Ash-flow Sheets Research Articles

The Gold Flat Tuff, Nevada: Insights into the evolution of peralkaline silicic magmas

The Gold Flat Tuff is the youngest (9.15 Ma) ash-flow sheet erupted from the Black Mountain Volcanic Centre, southwest Nevada, USA. This paper explores some aspects of the very complex nature of the tuff's magmatic plumbing system. The main body of the deposit is a mixed magma product, comprising pantelleritic and comenditic melts derived from independently evolving reservoirs, and antecrysts and enclaves derived from a range of basic to intermediate sources. Metre-scale cognate xenoliths point to the presence of alkali feldspar accumulation zones. The pantellerite contains phenocrysts of fluorite and chevkinite-(Ce). The inferred intermediate magma component contains perrierite-(Ce) phenocrysts. The pantellerite has unusually high contents of F (≤2.2 wt%), F + Cl (≤2.9 wt%) and ZrO2 (≤1.04 wt%). The high halogen contents may have influenced the evolution of the strongly peralkaline magma. The crystallization conditions are poorly constrained but those for the pantelleritic magma may have been close to water-saturation (>4 wt% melt water) at temperatures ~740 °C and fO2 around FMQ.

Fumarolic pipes in the Tshirege Member of the Bandelier Tuff on the Pajarito Plateau, Jemez Mountains, New Mexico

The objective of this study is to demonstrate the relationships among devitrification, vapor phase alteration, localization of gas emanations into fumarolic pipes, and initial deformation of the ash flow sheet during cooling and lithification. Utilizing a unique and temporary exposure of the Tshirege Member of the Bandelier Tuff near Los Alamos, New Mexico, we identify several zones of distinctly preserved fossil fumarolic activity. The fumarolic zones vary in width from a few centimeters to more than a meter. Almost ubiquitously, these zones demonstrate fines-depletion, induration of the margins, upward-flaring geometries, and intense fracturing of overlying geologic units. The fumaroles were preferentially located on post welding, early formed cooling joints that vented to the surface after the vapor phase alteration stage. The pipes were regularly spaced at distances of approximately 4.5 m (N–S) to 7 m (E–W). In turn the pipes were covered by a surge deposit and overlying tuff which rapidly lithified. The overlying tuff was then brecciated during continued fumarolic pipe emissions. Geochemical evaluations confirm the presence of high-temperature mineral (scapolite) indicative of transport of hot volcanic gases through these zones. The pipe centers and walls are depleted in SiO2, and enriched in Al2O3 and FeO. The overlying tuff breccia zones are enriched in Al2O3, FeO and MgO, and depleted in SiO2, NaO, and K2O. From comparison to other ignimbrite cooling histories, the fissures, fumaroles, and structures observed all likely formed in the first few decades after the deposition of the upper Tshirege subunits. This may have significant implications as to timing of initial cooling fractures and subsequent consolidation of gas emission pathways.

Dating the onset of volcanism at the Rum Igneous Centre, NW Scotland

Major volcanic activity on the Isle of Rum commenced with the eruption of thick (>100 m) intra-caldera rhyodacite ash-flow sheets fed from steep-sided feeder conduits in the proximity of the Main Ring Fault. Twenty plagioclase phenocrysts of the rhyodacite were analysed using single crystal 40 Ar/ 39 Ar laser dating, yielding a mean apparent age of 60.83 ± 0.27 Ma (MSWD=3.65). On an age v. probability plot the feldspars do not, however, show a simple Gaussian distribution, but a major peak at 60.33 ± 0.21 Ma and two smaller shoulders at c . 61.4 Ma and 63 Ma. These older ages are interpreted to represent recycled and largely re-equilibrated feldspars. The age peak at 60.33 ± 0.21 Ma is interpreted to represent the intrusion and eruption age of the rhyodacites. This new age constraint overlaps with that for the ultrabasic intrusion, implying that the latter was already forming at depth and supplying necessary heat during the early felsic activity phase, and quickly thereafter migrated upwards to shallow structural levels and intruded the volcano's earlier deposits. Combined with previously published ages, these new age data highlight an extremely rapid succession of events at the Rum centre, the whole sequence occurring in potentially <500 ka.

Deciphering Magma Chamber Dynamics from Styles of Compositional Zoning in Large Silicic Ash Flow Sheets

The understanding of the dynamic processes in magmatic systems has grown and changed markedly in the last decade. Old models for magmatic systems as vats of near-liquidus material have been revised by observations from seismology (Sinton and Detrick 1992), crystal chemistry and zoning (Davidson et al. 2007) and the geochronology and geochemistry of both plutonic and volcanic systems (Hildreth 2004; Charlier et al. 2007; Miller et al. 2007; Peressini et al. 2007; Walker et al. 2007). New views emphasise magmatic systems as temporally dominated by crystal mushes (magma bodies with a high fraction of solid particles; see definition in Miller and Wark 2008), that wax and wane in temperature and crystallinity, and are subject to significant open system processes (Charlier et al. 2007; Hildreth and Wilson 2007; Walker et al. 2007; Bachmann and Bergantz 2008). These processes, such as magma reintrusion, mixing, gas sparging, and subsequent thermal rejuvenation, may be significantly more important in producing the characteristics of a magmatic system than the previous closed-system, near-liquidus behaviour would predict. There are a number of recent reviews that summarize these observations (for example see Eichelberger et al. 2006; Bachmann et al. 2007b; Lipman 2007). Our aim here is to illustrate how this new perspective to magma dynamics is motivated by observations of heterogeneities (or lack thereof) in erupted rocks (and to a lesser amount in plutons). Owing to rapid withdrawal and quenching of magma during explosive volcanic eruptions (hours to a few days), large-volume (>1 km3) pyroclastic deposits (also referred to as ignimbrites or ash-flow tuffs ) provide an instant image of the state of the magma chamber evacuated during eruption. A first-order observation that characterizes these pyroclastic deposits of intermediate to silicic composition is that many do not tap into chemically …

Petrology of the Nohi Rhyolite and its related granitoids: A Late Cretaceous large silicic igneous field in central Japan

The Nohi Rhyolite (5000–7000 km 3) is one of the largest Late Cretaceous ash-flow fields, related to the Kula/Pacific Plate subduction, in central Japan. It consists mainly of thick, welded ash-flow sheets (a few 100 to > 1000 km 3) infilling cauldrons. Ash-flow tuffs of the Nohi Rhyolite are characterized as crystal-rich rhyodacite to rhyolite, fiammes of which have 30–45 vol.% phenocrysts (quartz, feldspars and minor mafic minerals). The crystal-rich rhyolitic magma sampled by some units of the Nohi Rhyolite formed mainly by fractional crystallization (plagioclase, pyroxene and ilmenite) from a parental crystal-rich rhyodacite. The parental magma of the largest Gero Ash-Flow Sheet (> 2200 km 3) is considered to have had a lower original water content than typical intermediate to evolved magmas produced in subduction zones. The Nohi Rhyolite has similar general petrological properties to the hypabyssal granitoid porphyries and the batholithic Naegi-Agematsu Granite, which is high-K, I-type, ilmenite-series dacitic to rhyolitic in composition and lacking mafic rocks. This petrological similarity, together with a close spatial and temporal relationship indicates that they formed as a series of large silicic magmatic events on the eastern margin of the Eurasian Continent during the Late Cretaceous. There are slight differences in trace element composition within subdivisions of the Nohi Rhyolite and granitoids, which probably relate to the region of magma emplacement rather than a temporal change in magma chemistry.

Identifying relationships among silicic magma batches by polytopic vector analysis: A study of the Topopah Spring and Pah Canyon ash-flow sheets of the southwest Nevada volcanic field

The chemically and mineralogically zoned Topopah Spring silicic ash-flow sheet (12.8 Ma, 1200 km 3) in the southwest Nevada volcanic field (SWNVF) contains two pumice fragment populations, a lower silica (LS) and a high-silica rhyolite (HSR) composition. Recent interpretations are that these magmas, represented by the pumice fragments, were not related by AFC processes within a single magma chamber. The emplacement of the Pah Canyon ash-flow sheet followed the Topopah Spring ash-flow sheet eruption. This smaller volume (∼ 35 km 3) ash-flow sheet contains pumice fragments that were previously interpreted as a hybrid magma formed by mixing of residual Topopah Spring LS and HSR magmas. Pumice fragments from these ash-flow sheets were analyzed by polytopic vector analysis (PVA), a multivariate statistical program that describes each sample in a dataset in terms of some proportion of each end member generated by the program. Therefore each sample, or pumice fragment, is uniquely described by some amount of each of the end members so that graphical analysis of the dataset allows the immediate recognition of separate magma batches, as pumice fragment samples cluster in discrete groups and show different variations in end member proportions. Recognition of hybrid magmas is also immediately apparent, as pumice fragments representing mixed magmas must plot between the parent magmas (pumice fragment groups). Our results confirm the existence of independently generated magmas involved in the formation of the Topopah Spring ash flow. However, PVA of Pah Canyon pumice fragments reveals that Pah Canyon cannot be described solely as a hybrid magma derived from Topopah Spring LS and HSR magmas. Furthermore, Pah Canyon and the Topopah Spring HSR pumice fragments have identical trends in the variation of end member proportions that most likely represent crystal fractionation or accumulation in both magmas that was previously undetected. This pattern is inconsistent with simple mixing between the TS-LS and TS-HSR magmas. Rather it is consistent with magma mixing followed by fractional crystallization.

U–Pb zircon geochronology of silicic tuffs from the Timber Mountain/Oasis Valley caldera complex, Nevada: rapid generation of large volume magmas by shallow-level remelting

Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 km3, 12.8 Ma)–Tiva Canyon (TC, 1,000 km3, 12.7 Ma) and Rainier Mesa (RM, 1,200 km3, 11.6 Ma)–Ammonia Tanks (AT, 900 km3, 11.45 Ma; all cited ages are previously published 40Ar/39Ar sanidine ages). Within each pair, eruptions are separated by only 0.1–0.15 My and produced tuffs with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with δ18O values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-δ18O AT cycle were earlier determined to exhibit ∼1.5‰ core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5–2.5‰. The following U–Pb isochron ages were calculated after correcting for the initial U–Pb disequilibria: AT (zircon rims: 11.7 ± 0.2 Ma; cores: 12.0 ± 0.1 Ma); pre-AT rhyolite lava: (12.0 ± 0.3 Ma); RM: 12.4 ± 0.3); TC: (13.2 ± 0.15 Ma); TS: (13.5 ± 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar–Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70–0.35 Ma older than the respective Ar–Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4–4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5–16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-δ18O AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the 40Ar/39Ar eruption age compared to core ages that are on average ∼0.2–0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz–zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-δ18O AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-δ18O magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-δ18O magma presently known.

Significance of whole-rock chemical analysis for ignimbrites : chemical comparison between essential clasts and matrix of the Late Cretaceous Setogawa Ash-Flow Sheet of the Nohi Rhyolite

Significance of whole-rock chemical analysis for ignimbrites : chemical comparison between essential clasts and matrix of the Late Cretaceous Setogawa Ash-Flow Sheet of the Nohi Rhyolite

Cooling and welding history of the Bishop Tuff in Adobe Valley and Chidago Canyon, California

A numerical model for cooling and welding of ash-flow tuffs was developed to obtain a quantitative understanding of the welding compaction that occurs during the cooling of ash-flow sheets and to evaluate the relative effects of emplacement temperature, thickness, and emplacement histories on compaction zonation (density profiles) of large ash-flow sheets. This model includes a set of two-phase, compressible Navier–Stokes equations for the transport of mass, momentum, and energy for a variable density volatile outfluxing through a porous media (ash-flow tuff). With this model we calculated theoretical compaction profiles (density vs. elevation) to match actual data measured in sections of Bishop Tuff. Compaction profiles from the Adobe basin indicate that there were 3 periods of non-deposition (5, 10, and 40 days) separating 4 distinct rapidly emplaced packets (REP) of pyroclastic flow deposition. In contrast, the 3 periods of quiescence (400, 20, and 800 days) between the 4 REPs in Chidago Canyon were considerably longer. These data constrain models of eruption for the Bishop Tuff by implying an eruption cycle that lasted longer than 3 years. The average calculated deposition temperatures for all 4 REPs in the Adobe basin are fairly uniform at 667±33 °C. In Chidago Canyon two of the REPs had a higher deposition temperature (∼660 °C) than the other two (534 °C).

Ongoing hydrothermal heat loss from the 1912 ash-flow sheet, Valley of Ten Thousand Smokes, Alaska

The June 1912 eruption of Novarupta filled nearby glacial valleys on the Alaska Peninsula with ash-flow tuff (ignimbrite), and post-eruption observations of thousands of steaming fumaroles led to the name ‘Valley of Ten Thousand Smokes’ (VTTS). By the late 1980s most fumarolic activity had ceased, but the discovery of thermal springs in mid-valley in 1987 suggested continued cooling of the ash-flow sheet. Data collected at the mid-valley springs between 1987 and 2001 show a statistically significant correlation between maximum observed chloride (Cl) concentration and temperature. These data also show a statistically significant decline in the maximum Cl concentration. The observed variation in stream chemistry across the sheet strongly implies that most solutes, including Cl, originate within the area of the VTTS occupied by the 1912 deposits. Numerous measurements of Cl flux in the Ukak River just below the ash-flow sheet suggest an ongoing heat loss of ∼250 MW. This represents one of the largest hydrothermal heat discharges in North America. Other hydrothermal discharges of comparable magnitude are related to heat obtained from silicic magma bodies at depth, and are quasi-steady on a multidecadal time scale. However, the VTTS hydrothermal flux is not obviously related to a magma body and is clearly declining. Available data provide reasonable boundary and initial conditions for simple transient modeling. Both an analytical, conduction-only model and a numerical model predict large rates of heat loss from the sheet 90 years after deposition.

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (ΔAn ≥ 10%) followed by gradual decrease in An-content (ΔAn ≤ 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.

Rapid generation of both high- and low-δ18O, large-volume silicic magmas at the Timber Mountain/Oasis Valley caldera complex, Nevada

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18O/ 16O evidence for an early, short-lived (∼10 yr), fumarolic event in the Topopah Spring Tuff near the proposed high-level nuclear waste repository within Yucca Mountain, Nevada, USA

18O/ 16O analyses of rock samples from the Exploratory Studies Facility (ESF) and from three other locations in the vicinity of Yucca Mountain, Nevada, suggest that the uppermost 5–20 m of the 12.8 Ma Topopah Spring Tuff displays a characteristic 18O/ 16O pattern that is indicative of short-lived (∼10 yr), high-temperature (∼500°C), fumarolic meteoric-hydrothermal activity. Hydrothermally altered rock is characterized by 1–3-m-thick zones of 18O-depleted groundmass (down to δ 18O=+5.5) located above and locally within the upper vitrophyre, as well as complementary, 2–4-m-thick, overlying zones of 18O-enriched (as high as δ 18O=+13.7), mineralogically altered, groundmass. Thirty-four samples of the Topopah Spring Tuff collected below the upper vitrophyre are not hydrothermally altered and correspondingly display a more uniform set of δ 18O values (δ 18O=+8.6 to +11.1). Sanidine phenocrysts from fumarolically altered tuff in the upper cooling unit are petrographically pristine and also display uniform δ 18O values (δ 18O=+7.80±0.45), which are in isotopic equilibrium with other phenocryst phases at magmatic temperatures. The 18O/ 16O signature displayed by the upper part of the Topopah Spring Tuff is analogous to that produced by short-lived (10–25 yr), high-temperature (500°C), fumarolic activity in the Bishop Tuff and in the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes, Alaska. The remarkable preservation of a fragile, fumarolic 18O/ 16O signature in the upper part of the Topopah Spring Tuff indicates that no subsequent hydrothermal alteration of this part of the tuff has occurred.

Origin and emplacement of the andesite of Burroughs Mountain, a zoned, large-volume lava flow at Mount Rainier, Washington, USA

Burroughs Mountain, situated at the northeast foot of Mount Rainier, WA, exposes a large-volume (3.4 km 3) andesitic lava flow, up to 350 m thick and extending 11 km in length. Two sampling traverses from flow base to eroded top, over vertical sections of 245 and 300 m, show that the flow consists of a felsic lower unit (100 m thick) overlain sharply by a more mafic upper unit. The mafic upper unit is chemically zoned, becoming slightly more evolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich and locally contains inclusions of quenched basaltic andesite magma that are absent from the upper unit. Widespread, vuggy, gabbronorite-to-diorite inclusions may be fragments of shallow cumulates, exhumed from the Mount Rainier magmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flow were the earliest products of the eruptive episode. The felsic–mafic–felsic progression in lava composition resulted from partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawal increased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic recharge stimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of the eruption. Pre-eruptive zonation resulted from combined crystallization–differentiation and intrusion(s) of less evolved magma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, at times, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complex recharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from large ash-flow sheets and other zoned lava flows.

Origin of silicic volcanic rocks in Central Costa Rica: a study of a chemically variable ash-flow sheet in the Tiribí Tuff

Chemical heterogeneities of pumice clasts in an ash-flow sheet can be used to determine processes that occur in the magma chamber because they represent samples of magma that were erupted at the same time. The dominant ash-flow sheet in the Tiribi Tuff contains pumice clasts that range in composition from 55.1 to 69.2 wt% SiO2. It covers about 820 km2 and has a volume of about 25 km3 dense-rock equivalent (DRE). Based on pumice clast compositions, the sheet can be divided into three distinct chemical groupings: a low-silica group (55.1–65.6 wt% SiO2), a silicic group (66.2–69.2 wt% SiO2), and a mingled group (58.6–67.7 wt% SiO2; all compositions calculated 100% anhydrous). Major and trace element modeling indicates that the low-silica magma represents a mantle melt that has undergone fractional crystallization, creating a continuous range of silica content from 55.1–65.6 wt% SiO2. Eu/Eu*, MREE, and HREE differences between the two groups are not consistent with crystal fractionation of the low-silica magma to produce the silicic magma. The low-silica group and the silicic group represent two distinct magmas, which did not evolve in the same magma chamber. We suggest that the silicic melts resulted from partial melting of relatively hot, evolved calc-alkaline rocks that were previously emplaced and ponded at the base of an over-thickened basaltic crust. The mingled group represents mingling of the two magmas shortly before eruption. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-001-0188-8.

18O/16O studies of fossil fissure fumaroles from the Valley of Ten Thousand Smokes, Alaska

At three sample sites where there are good exposures of the upper 15 m of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS), Alaska, ^(18)O/^(16)O studies indicate that fumarolic activity produced a very wide range of δ^(18)O values (−0.1 to +12.6; n=32) in the groundmass adjacent to fossil fissure fumaroles. This contrasts sharply with the uniformity of δ^(18)O in the groundmass away from fumarolic conduits (δ^(18)O=+5.9 to +7.1; n=7) and in all of the feldspar phenocrysts (δ^(18)O=+6.11 to +7.5 1 for 11 samples from this study and Hildreth 1987), independent of whether these were collected from fossil fumaroles or from unaltered tuff. Only one sample contained feldspars that were even slightly ^(18)O-enriched relative to the others (cloudy plagioclase δ^(18)O=+8.45). and this sample also contained the most ^(18)0-enriched groundmass of any of those analyzed (δ^(18)O=+12.6). This preservation of primary magmatic δ^(18)O values in the VTTS feldspar phenocrysts is clearly a consequence of the extremely short time span (i.e., 1912 to ≈1923) of vigorous, high-temperature, fumarolic activity in the 1912 ash-flow sheet. These ^(18)O/^(16)O systematica are strikingly similar to those discovered in the 2.8-Ma intracaldera Chegem Tuff (Gazis et al. 1996) and in the fossil fumaroles in the outflow sheet of the 0.76 Ma Bishop Tuff (Holt and Taylor 1998), thus confirming that a similar type of fumarolic meteoric-hydro-thermal activity occurred above the zone of intense welding in all three of these ash-flow tuffs. This is particularly important, because it provides a direct linkage between the older tuffs and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperatures (up to 645°C). The ^(18)O/^(l6)O effects in the VTTS can all be explained in terms of a two-stage history: (a) an early, 10- to 15-year-long, high-temperature (τ;450°C), fumarolic ^(18)O-depletion event (groundmass δ^(18)O=−0.1 to +4.8); and (b) a subsequent, much longer-lived, low-temperature (<150°C), ^(18)O-enrichment episode (groundmass as high as δ^(18)O=+12.6). Steam in these low-temperature fumaroles probably passed through various parts of the same hydrothermal system associated with the earlier, higher-temperature, fumarolic activity, and a weakened form of this low-temperature hydrothermal circulation continues to the present day (Keith et al. 1992; Lowell and Keith 1991). This low-temperature ^(18)O/^(16)O exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium-rich portions of feldspar phenocrysts during the waning (<150°C) stages of fumarolic activity (Spilde et al. 1993). The slight ^(18)O enrichment of apparently pristine, transparent feldspar phenocrysts (δ^(18)O=+7.51) in one of the ^(18)O-depleted, meteoric-hydrothermally altered fumarolic samples (whole-rock δ^(18)O=+4.8) probably indicates that this sample was incipiently altered at low temperatures as fumarolic activity waned, and thus may have had a whole-rock δ^(18)O value much lower than +4.87‰ prior to 1923.

Tephra sequences as indicators of magma evolution: 40Ar/ 39Ar ages and geochemistry of tephra sequences in the southwest Nevada volcanic field

Changes in rock chemistry with 40Ar/ 39Ar ages in tephra layers record the temporal and magmatic history of two volcanic systems in southwestern Nevada. Tephra layers from the Post-Grouse Canyon tephra sequence record three distinct groups. These groups are chemically distinct and have 40Ar/ 39Ar ages of 13.52±0.06, 13.31±0.18, and 12.95±0.10 Ma. The age groups correspond to three distinct chemical groups based on trace element distributions. These chemical groups cannot be related by any reasonable fractional crystallization or magma mixing model and are interpreted as distinct magma batches. The Pre-Rainier Mesa tephra sequence records two 40Ar/ 39Ar ages (12.79±0.10 and 11.84±0.18 Ma). The upper portion of this sequence is equivalent in age and chemistry to part of the overlying Rainier Mesa ash-flow sheet. The lower portion of the sequence is equivalent in age to the underlying Tiva Canyon ash-flow sheet but is chemically distinct from this sheet. The formation of this chemical group is consistent with mixing of low silica Tiva Canyon and high silica, low Th, Rainier Mesa magma. Post-Grouse Canyon magmas were most likely emplaced as a series of small, unrelated magma bodies, which allowed them to evolve independently. The mixed Pre-Rainier Mesa magma was produced by infilling of the Tiva Canyon magma chamber with Rainier Mesa-like magma after eruption of the Tiva Canyon ash-flow sheet at approximately 12.8 Ma. The upper portion of the Pre-Rainier Mesa tephra sequence represents eruption of Rainier Mesa magma less than 0.3 My. before that of the voluminous (1200 km 3) Rainier Mesa ash-flow sheet at approximately 11.71 Ma.

Magma evolution in the Purico ignimbrite complex, northern Chile: evidence for zoning of a dacitic magma by injection of rhyolitic melts following mafic recharge

The 1.3 Ma Purico complex is part of an extensive Neogene–Pleistocene ignimbrite province in the central Andes. Like most other silicic complexes in the province, Purico is dominated by monotonous intermediate ash-flow sheets and has volumetrically minor lava domes. The Purico ignimbrites (total volume 80–100 km3) are divided into a Lower Purico Ignimbrite (LPI) with two extensive flow units, LPI I and LPI II; and a smaller Upper Purico Ignimbrite (UPI) unit. Crystal-rich dacite is the dominant lithology in all the Purico ignimbrites and in the lava domes. It is essentially the only lithology present in the first LPI flow unit (LPI I) and in the Upper Purico Ignimbrite, but the LPI II flow unit is unusual for its compositional diversity. It constitutes a stratigraphic sequence with a basal fall-out deposit containing rhyolitic pumice (68–74 wt% SiO2) overlain by ignimbrite with dominant crystal-rich dacitic pumice (64–66 wt% SiO2). Rare andesitic and banded pumice (60–61 wt% SiO2) are also present in the uppermost part of the flow unit. The different compositional groups of pumice in LPI II flow unit (rhyolite, andesite, dacite) have initial Nd and Sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice (eNd=–6.7 to –7.2 and 87Sr/86Sr=0.7085–0.7090). However, two lines of evidence show that the andesite, dacite and rhyolite pumices do not represent a simple fractionation series. First, melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples. Second, mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre-eruptive temperatures (780–800 °C), whereas the rhyolites and andesites yield consistently higher temperatures (850–950 °C). Hornblende geobarometry and pressure constraints from H2O and CO2 contents in melt inclusions indicate upper crustal (4–8 km) magma storage conditions. The petrologic evidence from the LPI II system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than, and chemically unrelated to, the underlying dacite. We suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic Purico magma chamber. Rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite. Isotopic and compositional variations in the Purico system are typical of those seen throughout the Neogene ignimbrite complexes of the Central Andes. These characteristics were generated at moderate crustal depths (<30 km) by crustal melting, mixing and homogenization involving mantle-derived basalts. For the Purico system, assimilation of at least 30% mantle-derived material is required.

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km 3, respectively) chemically zoned (57–78 wt.% SiO 2) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.

Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado: Implications for magma-chamber processes during a large ash-flow eruption

The 27.8 Ma Fish Canyon Tuff, a vast ash-flow sheet (∼ 5000 km 3 ) of uniform phenocryst-rich dacite, is representative of “monotonous intermediate” eruptions from a magma chamber that lacked compositional gradients. Sparse small fragments of comagmatic granophyre in late-erupted tuff and postcaldera lava, having mineral compositions indistinguishable from phenocrysts in the tuff and precaldera lava-like rocks, record complex events in the Fish Canyon chamber just prior to eruption. Sanidine phenocrysts in the granophyre preserve zoning evidence of mingling with andesitic magma, then shattering by decompression and volatile loss accompanying early Fish Canyon eruptions before overgrowth by granophyre. The textural and chemical disequilibria indicate that the eruption resulted from batholith-scale remobilization of a shallow subvolcanic chamber, contrary to previous interpretations of magma storage and phenocryst growth in the lower crust.