KINETIC ARGON ISOTOPE FRACTIONATION IN DEGASSING MAGMA: A POSSIBLE CAUSE OF 39AR/40AR AGE BIAS IN QUATERNARY LAVAS

To obtain an ³⁹Ar/⁴⁰Ar age from a lava, geochronologists must distinguish radiogenic ⁴⁰Ar from excess ⁴⁰Ar. The ³⁹Ar/⁴⁰Ar age correction assumes excess argon has an atmospheric composition, yet young (<< 1 Ka) lavas actually have a strongly sub-atmospheric isotopic composition and appear to cluster around an apparent mass-dependent fractionation trend: ⁴⁰Ar/³⁶Ar = 2610 (38Ar/36Ar) - 209. Sub-atmospheric excess argon in lava could affect age estimates for volcanic rocks with relatively low concentrations of radiogenic ⁴⁰Ar, making them appear significantly younger.

We conducted HP-HT argon diffusion experiments in anhydrous rhyolite magma from Buck Mountain (Lassen, California). Later, we measured argon concentration and argon isotope fractionation vs distance traveled through the melt. Our results confirm that light argon does diffuse faster than heavy argon. OLS regressions calculated D_Ar = 4.77 and E_Ar = 3.7, confirming that argon is very mass selective compared mobile ions in the melt, which supports the theory that mobile ions in magma are very strongly associated with aluminosilicate network during diffusion.

FDM simulations indicate that direct gas exchange between flowing lava and the atmosphere is so slow that any isotopic alteration should attenuate within few millimeters of the surface. Simulations of argon diffusing between gas bubbles and magma predict that bubbles will generate chemical and isotopic gradients in response to any changes in the magma that cause bubble to grow or shrink. During decompression, rapidly growing bubbles may act like sinks for argon, which forms millimeter-scale gradients depleted in light argon around gas bubbles enriched in isotopically light argon. This may explain prior observations lava where if vesicles were removed analysis measured consistently heavier isotopic compositions. Buoyant mechanical separation of bubbles from denser gas poor magma should continually keep gas-rich magma slightly enriched in isotopically light argon. Assuming the gas-rich, buoyant magma is more likely to reach the surface, eruptions may tend to preferentially extract and solidify lavas with isotopically light compositions. This mechanism could resolve the age-disagreement between (younger) ³⁹Ar/⁴⁰Ar basalt ages and (older) U–Pb ash ziron ages from overlying ash layers.