Magma Heterogeneity as Evidenced by 87Sr/86Sr Disequilibrium in Plagioclase from Samoan Lavas

Abstract:

While it is widely accepted that the mantle is chemically and isotopically heterogeneous on a global scale, the extent to which these large-scale heterogeneities are distributed by convection and mixing remains unclear. Radiogenic isotopic data from whole rock lavas has shown heterogeneity even within the magma sampled by a single volcanic hand sample. Developments in microanalysis techniques have allowed the augmentation of whole rock data with crystalline and sub-crystalline analyses, with the goal of identifying geochemically and isotopically distinct melts contributing to a single magma plumbing system.

Radiogenic isotopic disequilibrium is caused by mixing of isotopically diverse magmas, and is evidenced by isotopic variation in phenocrysts and melt inclusions from a single young, fresh lava. Sr-isotopic values measured by LA-MC-ICPMS in clinopyroxenes (0.720830 to 0.720232) from a highly enriched Samoan hotspot lava differ significantly from the corresponding value of the surrounding ground mass (0.718592). Furthermore, the clinopyroxenes can be divided into two isotopically distinct populations, negating the argument that shallow assimilation (e.g. seawater) could be responsible for the isotopic disequilibrium. Many of the Samoan lavas exhibiting cpx-whole rock Sr-isotopic disequilibrium also exhibit Sr-isotopic disequilibrium between individual olivine-hosted melt inclusions.

We take advantage of the observation that, on a sub-grain scale, a single zoned plagioclase crystal can record multiple magma mixing events, with each zone reflecting the isotopic characteristics of the magma in which it formed. We report Sr-isotopic disequilibrium between zones of plagioclase crystals in the same Samoan lava that hosts extreme cpx-whole rock Sr-isotopic disequilibrium: the Sr-isotopic ratios extend to values higher than observed in the plagioclase. This implies a turbulent growth history involving magmas of differing isotopic compositions that mixed prior to eruption.