The dynamics of melting beneath Theistareykir, northern Iceland

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Abstract

[1] U-Th disequilibrium data on a suite of postglacial basalts (<12,000 years old) from Theistareykir, northern Iceland, show a relatively high degree of U series disequilibrium. For 21 out of 27 samples from Theistareykir, 230Th excesses are within 25 ± 5%. Four samples have 230Th excesses >30%. The two geochemically and isotopically most depleted samples (picrites) have excess 230Th of about 15%, similar to enriched basalts from locations adjacent to Theistareykir (Draugarhraun and Asbyrgi). The large range in chemical and isotopic variations of the Theistareykir samples provides an excellent opportunity to investigate the influence of source heterogeneity on the degree of U series disequilibrium. The lack of correlation between U-Th data and radiogenic isotope or chemical parameters indicates that source heterogeneity does not play an important role in creating the excess 230Th. Therefore the Theistareykir basalts provide evidence that the Th systematics result from variations in the timing of the melting and melt extraction process only. The relatively large and constant 230Th excesses for the bulk of the Theistareykir samples (20–30%) suggest that melting and melt extraction must be relatively reproducible, with the melting starting well within the garnet-stability field. Near fractional melting models suggest residual porosity ≤0.1%, upwelling rates ≤1cm/yr (corresponding to melting rates ≤1*10−4 kg/m3yr), and melt ascent velocities ≥3–4m/yr. The lower 230Th excesses of about 15% in both the most depleted lavas from Theistareykir and enriched lavas from locations adjacent to Theistareykir (Draugarhraun and Asbyrgi) indicate some variation in the melting and melt extraction process. The low 230Th excesses of the most depleted melts (picrites) could either be an effect of slow melt migration or be caused by incomplete melt aggregation, with melts created at the very bottom of the melting regime being underrepresented. The low 230Th excesses of the most enriched lavas are most likely due to slow melt migration for melts created at some distance from the center of the melting regime.

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