Origins of 210 Pb- 226 Ra disequilibria in basalts: New insights from the 1978 Asal Rift eruption
Résumé
There has been much debate as to whether 210 Pb-226 Ra 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 (210 Pb/ 226 Ra) t ratios are very low (0.2 to 0.6) and appear to correlate negatively with (226 Ra/ 230 Th). Invariant (230 Th/ 238 U) and (231 Pa/ 235 U) ratios require similar melting rates, porosities, and extents for all parental magmas. Thus, the range in (226 Ra/ 230 Th), which is negatively correlated with Th concentration, reflects fractional crystallization over millennia after the magmas were emplaced into the crust. This precludes the 210 Pb deficits from resulting from partial melting. Instead, the 210 Pb 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 210 Pb with respect to 226 Ra, 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 (210 Pb/ 226 Ra) ratios and sometimes 210 Pb excesses.
Domaines
Planète et Univers [physics]
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