Melting versus contamination effects on 238U-230Th-226Ra and 235U-231Pa disequilibria in lavas from São Miguel, Azores

Julie Prytulak*, Riccardo Avanzinelli, Govert Koetsier, Katharina Kreissig, Christoph Beier, Tim Elliott

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

14 Citations (Scopus)

Abstract

The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The 'enriched' signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U-230Th and 235U-231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U-230Th-226Ra and 235U-231Pa disequilibria are presented for sixteen mafic lavas (<50wt.% SiO2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230Th-226Ra disequilibria (~18% deficits to >300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D-UU and D-Th, coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.

Original languageEnglish
Pages (from-to)94-109
Number of pages16
JournalChemical Geology
Volume381
DOIs
Publication statusPublished - 14 Aug 2014

Keywords

  • Contamination
  • Dynamic melting
  • Pyroxenite/eclogite
  • São Miguel
  • U-series

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