Compositional variation and ²²⁶Ra‐²³⁰Th model ages of axial lavas from the southern Mid‐Atlantic Ridge, 8°48′S

Abstract

We present geological observations and geochemical data for the youngest volcanic features on the slow spreading Mid-Atlantic Ridge at 8848 0 S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km 2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have 226 Th disequilibria model ages of 1000 and 4000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 1008C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8848 0 S lava field support the model of low-frequency eruptions from single ascending magma batches that has been developed for slow spreading ridges.

High-resolution decadal-scale eruption age dating of young oceanic basalts at an active hydrothermal vent site

“…Currently, U-series dating provides the best age constraints for young mid-ocean ridge lavas. The application of U-series disequilibria to the dating of young submarine basalts is well established (e.g., Rubin and Macdougall, 1990;Goldstein et al, 1991;Volpe and Goldstein, 1993;Rubin et al, 1994;Lundstrom et al, 1995;Sturm et al, 2000;Sims et al, 2002;Cooper et al, 2003;Sims et al, 2003;Sims et al, 2008a;Standish and Sims, 2010;Waters et al, 2011;Waters et al, 2013a;Waters et al, 2013b;Haase et al, 2016;Scott et al, 2019;Sims et al, 2021). See Sims et al (2021) for a detailed overview of U and Th decay series dating methods.…”

“…Fortunately, 238 U-series disequilibria can provide robust eruption ages for young oceanic basalts (e.g., Rubin and Macdougall, 1990;Goldstein et al, 1991;Volpe and Goldstein, 1993;Rubin et al, 1994;Lundstrom et al, 1995;Sims et al, 1995;Sturm et al, 2000;Sims et al, 2002;Cooper et al, 2003;Sims et al, 2003;Sims et al, 2008a;Standish and Sims, 2010;Waters et al, 2011;Waters et al, 2013a;Waters et al, 2013b;Haase et al, 2016;Scott et al, 2019;Sims et al, 2021). The 238 U-decay series nuclides have a range of half-lives that are appropriate for dating a wide age range of young basalts, and the application of U-series disequilibria to the dating of young submarine basalts has been well established.…”

DataSheet1.docx

“…Currently, U-series dating provides the best age constraints for young mid-ocean ridge lavas. The application of U-series disequilibria to the dating of young submarine basalts is well established (e.g., Rubin and Macdougall, 1990;Goldstein et al, 1991;Volpe and Goldstein, 1993;Rubin et al, 1994;Lundstrom et al, 1995;Sturm et al, 2000;Sims et al, 2002;Cooper et al, 2003;Sims et al, 2003;Sims et al, 2008a;Standish and Sims, 2010;Waters et al, 2011;Waters et al, 2013a;Waters et al, 2013b;Haase et al, 2016;Scott et al, 2019;Sims et al, 2021). See Sims et al (2021) for a detailed overview of U and Th decay series dating methods.…”

“…Fortunately, 238 U-series disequilibria can provide robust eruption ages for young oceanic basalts (e.g., Rubin and Macdougall, 1990;Goldstein et al, 1991;Volpe and Goldstein, 1993;Rubin et al, 1994;Lundstrom et al, 1995;Sims et al, 1995;Sturm et al, 2000;Sims et al, 2002;Cooper et al, 2003;Sims et al, 2003;Sims et al, 2008a;Standish and Sims, 2010;Waters et al, 2011;Waters et al, 2013a;Waters et al, 2013b;Haase et al, 2016;Scott et al, 2019;Sims et al, 2021). The 238 U-decay series nuclides have a range of half-lives that are appropriate for dating a wide age range of young basalts, and the application of U-series disequilibria to the dating of young submarine basalts has been well established.…”