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

摘要

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 paleomagnetic 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 variability 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 variation can be explained by polybaric fractionation with local contributions from enriched sources. Remarkably, the extreme variations in crustal thickness, segment morphology, depth of the median valley, and even the presence of the Kane Transform Fault are not associated with compositionally distinct parental basaltic liquids. A comparison with other relatively isotopically 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 play a dominant role in the amount of compositional diversity in parental basalts generated at mid-ocean ridges.