ALBERT

Notes: Abstract Isotopic studies of rocks from oceanic island arcs such as the Marianas indicate that little, if any, recycling of continental material (e.g. oceanic sediments) occurs in these arcs. Because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs, an important question is whether the andesites of continental arcs are produced by a fundamentally different (more complex?) mechanism than the lavas of oceanic arcs. An excellent opportunity for study of this question is provided by the island of Sarigan, in the Mariana active arc, on which calc-alkaline andesites (including hornblende-bearing types) are exposed along with more mafic lavas. Available isotope data suggest the Sarigan lavas (including the andesites) were derived from mantle material with little or no involvement of continental components. Ratios of incompatible elements suggest that most of the Sarigan lavas were derived from similar source materials. Absolute abundances of incompatible elements vary irregularly within the eruptive sequence and indicate at least 5 distinct magma batches are represented on Sarigan. Major element data obtained on the lavas and mineral phases in them, combined with modal mineral abundances, suggest that the calc-alkaline nature of the volcanic rocks on Sarigan results from the fractional crystallization of titanomagnetite in combination with other anhydrous phases. Amphibole, although present in some samples, is mainly a late-crystallizing phase and did not produce the calc-alkline characteristics of these lavas. Gabbroic samples found in the volcanic sequence have mineralogc and geochemical characteristics that would be expected of residual solids produced during fractional crystallization of the Sarigan lavas. When combined, data on the lavas and the gabbros suggest the following crystallization sequence: olivine — plagioclase — clinopyroxene — titanomagnetite — orthopyroxene±hornblende, biotite and accessory phases. These results lead to the conclusion that calc-alkaline magmas can be generated directly from mantle sources.

Notes: Abstract Volcanic rocks exposed on Guam were erupted during the Late Middle Eocene (Facpi Fm.), Late Eocene-Oligocene (Alutom Fm.) and Miocene (Umatac Fm.). Four magma series are recognized: the boninite series (44 m.y.b.p.), the tholeiite and calc-alkaline series, which were erupted along with boninite series lavas at 32–36 m.y.b.p. and high-K lavas of the Umatac Fm. (14 m.y.b.p.). Isotope and and rare earth element (REE) characteristics of the four magma series are distinct. Boninite series lavas have U-shaped REE patterns, relatively low 143Nd/144Nd (0.51294–0.51298), and high 206Pb/204Pb (19.0–19.2). Tholeiite series lavas are LREE (light REE) depleted, and have high 143Nd/144Nd (0.51304–0.51306) and low 206Pb/204Pb (18.4–18.5). Calc-alkaline series lavas have Sr, Nd and Pb isotope ratios similar to tholeiite series lavas, but flat to U-shaped REE patterns. Umatac Fm. lavas are strongly LREE-enriched, and have higher 87Sr/ 86Sr (0.70375–0.70380) and 207Pb/204Pb relative to 206Pb/ 204Pb than Facpi and Alutom Fm. lavas. Boninite and tholeiite series magmas, erupted in the position of the Palau-Kyushu Ridge, were probably derived from distinct mantle sources having OIB and N-MORB-like isotopic characteristics, together with fluids derived from subducted Pacific plate basalt. Calc-alkaline series lavas were most likely derived from the tholeiite series by extensive crystal fractionation, wallrock contamination and magma mixing. Lavas of the Umatac Fm., erupted in the position of the West Mariana Ridge, may include up to 2–3% subducted sediment, similar to some active Mariana arc lavas.

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.