Ignimbrites to batholiths

Ignimbrites sample large magma reservoirs in the Earth’s upper crust, sometimes digging deep enough to link the volcanic realm with the plutonic world. Integrating textural, petrological, geochemical, and geochronological information on such deposits with geophysical signals suggest incremental growth and evolution of subvolcanic magma bodies that are dominated by high crystallinity mush zones, but sometimes remain sufficiently liquid to erupt. The eruptible upper portions are either extracted melt from the mush and constitute only a small volumetric fraction of the vertically extensive mushy batholithic magma body. The high-flux, ignimbrite flare-up phases are typically preceded by waxing magmatism that prime the crust to hold large, upper crustal silicic reservoirs where melt rich magma accumulates. Gas exsolution within such mushy reservoirs, and accumulation of the low density bubbles in the most melt-rich parts of the system, will also enhance eruptibility, emphasizing some of the observed chemical differences between evolved plutonic and volcanic rocks.https://digitalcommons.mtu.edu/techtalks/1020/thumbnail.jp

Sources of Hydrothermal Fluids Inferred from Oxygen and Carbon Isotope Composition of Calcite, Keweenaw Peninsula Native Copper District, Michigan, USA

The Mesoproterozoic North American Midcontinent Rift hosts the world’s largest accu-mulation of native copper in Michigan’s Keweenaw Peninsula. During a regional metamorpho-genic‐hydrothermal event, native copper was deposited along with spatially zoned main‐stage minerals in a thermal high. This was followed by deposition of late‐stage minerals including minor copper sulfide. Inferences from the oxygen and carbon isotopic composition of main‐stage hydrothermal fluids, as calculated from 296 new and compiled isotopic measurements on calcite, are consistent with existing models that low‐sulfur saline native copper ore‐forming fluids were domi-nantly derived by burial metamorphic processes from the very low sulfur basalt‐dominated rift fill at depth below the native copper deposits. Co‐variation of oxygen and carbon isotopic compositions are consistent with mixing of metamorphic‐derived fluids with two additional isotopically different fluids. One of these is proposed to be evolved seawater that provided an outside source of salinity. This fluid mixed at depth and participated in the formation of a well‐mixed hybrid metamorphic-dominated ore‐forming fluid. Secondary Ion Mass Spectrometry in‐situ isotopic analyses of calcite demonstrate a high degree of variability within samples that is attributed to variable degrees of shallow mixing of the hybrid ore‐forming fluid with sulfur‐poor, reduced evolved meteoric water in the zone of precipitation. The oxygen and carbon isotopic compositions of 100 new and compiled measurements on late‐stage calcite are mostly isotopically different than the main‐stage hydrothermal fluids. The late‐stage hydrothermal fluids are interpreted as various proportions of mixing of evolved meteoric water, main‐stage hybrid ore‐forming fluid, and shallow, evolved seawater in the relatively shallow zone of precipitation

Continental Magmatism and Uplift as the Primary Driver for First-Order Oceanic 87Sr/86Sr Variability with Implications for Global Climate and Atmospheric Oxygenation

Oceans cover 70% of Earth\u27s surface, setting it apart from the other terrestrial planets in the solar system, but the mechanisms driving oceanic chemical evolution through time remain an important unresolved problem. Imbalance in the strontium cycle, introduced, for example, by increases in continental weathering associated with mountain building, has been inferred from shifts in marine carbonate 87Sr/86Sr ratios. There are, however, uncertainties about the spatial and temporal patterns of crustal evolution in Earth\u27s past, particularly for the period leading up to the Cambrian explosion of life. Here we show that U-Pb age and trace element data from a global compilation of detrital zircons are consistent with marine carbonate 87Sr/86Sr ratios, suggesting changes in radiogenic continental input into Earth\u27s oceans over time. Increases in riverine Sr input were related to the break-up and dispersal of continents, with increased weathering and erosion of a higher proportion of radiogenic rocks and high-elevation continental crust. Tectonic processes exert a strong influence on the chemical evolution of the planet\u27s oceans over geologic time scales and may have been a key driver for concomitant increases in atmosphere-ocean oxygenation and global climate cooling

Building zoned ignimbrites by recycling silicic cumulates: insight from the 1,000km3 Carpenter Ridge Tuff, CO

The ~1,000km3 Carpenter Ridge Tuff (CRT), erupted at 27.55Ma during the mid-tertiary ignimbrite flare-up in the western USA, is among the largest known strongly zoned ash-flow tuffs. It consists primarily of densely welded crystal-poor rhyolite with a pronounced, highly evolved chemical signature (high Rb/Sr, low Ba, Zr, Eu), but thickly ponded intracaldera CRT is capped by a more crystal-rich, less silicic facies. In the outflow ignimbrite, this upper zone is defined mainly by densely welded crystal-rich juvenile clasts of trachydacite composition, with higher Fe-Ti oxide temperatures, and is characterized by extremely high Ba (to 7,500ppm), Zr, Sr, and positive Eu anomalies. Rare mafic clasts (51-53 wt% SiO2) with Ba contents to 4,000-5,000ppm and positive Eu anomalies are also present. Much of the major and trace-element variations in the CRT juvenile clasts can be reproduced via in situ differentiation by interstitial melt extraction from a crystal-rich, upper-crustal mush zone, with the trachydacite, crystal-rich clasts representing the remobilized crystal cumulate left behind by the melt extraction process. Late recharge events, represented by the rare mafic clasts and high-Al amphiboles in some samples, mixed in with parts of the crystal cumulate and generated additional scatter in the whole-rock data. Recharge was important in thermally remobilizing the silicic crystal cumulate by partially melting the near-solidus phases, as supported by: (1) ubiquitous wormy/sieve textures and reverse zoning patterns in feldspars and biotites, (2) absence of quartz in this very silicic unit stored at depths of >4-5km, and (3) heterogeneous melt compositions in the trachydacite fiamme and mafic clasts, particularly in Ba, indicating local enrichment of this element due mostly to sanidine and biotite melting. The injection of hot, juvenile magma into the upper-crustal cumulate also imparted the observed thermal gradient to the deposits and the mixing overprint that partly masks the in situ differentiation process. The CRT provides a particularly clear perspective on processes of in situ crystal-liquid separation into a lower crystal-rich zone and an upper eruptible cap, which appears common in incrementally built upper-crustal magma reservoirs of high-flux magmatic provinces

Outboard Onset of Ross Orogen Magmatism and Subsequent Igneous and Metamorphic Cooling Linked to Slab Rollback during Late-Stage Gondwana Assembly

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana

Spatial and temporal distribution of a rhyolite compositional continuum from wet-oxidizing to dry-reducing types governed by lower-middle crustal P-T-ƒO₂-ƒH₂O conditions in the Taupo Volcanic Zone, New Zealand.

A continuum of rhyolite compositions has been observed throughout the Taupo Volcanic Zone (TVZ) over the past 550 kyr. reflecting changes in the ƒH2O, ƒO₂, and P-T conditions in a lower crustal 'hot-zone' (10-30 km) where these evolved melts are generated by crystal fractionation of successively intruded basaltic magmas. The rhyolite compositional continuum is bound by two distinct end-member types: R1 is characterized by hydrous minerals (hornblende ± biotite), low FeO*/MgO (calc-alkaline series), low MREE, Y, and Zr, and high Sr; and R2 is characterized by anhydrous minerals (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), high MREE, Y, and Zr, and low Sr. Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type (R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted MREE) and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe-Ti oxides and ranges between -0.039 to +2.054 log units (ΔQFM; where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high fluid-flux typically correlates with the R1 and low fluid-flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3000 km³) ignimbrite 'flare-up' between ca. 340 and 240 ka. These magmas also display the highest K₂O and Pb isotopic compositions compared to those erupted more recently, and is consistent with a peak in slab-derived sediment input. The second cycle began roughly 180 ka, erupting ca. 800 km³ of magma, and continues to the present. The duration, rate, and composition of melt production within these cycles appears to be governed by the flux of fluid/sediment released from the subducting slab, while the distribution of melts may be governed more by extension along the central rift axis. The Matahina Ignimbrite (~160 km³ rhyolite magma; 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, TVZ. The outflow sheet is distributed primarily from the northeast to southeast and consists of a basal plinian fall member and three ash-flow members. Pumice clasts are separated into three groups defined by differences in bulk geochemistry and mineral contents: high CaO, MgO, Fe₂O₃T, TiO₂, and low Al₂O₃, +hornblende (A2), low CaO, MgO, Fe2O3T, TiO2, ±hornblende (A1), and a subset to A1, which has high-K, +biotite (B). Two types of crystal-rich mafic clasts were also deposited during the final stages of the eruption. The distinct A and B rhyolite magma types are petrogenetically related to corresponding type A and B andesitic magma by up to 50% crystal fractionation under varying ƒO₂-ƒH₂O conditions. Further variations in the low- to high-silica rhyolites can be accounted for by up to 25% crystal fractionation, again under distinct ƒO₂-ƒH₂O conditions. Reconstruction of the P-T-ƒO₂-ƒ’H₂O conditions of the andesite to rhyolite magmas are consistent with the existence of a compositional and thermal gradient prior to the eruption. Magma mingling/mixing between the basalt to andesite and main compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the bottom of the reservoir and effectively destroying the pre-eruptive gradients. A detailed examination of the diverse range of calcic-amphibole compositions from the ca. 330 ka Matahina eruption (ca. 160 km³ rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt to dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P: up to 0.6±0.06 GPa and T: up to 950°C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Ti-magnesiohornblende → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (up to 70°C), indicating new hotter magma periodically fluxed the crystal mush. (3) The dominant population of amphibole (magnesiohornblende) from the rhyolite is small and bladed and crystallized at low P-T conditions (P: 0.3 GPa, T: 765°C), equivalent to the eruptive P-T conditions. Amphibole (tschermakite-magnesiohornblende) from the dacitic and low-silica rhyolitic pumice form two distinct populations, which nucleated at two different T (High: 820°C and Low: 750°C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive T. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals+liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper-crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed

Genesis of rhyolitic melts in the upper crust: Fractionation and remobilization of an intermediate cumulate at Lake City caldera, Colorado, USA

Lake City Caldera (22.93 ± 0.02 Ma) is the youngest of 25 Tertiary calderas within the Southern Rocky Mountain Volcanic Field (SRMVF) and offers an opportunity to study the relationship between plutonic rocks and their volcanic equivalents. Extreme topographical relief of the area reveals the three-dimensional exposure of a complex, high-K calc-alkaline, magmatic system. Lake City caldera is comprised of two principal units: 1) a resurgent quartz syenite intrusion, and 2) the Sunshine Peak Tuff (Lower, Middle, and Upper). The Lower and Middle Sunshine Peak Tuff (LSPT and MSPT) are crystal-poor rhyolite, while the Upper Sunshine Peak Tuff (USPT) is a crystal-rich trachyte. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by melt extraction from a long-lived quartz syenitic magma reservoir at intermediate crystallinity (~50–70% crystals). A slight compositional zoning from the Lower Sunshine Peak Tuff (~76 wt% SiO2) to the Middle Sunshine Peak Tuff (~74 wt% SiO2) ignimbrites indicates that further modification by either mixing or additional crystal fractionation might have occurred following melt extraction from the mushy syenitic reservoir. Geochemical and textural analyses show that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the left-over syenite cumulate. Quartz syenite crystal size distributions of potassium feldspar show the characteristic shape of crystal accumulation, and both volcanic and plutonic units contain abundant glomerocrysts. The presence of mafic enclaves within both the syenite and USPT trachyte, coupled with large increases of Ba content in rims of the K-feldspars from the trachyte suggest that the injection of less evolved magma into the host reservoir caused the re-melting of the syenite cumulate mush, allowing for the eruption of the full SPT sequence. Titanium-in-quartz thermobarometry shows that the majority of grains in all units formed below ~800 °C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera

Quartz crystals in Toba rhyolites show textures symptomatic of rapid crystallization

Textural and chemical heterogeneities in igneous quartz crystals preserve unique records of silicic magma evolution, yet their origins and applications are controversial. To improve our understanding of quartz textures and their formation, we examine those in crystal-laden rhyolites produced by the 74 ka Toba supereruption (\u3e2800 km3) and its post-caldera extrusions. Quartz crystals in these deposits can reach unusually large sizes (10–20 mm) and are rife with imperfections and disequilibrium features, including embayments, melt inclusions, titanomagnetite and apatite inclusions, spongy morphologies, hollow faces, subgrain boundaries, multiple growth centers, and Ti-enriched arborescent zoning. Using a combination of qualitative and quantitative analyses (petrography, CL, EBSD, X-ray CT, LA-ICPMS), we determine that those textures commonly thought to signify crystal resorption, crystal deformation, synneusis, or fluctuating P–T conditions are here a consequence of rapid disequilibrium crystal growth. Most importantly, we discover that an overarching process of disequilibrium crystallization is manifested among these crystal features. We propose a model whereby early skeletal to dendritic quartz growth creates a causal sequence of textures derived from lattice mistakes that then proliferate during subsequent stages of slower polyhedral growth. In a reversed sequence, the same structural instabilities and defects form when slow polyhedral growth transitions late to fast skeletal-dendritic growth. Such morphological transitions result in texture interdependencies that become recorded in the textural-chemical stratigraphy of quartz, which may be unique to each crystal. Similar findings in petrologic experimental studies allow us to trace the textural network back to strong degrees of undercooling and supersaturation in the host melt, conditions likely introduced by dynamic magmatic processes acting on short geologic timescales. Because the textural network can manifest in single crystals, the overall morphology and chemistry of erupted quartz can reflect not only its last but its earliest growth behavior in the melt. Thus, our findings imply that thermodynamic disequilibrium crystallization can account for primary textural and chemical heterogeneities preserved in igneous quartz and may impact the application of quartz as a petrologic tool

Origin and quantification of diffuse CO2 and H2S emissions at Crater Hills, Yellowstone National Park

We characterized volatile emissions based upon diffuse soil degassing measurements and fumarolic gas chemistry at Crater Hills, a thermally-altered area adjoining the Sour Creek resurgent dome that is located within the Yellowstone Caldera. The objective of this study was to investigate the source and flux of CO2 and H2S gases to improve our understanding of both the total emissions and origin of the spatial distribution. The total emission of CO2 estimated using the sequential Gaussian simulation method (sGs) was 66 to 109 t day−1 with 95% confidence, which is an underestimation due to the: (1) inability to measure a high flux area on a steep slope, and (2) absence of measurements from fumarole and hot pool emissions. Based on gas chemistry data obtained for a fumarole at Crater Hills in 2007, the proportion of CO2 calculated to be derived from magma would be at least 38%, but could be as high as 50%. The spatial distribution of prominent geothermal features with the highest gas flux are broadly consistent with the regional fault pattern and, therefore, likely reflect the pattern of blind faults and/or fractures covered by overlying alluvium. The estimated emission of H2S was 0.39 t day−1, based on the linear correlation between H2S and CO2. The heat output was also estimated to be ~35 MW with an average heat flux of ~100 W m−2 based upon CO2-H2O-heat relations