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Does melt (dry or hydrous) influence cation diffusion in plagioclase?

Thomas Shea1,2, Michel Pichavant2, Julia Hammer1, Kenneth Koga2, Michael Jollands3, Saskia Erdmann2, Rémi Champallier2

  • Affiliations: 1 Earth Sciences dept., University of Hawai\'i at Manoa, United States 2 ISTO, Université d\'Orleans, France 3 Gemological Insitute of America, New York, United States

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 253

  • Programme No: 1.1.39

  • Theme 1 > Session 1

Abstract

Plagioclase feldspar forms in most magmas. Its chemistry is sensitive to T, P, and melt composition (including H2O), making it an important tool to track magma storage and differentiation conditions. It is also exploited to track timescales of magmatic processes using diffusion chronometry. A recent investigation of cation diffusion in olivine has shown diffusivities to be potentially sensitive to the presence of melt around the mineral. In this study, we test whether (1) the presence of melt or solids around plagioclase and (2) the presence/absence of substantial H2O in the system have any bearing on diffusion rates of Mg, Fe, or Sr. We carried out experiments in gas-mixing and IHPV furnaces at conditions of 0.1--85 MPa, up to ~3.5 wt.% H2O, and 1025-1080°C. Two experimental geometries were employed: plagioclase seeds cut into parallelepipeds surrounded by basaltic-andesite or mineral powders (augite, SiO2, diopside, plagioclase), or plagioclase crucibles filled with powder. Preliminary results show little sensitivity of Mg to the type of diffusant source (powder vs. melt), to crystallographic orientation, or to H2O in the system. Mg diffusivities are consistent with prior studies (Van Orman et al. 2014). Fe diffuses faster than Mg and therefore has the potential to resolve shorter timescales difficult to record with other elements. Importantly, our experiments all resulted in gradients in Ca-Na not accompanied by gradients in Al-Si. These gradients are often longer than gradients in Mg, implying that Ca and Na may be considerably more mobile than previously recognized.