The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations

The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydritebearing pumices

The calculation of volumes of fallout tephra layers is difficult because of the nonlinear dependence of thickness on area and because of the extrapolations required at the vent and in distal regions. Calculation using the trapezoidal rule, straight lines on log-log plots of area versus thickness, straight lines on plots of log thickness versus area1/2, and the crystal-concentration method are reviewed and the problems with each method discussed. The method using straight lines on plots of log thickness versus area1/2 is the most geologically reasonable because most deposits thin exponentially from source and therefore plot as straight lines using these coordinates. Errors and uncertainties in previous derivations for using this method are discussed and more general formulas presented. The method is also used to gain perspective on the “missing” distal volumes calculated by the crystal-concentration method compared to those calculated based only on isopach data.

Another look at the calculation of fallout tephra volumes

Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr  0 70406---0 70719; Nd/Nd  0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

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Relationships between mafic and peralkaline silicic magmatism in continental rift settings: A petrological, geochemical and isotopic study of the Gedemsa volcano, Central Ethiopian rift

An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature

The Los Chocoyos Ash, guatemala: A major stratigraphic marker in middle America and in three ocean basins

Trace-element sanidine/glass distribution coefficients for peralkaline silicic rocks and their implications to peralkaline petrogenesis

Three composite cones have grown on the southern edge of the previously existing Atitlan Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlan, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Toliman are mantled by more silicic lava flows. On Toliman, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlan’s rocks and late lavas from Toliman have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Silicic Plinian tephra units representing more than 30 Quaternary eruptions blanket Guatemala and El Salvador. They were erupted mainly from 5 principal sources, all of them calderas. Several of the eruptions were accompanied by ash flows. These eruptions also have the most extensive tephra deposits. The total volume of material erupted is equivalent to 300–500 km3 of dense rock. A major uncertainty is the volume of tephra scattered very far from the source. The volume of silicic magma erupted in the Quaternary is similar to the volumes of mafic lava produced at the volcanic front. The basaltic and andesitic cones of the volcanic front parallel the offshore Middle America trench and the active underthrust zone. The five caldera sources form a trend parallel to the volcanic front, on the side opposite the trench, where the older continental crust abuts the volcanic zone. The ages of silicic volcanism precede and overlap with the age of mafic volcanic front, which is largely younger than 50,000 years. All of the calderas have multiple eruptions which span at least many tens of thousands of years. Between the calderas the interfingering of ashes has allowed a network of relative ages to be established. We used a variety of techniques to characterize these units. They can be readily distinguished from units from many other provinces, but considerable effort is required to distiguish among the local units. Standard field and petrographic observations (stratigraphic data, thicknesses, grain size, lithic content, mineralogy) establish the critical framework which disallows most erroneous correlations. Geochemical analysis, particularly trace elements, provide a rapid means of ruling out many more possible corrections. Qualitative mineralogical analysis by electron microprobe of hornblende and Fe-Ti oxides was a very effective last resort for correlation.

Quaternary Tephra of Northern Central America

Intermediate lavas from the middle Miocene McDermitt caldera complex, Nevada-Oregon, have very high FeO* (= total Fe as FeO; 9.1 to 10.2 wt %) and low MgO (0.4 to 2.1 wt %) contents at SiO 2 contents of 59.5 to 62 wt %. With the exception of significantly higher K 2 O contents (3.1 to 4.7 wt %), these lavas are compositionally very similar to icelandite (“tholeiitic andesite”) from oceanic areas. In addition to the voluminous comendite tuffs of the complex, a thin unit of densely welded crystal-rich pantellerite tuff of probable air-fall origin is exposed on the northern wall of the McDermitt caldera. The pantellerite tuff contains 1.5% aenigmatite phenocrysts. The high FeO*/MgO ratios of the icelandites and the peralkaline nature of the silicic rocks suggest differentiation under conditions of low f O 2 and f H 2 O. The existence at McDermitt of a continuous series of rocks ranging from potassic icelandite to comendite and pantellerite is consistent—as are other geologic, geochemical, and geophysical data—with the derivation of the icelandites and ultimately of the voluminous peralkaline silicic rocks of the complex from mafic magma generated from upwelling mantle material. Labile U released during the crystallization of late subvolcanic bodies of peralkaline silicic magma may have provided an appreciable proportion of the epigenetic U now present in the upper part of the caldera complex. Reducing conditions produced by the very Fe-rich glassy icelandites may have contributed to the fixation of U at the Aurora and Bretz prospects.

John W. Drexler

Papers

The Los Chocoyos Ash, guatemala: A major stratigraphic marker in middle America and in three ocean basins

Trace-element sanidine/glass distribution coefficients for peralkaline silicic rocks and their implications to peralkaline petrogenesis

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Quaternary Tephra of Northern Central America

Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon