Origins of 210Pb-226Ra disequilibria in basalts: New insights from the 1978 Asal Rift eruption
Article first published online: 6 JUL 2012
©2012. American Geophysical Union. All Rights Reserved.
Geochemistry, Geophysics, Geosystems
Volume 13, Issue 7, July 2012
How to Cite
2012), Origins of 210Pb-226Ra disequilibria in basalts: New insights from the 1978 Asal Rift eruption, Geochem. Geophys. Geosyst., 13, Q07002, doi:10.1029/2012GC004173., , , and (
- Issue published online: 6 JUL 2012
- Article first published online: 6 JUL 2012
- Manuscript Accepted: 6 JUN 2012
- Manuscript Revised: 4 JUN 2012
- Manuscript Received: 28 MAR 2012
- Asal Rift;
- U-series disequilibria;
 There has been much debate as to whether 210Pb-226Ra 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 (210Pb/226Ra)t ratios are very low (0.2 to 0.6) and appear to correlate negatively with (226Ra/230Th). Invariant (230Th/238U) and (231Pa/235U) ratios require similar melting rates, porosities, and extents for all parental magmas. Thus, the range in (226Ra/230Th), which is negatively correlated with Th concentration, reflects fractional crystallization over millennia after the magmas were emplaced into the crust. This precludes the 210Pb deficits from resulting from partial melting. Instead, the 210Pb 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 210Pb with respect to 226Ra, 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 (210Pb/226Ra) ratios and sometimes 210Pb excesses.