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.