Analysis of 30 individual pumice blocks, together with bulk samples from the ash-flow member of the Los Chocoyos Ash within the Quezaltenango Valley, Guatemala, demonstrates that prior to its eruption, its associated magma-chamber was zoned. Eruption of a high-K (K 2 O/Na 2 O > 1), crystal-poor, biotite-bearing rhyolite with crystal equilibration temperatures of less than 800 °C produced the widespread H-tephra member and the initial phases of the ash-flow member. As the ash-flow eruption continued, a more-heterogeneous, low-K, crystal-rich, cummingtonite- and hornblende-bearing rhyolite became predominant; its phenocrysts had equilibrated at temperatures of about 950 °C. The water content of the high-K rhyolite was several percent, whereas the low-K rhyolite was much drier. Bulk samples of the ash-flow member are homogenized mixtures of matrix shards that represent either the high-K or low-K rhyolite magmas; the overall ratio for the ash-flow member is 60% high-K and 40% low-K type. The 87 Sr/ 86 Sr ratios for both high-K and low-K magma types are identical and average 0.70405 ± 0.00003. This value is nearly the same as all basaltic, all andesitic, and most rhyolitic Quaternary volcanic rocks tested in Guatemala so far. The 87 Sr/ 86 Sr ratios for bulk samples of the ash are significantly higher and more variable (0.70426 ± 0.00009), probably because of xenocrystic contamination. Detailed mixing and Rayleigh calculations using observed mineral phases in the ash show that the concentrations of 8 major and 17 minor elements in the ash are consistent with the derivation of high-K rhyolite from low-K magma by crystal fractionation at shallow depths. The time required for such fractionation is at least 10 4 yr. The absence of a continuum of compositions from low-K to high-K rhyolite and the differences in p H 2 O and temperature suggest that the two magmas were separated during fractionation. The Los Chocoyos Ash is the most silicic major Quaternary unit in the Guatemalan Highlands; the volume of magma from which it was derived is far greater than that of all other Quaternary volcanic rock units in the area.

Persistent lava extrusion at the Santiaguito dome complex (Guatemala) results in continuous lahar activity and river bed aggradation downstream of the volcano. We present a simple method that uses vegetation indices extracted from Landsat Thematic Mapper (TM) data to map impacted zones. Application of this technique to a time series of 21 TM images acquired between 1987 and 2000 allow us to map, measure, and track temporal and spatial variations in the area of lahar impact and river aggradation. In the proximal zone of the fluvial system, these data show a positive correlation between extrusion rate at Santiaguito (E), aggradation area 12 months later (A prox ), and rainfall during the intervening 12 months (Rain12): A prox = 3.92 + 0.50 E + 0.31 ln(Rain12) (r 2 = 0.79). This describes a situation in which an increase in sediment supply (extrusion rate) and/or a means to mobilize this sediment (rainfall) results in an increase in lahar activity (aggraded area). Across the medial zone, we find a positive correlation between extrusion rate and/or area of proximal aggradation and medial aggradation area ( A med ): A med = 18.84 - 0.05 A prox - 6.15 Rain12 ( r 2 = 0.85). Here the correlation between rainfall and aggradation area is negative. This describes a situation in which increased sediment supply results in an increase in lahar activity but, because it is the zone of transport, an increase in rainfall serves to increase the transport efficiency of rivers flowing through this zone. Thus, increased rainfall flushes the medial zone of sediment. These quantitative data allow us to empirically define the links between sediment supply and mobilization in this fluvial system and to derive predictive relationships that use rainfall and extrusion rates to estimate aggradation area 12 months hence.

Springs encircling Santa María volcano in Guatemala generally contain bicarbonate waters. Bicarbonate waters southwest of the persistently active Santiaguito lava dome are characterized by high Mg/Ca. Other springs contain acid sulfate or chloride waters. Most acid sulfate and chloride waters are spatially confined to springs, wells, and streams of the Zunil and Zunil-II (Sulfur Mountain) geothermal fields on the flanks of the Cerro Quemado dome complex, 5 km to the east-northeast of Santa María. Some acid sulfate waters have unusually high S/Cl ratios (20–70). Chloride waters are dilute versions of typical geothermal brines. The δ 13 C ratios of all waters fall in a very narrow range (−11.5‰ to −8.5‰). The nonreactive gas compositions from fumaroles encircling Santa María are typical of those sampled at other subduction zone volcanoes. Most fumarolic gases from Santa María have notably lighter δ 13 C ratios compared to gases sampled from elsewhere on the Central American volcanic front. The He and C isotopic values of fumarolic gas samples from the Santa María region indicate significant mantle input. Estimated magmatic δ 13 C ratios for Zunil and Zunil-II, however, are lighter than accepted mantle values (−11‰ to −14‰). This is most likely caused by shallow crustal contamination. All of the spring waters from the Santa María region represent variable interactions between magmatic/hydrothermal fluids and meteoric waters. There is, however, only limited mixing between bicarbonate, acid sulfate, and chloride waters. Surface discharges of chloride waters are inhibited by high precipitation rates. The high S/Cl ratios of some of the acid sulfate waters from Zunil/Zunil-II reflect extensive scrubbing by the underlying hydrothermal system. High Mg/Ca bicarbonate waters from springs south-southwest of the Santiaguito dome complex have experienced enhanced water-rock interaction, and their slightly heavier δ 13 C ratios (−9.5‰ to −8‰) hint at a small distinction between magmatic δ 13 C at Zunil/Zunil-II and Santa María. This supports previous suggestions that the hydrothermal system beneath the Zunil area is independent of Santa María. Gas samples from Zunil/Zunil-II and Cerro Quemado, on the other hand, do share similar δ 13 C ratios, strengthening the notion that magmatism at the latter is propelling the hydrothermal system northeast of Santa María. Hence, monitoring of the springs and fumaroles at Zunil/Zunil-II could prove useful in forecasting of future activity at Cerro Quemado.