Origins of 210Pb-226Ra disequilibria in basalts: New insights from the 1978 Asal Rift eruption

There has been much debate as to whether Pb-210-Ra-226 disequilibria in young volcanic rocks result from partial melting, cumulate interaction or magma degassing. Here we present new data from basalts erupted in 1978 from Ardoukoba volcano in the Asal Rift. The (Pb-210/Ra-226)(t) ratios are very low (0.2 to 0.6) and appear to correlate negatively with (Ra-226/Th-230). Invariant (Th-230/U-238) and (Pa-231/U-235) ratios require similar melting rates, porosities, and extents for all parental magmas. Thus, the range in (Ra-226/Th-230), which is negatively correlated with Th concentration, reflects fractional crystallization over millennia after the magmas were emplaced into the crust. This precludes the Pb-210 deficits from resulting from partial melting. Instead, the Pb-210 deficits must have formed subsequent to magma differentiation and are interpreted to reflect several decades of magma degassing. Many young basalts erupted in a variety of tectonic settings are similarly depleted in Pb-210 with respect to Ra-226, suggesting that they continuously degas over a period of a few to several decades, perhaps reflecting the time required to rise to the surface from deeper reservoirs. In some of these basalts, gas accumulation leads to the shallowest, most evolved, and earliest erupting magmas having the highest (Pb-210/Ra-226) ratios and sometimes Pb-210 excesses.