Textural, geochemical, and isotopic variations in gabbroic shear zones from the MARK area

Abstract

Gabbros recovered during Ocean Drilling Program Leg 153 from the Mid-Atlantic Ridge, south of the Kane Fracture Zone (MARK area) display remarkable textural and compositional diversity over meter to decameter scales. The textures in core samples for this study can be broadly categorized into weakly and moderately recrystallized, Porphyroclastic and mylonitic. These divisions are based primarily on varying degrees of static and dynamic recrystallization in Plagioclase, olivine, and cli- nopyroxene. Textural relations indicate that some samples were deformed under hypersolidus conditions. Whole-rock geochemical and isotopic variations have been examined to investigate the possible correlations between strain localization, melt migration, and hydrothermal alteration. Preliminary results show that there are no systematic variations of strontium-iso- tope ratios with textural variations or with depth. This may indicate only limited seawater-rock interaction. Alternatively, if deformation occurred at mid-to-lower crustal depths, the seawater composition may have been buffered by the overlying rock column. Neodymium-isotope ratios are consistent with primary magmatic values. Whole-rock lead isotopes have significantly more radiogenic lead-isotope ratios than primary magmatic values, and may therefore be more sensitive to alteration. Mineral chemistry indicates highly heterogeneous Plagioclase compositions (mm to decameter scales) generated during dynamic recrystallization and post-kinematic alteration. Deformed samples tend to have a lower anorthite content than undeformed sam- ples, but there are several inconsistencies. Downhole variations in incompatible elements indicate that concentrations of late- stage melt fractions correspond to intervals 0-30 mbsf at each drill site and 60-70 mbsf at Holes 92IE and 923A. High-temper- ature shear-zone fabrics (amphibolite to granulite facies) are concentrated in these intervals, but there are no systematic geochemical variations with the intensity of deformation within these intervals. Strain localization may have been promoted by zones of high residual melt porosity. If late-stage melt migration was focused by shearing, consequent local temperature eleva- tions may have accelerated recovery mechanisms and promoted dislocation creep. A higher bulk Plagioclase (and in some cases quartz) content, transient low effective stresses, and melt assisted diffusion creep may also have weakened these zones. INTRODUCTION