Brandon L. Browne

The nature and timing of caldera collapse as indicated by accidental lithic fragments from the AD ∼1000 eruption of Volcán Ceboruco, Mexico

One way to determine the mechanics of caldera formation is through quantitative component analysis of accidental lithic fragments in pyroclastic deposits erupted before and during caldera collapse. All previous studies of this sort, however, are based on pyroclastic deposits from large-volume (>10 km3) caldera eruptions. In this study, we use quantitative component analysis of lithic fragments to determine the mechanics of the AD ∼1000 small-volume (3–4 km3 DRE) caldera eruption of Volcan Ceboruco, located in western Mexico. During this eruption, caldera collapse occurred in such a way that lithic fragments of decreasing depths were preferentially erupted with time. Prior to caldera collapse, deep-origin lithics (∼6 km depth) and vent-derived lithics were erupted. Deposits emplaced during collapse, however, contain a distinctly different population of lithic fragments, such as lithics from mid-depth origin (∼1 km depth) and from the vent. By the end of the eruption, the total amount of lithic material in the deposits had increased from <15 wt% before collapse to as much as 90 wt%, yet this material contains essentially no lithics of deep origin. This suggests that collapse resulted in the obstruction of both deep-origin lithics and magma from reaching the surface. Based on these data, we suggest that collapse occurred as a result of the fracturing and subsequent failure of the overlying magma reservoir roof that occurred along inward dipping faults. The collapse forced the fragmentation depth and progressive erosion of lithic material from deep to shallow depths, which subsequently sealed the conduit, stopping the eruption.

Pre-eruptive storage conditions of the Holocene dacite erupted from Kizimen Volcano, Kamchatka

This study describes an investigation of the pre-eruptive conditions (T, P and fO2) of dacite magma erupted during the KZI cycle (12,000–8400 years ago) of Kizimen Volcano, Kamchatka, the earliest, most voluminous and most explosive eruption cycle in the Kizimen record. Hydrothermal, water-saturated experiments on KZI dacite pumice coupled with titanomagnetite-ilmenite geothermometry calculations require that the KZI dacite existed at a temperature of 823 ± 20°C and pressures of 125–150 MPa immediately prior to eruption. This estimate corresponds to a lithologic contact between Miocene volcaniclastic rocks and Pliocene-Pleistocene volcanic rocks located at a depth of 5–6 km beneath the Kizimen edifice, which may have facilitated the accumulation of atypically large volumes of gas-rich dacite during the KZI cycle.

Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California

Red Cones are a pair of basaltic cinder cones located 5 km SSW of Mammoth Mountain at the southern end of the MonoInyo volcanic chain, in eastern California. Charcoal recovered at two separate locations beneath the Red Cones scoria-fall deposits indicates that the eruption most likely occurred shortly after 8490 ± 90 14 C yr B.P. and no later than 9325 ± 83 14 C yr B.P., which implicates Red Cones as the most recent eruption of basalt in the Mono-Inyo volcanic chain. Results from geologic fi eld mapping combined with geochemical and petrologic analysis suggest that the ca. 8500 yr B.P. eruption produced 10.1 ◊ 10 6 m 3 of magma, possibly beginning from south Red Cone and later from north Red Cone via Hawaiian, Strombolian, and violent Strombolian eruptions over a minimum of 28 days. All deposits contain plagioclase, olivine, clinopyroxene, chrome-spinel, and titanomagnetite. Material erupted from each cone can be classifi ed as high-aluminum basalts that exhibit calc-alkaline differentiation trends and belong to the medium-K series. Red Cone basalt samples are generally similar in terms of many major and trace element concentrations, but south Red Cone samples typically contain more SiO 2 , Sr, Zr, Rb, and Ba, and less MgO, FeO, CaO, Ni, and Cr than north Red Cone samples. Clinopyroxene-liquid thermo barometry calculations indicate that the majority of Red Cones clinopyroxene crystal cores crystallized at temperatures of 1160‐1210 °C and pressures equivalent to 10‐25 km depth, which supports the possibility of a basaltic dike and sill plexus located 10‐25 km beneath the west and southwest fl anks of Mammoth Mountain.

Quenched mafic inclusions in ≤2200 years B.P. deposits at Augustine Volcano, Alaska

This article describes results from morphological, textural, mineralogical, and compositional analyses of lava clasts and quenched mafic inclusions from 10 Holocene debris-avalanche deposits (2200–125 years B.P.) that form the lower flanks of Augustine Volcano, Alaska. Mafic inclusions collected from the Rocky Point pyroclastic-flow deposit emplaced during the 2006 eruption are included for comparison. All deposits contain evidence for mixing between basalt and high-silica andesite prior to and during eruption, including compositional banding, disequilibria phenocryst assemblages, linear variation in major- and trace-element concentrations, and quenched mafic inclusions (51.3–57.3 wt.% SiO2) hosted by andesite lavas (59.1–62.6 wt.% SiO2). Generally, inclusions from all deposits share many morphologic and petrologic characteristics. Inclusions range in diameter from <1 to >36 cm, although 87% of the 959 inclusions analysed are less than 5 cm in diameter. Mafic inclusions account for an average of 8 vol.% of host andesitic lava clasts included in the oldest debris-avalanche deposits compared with subsequently emplaced deposits that contain an average of only 1–3 vol.%. Inclusions contain phenocrysts of plagioclase, amphibole, clinopyroxene, olivine, and rare orthopyroxene as well as microphenocrysts of plagioclase, amphibole, clinopyroxene, olivine, magnetite, ilmenite, and apatite in a glassy, vesicular, and acicular groundmass. Plagioclase phenocrysts in inclusions typically have a 30–150 μm-thick, fine-grained ‘dusty sieved’ rim superimposed over oscillatory zoned texture, suggesting that they existed in the host andesite magma prior to basaltic intrusion, but were engulfed during the intrusion and inclusion formation process. Mafic inclusions are calc-alkaline, low-K (0.45–0.82 wt.% K2O) basalts to basaltic andesites that are altogether different in terms of mineralogy and composition from olivine basalt contained in late Pleistocene-age fragmental deposits described by Plank et al. (2006, The Augustine basalt: Eos (American Geophysical Union Transactions), v. 87, Abstract V42B-06) and Larsen et al. (2010, Petrology and geochemistry of the 2006 eruption of Augustine Volcano, in Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 eruption of Augustine Volcano, Alaska: US Geological Survey Professional Paper 1769). Thus, the most reliable approximation of the basaltic endmember composition involved in magma mixing processes at Augustine Volcano over the past ∼2200 years are the most mafic inclusions, which consistently possess the (1) highest abundance of vesicles, (2) fewest number of phenocryst-sized plagioclase, and (3) largest volume of microlites compared with more contaminated inclusions.

Rates of Magma Ascent and Storage

Abstract The manner in which a volcano erupts is largely controlled by the rate of magma ascent and the geometry of the volcanic conduit connecting the volcanic vent at the surface to the magma reservoir region in the shallow crust. Factors influencing the rate in which magma ascends to the surface during eruptions include both physical and chemical properties of the magma, such as its temperature, composition, volatile budget, and crystallinity, all of which affect its viscosity and density, as well as the permeability of the conduit walls. This chapter offers a summary of different techniques used to examine and constrain the rates of magmatic processes that occur in shallow crustal reservoirs as well as in conduits while magma ascends to the surface during eruption. Whereas compositional zoning patterns in phenocrysts and diffusion modeling indicate that relatively short-lived magmatic processes like magma mixing occur over timescales of 10 0 –10 2 days, processes like magma assimilation and crystal-melt fractionation persist for 10 3 –10 6 years based on studies that utilized radioactive isotopes from whole-rock or mineral separates. Thus, such timescales must also be required for the accumulation and storage of magma in the shallow crust, particularly in the case of silicic magmas emplaced as a result of voluminous caldera-forming eruptions. Techniques aimed at understanding syneruptive magma ascent involve magma extrusion rate studies as well as experimentally replicating a variety of decompression-induced mineral-melt reactions, such as bubbles, microlites, and reaction rims. Results from these studies suggest a general relationship between magma ascent rates in excess of ∼0.2 m/s and explosive eruptions. Moreover, magma ascent rates appear to be relatively consistent over a wide range of magma compositions.

Major and trace element zoning in plagioclase from Kizimen Volcano (Kamchatka): Insights into magma-chamber processes

The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare-earth elements of acidic lavas and basalt inclusions intersect; (4) Sr-Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and thermal interaction of melts or exclusively thermal ones. Study of the major- and trace-element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber.