Evidence for Remobilization of Barite Affecting Radiometric Dating Using ~(228)Ra, ~(228)Th, and ~(226)Ra/Ba Values:Implications for the Evolution of Sea-Floor Volcanogenic Massive Sulfides

摘要

Samples of chimneys (black smokers) and other volcanogenic massive sulfide (VMS) mineralization were collected from hydrothermal vent fields associated with submarine arc and back-arc volcanoes in the western Pacific. Each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site, information that is vital when establishing the time elapsed to amass an ore deposit. In each case, the radiometric dating relied on the radium isotopes ~(228)Ra and ~(226)Ra with half-lives of 5.75 and 1,600 years, respectively, which coprecipitate with barite and then decay. We began dating VMS mineralization 15 years ago, knowing that the ~(226)Ra/Ba decreases with time, with the Ba and Ra assumed to be from a common source such that the initial ~(226)Ra/Ba value, i.e., the value at the onset of mineralization, was constant. Thus, the age of an "old" chimney (up to 15,000 yr) was calculated by comparing its ~(226)Ra/Ba value with the initial value established from nearby "young" chimneys (<35 yr) that were active or still contained ~(228)Ra. Since then, we have discovered that chimneys and other forms of sea-floor VMS mineralization can contain barite from two or more hydrothermal events. That is, older barite can be dissolved and reprecipitated (remobilized) from earlier mineralization deposited subseafloor, or earlier sea-floor mineralization overprinted (crosscut) by new. In many cases, the ~(226)Ra/Ba dating did not provide an absolute age for the older material but, rather, a difference in age between two samples that had incorporated varying proportions of older barite that was low or "dead" (i.e., > 15,000 yr) with respect to ~(226)Ra, due to radioactive decay.Remobilizing of older barite is consistent with zone refining of metals in large VMS deposits, but we have only considered this recently. However, our investigations show that this can affect the initial ~(226)Ra/Ba values for VMS mineralization from arc and back-arc volcanoes, and also from mid-ocean ridges. Where sufficient samples contained ~(228)Ra, we estimated an age for the remobilized barite by plotting a ~(226)Ra/Ba versus ~(228)Ra/ Ba mixing line for the VMS at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for > 15,000 years. Such long histories of hydrothermal activity indicate the propensity for producing large metal-rich deposits on the seafloor.