Investigating the origin of anorthitic plagioclase through a combination of experiments and natural observations

The origin of anorthitic plagioclase in volcanic rocks worldwide remains an enigma. Arenal volcano, Costa Rica, provides a good example of the problem: up to An₉₆ plagioclase are found within moderately evolved basaltic andesites. We follow up a recent suggestion [Lundstrom, C.C., Boudreau, A., Pertermann, M., 2005. Diffusion-reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering. Earth Planet. Sci. Lett. 237, 829-854] that diffusion-reaction processes occurring between ascending magmas and gabbroic wall rocks might provide a means of producing anorthitic plagioclase without requiring exotic, ultra-calcic melt compositions by integrating observations from three analytical methods: (1) laboratory diffusion-reaction experiments in the diopside-albite-anorthite (DAA) system; (2) transmission electron microscopy (TEM) of plagioclase grains from both a diffusion-reaction experiment and an Arenal lava; and (3) Sr isotope microstratigraphy of phenocrysts and megacrysts from natural lavas from the current eruption at Arenal. Two plagioclase saturated DAA melts, juxtaposed in experiments at either 1 bar pressure (anhydrous) or at 0.5 GPa pressure (hydrous) for 3-5 days at 1244 or 1125 °C, illustrate the dramatic effect of H₂O on facilitating plagioclase reaction. In the presence of wet melt, enrichment in anorthitic plagioclase occurs at the material interface while in the presence of the dry 1 bar melt, plagioclase shows no reaction. TEM analyses of a homogeneous anorthitic plagioclase from a diffusion-reaction experiment indicates heterogeneous columns of more anorthite-rich plagioclase at the < 100 nm scale while a homogeneous anorthitic plagioclase phenocryst from a 1968 lava from Arenal found much of the crystal to be composed of 20-50 μm diameter subgrains, having ∼ 1 μm diameter tubes filled with Fe, K, Si-rich glass. Large core-rim variations in ⁸⁷Sr/⁸⁶Sr occur within three plagioclase phenocrysts from both 1968 and 1999 Arenal lavas. Although core compositions vary widely (0.7038-0.7052), all three phenocrysts have similar ⁸⁷Sr/⁸⁶Sr rim compositions (0.7043). The variability in ⁸⁷Sr/⁸⁶Sr at the phenocryst scale but homogeneity in ⁸⁷Sr/⁸⁶Sr at the bulk scale in Arenal lavas [Ryder, C.H., Gill, J.B., Tepley III, F.J., Ramos, F., Reagan, M., this volume. Closed to open differentiation at Arenal volcano (1968-2003). J. Volcanol. Geotherm. Res.] could be explained by erupted lavas representing a near steady state balance between ascending magmas and a flux derived from diffusion-reaction processes occurring with surrounding crustal gabbros.