Large Silicic Magma Research Articles

Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U–230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.

The effects of magma flux variations on the formation and lifetime of large silicic magma chambers

Incremental emplacement of large silicic intrusions limits the size and lifetime of any magma chamber if earlier injected magma pulses solidify before the next pulse is emplaced. Geochronology indicates that long‐term average magma fluxes of plutons are too low for the accumulation of magma volumes larger than a single pulse. To better constrain the formation of large‐volume magma chambers, we investigate the influence of a changing emplacement rate of successive sill injections that form a composite intrusion in the upper crust. A thermal model with an explicit finite difference scheme simulates periods of transient high magma fluxes keeping the long‐term average flux small. Several scenarios regarding how fluxes vary were analyzed. A progressive increase in flux does not result in magma accumulation. Only a step‐like flux increase of at least one order of magnitude above the background flux produces a large magma reservoir. To generate a magma chamber of a melt‐crystal mix above solidus temperature with a volume of 500–2000 km3, transient high fluxes of at least 1.25 × 10 − 2km3/a, equivalent to vertical, one‐dimensional accretion rates of a few cm/a, are required. The transient high flux range where magma accumulates is largest for model scenarios with one period of transient high flux, and therefore, this emplacement style is favored for the built‐up of substantial magma chambers during pluton growth. Intrusion scenarios with three and four periods of transient high fluxes do not generate reservoirs of mobile magma, but magma mushes can be present for some hundred thousand years.

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems.

Silicic calderas in arc settings: Characteristics, distribution, and tectonic controls

Silicic calderas represent the surface expressions of large silicic magma bodies. Although development of sizable volumes of granite, sensu lato, is considered key in stabilizing continental crust, the factors that contribute to its origin are debated. Based on a comprehensive literature review, we analyze the characteristics, tectonic settings, and spatial distributions of 108 large (≥5 km diameter), Quaternary, silicic (>63 wt% SiO 2 ) calderas in arc settings. Generally, arcs associated with trench-normal convergence rates between 70 and 95 mm/yr are more likely to host silicic calderas, indicating that a larger flux of basalt favors formation of evolved magma, although New Zealand is a notable exception. The nature of the crust controls the types of calderas that develop; large and rhyolitic calderas tend to occur on old, thick, continental crust near preexisting structures and/or areas under local extension. In continental margins, silicic calderas are distributed more widely behind the volcanic front than are typical arc volcanoes. In addition, arcs with greater caldera densities tend to be young or have migrated, suggesting that the overlying crustal column can be depleted of a low–melting-temperature component over time. These results allow geologists studying batholiths to use silica content, size, and distribution of plutons to infer the tectonic properties of ancient arcs.

Microanalysis of large volume silicic magma in continental and oceanic arcs: Melt inclusions in Taupo Volcanic Zone and Kermadec Arc rocks, South West Pacific

The petrogenesis of large volume silicic arc magmas are investigated through microanalysis of four major eruptives of variable size from the continental Taupo Volcanic Zone and one from the oceanic Kermadec Arc. Orthopyroxene, plagioclase and quartz-hosted melt inclusions and groundmass glass display a range in major element (e.g. SiO 2 = 74–79 wt.%, CaO = 0.2–2.5 wt.% and FeO = 0–3 wt.%) and trace element compositions (e.g. Sr = 17–180 ppm and Ba = 140–1500 ppm). Healy melt inclusions are lower in K 2O and Ce/Yb relative to Taupo Volcanic Zone melt inclusions reflecting the lack of continental lithosphere in the oceanic setting. Quantitative trace element modelling of Healy melt inclusions indicates fractional crystallisation is the dominant process responsible for the generation of silicic magma at Healy seamount, although remobilisation of a crystal mush body and/or crustal anatexis cannot be discounted. The chemical heterogeneity of melt inclusions in the continental Taupo Volcanic Zone indicates that the observed crystal populations present in silicic magmas are composed of both phenocrysts and antecrysts and thus derived from polygenetic sources, including mature crystal mush zones that are amalgamated together prior to eruption. This is further enforced through cathodoluminescence imaging of a selection of quartz hosts from the Whakamaru Ignimbrite that allows the location of melt inclusions to be established. This reveals that the most evolved Whakamaru melt inclusions are located in the core of quartz crystals and the least evolved Whakamaru melt inclusions are co-genetic with the groundmass glass in the rim of the crystals. In contrast, Taupo melt inclusions and groundmass glasses are all co-genetic indicating that all crystals grew from this final magma batch prior to eruption. A general trend of increasing magma evolution (to higher SiO 2, Rb/Sr) with erupted volume suggests that the larger magma bodies present in continental crust incorporate more crustal material through assimilation and/or as partial melting of crust.

Evolution of Large Silicic Magma Systems: New U-Pb Zircon Data on the NW Permian Athesian Volcanic Group (Southern Alps, Italy)

Abstract During the Permian, the Southern Alps were affected by prominent igneous activity that produced voluminous basic to acidic volcanic and plutonic rocks. The magmatism originated during a period in which postorogenic extension and transtensional faulting controlled magma ascent and emplacement. Within this scenario, the Athesian Volcanic Group (AG), with lithologies ranging from basaltic andesites to rhyolites, constitutes a major part of the Permian magmatism in the central and eastern Southern Alps. We present an up-to-date stratigraphic reconstruction of the northwestern sector of the AG volcanic sequence combining detailed field mapping and new geochronological data. Single-zircon U-Pb age data suggest that the volcanism in the AG lasted ca. 10 m.yr., between 285 and 274 Ma, with a deposition rate that increased throughout the eruptive cycle. The processes of magma deposition and basin formation were strongly influenced by tectonic activity over the entire history of the AG. At least two major ...

Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand

Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km3 in a total magma volume of ~530 km3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from 87Sr/86Sr = 0.70547 to 0.70657, with compositions of An43 to An78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87Sr/86Sr = 0.70764 and An78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction.

Eruptive history, geochronology, and magmatic evolution of the Puyehue-Cordon Caulle volcanic complex, Chile

Forty-three 40 Ar/ 39 Ar age determinations of lava flows, domes, ignimbrites, and dikes, plus 14 C dates from seven distal tephra layers, combined with stratigraphy, geochemistry, and Sr and Th isotope data, establish an eruptive chronology for the Puyehue-Cordon Caulle volcanic complex at 40.5° S in the Andean southern volcanic zone (SVZ). The complex preserves ~131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space. Approximately 80% of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka. The modern Puyehue stratovolcano was built on the southerly shield during the past 69 k.y. following a hiatus of 25 k.y. Puyehue has erupted ~15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern SVZ and evolved via at least six phases including: (1) basaltic andesitic to dacitic lavas between 69 and 32 ka, (2) a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of MgO-rich basaltic andesite at 34 ka, (3) dacitic to rhyolitic flows and domes from 19 to 12 ka, (4) basaltic to basaltic andesitic flows between 15 and 12 ka, (5) subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka, followed by (6) a powerful series of phreatomagmatic and sub-Plinian eruptions at ca. 1.1 ka that obliterated the preceding rhyolite domes and formed the present 2.5-km-diameter, 280-m-deep summit crater. Along the Cordon Caulle fissure zone, ~5 km 3 of rhyodacitic to rhyolitic lavas, domes, and cones have formed during the past ~16.5 k.y., including explosive and effusive eruptions in 1921–1922 and 1960. Eruptive rates were nonuniform over time, with background growth at 0.04 km 3 /k.y. or less, punctuated by spurts at up to 0.90 km 3 /k.y. The time-averaged rate, 0.42 km 3 /k.y., is nearly double that at the Tatara-San Pedro complex 500 km to the north during the past 300 k.y. These findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable. The last three stratocone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. Puyehue basalt exhibits subtle changes in 238 U- 230 Th, 87 Sr/ 86 Sr, and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge, or the extent to which basalt was modified by assimilation of heterogeneous crustal melts. The complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt. Despite the high flux of basalt during the past 300 k.y., no large silicic magma reservoir formed in the upper crust. Instead, 238 U- 230 Th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, promoted perhaps by conduits that reflect strike-slip faulting beneath the complex.

Petrologic Reconstruction of Magmatic System Variables and Processes

Explosive volcanic eruptions constitute a major class of natural hazard with potentially profound economic and societal consequences. Although such eruptions cannot be prevented and only rarely may be anticipated with any degree of accuracy, better understanding of how explosive volcanoes work will lead to improved volcano monitoring and disaster mitigation. A major goal of modern volcanology is linking of surface-monitored signals from active volcanoes, such as seismicity, ground deformation and gas chemistry, to the subterranean processes that generate them. Because sub-volcanic systems cannot be accessed directly, most of what we know about these systems comes from studies of erupted products. Such studies shed light on what happens underground prior to and during eruptions, thereby providing an interpretative framework for post hoc evaluation of monitoring data. The aim of this review is to present some of the current petrological techniques that can be used for studying eruptive products and for constraining key magmatic variables such as pressure, temperature, and volatile content. We first review analytical techniques, paying particular attention to pitfalls and strategies for analyzing volcanic samples. We then examine commonly used geothermometry schemes, evaluating each by comparison with experimental data not used in the original geothermometer calibrations. As there are few mineral-based geobarometers applicable to magma storage regions, we review other methods used to determine pre-eruptive magma equilibration pressures. We then demonstrate how petrologically-constrained parameters can be compared to the contemporaneous monitoring record. These examples are drawn largely from Mount St. Helens volcano, for which there are abundant petrological and monitoring data. However, we emphasize that our approaches can be applied to any number of active volcanoes worldwide. Finally, we illustrate the application of these techniques to two different types of magmatic systems—large silicic magma chambers and small intermediate-composition magma storage regions—with particular focus on the combined evolution of melt …

Effects of stress in the evolution of large silicic magmatic systems: An example from the Miocene felsic volcanic field at Kii Peninsula, SW Honshu, Japan

Miocene felsic volcanic field at the Kii Peninsula (MFVK) in southwest Japan allows us to reveal temporal and spatial change of a large felsic volcanic system in response to changes in the local and regional stress field. The MFVK was active episodically ejecting more than a few hundred cubic kilometers of magma in a span of hundred thousand to million years. Remnant of this large-scale volcanism is characterized by a broad area of deeply incised and complex cluster of collapse calderas in the Kii Peninsula. Major conduits for ash–flow tuff eruptions lie on the margins of the MFVK. Paleostress orientation of the σ Hmax was inferred from lithologic distribution and structure in rocks in the MFVK. The optimal tectonic stress orientation at the time of the MFVK formation was NNE–SSW to the present azimuth. Approximately 1500 km 3 of magma were erupted from the MFVK during a duration of 0.1 million to 1.6 Myr. This suggests an area normalized eruption rate of 10 –2 to 10 –3 km 3/ka km 2. This long-term eruption rate of the MFVK is at the high end of rates for other large felsic volcanic fields (LFVFs), and ranks as one of the larger rates of LFVF worldwide. The large eruption rate of the MFVK and the formation of a large magma chamber is discussed in relation to tectonic stress. Although a compressional stress field favors the formation of a sill-like large magma chamber, a larger magma supply rate is more essential. We believe that a large rate of magma supply in combination with local and regional stress conditions in the early development of the MFVK promoted magma accumulation over eruption leading to the development of a large silicic magma chamber. We suggest that such a large magma chamber can generate excess pressure inside the chamber itself that results in a local tumescence and counteracts the tectonic compressional stress field, allowing magma to reach the surface. This scenario is the most plausible explanation for the large eruption rates of caldera volcanoes under a compressional stress field. The case of the MFVK is similar to the Toba caldera (Indonesia), which lies adjacent the geologic suture zone of wrench faults (the Sumatra faults).

Spatiotemporal evolution of a marine caldera-forming eruption, generating a low-aspect ratio pyroclastic flow, 7.3 ka, Kikai caldera, Japan: Implication from near-vent eruptive deposits

The VEI 7, 7.3 ka caldera-forming eruption of the Kikai caldera occurred in a shallow sea, and caused devastating damage to the prehistoric human settlements of southern Kyushu, Japan. This Holocene activity at this volcano records eruptions with compositional range of 56 to 74 wt.% in SiO 2 spanning with the climactic eruption. In this study, stratigraphy, component, and lithology of the pyroclastic deposits were analyzed at near-vent islands (Satsuma Iwo-jima and Take-shima) in order to reconstruct this eruption. Stratigraphic sections are characterized by plinian pumice-fall deposits (Unit A), intraplinian flow deposits (Unit B), climactic pyroclastic flow deposits (Units C1–C3), and co-ignimbrite ash-fall deposits (Unit D). In total the estimated magma volume in the system was 70–80 km 3 (DRE) and the eruption therefore represents the evacuation of a major silicic magmatic system. The plinian stage (Phase 1) is subdivided into an initial small phase and a second large one. The column height in the second phase was estimated to be 40–43 km. The total tephra volume of this stage was estimated to be 40 km 3. The magma discharge rate has been calculated from the column height data to be 2 × 10 8 kg/s. The eruption duration is also estimated to be a minimum of approximately 28 h. Collapse of the column (Phase 2) produced Unit B, which consists of multiple thin lithic-rich or pumice-rich layers or pods, including welded pumice-fall layers. The deposits are characterized by stratified or cross-stratified facies and display various degree of welding. These sedimentary characteristics indicate that, during the plinian column collapse, high temperature turbulent density currents were generated where dense pyroclasts were well segregated, resulting in the lithic-rich layers or pods. Phase 3 is characterized by Units C1–C3. Unit C1 shows non-welded stratified facies, which consist of lithic and crystals, including quenched juvenile materials as a minor constituent. Unit C2 displays welded stratified facies, which consist of lithic-rich layers and pumice-rich layers. These two subunits occur only in topographic lows in Satsuma Iwo-jima. Unit C3 is thickest and poorly-sorted non-welded massive deposit, which includes fragments of welded tuff from underlying units in proximal regions. These facts indicate that multiple pyroclastic density currents produced Units C1 and C2 in the near-vent area, and were followed by the main sustained current producing Unit C3, a low-aspect ratio ignimbrite, distributed over a wide area of southern Kyushu across the sea. Varying extents of magma–water interactions started during Phase 2, continuing during the early stages of Phase 3, and diminished during the climactic C3 ignimbrite stage. In addition, collapse of the caldera may have started before Unit C deposition, based on the evidence of a fault overlain by Unit C on the caldera rim. The collapse may have initiated water access to the magma. The source appears to have been biased toward the western side of the caldera. The Holocene evolution of the Kikai volcano records the existence of a large silicic magma system at depths of about 7 km that coexisted with or was regularly recharged with mafic magma.

Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado

Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y ( 40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing four- to six-fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass ~30% of those in whole rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (~4%) + titanite (~0.5–1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. Sr/Y in Fish Canyon glass (13–18) is lower than the typical “adakitic” value (>40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated.

A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions

The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 105 to 106 years. We address the question of why buoyant and otherwise eruptible high-silica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 1018 to 1020 Pa s–1, dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>102 km3) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The long-term magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of small-volume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits.

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1–0.78 Ma), and post-caldera rhyolites (0.65–0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of δ18O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of δ18O(quartz). The youngest domes of Glass Mountain are similar to BT in δ18O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The δ18O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant δ18O(melt)=7.8±0.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of δ18O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of δ18O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal δ18O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale δ18O homogeneity of BT and other large magma chambers as evidence of their longevity (>105 years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (102–104 years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable δ18O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 103–104 years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.

Effusive eruptions from a large silicic magma chamber: the Bearhead Rhyolite, Jemez volcanic field, NM

Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.40Ar/39Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540ka interval between 7.06 and 6.52Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200km2), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.

Genetic links between ash-flow calderas and associated ore deposits as revealed by large-scale thermo-mechanical modeling

In addition to a number of epithermal ore deposits, some porphyry-type ore deposits are also hosted by ash-flow calderas. Spatial correlation between ash-flow calderas and these deposits is observed in several parts of the world, but temporal correlation between the mineralization and the episode of caldera formation remains unclear, especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation. Because caldera collapse represents only a very short event in a long time-scale evolution between a silicic magma chamber and its host rocks, we have developed a thermo-mechanical model of upper crustal and magmatic system behavior during the pre- and post-caldera formation stages. Major petrophysical properties of ash-flow caldera related rocks are incorporated into the model, which accounts for brittle–elasto-ductile rheology and heat transfer, and reproduces fault initiation and various caldera collapse scenarios. Large-scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described. In particular, we found that the outer sides of caldera border faults are associated with both anomalous heat-transfer and a high fracture density. The finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults. In most cases, the model predicts “Y-shaped” fault geometry, which is surprisingly similar to that of mineralized faults in the field. Variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20MPa, with deep overpressured zones close to the inner edge of the chamber corners. Despite the fact that hydrothermal convection and fluid geochemistry are not included in the model, these thermal and mechanical results are believed to constrain the conditions for mineralization processes. Our results show that in the presence of regional extension, the development of caldera border faults may be suppressed to the advantage of deep-seated fractures, clustered above the central part of the magma chamber, thus favoring conditions for the formation of porphyry-type ore deposits. Finally, we demonstrate that well before the eruption, the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust, resulting in breakup and pre-eruptive normal faulting and thus allowing for conditions of pre-caldera mineralization.

Tectonic controls on magmatism in The Geysers–Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States

Laser probe Ar-40/Ar-39 dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 +/- 1 to 761 +/- 1 ha, Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 +/- 0.03 to 2.3 +/- 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts similar to 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

The Geysers-Clear Lake area, California; thermal waters, mineralization, volcanism, and geothermal potential

Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northern California include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization. The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults.Many of the waters of the region are nonmeteoric as defined by their isotopic signature. One type of isotopically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved connate in origin. A second type, evolved meteoric water, has moderate chloride contents, high boron contents, and high B/Cl ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine, where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers geothermal field when intrusion of silicic magma occurred, because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km 3 of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and the McLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by the small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and progressively older volcanic centers occur to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field, providing the heat for the geothermal field.Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a preexisting hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but xather of boiled-down, boron-rich Franciscan evolved meteoric water. This water is likely to be present in limited quantities and will not provide a significant hot water resource for geothermal power production at The Geysers field or from the main Clear Lake volcanic field.

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

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.