Multicomponent diffusion in K-feldspar: Sr, Ba, Ti and P diffusion experiments in sanidine

Diffusion is a key to understand the timescales of magma dynamics, and thus the evolution of igneous systems. As K-feldspar is a major mineral in felsic plutonic rocks, investigating the diffusion behaviour of trace elements in K-feldspar provides valuable information on magma dynamics. K-feldspar has been used to determine timescales of crystal growth or residence times for volcanic and plutonic systems (e.g., [1], [2]).

To provide better constraints on the duration of magmatic processes (e.g., magma replenishment, magma mixing, etc.), there is a need for accurate diffusion coefficients. Therefore, our study focuses on the results of experiments that aim to constrain diffusivities of Sr, Ba, Ti and P in sanidine (Or₉₈). We performed experiments at 1 atm pressure, between 825 and 1050 °C, for diffusion normal to (010) at controlled aSiO₂. The polished sections of sanidine were surrounded by source powders in Pt capsules and annealed in air, from times ranging from 1 month to 6 months. Sources of diffusant were SrO-, BaO-, TiO₂- and P₂O₅-doped cristobalite or SrO- and BaO-doped cristobalite (made using the sol-gel method) mixed with finely ground sanidine (1:1), or with finely ground sanidine and apatite (1:1:1). The experimental products were then analysed by secondary ion mass spectrometry (SIMS) depth profiling. The absolute depth of the profiles was determined with white light interferometry. Diffusion rates in sanidine determined in this study are different from earlier studies ([3], [4], [5]). Sr diffusion rate is ~1.5–2 orders of magnitude slower than previously determined, while Ba, Ti and P did not show any measurable diffusion profiles, indicating that Ba and Ti likely diffuse more slowly than previously determined. Zoning in Ba in natural K-feldspar megacrysts thus predominantly records crystal growth processes that are weakly affected by diffusion.

 

[1] Chamberlain, Morgan, Wilson (2014) Contrib Mineral Petrol; [2] Rout, Blum-Oeste & Wörner (2021) Journal of Petrology; [3] Cherniak (1996) Geochimica et Cosmochimica Acta; [4] Cherniak (2002) Geochimica et Cosmochimica Acta; [5] Cherniak & Watson (2020) American Mineralogist.