40Ar/39Ar dating of volcanic fallout under seawater: influence of glass

Abstract

Accurate dating of submarine volcanic rocks is crucial for understanding many geological processes. However, some submarine volcanic rocks yield inaccurate young ⁴⁰Ar/³⁹Ar ages. To address this issue, we conducted ⁴⁰Ar/³⁹Ar, electron microprobe and energy-dispersive X-ray spectroscopy analyses on hornblende, plagioclase and glass samples of the fallout tephra from borehole U1396A of the Integrated Ocean Drilling Program (IODP) 340 Expedition. Based on the ⁴⁰Ar/³⁹Ar analyses, 3 samples yield reasonable ages for the tephra, whereas the other 6 samples exhibit abnormally young ages. Remarkably, glass is widely present in hornblendes and plagioclases, as shown in the field emission scanning electron microscopy (FESEM) images. These glass samples have much higher K contents than the hornblende and plagioclase samples. Based on the K distribution of the glass, we propose that seawater can significantly increase the K content in some samples containing glass during the post-eruption period. Due to the extra addition of K, the determined ⁴⁰Ar/³⁹Ar ages of these submarine samples are predicted to be younger and are incorrect. In this study, an effective pretreatment method is proposed to reduce the effect of glass on the ⁴⁰Ar/³⁹Ar dating of the hornblende samples. The three hornblende samples subjected to our pretreatment method have all yielded more reliable ⁴⁰Ar/³⁹Ar ages, which suggests that our pretreatment method has the potential to enable more submarine volcanic rock samples to produce trustworthy ⁴⁰Ar/³⁹Ar age determinations in the future.