Geochemical zoning, mingling, eruptive dynamics and depositional processes; the Campanian Ignimbrite, Campi Flegrei caldera, Italy

In June 1912, the world9s largest twentieth century eruption broke out through flat-lying sedimentary rocks of Jurassic age near the base of Trident volcano on the Alaska Peninsula. The 60 h ash-flow and Plinian eruptive sequence excavated and subsequently backfilled with ejecta a flaring funnel-shaped vent since called Novarupta. The vent is adjacent to a cluster of late Quaternary stratocones and domes that have released about 140 km 3 of magma in the past 150 k.y. Although the 1912 vent is closest to the Trident group and is also close to Mageik and Griggs volcanoes, it was the summit of Mount Katmai, 10 km east of Novarupta, that collapsed during the eruption to form a 5.5 km 3 caldera. Many earthquakes, including 14 in the range M 6−7, took place during and after the eruption, releasing 250 times more seismic energy than the 1991 caldera-forming eruption of the Philippine volcano, Pinatubo. The contrast in seismic behavior may reflect the absence of older caldera faults at Mount Katmai, lack of upward (subsidence opposing) magma flow owing to lateral magma withdrawal in 1912, and the horizontally stratified structure of the thick shale-rich Mesozoic basement. The Katmai caldera compensates for only 40% of the 13 km 3 of 1912 magma erupted, which included 7–8 km 3 of slightly zoned high-silica rhyolite and 4.5 km 3 of crystal-rich dacite that grades continuously into 1 km 3 of crystal-rich andesite. We have now mapped, sampled, and studied the products of all 20 components of the Katmai volcanic cluster. Pyroxene dacite and silicic andesite predominate at all of them, and olivine andesite is also common at Griggs, Katmai, and Trident volcanoes, but basalt and rhyodacite have erupted only at Mount Katmai. Rhyolite erupted only in 1912 and is otherwise absent among Quaternary products of the cluster. Pleistocene products of Mageik and Trident and all products of Griggs are compositionally distinguishable from those of 1912 at Novarupta. Holocene products of Mount Martin and Trident are closer in composition to the andesite-dacite array of 1912, but they reveal consistent differences. The affinity of the 1912 suite is closest with the array of products erupted by the Southwest Katmai cone, the edifice that had produced the only pre-1912 rhyodacite as well as the largest prehistoric Plinian eruption in the cluster. It is doubtful that any 1912 magma had been stored beneath Novarupta or Trident, and there is no evidence that more than one magma chamber erupted in 1912. Despite a compositional gap separating the aphyric rhyolite from the very crystal-rich andesite-dacite continuum, isotopic and chemical affinities linking all the 1912 ejecta and the continuity of all those ejecta in magmatic temperature and oxygen fugacity suggest that the rhyolite originated principally by incremental upward expulsion of interstitial melt from subjacent andesite-dacite mush. A large reservoir of such hot crystal mush is required both as the residue of rhyolitic melt separation and as a proximate heat source to thermally sustain the nearly aphyric condition of the overlying rhyolite. A model is presented for a unitary zoned chamber beneath Mount Katmai.

Katmai volcanic cluster and the great eruption of 1912

Reconstructing faunal migrations using intra-tooth sampling and strontium and oxygen isotope analyses: a case study of modern caribou (Rangifer tarandus granti)

http://core.ecu.edu/geology/heimanna/Heimann%20et%20al_2008_GCA_Fe%20isotopes%20in%20siliceous%20IR.pdf

The role of volatile exsolution and sub-solidus fluid/rock interactions in producing high 56Fe/54Fe ratios in siliceous igneous rocks

The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial 87Sr/86Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest 87Sr/86Sr, associated glass has higher 87Sr/86Sr, and biotite phenocrysts have the highest 87Sr/86Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe−Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing 87Sr/86Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing 87Sr/86Sr is caused primarily by radioactive decay of 87Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb−Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars ( 3x105 years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10-3km3/y. The growth rate of alkali feldspar is estimated to be about 10-14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (>50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.

/pdf/time-scales-of-large-volume-silicic-magma-systems-sr-191d2pescv.pdf

Time scales of large volume silicic magma systems: Sr isotopic systematics of phenocrysts and glass from the Bishop Tuff, Long Valley, California

Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO2, fiamme group 7) had δ18O= +8.5, initial 87Sr/86Sr=0.7099, eNd, and 206Pb/204Pb=17.80, suggesting that the magma was produced by ∼50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO2, fiamme group 1) depart from the mafic “base-level” composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. δ18O values decrease by as much as 1.5‰ from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. δ18O values abruptly inerease by up to 1.5‰ between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial 87Sr/86Sr ratios in phenocrysts are as high as 0.7170. e values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. 206Pb/204Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.

Non-monotonic chemical and O, Sr, Nd, and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff, Colorado

Christopher J. Fridrich

Papers

Non-monotonic chemical and O, Sr, Nd, and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff, Colorado