Large-volume Silicic Magma Research Articles

Copper-mineralised porphyries sample the evolution of a large-volume silicic magma reservoir from rapid assembly to solidification

Volatile-rich intermediate to silicic magmatic systems can feed devastating volcanic eruptions but also generate valuable magmatic-hydrothermal ore deposits that supply most of the world's copper. Understanding the geometry, dynamics and timescales of these magmatic systems is critical in developing models for predicting the occurrence of ore deposits and future large volcanic eruptions. Here, we use zircon petrochronology from an equigranular monzonite and successively emplaced porphyry dykes to reconstruct the time – temperature – composition evolution of the magma that sourced the giant Cu-Mo-Au deposit at Bingham Canyon (USA). Combining high-precision CA-ID-TIMS geochronology with in-situ trace element analyses by LA-ICP-MS shows the intra-grain, inter-sample and temporal geochemical changes recording the evolution of the magmatic system over 817 ± 62 kyr. Systematic variation of zircon chemistry with time indicates crystallisation from a coherent magma reservoir. After reservoir assembly its thermal and chemical state was controlled by protracted monotonous crystallisation over ∼650 kyr with rapid cooling over the first 200 kyr followed by a longer period approaching the granite solidus. Porphyry Cu-Au ore formation occurred after the early drop in magma temperature that resulted in large-scale fluid saturation and expulsion into the sub-volcanic environment but main Mo-mineralisation occurred after protracted low-temperature magma storage and the emplacement of the last porphyry. Zircons do not quantify the depth of this reservoir but integrating independent geophysical evidence with 2-D thermal modelling indicates that the time – temperature evolution recorded by the zircons is consistent with rapid incremental assembly of this large pluton (magma emplacement rate ≳ 0.0065 km3/yr) by initially zircon-undersaturated monzonitic magma into pre-heated upper crust. Our results indicate that massive fluid expulsion from rapidly-formed, large magma reservoirs containing mobile but mushy magma (>40 wt.% melt) can occur in the upper crust, favouring the formation of giant porphyry copper deposits.

Rapid assembly and crystallization of a fossil large-volume silicic magma chamber

The rates at which large volumes of eruptible, silicic (>65 wt% SiO2) magma (magma chambers) are assembled, as well as their longevity in the upper crust, remain controversial. This controversy is due, in part, to a missing record of granitoid plutonic complexes that represent large fossil upper crustal magma chambers. We present new geologic mapping and high-precision U-Pb zircon geochronology from the Eocene Golden Horn batholith in Washington State, USA. These data reveal that the batholith was constructed as a series of sills over 739 ± 34 k.y. Topographic relief of >2 km permits volume estimates for 4 of the sills, the largest of which, a >424 km3 rapakivi granite, was emplaced over 26 ± 25 k.y. at a rate of ∼0.0125 km3/yr. This rate exceeds those needed to build large, silicic magma chambers in thermal models, and we suggest that that this unit may represent the first fossil magma chamber of this type recognized in the geologic record.

Rapid magma evolution constrained by zircon petrochronology and 40Ar/39Ar sanidine ages for the Huckleberry Ridge Tuff, Yellowstone, USA

Understanding the time scales of magmatic differentiation, storage, and eruption of large-volume silicic magmas is a primary goal of igneous petrology. Within the Huckleberry Ridge Tuff (HRT; Idaho, USA), representing the earliest and largest caldera-forming eruption associated with Yellowstone volcanic activity, zircon morphological zoning patterns coupled to strongly correlated changes in Ti-in-zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the HRT member B magma body. Tandem in situ and isotope dilution U-Pb dating of single zircon crystals demonstrates an absence of pre-Pleistocene xenocrysts, but reveals the presence of antecrysts recycled from pre-caldera rhyolites in the HRT magma. The petrochronologic interpretation of autocrystic zircon thermal, chemical, and temporal characteristics suggests that HRT member B differentiated over ∼10 k.y. prior to eruption at 2.0794 ± 0.0046 Ma as defined by new astronomically calibrated, single-crystal total fusion 40 Ar/ 39 Ar sanidine analyses. This refined eruption age demonstrates that the transitional polarity preserved by HRT member B does not record the Reunion subchron, but rather a separate, younger geomagnetic event. Our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma.

Tracking the evolution of large-volume silicic magma reservoirs from assembly to supereruption

The most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs. The thermal evolution of these subcaldera magma reservoirs controls the volume of eruptible magma and eruptive style. Here we combine high-precision zircon U-Pb geochronology, trace element analyses of the same mineral grains, and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the Oligocene Fish Canyon Tuff magma reservoir (Colorado, United States) as a function of absolute time. Systematic compositional variations in U-Pb dated zircons record ~440 k.y. of magma evolution. An early phase of volumetric growth was followed by a period of cooling and crystallization, during which the Fish Canyon magma approached complete solidifi cation. Subsequent remelting, due to underplated andesitic recharge magmas, began 219 ± 45 k.y. prior to eruption, and led to the generation of ~5000 km3 of eruptible crystal-rich (~45 vol%) dacite. Age-equivalent, but compositionally different, zircons in an andesite enclave from late-erupted Fish Canyon Tuff tie the growth and thermal evolution of the upper-crustal reservoir to a lower-crustal magma processing zone. Our results demonstrate that the combination of high-precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fl uid dynamic modeling.

Tectonic and climate history influence the geochemistry of large-volume silicic magmas: New δ18O data from the Central Andes with comparison to N America and Kamchatka

New δ18O data from magmatic quartz, plagioclase and zircon crystals in Neogene large-volume, rhyodacitic ignimbrites from the Central Andean Ignimbrite Province reveal uniformly high-δ18O values (δ18O(Qtz) from +8.1 to +9.6‰ — 43 analyses from 15 ignimbrites; δ18O(Plag) from +7.4 to +8.3‰ — 10 analyses from 6 ignimbrites; δ18O(Zrc) from +6.7 to +7.8‰ — 5 analyses from 4 ignimbrites). These data, combined with crustal radiogenic isotopic signatures of Sr, Nd and Pb, imply progressive contamination of basaltic magmas with up to 50vol.% upper crust in these large volume silicic systems. The narrow range of δ18O values also demonstrate that surprising homogeneity was achieved through space (100's km) and time (~10Ma to recent) in these large-volume magmas, via residence in their parental middle to upper crustal bodies. Low-δ18O values of many large volume (>10km3) silicic magmas in North America and Kamchatka, discussed here for comparison, reflect the influence of meteoric-hydrothermal events and glaciations in lowering these δ18O values via the assimilation of hydrothermally-altered crustal material. Conversely, there is a scarcity of a low-δ18O signature in the Central Andes and subduction-related or influenced systems in North America, such as the Oligocene Great Basin of Nevada and Utah, the Southern Rocky Mountain Volcanic Field of Colorado, and the SW Nevada volcanic field system. In these regions, the generally heavy-δ18O magmatic signature is interpreted as a reflection of how a broadly compressional regime, high elevation, aridity and evaporation rates limit availability and infiltration of large amounts of surface meteoric water and hydrothermal alteration of the shallow crust. This leads us to speculate that the δ18O values of large volume silicic magmas in these areas record a paleoelevation and paleoclimate signal. If this is the case, δ18O values of ignimbrites can potentially be used to track the effects of a meteoric-hydrothermal derived δ18O signature from upper crustal rocks that are subsequently assimilated to produce these magma types.

Quartz crystal size distribution of the Ordovician Millbrig K-bentonite

Crystal size distribution (CSD) analysis has been applied to quartz crystals of the Ordovician Millbrig K-bentonite, which represents one of the largest known fallout ash deposits in the Phanerozoic Era, to establish crystal growth histories and conditions in the magma chamber prior to eruption. Specific CSDs were examined for crystal growth conditions. All Millbrig samples exhibit lognormal concave-down shapes in agreement with previously reported CSDs on large silicic systems but in contrast to more mafic systems characterized by linear CSDs. Concave-down CSDs are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature [Bindeman, I.N., 2003. Crystal sizes in evolving silicic magma chambers. Geology, 31: 367–370.]. Quartz crystals of the Millbrig must have been produced by surface-controlled, size-dependent growth with a minor Ostwald ripening component, which could have easily changed to supply-controlled growth due to a limited supply of nutrients, as the crystals grew larger. CSDs were also used to examine the possible origin of multiple ash beds in the Millbrig. These ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches, each with slightly different crystal growth conditions.

Zircon crystallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ∼60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; [Smith et al., 1996][1]) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). [1]: #ref-32

The Cougar Point Tuff: Implications for Thermochemical Zonation and Longevity of High-Temperature, Large-Volume Silicic Magmas of the Miocene Yellowstone Hotspot

The 12.7-10.5 Ma Cougar Point Tuff in southern Idaho, USA, consists of 10 large-volume (>10²-10³ km³ each), high-temperature (800-1000 °C), rhyolitic ash-flow tuffs erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot. These tuffs provide evidence for compositional and thermal zonation in pre-eruptive rhyolite magma, and suggest the presence of a long-lived reservoir that was tapped by numerous large explosive eruptions. Pyroxene compositions exhibit discrete compositional modes with respect to Fe and Mg that define a linear spectrum punctuated by conspicuous gaps. Airfall glass compositions also cluster into modes, and the presence of multiple modes indicates tapping of different magma volumes during early phases of eruption. Equilibrium assemblages of pigeonite and augite are used to reconstruct compositional and thermal gradients in the pre-eruptive reservoir. The recurrence of identical compositional modes and of mineral pairs equilibrated at high temperatures in successive eruptive units is consistent with the persistence of their respective liquids in the magma reservoir. Recurrence intervals of identical modes range from 0.3 to 0.9 Myr and suggest possible magma residence times of similar duration. Eruption ages, magma temperatures, Nd isotopes, and pyroxene and glass compositions are consistent with a long-lived, dynamically evolving magma reservoir that was chemically and thermally zoned and composed of multiple discrete magma volumes.

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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Rejuvenation of the Fish Canyon magma body: A window into the evolution of large-volume silicic magma systems

Voluminous, unzoned, phenocryst-rich pyroclastic deposits, considered as erupted batholiths, provide a unique opportunity to investigate magmatic processes in silicic magmas. The Fish Canyon Tuff, a well-documented example of these monotonous ignimbrites, displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near-isobaric reheating from ;720 to 760 8C. These observations, along with a high crystallinity (45%) and near-solidus mineral assemblage, suggest that the Fish Canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption. Rejuvenation was triggered by intrusion of water-rich mafic magmas at the base of the Fish Canyon mush, but the mechanisms of heat transfer remain poorly understood. The growth of amphibole during reheating requires addition of mafic components, but the absence of any measurable gradients and the paucity of mafic enclaves in the Fish Canyon magma rule out a reheating event dominated by convective mixing with a mafic magma. Closed-system processes, such as heat conduction and convective self-mixing, could not account for the transport of externally derived mafic components. We performed numerical simulations of upward percolation of a hot, low-density H2O-CO2 fluid phase (gas sparging) through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems. Sparging by ;20‐40 km 3 of gas extracted from ;3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by .40 8C in 150‐200 k.y. Moreover, the vertical thermal gradient after 150 k.y. in most of the mush is small (;25 8C in the upper 65%). Gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal-poor rhyolites by expelling interstitial melt. However, our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites.

A model that helps explain Sr-isotope disequilibrium between feldspar phenocrysts and melt in large-volume silicic magma systems

Feldspar phenocrysts of silicic volcanic rocks are commonly in Sr-isotopic disequilibrium with groundmass. In some cases the feldspar is more radiogenic, and in others it is less radiogenic. Several explanations have been published previously, but none of these is able to accommodate both senses of disequilibrium. We present a model by which either more- or less-radiogenic feldspar (or even both within a single eruptive unit) can originate. The model requires a magma body open to interaction with biotite- and feldspar-bearing wall rock. Magma is incrementally contaminated as wall rock melts incongruently. Biotite preferentially melts first, followed by feldspar. Such melting behavior, which is supported by both field and experimental studies, first contaminates magma with a relatively radiogenic addition, followed by a less-radiogenic addition. Feldspar phenocrysts lag behind melt (groundmass of volcanic rock) in incorporating the influx of contaminant, thus resulting in Sr-isotopic disequilibrium between the crystals and melt. The sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process. This model is testable by isotopic fingerprinting of individual feldspar crystals. For a given set of geologic boundary conditions, specific core-to-rim Sr-isotopic profiles are expectable. Moreover, phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history. Initial results of Sr-isotopic fingerprinting of sanidine phenocrysts from the Taylor Creek Rhyolite are consistent with the model. More tests of the model are desirable.

The Origin and Evolution of Large-Volume Silicic Magma Systems: Long Valley Caldera

Despite extensive study, the origin of large-volume silicic magma systems remains poorly constrained. We review the source regions and processes involved in the generation, differentiation, and eruption of caldera-related silicic magma with particular reference to the Bishop Tuff erupted from Long Valley Caldera, California. Nd-isotopic compositions of the earliest-erupted rhyolites (between 2,1 and 1.2 Ma) at Glass Mountain, which may be associated with the Bishop Tuff magma chamber, are consistent with extensive fractional crystallization of basaltic magmas derived from an enriched lithospheric mantle source. In contrast, Nd-isotopic compositions of late Glass Mountain lavas (1.2 to 0.8 Ma) and the Bishop Tuff (0.76 Ma) suggest significant incorporation of continental crust. Shallow-crustal residence histories inferred from Sr-isotopic studies at Long Valley suggest that large-volume silicic magmas reflect either: (1) continuous growth with episodic eruption from a single, large, long-lived magma chamber; or (2) rapid generation and eruption of separate, short-lived magma batches. Residence histories that require several rapid differentiation events (≤104 yr) followed by extended periods (≥105 yr) over which magmas are maintained without significant cooling and crystallization are difficult to reconcile with numerical models of the thermal evolution of shallow-crustal magma reservoirs. Sr-isotopic compositions of the Bishop Tuff and precursor Glass Mountain lavas alternatively may reflect assimilation of wall-rock melts undergoing high-87Sr/86Sr, low-Sr concentration dehydration during convective sidewall fractional crystallization of parental rhyolites. If so, the reported apparent Sr-isochronal relationships may have little or no age significance. Sr-isotopic systematics of the Bishop Tuff also provide insights into the influence of eruption dynamics on compositional gradients recorded in pyroclastic deposits. Sr-isotopic compositions of individual fall and flow pumice are consistent with recent field studies that suggest that the Bishop ignimbrite is intraplinian, and that fall and flow deposits previously interpreted to be sequential are largely coeval. Moreover, the distribution of Sr-isotopic compositions of pumice as a function of stratigraphic height suggests that magmas from different depths with different 87Sr/86Sr ratios were intimately mixed before exiting the vent and can be qualitatively predicted from numerical models of magma withdrawal from zoned crustal reservoirs.

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initialɛNd values (∼1ɛNd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similarɛNd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higherɛNd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from lowɛNd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The lowɛNd magmas most likely formed via the incorporation of lowδ18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higherδ18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have lowɛNd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing “basification” of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.