When do we need pan-global freeze to explain ^(18)O-depleted zircons and rocks?

Rocks with δ^(18)O values of less than 5‰ SMOW (Standard Mean Ocean Water) contain oxygen derived from ∼0‰ seawater or meteoric (rain or melted snow, <0‰) waters. As δ^(18)O_(precipitation) values decrease with increasing latitude, altitude, and toward the interior of continents, the low δ^(18)O values (<5‰) of hydrothermally altered rocks can potentially serve as a proxy for the δ^(18)O values of the altering water and as a proxy for climates (Fig. 1). Hydrothermal exchange of rocks with large quantities of meteoric waters presents the most viable opportunity to imprint low-δ^(18)O water values on the protolith (Fig. 2). Such processes typically require shallow depths of a few kilometers (where water circulates through open cracks and porous rocks), a heat source to drive meteoric-hydrothermal systems, and appropriate hydrogeologic conditions for water refill. These conditions are most commonly found in caldera and rift settings, such as in Yellowstone (Wyoming, United States) and Iceland. Oxygen—as the major element—is not significantly affected by subsequent metamorphism and melting (by more than ~1 ‰), and metamorphism often creates large, refractory metamorphic minerals (garnets, omphacites, zircons) that lock the protolith's oxygen isotopic values permanently in the geologic record

Time constraints on the origin of large volume basalts derived from O-isotope and trace element mineral zoning and U-series disequilibria in the Laki and Grímsvötn volcanic system

The 1783–1784 AD fissure eruption of Laki (Iceland) produced 15 km^3 of homogeneous basaltic lavas and tephra that are characterized by extreme (3‰) ^(18)O-depletion relative to normal mantle. Basaltic tephra erupted over the last 8 centuries and as late as in November 2004 from the Grímsvötn central volcano, which together with Laki are a part of a single volcanic system, is indistinguishable in δ^(18)O from Laki glass. This suggests that all tap a homogeneous and long-lived low-δ^(18)O magma reservoir. In contrast, we observe extreme oxygen isotope heterogeneity (2.2–5.2‰) in olivine and plagioclase contained within these lavas and tephra, and disequilibrium mineral-glass oxygen-isotope fractionations. Such low-δ^(18)O_(glass) values, and extreme 3‰ range in δ^(18)O_(olivine) have not been described in any other unaltered basalt. The energy constrained mass balance calculation involving oxygen isotopes and major element composition calls for an origin of the Laki–Grímsvötn quartz tholeiitic basaltic melts with δ^(18)O = 3.1‰ by bulk digestion of low-δ^(18)O hydrated basaltic crust with δ^(18)O = − 4‰ to + 1‰, rather than magma mixing with ultra-low-δ^(18)O silicic melt. The abundant Pleistocene hyaloclastites, which were altered by synglacial meltwaters, can serve as a likely assimilant material for the Grímsvötn magmas. The (^(226)Ra /^(230)Th) activity ratio in Laki lavas and 20th century Grímsvötn tephras is 13% in-excess of secular equilibrium, but products of the 20th century Grímsvötn eruptions have equilibrium (^(210)Pb /^(226)Ra). Modeling of oxygen isotope exchange between disequilibrium phenocrysts and magmas, and these short-lived U-series nuclides yields a coherent age for the Laki–Grímsvötn magma reservoir between 100 and 1000 yrs. We propose the existence of uniquely fingerprinted, low-δ^(18)O, homogeneous, large volume, and long-lived basaltic reservoir beneath the Laki–Grímsvötn volcanic system that has been kept alive in its position above the center of the Icelandic mantle plume. Melt generation, crustal assimilation, magma storage and homogenization all took place in only a few thousands of years at most

Volcanic sulfate aerosol formation in the troposphere

International audienceThe isotopic composition of volcanic sulfate provides insights into the atmospheric chemical processing of volcanic plumes. First, mass-independent isotopic anomalies quantified by Δ17O and to a lesser extent Δ33S and Δ36S in sulfate depend on the relative importance of different oxidation mechanisms that generate sulfate aerosols. Second, the isotopic composition of sulfate (δ34S and δ18O) could be an indicator of fractionation (distillation/condensation) processes occurring in volcanic plumes. Here we present analyses of O-and S isotopic compositions of volcanic sulfate absorbed on very fresh volcanic ash from nine moderate historical eruptions in the Northern Hemisphere. Most of our volcanic sulfate samples, which are thought to have been generated in the troposphere or in the tropopause region, do not exhibit any significant mass-independent fractionation (MIF) isotopic anomalies, apart from those from an eruption of a Mexican volcano. Coupled to simple chemistry model calculations representative of the background atmosphere, our data set suggests that although H2O2 (a MIF-carrying oxidant) is thought to be by far the most efficient sulfur oxidant in the background atmosphere, it is probably quickly consumed in large dense tropospheric volcanic plumes. We estimate that in the troposphere, at least, more than 90% of volcanic secondary sulfate is not generated by MIF processes. Volcanic S-bearing gases, mostly SO2, appear to be oxidized through channels that do not generate significant isotopically mass-independent sulfate, possibly via OH in the gas phase and/or transition metal ion catalysis in the aqueous phase. It is also likely that some of the sulfates sampled were not entirely produced by atmospheric oxidation processes but came out directly from volcanoes without any MIF anomalies

Rhyolite generation prior to a Yellowstone supereruption: insights from the Island Park-Mount Jackson rhyolite series

The Yellowstone volcanic field is one of the largest and best-studied centres of rhyolitic volcanism on Earth, yet it still contains little-studied periods of activity. Such an example is the Island Park–Mount Jackson series, which erupted between the Mesa Falls and Lava Creek caldera-forming events as a series of rhyolitic domes and lavas. Here we present the first detailed characterisation of these lavas and use our findings to provide a framework for rhyolite generation in Yellowstone between 1·3 and 0·6 Ma, as well as to assess whether magmatic evolution hints at a forthcoming super-eruption. These porphyritic (15–40% crystals) lavas contain mostly sanidine and quartz with lesser amounts of plagioclase (consistent with equilibrium magmatic modelling via rhyolite-MELTS) and a complex assemblage of mafic minerals. Mineral compositions vary significantly between crystals in each unit, with larger ranges than expected from a single homogeneous population in equilibrium with its host melt. Oxygen isotopes in quartz and sanidine indicate slight depletions (δ18Omagma of 5·0–6·1‰), suggesting some contribution by localised remelting of hydrothermally altered material in the area of the previous Mesa Falls Tuff-related caldera collapse. The preservation of variable O isotopic compositions in quartz requires crystal entrainment less than a few thousand years prior to eruption. Late entrainment of rhyolitic material is supported by the occurrence of subtly older sanidines dated by single-grain 40Ar/39Ar geochronology. The eruption ages of the lavas show discrete clusters illustrating that extended quiescence (&gt;100 kyr) in magmatic activity may be a recurring feature in Yellowstone volcanism. Ubiquitous crystal aggregates, dominated by plagioclase, pyroxene and Fe–Ti oxides, are interpreted as cumulates co-erupted with their extracted liquid. Identical crystal aggregates are found in both normal-δ18O and low-δ18O rocks from Yellowstone, indicating that common petrogenetic processes characterise both volcanic suites, including the late-stage extraction of melt from an incrementally built upper crustal mush zone

Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

The diffusion of molecular water (H2Om) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δDH2O) to the glass (δDgl) offset by a glass-H2O fractionation factor (ΔDgl-H2O = δDgl – δDH2O) which is approximately -33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H2O during eruption, transport, and emplacement. To aid in the interpretation of δDgl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 oC for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 oC and below 250 oC. Lower temperature experiments yield raw ΔDgl-H2O values in the range of -33 ± 11‰. Experiments at 225 oC using both positive and negative initial ΔDgl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H2O vapor (103lnαgl-H2O) below 250 oC. Variation in ΔDgl-H2O (-33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 oC yield unexpectedly low δDgl values with ΔDgl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H2O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H2Om and 1.0 wt% hydroxl (OH-) in the highest P-T experiment at 375 oC and 21.1 MPa. Therefore, we employ a 1D isotope diffusion– reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 103lnαgl-H2O values of -33‰± 5‰and -25‰± 5‰at 175 oC and 225 oC, respectively. At higher temperatures, ΔDgl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δDgl value is highly responsive to changes in the δDgl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δDgl fractionation with progressive glass hydration. These results indicate that the observed ΔDgl-H2O of ~-33 ± 11‰ can be cautiously applied as an equilibrium 103lnαgl-H2O value to natural silicic glasses hydrated below 250 oC to identify hydration sources. This approximate ΔDgl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H2O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface

Multi-cyclic and isotopically diverse silicic magma generation in an arc volcano : Gorely Eruptive Center, Kamchatka, Russia

The Kamchatka Peninsula is home to some of the most frequent and prolific subduction-related volcanic activity in the world, with the largest number of caldera-forming eruptions per length of the volcanic arc. Among those, Gorely volcano has a topographically prominent Late Pleistocene caldera (13 km × 12 km, estimated to have produced >100 km3 of magma), which is now almost completely filled by a central cone. We report new 40Ar/39Ar ages and geochemical and isotopic data for newly recognized Mid-Pleistocene ignimbrite units of large but unknown volume sourced from the Gorely eruptive center, most of which were deposited in marginal glacial conditions. These ignimbrites have crystallinities of 9–24% and most are quartz-, amphibole-, and zircon-undersaturated. Additionally, we studied 32 eruptive units, including stratigraphically constrained Holocene tephra, and pre- and post-caldera lava sequences, to understand the petrogenetic and temporal evolution of this long-lived, multi-cyclic, arc volcano. Material erupted prior to the formation of the modern Gorely edifice, including the voluminous ignimbrites and eruptions of the ‘pra-Gorely’ stage, consist primarily of dacite and andesite, whereas sequences of the modern Gorely edifice are represented by basalt to basaltic andesite. MELTS and EC-AFC modeling shows that it is possible to obtain silicic compositions near those of the evolved ignimbrite compositions through 60–75% fractional crystallization at 1 kbar and nickel–nickel oxide (NNO) oxygen fugacity. However, our newly compiled major and trace element data for Gorely yield two separate bimodal peaks in a SiO2–frequency diagram, showing a prominent Daly Gap, with a deficiency in andesite. Trace element concentrations define two separate trends, one for more silicic and another for more mafic sequences. Additionally, δ18Omelt values reconstructed from coexisting plagioclase and clinopyroxene phenocrysts range from a low value of 4·85‰ to a normal value of 6·22‰. The low δ18O values range throughout the known lifespan of Gorely, with the lowest value being from the first known ignimbrite to erupt, indicating episodic but temporally decreasing crustal assimilation of previously hydrothermally altered material. 87Sr/86Sr and 143Nd/144Nd ratios show wide ranges from 0·70328 to 0·70351 and from 0·51303 to 0·51309 respectively, also suggesting incorporation of surrounding crust, although there are less clear trends throughout the lifespan of Gorely. The combination of light and diverse δ18O values with elevated 87Sr/86Sr and low 143Nd/144Nd ratios suggests contamination by older and isotopically diverse, low-δ18O country-rocks, such as the neighboring 11 Ma Akhomten granitic massif, which shows ranges in δ18O, 87Sr/86Sr, and 144Nd/143Nd values overlapping with the Gorely magmas. In addition, the presence of glomerocrysts and mafic enclaves in the majority of Gorely dacites indicates a period of crystal settling and subsequent intrusion of hot, primitive basalt that probably triggered eruption. Finally, elevated Nb concentrations relative to other Kamchatkan volcanoes suggest that Gorely magmas may involve an enriched component, probably caused by delamination of a lower crustal root. Our results argue for an incremental view of silicic magma generation at so-called ‘long-term eruptive centers’, in Kamchatka and worldwide, consisting of alternating episodes of magmatic and hydrothermal activity, and glacial advances and retreats. We demonstrate that large-volume, isotopically distinct, silicic magma can be generated rapidly between cone-building phases of volcanic activity through a combination of fractional crystallization, assimilation of older country rocks, and shallow assimilation of hydrothermally altered but otherwise petrochemically similar older intracaldera tuffs and intrusions. These transient shallow silicic magma chambers empty nearly completely in ignimbrite-forming eruptions after 103–105 years of assembly, partially triggered by glacial surface dynamics

Insights on lava–ice/snow interactions from large-scale basaltic melt experiments

Quantitative measurements of interactions between lava and ice/snow are critical for improving our knowledge of glaciovolcanic hazards and our ability to use glaciovolcanic deposits for paleoclimate reconstructions. However, such measurements are rare because the eruptions tend to be dangerous and not easily accessible. To address these difficulties, we conducted a series of pilot experiments designed to allow close observation, measurements, and textural documentation of interactions between basaltic melt and ice. Here we report the results of the first experiments, which comprised controlled pours of as much as 300 kg of basaltic melt on top of ice. Our experiments provide new insights on (1) estimates for rates of heat transfer through boundary layers and for ice melting; (2) controls on rates of lava advance over ice/snow; (3) formation of lava bubbles (i.e., Limu o Pele) by steam from vaporization of underlying ice or water; and (4) the role of within-ice discontinuities to facilitate lava migration beneath and within ice. The results of our experiments confirm field observations about the rates at which lava can melt snow/ice, the efficacy with which a boundary layer can slow melting rates, and morphologies and textures indicative of direct lava-ice interaction. They also demonstrate that ingestion of external water by lava can create surface bubbles (i.e., Limu) and large gas cavities. We propose that boundary layer steam can slow heat transfer from lava to ice, and present evidence for rapid isotopic exchange between water vapor and melt. We also suggest new criteria for identifying ice-contact features in terrestrial and martian lava flows

Geochemistry of the Late Holocene rocks from the Tolbachik volcanic field, Kamchatka: Quantitative modelling of subduction-related open magmatic systems

We present new major and trace element, high-precision Sr-Nd-Pb (double spike), and Oisotope data for the whole range of rocks from the Holocene Tolbachik volcanic field in the Central Kamchatka Depression (CKD). The Tolbachik rocks range from high-Mg basalts to low-Mg basaltic trachyandesites. The rocks considered in this paper represent mostly Late Holocene eruptions (using tephrochronological dating), including historic ones in 1941, 1975-1976 and 2012-2013. Major compositional features of the Tolbachik volcanic rocks include the prolonged predominance of one erupted magma type, close association of middle-K primitive and high-K evolved rocks, large variations in incompatible element abundances and ratios but narrow range in isotopic composition. We quantify the conditions of the Tolbachik magma origin and evolution and revise previously proposed models. We conclude that all Tolbachik rocks are genetically related by crystal fractionation of medium-K primary magmas with only a small range in trace element and isotope composition. The primary Tolbachik magmas contain ~14 wt% MgO and ~4% wt% H2O and originated by partial melting (~6%) of moderately depleted mantle peridotite with Indian-MORB-type isotopic composition at temperature of ~1250oC and pressure of ~2 GPa. The melting of the mantle wedge was triggered by slab-derived hydrous melts formed at ~2.8 GPa and ~725oC from a mixture of sediments and MORB- and Meiji- type altered oceanic crust. The primary magmas experienced a complex open-system evolution termed Recharge-Evacuation-Fractional Crystallization (REFC). First the original primary magmas underwent open-system crystal fractionation combined with periodic recharge of the magma chamber with more primitive magma, followed by mixing of both magma types, further fractionation and finally eruption. Evolved high-K basalts, which predominate in the Tolbachik field, and basaltic trachyandesites erupted in 2012-2013 approach steady-state REFC liquid compositions at different eruption or replenishment rates. Intermediate rocks, including high-K, high-Mg basalts, are formed by mixing of the evolved and primitive magmas. Evolution of Tolbachik magmas is associated with large fractionation between incompatible trace elements (e.g., Rb/Ba, La/Nb, Ba/Th) and is strongly controlled by the relative difference in partitioning between crystal and liquid phases. The Tolbachik volcanic field shows that open-system scenarios provide more plausible and precise descriptions of long-lived arc magmatic systems than simpler, but often geologically unrealistic, closed-system models

Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr–Nd–Pb–Hf–O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low δ18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high δ18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc