V41F-01 INVITED
The Growth of Magma Bodies by Amalgamation of Discrete Sheet Intrusions: Implications for the Formation of Magma Chambers
Until recently, igneous bodies (plutons and magma chambers) were commonly considered to be approximately spherical bodies, rapidly emplaced into the crust. However, field, structural, geophysical, and geochronological studies indicate that many plutons are low aspect-ratio tabular bodies (sills) that are formed by the amalgamation of successive discrete magma pulses. The thermal evolution of an igneous body that grows by accretion of thin magma sheets is fundamentally different from the evolution of a rapidly emplaced magma sphere or of a single thick magma sill. In thin sheet intrusions, the heat loss is through the walls of the sheets and the temperatures within the intrusions do not depend on the volumes injected but on the one-dimension sheets emplacement rate. The first sheets injected in a cold crust rapidly cool down and solidify. The ability of successive intrusions to stay at high temperature and eventually build up a long-lived magma chamber is controlled by the emplacement rate. Heat transfer modeling applied in the context of a volcanic arc shows that average emplacement rates of at least several centimeters per year and an incubation time of tens thousands of years are needed for a persistent magma chamber to form. During the incubation time, the intrusions solidify and when a chamber of high melt fraction magma eventually grows, the volume of eruptible magma only form a small part of the total intruded volume. The emplacement rate of plutons is controversial. Geochronological data suggest that some plutons may be emplaced over millions years. For a pluton that is assembled at a slow rate of a few millimeters per year, millions of years are needed, over which kilometric thicknesses are intruded, before a volume of magma larger than the size of a single intrusion becomes mobile and eruptible. In many cases, volcanic products may come from a deep source without being associated with a long-lived upper crust magma chamber. If volcanism is associated with the assembly of plutons over millions years, an active magma chamber only represents a small part of the total intruded volume. The formation of the magma chamber occurs at a late stage in the growth of the pluton unless transient accelerations of the emplacement rate affect the pluton assembly.
V41F-02
Why do magmas stall? Insights from petrologic and geodetic data
Magmas stall at various depths in the crust due to their internal properties (magma viscosity, buoyancy) and external crustal controls (local stress regime, wallrock strength). Annen et al. (JPet 2006) propose a petrological model in which buoyant magma ascends through the crust until the depth of water saturation, after which it crystallizes catastrophically and stalls due to the large increase in magma viscosity. Magmas may erupt from this storage region, or viscous death may result in pluton formation. In order to test this model, and constrain magma storage depths, we combine petrological and geodetic data for several active volcanoes along the Aleutian-Alaska arc. We analyzed glassy, primarily olivine-hosted melt inclusions by SIMS in tephra samples for their pre-eruptive volatile contents, which can be related to the depth of entrapment via pressure-dependent H2O-CO2 solubility models (e.g., VolatileCalc). Melt inclusions are not in equilibrium with pure water vapor (all will contain S and C species), but >50% of the inclusion population are in equilibrium with a vapor containing >85% H2O. Geodetic data (InSAR, GPS) record surface deformation related to volcano inflation/deflation, and can be inverted to solve for the depths of volume change (magma storage) in the crust. In the Aleutians, we find that the maximum melt inclusion trapping depths and geodetic depths correlate, suggesting both techniques record crustal magma storage and crystallization. Melt inclusions from the 1997 Okmok eruption are trapped at ≤3 km; deformation during the eruption and subsequent inflation occurred at 3±0.5 km (Miyagi et al., EPSL 2004; Lu & Masterlark, JGR 2005). At Akutan, melt inclusions and GPS data indicate magma storage at ~5-7 km. Inclusions from flank cones of Makushin yield depths of 7 km, similar to inflation observed beneath the main edifice (6.8 km, Lu et al., JGR 2002). Pleistocene inclusions from Augustine volcano indicate magma storage at 10-18 km, in accord with a deep magma source proposed for the 2006 eruption. Melt inclusions from Shishaldin are trapped at depths up to 4 km, coincident with the base of the conduit (Vergnoille & Caplan Auerbach, BVolc 2006). Other volcanoes record similar depths of melt inclusion entrapment and deformation, including Mt. St. Helens, Irazú, Soufriere Hills, Vesuvius, and Etna. Clearly, crystallization will occur where magmas stall, cool, and degas, so it may not be surprising that the depths of deformation correlate with the depths of melt inclusion entrapment. But the question of why magmas stall at various depths remains. In the Aleutians, maximum H2O contents of melt inclusions (from 2 wt% at Shishaldin to 7 wt% at Augustine) negatively correlate with measures of the degree of mantle melting (Ti6.0 and Y6.0), which is expected if water drives mantle melting beneath arcs (e.g. Kelley et al. JGR 2006; Portnyagin et al EPSL 2007). Thus, if magmas stall near the depths where they reach H2O-saturation, as predicted by Annen et al. and observed here, then magma chamber and pluton depths may ultimately be controlled by the primary magmatic water contents set in the mantle.
V41F-03
Modeling Magma Mixing: Evidence from U-series age dating and Numerical Simulations
Magma mixing and recharge is an ubiquitous process in the shallow crust, which can trigger eruption and cause magma hybridization. Phenocrysts in mixed magmas are recorders for magma mixing and can be studied by in- situ techniques and analyses of bulk mineral separates. To better understand if micro-textural and compositional information reflects local or reservoir-scale events, a physical model for gathering and dispersal of crystals is necessary. We present the results of a combined geochemical and fluid dynamical study of magma mixing processes at Volcan Quizapu, Chile; two large (1846/47 AD and 1932 AD) dacitic eruptions from the same vent area were triggered by andesitic recharge magma and show various degrees of magma mixing. Employing a multiphase numerical fluid dynamic model, we simulated a simple mixing process of vesiculated mafic magma intruded into a crystal-bearing silicic reservoir. This unstable condition leads to overturn and mixing. In a second step we use the velocity field obtained to calculate the flow path of 5000 crystals randomly distributed over the entire system. Those particles mimic the phenocryst response to the convective motion. There is little local relative motion between silicate liquid and crystals due to the high viscosity of the melts and the rapid overturn rate of the system. Of special interest is the crystal dispersal and gathering, which is quantified by comparing the distance at the beginning and end of the simulation for all particle pairs that are initially closer than a length scale chosen between 1 and 10 m. At the start of the simulation, both the resident and new intruding (mafic) magmas have a unique particle population. Depending on the Reynolds number (Re) and the chosen characteristic length scale of different phenocryst-pairs, we statistically describe the heterogeneity of crystal populations on the thin section scale. For large Re (approx. 25) and a short characteristic length scale of particle-pairs, heterogeneity of particle populations is large. After one overturn event, even the "thin section scale" can contain phenocrysts that derive from the entire magmatic system. We combine these results with time scale information from U-series plagioclase age dating. Apparent crystal residence times from the most evolved and therefore least hybridized rocks for the 1846/47 and 1932 eruptions of Volcan Quizapu are about 5000 and about 3000 yrs, respectively. Based on whole rock chemistry as well as textural and crystal-chemical data, both eruptions tapped the same reservoir and therefore should record similar crystal residence times. Instead, the discordance of these two ages can be explained by magma mixing as modeled above, if some young plagioclase derived from the andesitic recharge magma which triggered the 1846/47 AD eruption got mixed into the dacite remaining in the reservoir after eruption, thus lowering the apparent crystal residence time for magma that was evacuated from the reservoir in 1932.
V41F-04
Constraints on the Rates of Replenishment, Magma Mixing, and Crystal Recycling at Santorini Volcano, Greece
Santorini is a young, active volcano, which preserves abundant evidence for open-system processes such as magma replenishment and crystal recycling, and thus represents an ideal system in which to study magma chamber dynamics. Santorini is the largest volcanic centre in the Aegean arc, with an eruptive history spanning more than 250,000 years over two eruptive cycles. The cycles are dominated by extended periods of effusive shield-building activity with occasional large-magnitude explosive eruptions, the Minoan eruption of ~3600 years ago being the most recent. Current activity consists of a phase of post-caldera reconstruction, focused recently on the intra-caldera Kameni islands. Microsampling to measure 87Sr/86Sr ratios of plagioclase cores indicates the presence of a complex plumbing system beneath Santorini. Large rhyodacitic deposits typically contain a mafic component, interpreted as the eruption trigger. In some cases, the mafic magma groundmass and phenocrysts are isotopically similar to their rhyodacite host; other deposits show the opposite, implying the coexistence of isotopically distinct magma batches. To add further complexity, plagioclase phenocrysts are in some cases in equilibrium with their groundmass while others show the reverse, implying modification due to crystal recycling or shallow mixing processes prior to eruption. Mafic enclaves in the recent Kameni lavas, again interpreted as the probable eruption trigger, provide some constraints on the rates of these recycling, mixing, and triggering processes. Glomerocrysts and xenocrysts of recycled gabbroic cumulate material are present in a number of Kameni enclaves. Isotopic and chemical disequilibrium between the cumulate crystals and the host indicate that these fragments are derived from pre- existing crystal mush piles pervaded by the replenishing melts as they migrated to shallow levels, creating disequilibrium between the cumulate mineral cores and the replenishing melts. 87Sr/86Sr isotope ratios of plagioclase xenocryst cores suggest crystal recycling from a pre-Minoan source is probable. Olivine xenocrysts in the enclaves possess narrow (10-30 μm) Fe-Mg diffusion profiles, due to interaction with enclave magma groundmass, which can be used to estimate the interval between entrainment and eruption. Initial modelling of diffusion profiles from more than 60 crystals suggests short timescales, from 15 to 45 days, for the combined migration-replenishment-eruption cycle at Kameni.
V41F-05
Patterns in Long-Lived Continental Magmatism; Crustal Modulation of Mantle-Derived Input
Patterns in volumetric eruption rate of continental intermediate to silicic magmatic systems have implications for how heat is delivered and processed in the crust, and reveal the evolution of crustal scale magmatic systems. Drawing on the Neogene volcanic rocks of the Altiplano Puna Volcanic Complex (APVC) and the Aucanquilcha Volcanic Cluster (AVC) of the Central Andes, we compare their histories to other long-lived crustal magmatic complexes. Erupted volumes of such systems vary from a few tens of km3 to tens of thousands of km3 of magma with associated plutonic volumes several times that. Despite differences in volume, these complexes share a general family resemblance. They have lifespans of about 10 million years. Where resolution allows, the record of activity may reveal distinct pulses, lasting a few hundred thousand to ~1 to 2 m.y., demonstrating the composite nature of the magmatic systems. These complexes bear isotopic and compositional evidence of crustal and mantle involvement in the origin of the magmas and mineralogic evidence for subsequent equilibration at shallow crustal levels. Many, but not all, have abundant ignimbrites. Most strikingly they all have a three stage evolutionary history that is scale independent. An early waxing stage characterized by low volume, low flux volcanism that is compositionally diverse and may be dispersed; a climactic stage of dramatically higher flux, that is compositionally more focused and may be spatially more focused; and a final waning stage of small eruptions. We interpret these patterns to be the result of long-lived thermal pulses delivered from the mantle in the form of basaltic magma. A fundamental question is to what extent does the surface pattern reflect the mantle input. We suggest that the thermal signal is modulated by interaction and thermal incubation in the crust that leads to development of a large crustal magma reservoir that in turn modulates the composition of erupted magma. The interplay between mantle power input, heat advection, thermal and mechanical state of the crust, silicic magma production and intrusion rates among other factors need to be explored to resolve the nature and time scales of modulation.
V41F-06
Magmatic Genesis Versus Evolution: the Tale of the Miocene - Pliocene Tuscan Magmatic Province
Miocene-Pliocene Tuscan Magmatic Province (TMP, Central Italy) reveals a complex magmatic setting with a large variety of volcanic and plutonic rock types closely associated in space and time (8-1 Ma). The main rock associations are represented by three group of rocks with different degree of evolution: i) mafic rocks with lamproitic, high-potassium calc-alkaline, and shoshonitic affinity; ii) intermediate rocks showing strong field, petrographic, and geochemical evidence of magma interaction processes; iii) felsic volcanics and intrusives showing petrological and geochemical characteristics of both extreme evolved and pure anatectic melts. Geochemical and thermodynamical characteristics of volcanic and plutonic rocks of TMP suggest a complex petrological evolution in which interaction between mafic and felsic magmas played a major role. The development of interaction processes between mafic and felsic magmas involves several evolutionary stages including the evolution of mafic magmas by Contamination and Fractional Crystallization (CFC) processes, once injected into the felsic reservoirs, followed by widespread mixing processes in magma chambers. Petrologic data indicate that felsic end-members are crustal anatectic melts derived by partial melting at ca 4-6 Kbar of a metasedimentary protolith constituting the Tuscan basement. Residual assemblages of the partial melting process, calculated by geochemical models, are consistent with experimental petrology data, and constraint the levels of emplacement for intrusive complexes, and the paths of extrusion for the effusive ones. Mafic end-members are compositionally akin to basic-intermediate magmas cropping out on the island of Capraia and to lamproites; such a variety of mafic magmas are found associated even in a single volcanic or plutonic complex, testifying for an extremely heterogeneous mantle source. The petrological model proposed in this work fits well with geophysical data. Starting from Middle Pliocene, western Tuscany underwent a strong regional uplift caused by an asthenospheric intrusion that thinned the crustal stack and completely restructured the crust-mantle boundary with possible intercalation of stack of high- level crust even in the lower crust. In response to the increasing heat flow both asthenosphere and heterogeneously metasomatized lithosphere melted to give basaltic magmas that either extruded or intruded and mixed with anatectic melts, the latter generated in the interlayered metasedimentary crustal stacks.
V41F-07
Scope of Silicic Magmatism Associated With the Snake River Plain-Yellowstone (SRPY) "Hotspot" Track
Eruptive volumes of silicic volcanic rocks provide indirect albeit minimal indications of the scale of magmatism associated with the SRPY hotspot track. Coherent eruptive centers at Yellowstone produced ca. 6000 km3 of high silica rhyolite over &~2 m.y. whereas the Bruneau-Jarbidge center in the central SRP produced as much as 10,000 km3 between 12.7 and 8 Ma. Total magma volumes could be significantly larger. About half erupted as 'supervolcano class` ignimbrites (i.e., exceeding 103 km3). Implicitly, such volumes must be on-tap at least periodically during the lifetime of an eruptive center but heterogeneities in mineral populations imply that magma may be derived from a plexus of isolated pockets in the crust (`crustal sponge`) rather than a well-mixed chamber. Magmatism was strongly bimodal - ‘A-type' high silica rhyolite and basalt - intermediate composition lavas are rare. Moreover, each center produced rhyolite for 2-3 m.y. prior to the onset of basaltic volcanism which dominated subsequent activity. Nearly all SRPY rhyolites carry anhydrous mineral assemblages and mineral thermometry indicates high magmatic temperatures (typically >850-900°C). Radiogenic isotopes (Sr-Nd-Pb), trace element patterns, and low 18O in many SRPY rhyolites implicate a crustal source, although Nd isotopic data preclude large contributions from Archean crust. Silicic volcanism initiated in N-central Nevada ca. 16 Ma and migrated to Yellowstone by 2 Ma. However, the pattern of silicic eruptions was not simply progressive in space and time. Between 11.5-10 Ma major silicic eruptions occurred over a swath of more than 400 km - signifying availability of diverse rhyolite magmas beneath much of the SRP. Assuming that crustal melting was driven by basaltic intrusions, such magmas must have been generated beneath much of the province prior to 10 Ma – in part, well in advance of the postulated position of the Yellowstone hotspot at that time. The quantity of basalt needed to power SRPY silicic magmatism is estimated as equivalent to an underplate of 10-15 km thickness. Available geophysical data suggest that this mass was confined roughly within the physiographic footprint of the province. Because crustal thickness appears to be nearly constant, the crust must have deformed to accommodate emplacement of such large volumes of basalt. It is suggested that deformation was dominantly extensional on Basin and Range style structures. As a consequence, the crust must have become denser and interlaced with significant proportions of ‘juvenile' material over time. It is suggested that crustal evolution influenced eruptive activity, and particularly onset of basaltic volcanism.
V41F-08 INVITED
Dynamical Constraints on the Life Cycle of Voluminous Silicic Systems: How to Build, Maintain, and Destroy Shallow Silicic Magma Bodies
The generation, accumulation and eruption of silicic magmas in the shallow crust occurs on a wide range of length and timescales. Here we focus on two questions related to these interrelated processes: 1.) What are the energy and dynamic constraints for the relative proportions of crustal and mantle melts in the production of silicic systems? 2.) What processes can alleviate the so called "room problem", and how does this contribute to the longevity of the system? We address the first two questions through a coupled finite volume and finite element approach to determine the evolving thermal and stress fields in response to magma injection in the upper crust. We perform a suite of two- dimensional simulations over a range of magmatic fluxes and different regional stresses. To constrain the volume and depth of assimilation of crustal materials we also perform oxygen isotope calculations that are coupled with the evolving thermal and permeability structure of the crust. The rate of δ18O-depletion of the crust is controlled by both the supply of low δ18O meteoric waters at depth and by the temperature dependent kinetics of oxygen exchange between the water and the permeable crystal framework. We find that extensive crustal melting and significant δ18O-depletion, such as is found in many Snake River rhyolites, can be explained by a self-consistent combination of elevated basaltic flux, enhanced upper crustal permeability, and magmatic volume accommodation, such as by extension. This stands in contrast to low flux silicic systems such as the Kos Plateau Tuff in the Aegean arc that are isotopically very mantle-like.