Abstract The concentrations and isotopic distribution of uranium and thorium in four fresh, young (< 100,000 yr) volcanic rocks and their mineral separates from Guatemala and Sumatra suggest that uranium and possibly thorium are susceptible to mobilization from phenocrysts. Phases low in uranium show evidence of significant uranium loss, possibly related to siting of the uranium in defects. Because major loss of uranium from individual low-uranium phases represents only a minor loss of uranium from the whole-rock system, whole rocks, despite alteration, exhibit apparently undisturbed U abundances and isotopic ratios. Limited data indicate that 230 Th- 238 U radioactive disequilibrium-series dating is not a reliable geochronometer in such rocks, but the same data can be applied to estimate relative U mobility in phenocrysts of young volcanic rocks.

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.

Upper Pleistocene rhyolitic ash-flow and air-fall tuffs, erupted from several centers, were sampled in 23 pumice-filled basins over an area of 16,000 km 2 . Fifty pumice-matrix samples were analyzed for as many as 20 trace elements. Ba, Fe, Hf, Rb, Sm, Sr, Th, Ti, and Zr were particularly useful in “fingerprinting” correlations between basins and in corroborating the stratigraphy previously established within individual basins. On the basis of similar trace elements, a tephra and an overlying ash-flow sheet (together, a unit here named the Los Chocoyos Ash) appear to have formed from a multiphase eruption. The tephra, whose volume exceeded 100 km 3 , blanketed an area greater than 1 × 10 6 km 2 . The second phase of the eruption produced an ash flow of greater than 200 km 3 . Areal geochemical patterns within the ash-flow sheet are probably related to sequentially less explosive eruptions of progressively more mafic ash flows. Changes in chemical composition, size of pumice and lithic fragments, thickness, and elevation all suggest a source for the Los Chocoyos Ash in the Lake Atitlán cauldron. Chemical data suggest correlation of the H-tephra member of the Los Chocoyos Ash with the most prominent D layer of the Worzel ash of the equatorial Pacific.