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

Arc Magmas from Slab to Eruption: The Case of Kliuchevskoy Volcano

Arc magmas are generated by a number of mantle and crustal processes. Our multidisciplinary, long-term research is aimed at deciphering these processes for a single arc volcano, Kliuchevskoy volcano in Kamchatka. Some key results of the study follow: 1) Modeling of trace element and H2O contents in melt inclusions suggests that the primary magmas originate via hydrous flux-melting of the mantle wedge at temperatures close to the dry peridotite solidus. The role of decompression melting is minor or absent at Kliuchevskoy and other arc volcanoes built on relatively thick crust. 2) Geochemistry of high-Mg olivine suggests that primary Kliuchevskoy magmas have substantial contribution from olivine-free pyroxenite (up to 30 %), which could be formed by reaction of slab melts (or supercritical fluids) with mantle wedge peridotite. 3) Parental Kliuchevskoy melts start to crystallize as deep as the Moho boundary, and the erupted magmas reflect multistage and complex processes of crystallization, magma mixing and crustal assimilation. None of the Kliuchevskoy rocks analyzed thus far represent true primary melt compositions. 4) The Kliuchevskoy Holocene eruptive history is not steady-state in terms of eruption rate and geochemistry. There are two millenial cycles with major and trace element and OSr- Nd-Pb and U-series isotope compositions of the magmas changing gradually from more to less affected by crustal (?) assimilation. The onset of the cycles correlates with periods of enhanced volcanic activity in Kamchatka, suggesting that the extent of magma-crust interaction is inversely related to magma production rate and thus magma flux from the mantle

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

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

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

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 dynamic

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

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

Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios ^(17)O/^(16)O, ^(18)O/^(16)O, ^(34)S/^(32)S, and ^(33)S/^(32)S, ^(36)S/^(32)S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km^3), Mt. Spurr (Alaska, 1953, <1 km3), Gjalp (Iceland, 1996, 1998, <1 km^3), Pinatubo (Phillipines, 1991, 10 km^3), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km^3), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km^3), and Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km^3 and 2.04 Ma 2500 km^3, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage. This global dataset spans a significant range in δ^(34)S, δ^(18)O, and Δ^(17)O of sulfate (29‰, 30‰, and 3.3‰, respectively) with oxygen isotopes recording mass-independent (Δ^(17)O > 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ^(17)O > 0.2‰ is present as 1–10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ^(17)O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ^(34)S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ^(33)S/^(32)S vs. ^(34)S/^(32)S and ^(36)S/^(32)S vs. ^(34)S/^(32)S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO_2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO_2 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO_2 in volcanic plumes. Above-zero Δ^(17)O values and their positive correlation with δ^(18)O in sulfate can be explained by oxidation by high-δ^(18)O and high-Δ^(17)O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ^(17)O values, and the largest range of δ^(18)O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3–8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion

Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5- 38.0°S): Constraints on Mantle Wedge and Input Compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O-Sr-Nd-Hf-Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5-38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile Trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9-5.6 and backarc = 5.0-5.4 per mil) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf-Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab