Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area

Mass spectrometric measurements of Th-230-Ra-226, U-235-Pa-231 and U-238-Th -230 disequilibria are used to determine eruption ages for four mid-ocean r idge basalts from the median valley of the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK area). Three samples were collected across-axis on the Axial Volcanic Ridge (i.e. the Neovolcanic Ridge) near the Snake Pit h ydrothermal mound, and one sample was collected near the crest of Serocki V olcano similar to 50 km south of Snake Pit. (Ra-226)/(Th-230) and (Th-230)/ (U-238) activity ratios are low and uniform for all four samples, while (Pa -233)/(U-235) activity ratios are elevated and somewhat more variable. Age constraints suggest that these lavas, from the most robust volcanic edifice s in the MARK area, are 10 000-20 000 yr old. The age data are used to eval uate the efficacy of commonly used age estimate scales based on qualitative indicators (e.g. sediment cover, glass quality) and to begin to quantify t he temporal and spatial dependence of volcanic, tectonic and hydrothermal p rocesses at slow-spreading oceanic ridges. (C) 2000 Elsevier Science B.V. A ll rights reserved.