Notes: Twenty five elements in twenty six geochemical reference standards analyzed by X-ray fluorescence spectrometry yield major and trace element concentrations close to consensus values. Ninety percent of our analyses agree with consensus values for standards to within pm 0.5 to 1.5 relative percent for major elements and pm 3 to 5 relative percent or pm 1-3 ppm, whichever is higher, for trace elements. Consistent divergences from consensus values of trace elements are noted.

Notes: Abstract Arenal volcano in Costa Rica has been erupting nearly continuously, but at a diminishing rate, since 1968, producing approximately 0.35 km3 of lavas and tephras that have shown consistent variations in chemistry and mineralogy. From the beginning of the eruption in July 1968 to early 1970 (stage 1, vol.=0.12 km3) tephras and lavas became richer in Ca, Mg, Ni, Cr, Fe, Ti, V, and Sc and poorer in Al2O3 and SiO2. Concentrations of incompatible trace elements (including Sr) decreased by 5%–20%. Phenocryst contents increased 20–50 vol%. During stage 2 (1970–1973, vol. = 0.13 km3) concentrations of compatible trace elements rose, and concentrations of incompatible trace elements either remained constant or also rose. Al2O3 contents decreased by 1 wt%. Phenocryst content increased slightly, principally due to increased orthopyroxene. During stage 3 (mid-1974 to the present, vol.= 0.10 km3) concentrations of SiO2 increased by 1 wt%, compatible trace elements decreased slightly, and incompatible trace element concentrations increased by 5% to 10%. Although crystals increased in size during stage 3, their overall abundance stayed roughly constant. Our modeling suggests that early stage-1 magmas were produced by boundary layer fractionation under high-p H2O conditions of an unseen basaltic andesitic magma that intruded into the Arenal system after approximately 500 B.P. Changes in composition during stage 2 resulted from mixing of this more mafic original magma with new magma that had a similar SiO2 content, but higher compatible and incompatible element concentrations. The changes during stage 3 resulted from continued influx of the same magma plus crystal removal. We conclude that the eruption proceeded in the following way. Before 1968 zoned stage-1 magma resided in the deep crust below Arenal. A new magma intruded into this chamber in July 1968 causing ejection of the stage-1 magmas. The intruding magma mixed with mafic portions of the original chamber producing the mixed lavas of stage 2. Continued mixing plus crystal fractionation along the chamber and conduit walls produced stage-3 lavas. The time scales of crustal level magmatic processes at Arenal range 100–103 years, which are 3–6 orders of magnitude shorter than those of larger, more silicic systems.