Radium isotopes for groundwater age and sustainability in the highland of Antananarivo, Madagascar

Activities of 223Ra and 226Ra in Fluids From the Lost City Hydrothermal Field Require Short Fluid Residence Times

“…We developed a model for the LCHF based on studies of Ra in groundwater (e.g., Kiro et al, 2013Kiro et al, , 2012Krishnaswami et al, 1982;Rajaomahefasoa et al, 2019;Weinstein et al, 2021); terrestrial geothermal systems (Hammond et al, 1988); and marine hydrothermal systems (Cochran & Kadko, 2008). Our model assumes: (a) more than 99% of the U in the serpentinite was deposited from seawater as the serpentine formed (Seitz & Hart, 1973), (b) the serpentinite at the LCHF formed at least 5 Ky ago and possibly as early as 1 My (Kelley et al, , 2005, (c) the only source of radium into the fluids is from the serpentinite and is due to alpha recoil from 230 Th and 231 Pa daughters that have been generated through U decay, and (d) the system is at steady state on a time scale of a few years.…”

“…Equations can be written for both 223 Ra and 226 Ra. Dividing these equations results in: The recoil rate constant is related to the recoil efficiency ( 𝐴𝐴 𝐴𝐴 , fraction of recoil events that displace the daughter toward the surface), the recoil range, and the characteristic rock surface, which is related to surface area (Rajaomahefasoa et al, 2019).…”

“…We start with an equation from Rajaomahefasoa et al (2019) developed for the production of mobile Ra isotopes in groundwater, neglecting rock alteration:…”