not associated with compositionally disti nct parental basaltic liquids. A comparison with other relatively isotopica lly homogeneous ridge segments reveals that parental basalts from near the Kane Transform Fault are not uniquely homogeneous, This comparison suggests that crustal morphology and spreading rate do not Clay a dominant role in the amount of compositional diversity in parental basalts generated at mid- ocean ridges. (C) 2001 Elsevier Science B.V. All rights reserved.