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 (>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

Alteration of volcaniclastic deposits at Minna Bluff : geochemical insights on mineralizing environment and climate during the Late Miocene in Antarctica

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 3258–3280, doi:10.1002/2014GC005422.Secondary minerals in volcaniclastic deposits at Minna Bluff, a 45 km long peninsula in the Ross Sea, are used to infer processes of alteration and environmental conditions in the Late Miocene. Glassy volcaniclastic deposits are altered and contain phillipsite and chabazite, low to high-Mg carbonates, chalcedony, and clay. The δ18O of carbonates and chalcedony is variable, ranging from −0.50 to 21.53‰ and 0.68 to 10.37‰, respectively, and δD for chalcedony is light (−187.8 to −220.6‰), corresponding to Antarctic meteoric water. A mean carbonate 87Sr/86Sr ratio of 0.70327 ± 0.0009 (1σ, n = 12) is comparable to lava and suggests freshwater, as opposed to seawater, caused the alteration. Minerals were precipitated at elevated temperatures (91 and 104°C) based on quartz-calcite equilibrium, carbonate 13C-18C thermometry (Δ47 derived temperature = 5° to 43°C) and stability of zeolites in geothermal systems (>10 to ∼100°C). The alteration was a result of isolated, ephemeral events involving the exchange between heated meteoric water and glass during or soon after the formation of each deposit. Near-surface evaporative distillation can explain 18O-enriched compositions for some Mg-rich carbonates and chalcedony. The δ18Owater calculated for carbonates (−15.8 to −22.9‰) reveals a broad change, becoming heavier between ∼12 and ∼7 Ma, consistent with a warming climate. These findings are independently corroborated by the interpretation of Late Miocene sedimentary sequences recovered from nearby sediment cores. However, in contrast to a cold-based thermal regime proposed for ice flow at core sites, wet-based conditions prevailed at Minna Bluff; a likely consequence of high heat flow associated with an active magma system.This research was funded by a collaborative grant NSF OPP 05-38033. It also was supported by UNED/NSF 250550001146, NSF grants EAR-0949191, ARC-1215551, EAR-1325054, EAR-1352212, EAR-1049351, ACS grant 51182-DNI2, DOE grants DE-FG02-13ER16402, and DE-SC0010288, a Hellman Fellowship, and a Katzner grant (BGSU).2015-02-1

The Systematics of Olivine CaO + Cr-Spinel in High-Mg# Arc Volcanic Rocks: Evidence for in-Situ Mantle Wedge Depletion at the Arc Volcanic Front

We investigated the state of the arc background mantle (i.e. mantle wedge without slab component) by means of olivine CaO and its Cr-spinel inclusions in a series of high-Mg# volcanic rocks from the Quaternary Trans-Mexican Volcanic Belt. Olivine CaO was paired with the Cr# [molar Cr/(Cr + Al) *100] of Cr-spinel inclusions, and 337 olivine+Cr-spinel pairs were obtained from 33 calc-alkaline, high-K and OIB-type arc front volcanic rocks, and three monogenetic rear-arc basalts that lack subduction signatures. Olivine+Cr-spinels display coherent elemental and He–O isotopic systematics that contrast with the compositional diversity of the bulk rocks. All arc front olivines have low CaO (0.135 ± 0.029 wt %) relative to rear-arc olivines which have the higher CaO (0.248 ± 0.028 wt %) of olivines from mid-ocean ridge basalts. Olivine 3He/4He–δ18O isotope systematics confirm that the olivine+Cr-spinels are not, or negligibly, affected by crustal basement contamination, and thus preserve compositional characteristics of primary arc magmas. Variations in melt H2O contents in the arc front series and the decoupling of olivine CaO and Ni are inconsistent with controls on the olivine CaO by melt water and/or secondary mantle pyroxenites. Instead, we propose that low olivine CaO reflects the typical low melt CaO of high-Mg# arc magmas erupting through thick crust. We interpret the inverse correlation of olivine CaO and Cr-spinel Cr# over a broad range of Cr# (~10–70) as co-variations of CaO, Al and Cr of their (near) primary host melts, which derived from a mantle that has been variably depleted by slab-flux driven serial melt extraction. Our results obviate the need for advecting depleted residual mantle from rear- and back-arc region, but do not upset the larger underlying global variations of melt CaO high-Mg# arc magmas worldwide, despite leading to considerable regional variations of melt CaO at the arc front of the Trans-Mexican Volcanic Belt

New Biotite and Muscovite Isotopic Reference Materials, USGS57 and USGS58, for δ2H Measurements–A Replacement for NBS 30

The advent of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) coupled with a high temperature conversion (HTC) system enabled faster, more cost effective, and more precise δ2H analysis of hydrogen-bearing solids. Accurate hydrogen isotopic analysis by on-line or off-line techniques requires appropriate isotopic reference materials (RMs). A strategy of two-point calibrations spanning δ2H range of the unknowns using two RMs is recommended. Unfortunately, the supply of the previously widely used isotopic RM, NBS 30 biotite, is exhausted. In addition, recent measurements have shown that the determination of δ2H values of NBS 30 biotite on the VSMOW-SLAP isotope-delta scale by on-line HTC systems with CF-IRMS may be unreliable because hydrogen in this biotite may not be converted quantitatively to molecular hydrogen. The δ2HVSMOW-SLAP values of NBS 30 biotite analyzed by on-line HTC systems can be as much as 21 mUr (or ‰) too positive compared to the accepted value of −65.7 mUr, determined by only a few conventional off-line measurements. To ensure accurate and traceable on-line hydrogen isotope-ratio determinations in mineral samples, we here propose two isotopically homogeneous, hydrous mineral RMs with well-characterized isotope-ratio values, which are urgently needed. The U.S. Geological Survey (USGS) has prepared two such RMs, USGS57 biotite and USGS58 muscovite. The δ2H values were determined by both glassy carbon-based on-line conversion and chromium-based on-line conversion, and results were confirmed by off-line conversion. The quantitative conversion of hydrogen from the two RMs using the on-line HTC method was carefully evaluated in this study. The isotopic compositions of these new RMs with 1-σ uncertainties and mass fractions of hydrogen are: USGS57 (biotite) δ2HVSMOW-SLAP = −91.5 ± 2.4 mUr (n =24) Mass fraction hydrogen = 0.416 ± 0.002% (n=4) Mass fraction water = 3.74 ± 0.02% (n=4) USGS58 (muscovite) δ2HVSMOW-SLAP = −28.4 ± 1.6 mUr (n =24) Mass fraction hydrogen = 0.448 ± 0.002% (n=4) Mass fraction water = 4.03 ± 0.02% (n =4). These δ2HVSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle

Isotopic and Petrologic Investigation, and a Thermomechanical Model of Genesis of Large-Volume Rhyolites in Arc Environments: Karymshina Volcanic Complex, Kamchatka, Russia

The Kamchatka Peninsula of eastern Russia is currently one of the most volcanically active areas on Earth where a combination of >8 cm/yr subduction convergence rate and thick continental crust generates large silicic magma chambers, reflected by abundant large calderas and caldera complexes. This study examines the largest center of silicic 4-0.5 Ma Karymshina Volcanic Complex, which includes the 25 × 15 km Karymshina caldera, the largest in Kamchatka. A series of rhyolitic tuff eruptions at 4 Ma were followed by the main eruption at 1.78 Ma and produced an estimated 800 km3 of rhyolitic ignimbrites followed by high-silica rhyolitic post-caldera extrusions. The postcaldera domes trace the 1.78 Ma right fracture and form a continuous compositional series with ignimbrites. We here present results of a geologic, petrologic, and isotopic study of the Karymshina eruptive complex, and present new Ar-Ar ages, and isotopic values of rocks for the oldest pre- 1.78 Ma caldera ignimbrites and intrusions, which include a diversity of compositions from basalts to rhyolites. Temporal trends in δ18O, 87Sr/86Sr, and 144Nd/143Nd indicate values comparable to neighboring volcanoes, increase in homogeneity, and temporal increase in mantle-derived Sr and Nd with increasing differentiation over the last 4 million years. Data are consistent with a batholithic scale magma chamber formed by primarily fractional crystallization of mantle derived composition and assimilation of Cretaceous and younger crust, driven by basaltic volcanism and mantle delaminations. All rocks have 35–45% quartz, plagioclase, biotite, and amphibole phenocrysts. Rhyolite-MELTS crystallization models favor shallow (2 kbar) differentiation conditions and varying quantities of assimilated amphibolite partial melt and hydrothermally-altered silicic rock. Thermomechanical modeling with a typical 0.001 km3/yr eruption rate of hydrous basalt into a 38 km Kamchatkan arc crust produces two magma bodies, one near the Moho and the other engulfing the entire section of upper crust. Rising basalts are trapped in the lower portion of an upper crustal magma body, which exists in a partially molten to solid state. Differentiation products of basalt periodically mix with the resident magma diluting its crustal isotopic signatures. At the end of the magmatism crust is thickened by 8 km. Thermomechanical modeling show that the most likely way to generate large spikes of rhyolitic magmatism is through delamination of cumulates and mantle lithosphere after many millions of years of crustal thickening. The paper also presents a chemical dataset for Pacific ashes from ODDP 882 and 883 and compares them to Karymshina ignimbrites and two other Pleistocene calderas studied by us in earlier works

In situ probing of the present-day zircon-bearing magma chamber at Krafla, Northeastern Iceland

Active felsic magmatism has been rarely probed in situ by drilling but one recent exception is quenched rhyolite sampled during the 2009 Iceland Deep Drilling Project (IDDP). We report finding of rare zircons of up to ∼100 µm in size in rhyolite glasses from the IDDP-1 well products and the host 1724 AD Viti granophyres. The applied SHRIMP U-Th dating for both the IDDP and the Viti granophyre zircons gives zero-age (±2 kyr), and therefore suggests that the IDDP-1 zircons have crystallized from an active magma intrusion rather than due to the 20–80 ka post-caldera magmatic episodes recorded by nearby domes and ridges. Ti-in-zircon geothermometer for Viti granophyre reveals zircon crystallization temperatures ∼800°C–900°C, whereas IDDP-1 rhyolite zircon cores show Ti content higher than 100 ppm, corresponding to temperatures up to ∼1,100°C according to the Ti-in-zircon thermometer. According to our thermochemical model at such elevated temperatures as 1,100°C, rhyolitic magma cannot be saturated with zircon and zircon crystallization is not possible. We explain this controversy by either kinetic effects or non-ideal Ti incorporation into growing zircons at low pressures that start to grow from nucleus at temperatures ∼930°C. High temperatures recorded by IDDP-1 zircon together with an occurrence of baddeleyite require that the rhyolite magma formed by partial melting of the host granophyre due to basaltic magma intrusion. Zr concentration profiles in glass around zircons are flat, suggesting residence in rhyolitic melt for >4 years. In our thermochemical modeling, three scenarios are considered. The host felsite rocks are intruded by: 1) a basaltic sill, 2) rhyolite magma 3) rhyolite sill connected to a deeper magmatic system. Based on the solution of the heat conduction equation accounting for the release of latent heat and effective thermal conductivity, these data confirm that the rhyolite magma could be produced by felsic crust melting as a result of injection of a basaltic or rhyolite sill during the Krafla Fires eruption (1975 AD)

Oxygen isotope heterogeneity of the mantle beneath the Canary Islands : insights from olivine phenocrysts

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 162 (2011): 349-363, doi:10.1007/s00410-010-0600-5.A relatively narrow range of oxygen isotopic ratios (δ18O = 5.05.4‰) is preserved in olivine of mantle xenoliths, mid-ocean ridge (MORB) and most ocean island basalts (OIB). The values in excess of this range are generally attributed either to the presence of a recycled component in the Earth’s mantle or to shallow level contamination processes. A viable way forward to trace source heterogeneity is to find a link between chemical (elemental and isotopic) composition of the earlier crystallized mineral phases (olivine) and the composition of their parental magmas, then using them to reconstruct the composition of source region. The Canary hotspot is one of a few that contains ~1-2 Ga old recycled ocean crust that can be traced to the core-mantle boundary using seismic tomography and whose origin is attributed to the mixing of at least three main isotopically distinct mantle components i.e., HIMU, DMM and EM. This work reports ion microprobe and single crystal laser fluorination oxygen isotope data of 148 olivine grains also analyzed for major and minor elements in the same spot. The olivines are from 20 samples resembling the most primitive shield stage picrite through alkali basalt to basanite series erupted on Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro, Canary Islands, for which shallow level contamination processes were not recognized. A broad range of δ18Oolivine values from 4.6 to 6.1‰ was obtained and explained by stable, long-term oxygen isotope heterogeneity of crystal cumulates present under different volcanoes. These cumulates are thought to have crystallized from mantle derived magmas uncontaminated at crustal depth, representing oxygen isotope heterogeneity of source region. A relationship between Ni×FeO/MgO and δ18Oolivine values found in one basanitic lava erupted on El Hierro, the westernmost island of the Canary Archipelago, was used to estimate oxygen isotope compositions of partial melts presumably originated from peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, ~12 Ga, recycled ocean crust) and pyroxenite (young, <1 Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9±0.3‰ for peridotite and pyroxenite derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB δ18O values. This difference together with the broad range of δ18O variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.This work was supported by the CNRS “poste rouge” grant to AG, the NSF EAR-CAREER-0844772 grant to IB and the CRPG-CNRS and at its initial stage by the DFG (grant SCHM 250/64) and the Alexander von Humboldt Foundation (Wolfgang Paul Award to A.V. Sobolev who provided access to the electron microprobe at the Max Planck Institute, Mainz, Germany)