Multicomponent diffusion of F, Cl and OH in apatite with application to magma ascent rates

Abstract

Chemical zoning in igneous minerals is a potential record of the time, processes, and thermal evolution during the lifetime of a given magma reservoir. Abundances of volatiles (OH, Cl and F) in apatite from terrestrial and extraterrestrial plutonic and volcanic rocks have been used to study volatile behavior in magmas, however, volatile diffusivities in apatite are poorly constrained. Here we report new experimental results on Cl, F and OH diffusivities in apatite and apply them to estimate magma ascent times and rates. The experiments were carried out on oriented natural Durango fluorapatite crystals at 800–1100°C, 1-atm, and oxygen fugacity at the wüstite-magnetite buffer. Experimental charges and chemical profiles were investigated with a variety of methods, including scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, secondary ion mass spectrometry, and nuclear reaction analysis.We find that the concentration profiles of Cl show evidence of uphill diffusion that is likely related to the co-existence of three monovalent anions, i.e., OH−, Cl−, F−, at the same site of the apatite structure. Chemical gradients of OH, Cl and F were reproduced using a multicomponent diffusion model to extract the tracer diffusion coefficient (Di⁎) of each component (i). The calculated values of Di⁎ parallel to the c-axis show a general relation of DF⁎>DCl⁎>DOH⁎, and define the following Arrhenius relations (parallel to the c-axis, at 1 bar) as:DCl⁎=7(+12−4)×10−5×[exp(−294(±12) kJ⋅mol−1RT)] m2⋅s−1DF⁎=5(+49−5)×10−4×[exp(−308(±76) kJ⋅mol−1RT)] m2⋅s−1DOH⁎=4(+11−3)×10−2×[exp(−401(±39) kJ⋅mol−1RT)] m2⋅s−1The activation energy for Cl diffusion that we determined (294 kJ⋅mol−1) is within the range of that reported by Brenan (1994), but the pre-exponential factor is smaller and thus we obtain in general slower diffusivities than Brenan (1994). DCl⁎ and DOH⁎ parallel to the a-axis are 1 to 2 orders of magnitude slower than those parallel to the c-axis, indicating anisotropic diffusion of Cl and OH. Preliminary results on S diffusivity (parallel to the c-axis) at 800–900°C show values between those of Cl and OH. The diffusion coefficients and model proposed in this study can be used to estimate the timescales of volatile re-equilibration in apatite in a variety of contexts from plutonic rocks and layered intrusions, to volcanic rocks and meteorites. We show that, for example, magma ascent rates can be determined by modelling Cl zoning in volcanic apatite. These applications provide new opportunities for understanding the influence of magma ascent rates on the eruption styles of volcanoes, thus having potential contributions to improving volcano forecasting and hazard assessments.