Generation and evolution of magma beneath the East Pacific Rise: Constraints from U-series disequilibrium and plagioclase-hosted melt inclusions

Guo Liang Zhang, Zhi Gang Zeng*, Christoph Beier, Xue Bo Yin, Simon Turner

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)

Abstract

Mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR 12°50'N) were analyzed for U-series isotopes and compositions of plagioclase-hosted melt inclusions. The 226Ra and 230Th excesses are negatively correlated; the 226Ra excess is positively correlated with Mg# and Sm/Nd, and is negatively correlated with La/Sm and Fe8; the 230Th excess is positively correlated with Fe8 and La/Sm and is negatively correlated with Mg# and Sm/Nd. Interpretation of these correlations is critical for understanding the magmatic process. There are two models (the dynamic model and the "two-porosity" model) for interpreting these correlations, however, some crucial parameters used in these models are not ascertained. We propose instead a model to explain the U-series isotopic compositions based on the control of melt density variation. For melting either peridotite or the "marble-cake" mantle, the FeOt content, 230Th excess and La/Sm ratio increases and Sm/Nd decreases with increasing pressure. A deep melt will evolve to a higher density and lower Mg# than a shallow melt, the former corresponds to a long residence time, which lowers the 226Ra excess significantly. This model is supported by the existence of low 226Ra excesses and high 230Th excesses in MORBs having a high Fe8 content and high density. The positive correlation of 226Ra excess and magma liquidus temperature implies that the shallow melt is cooled less than the deep melt due to its low density and short residence time. The correlations among Fe8, Ti8 and Ca8/Al8 in plagioclase-hosted melt inclusions further prove that MORBs are formed from melts having a negative correlation in melting depths and degrees. The negative correlation of 226Ra excess vs. chemical diversity index (standard deviation of Fe8, Ti8 and Ca8/Al8) of the melt inclusions is in accordance with the influence of a density-controlled magma residence time. We conclude that the magma density variation exerts significant control on residence time and U-series isotopic compositions.

Original languageEnglish
Pages (from-to)1-17
Number of pages17
JournalJournal of Volcanology and Geothermal Research
Volume193
Issue number1-2
DOIs
Publication statusPublished - Jun 2010

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