Thor H. Hansteen

An alternative data acquisition and evaluation strategy for improved isotope ratio precision using LA-MC-ICP-MS applied to stable and radiogenic strontium isotopes in carbonates

Strontium isotopes in various marine carbonates were determined using an “AXIOM” MC-ICP-MS in combination with a NewWave UP193 laser ablation unit. Using a modified measurement and data reduction strategy, an external reproducibility of 87Sr/86Sr ratios in carbonates of about 19 ppm (RSD) was achieved. For recent and sub-recent marine carbonates a mean radiogenic strontium isotope ratio 87Sr/86Sr of 0.709170 ± 0.000007 (2SE) was determined, which agrees well with the value of 0.7091741 ± 0.0000024 (2SE) reported for modern sea water (J. M. McArthur, D. Rio, F. Massari, D. Castrodi, T. R. Bailey, M. Thirlwall and S. Houghton, Palaeogeogr. Palaeoclimatol. Palaeoeco., 2006, 242(126), 2006). Compared to published laser-based methods, an improved accuracy and precision was achieved by applying a new data reduction protocol using the simultaneous responses of all isotopes measured. The latter is considered as a new principal approach for isotope ratio evaluation using LA-MC-ICP-MS. A major advantage of the presented method is the direct determination of the stable strontium isotope fractionation. Providing reproducible sample ablation, introduction into the plasma and stable plasma condition, this method excludes the efforts of a quantitative strontium recovery after ion chromatographic separation to avoid additional fractionation of the sample strontium due to chemical pre-treatment/separation (ion chromatography and solution preparation), and is therefore, together with the quicker sample preparation and spatially resolved analysis, advantageous when compared to published solution–nebulization bracketing-standard MC-ICP-MS methods for stable strontium isotope determination.

The effects of flank collapses on volcano plumbing systems

The growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches, explosive eruptions, and tsunamis. El Hierro, the youngest island of the Canary Archipelago, has been repeatedly affected by such mass-wasting events in the last 1 Ma. Our field observations and petrological data suggest that the largest and most recent of these flank collapses—the El Golfo landslide—likely influenced the magma plumbing system of the island, leading to the eruption of higher proportions of denser and less evolved magmas. The results of our numerical simulations indicate that the El Golfo landslide generated pressure changes exceeding 1 MPa down to upper-mantle depths, with local amplification in the surroundings and within the modeled magma plumbing system. Stress perturbations of that order might drastically alter feeding system processes, such as degassing, transport, differentiation, and mixing of magma batches.

Samples from the Jurassic ocean crust beneath Gran Canaria, La Palma and Lanzarote (Canary Islands)

Gabbro and minor metabasalt fragments of MORB composition were found on three of the seven Canary Islands. On Gran Canaria, they occur as metamorphosed (greenschist facies) metabasalt and metagabbro clasts in Miocene fanglomerates and sandstones overlying the shield basalts. On Lanzarote and La Palma, MORB gabbros occur as xenoliths in Pleistocene and historic basanite scoria cones and lava flows. The MORB xenoliths are interpreted as fragments of layers 2 and 3 of the underlying Mesozoic oceanic crust, based on mineral compositions (An-rich plagioclase, Ti- and Al-poor clinopyroxene, ± orthopyroxene ± olivine), depleted major and trace element signatures, and Jurassic ages (ca. 180 Ma) determined on single primary plagioclase and secondary amphibole crystals using the 40Ar/39Ar laser technique. The Lanzarote gabbros are very mafic (mg# 87 to 89 in clinopyroxene), moderately deformed, and highly depleted. Gran Canaria gabbros are more evolved (mg# 69 to 83 in clinopyroxene) and texturally mostly isotropic. La Palma MORB gabbros have a range of compositions (mg# 68 to 83 in clinopyroxene), some rocks being strongly metasomatized by interaction with basanite magma. The occurrence of MORB fragments on Lanzarote provides definite evidence that oceanic crust beneath the Canary Island archipelago continues at least as far east as the eastern Canary Islands. We postulate that MORB gabbros on Lanzarote which are commonly associated with peridotite xenoliths, represent the base of oceanic layer 3 where gabbros and peridotites were possibly tectonically interleaved. Such tectonic mixing would explain the enigmatic seismic velocities in this area. Gabbro xenoliths from La Palma were derived from within layer 3, probably from wall rock close to magma reservoirs emplaced during the Pleistocene/Holocene growth of La Palma. The Gran Canaria xenoliths are interpreted to represent the metamorphosed layer 2 and upper layer 3. The abundance of lower crustal xenoliths emphasizes the importance of the lower crust and crust-mantle boundary zone as a major level of magma accumulation.

Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas

Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (δ18O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ18O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ18O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ18O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ18O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics.

Combined bromine and chlorine release from large explosive volcanic eruptions: A threat to stratospheric ozone?

Large explosive volcanic eruptions inject gases, aerosols, and fi ne ashes into the stratosphere, potentially infl uencing climate. Emissions of chlorine (Cl) and bromine (Br) from such large eruptions play an important role for catalytic destruction of ozone in the stratosphere, but hitherto the global effects of simultaneous catastrophic release of volcanic Br and Cl into the stratosphere have not been investigated. The Br release from 14 large explosive eruptions throughout Nicaragua covering an entire subduction zone segment in the past 70 ka was determined with petrologic methods. Melt inclusions in volcanic phenocrysts were analyzed using a new optimized synchrotron‐X-ray fl uorescence microprobe set-up. Single eruptions produced Br outputs of 4‐600 kt, giving an average Br emission of 27 kt per eruption. Using the assumption that 10% of the emitted halogens reach the stratosphere, the average Br and Cl loading to the stratosphere would be 3 ppt and 1500 ppt, respectively, which together would account for 185% of the preindustrial equivalent effective stratospheric Cl loading. We thus conclude that many large tropical volcanic eruptions had and have the potential to substantially deplete ozone on a global scale, eventually forming future ozone holes.

Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands: Some repercussions of giant landslides on ocean island magmatism

Large-scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing >20–25 km3 of the volcanos north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large-scale landslides and subsequent volcanism/magmatism during Tenos evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Tenos most evolved, low-density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low-Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long-term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano-tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near-surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse-induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short-term compositional variations may also directly relate to major flank collapse events.

Phenotypic plasticity of coralline algae in a High CO2 world.

It is important to understand how marine calcifying organisms may acclimatize to ocean acidification to assess their survival over the coming century. We cultured the cold water coralline algae, Lithothamnion glaciale, under elevated pCO2 (408, 566, 770, and 1024 μatm) for 10 months. The results show that the cell (inter and intra) wall thickness is maintained, but there is a reduction in growth rate (linear extension) at all elevated pCO2. Furthermore a decrease in Mg content at the two highest CO2 treatments was observed. Comparison between our data and that at 3 months from the same long-term experiment shows that the acclimation differs over time since at 3 months, the samples cultured under high pCO2 showed a reduction in the cell (inter and intra) wall thickness but a maintained growth rate. This suggests a reallocation of the energy budget between 3 and 10 months and highlights the high degree plasticity that is present. This might provide a selective advantage in future high CO2 world.

Gas emissions from five volcanoes in northern Chile and implications for the volatiles budget of the Central Volcanic Zone

This study performed the first assessment of the volcanic gas output from the Central Volcanic Zone (CVZ) of northern Chile. We present the fluxes and compositions of volcanic gases (H2O, CO2, H2, HCl, HF, and HBr) from five of the most actively degassing volcanoes in this region—Lascar, Lastarria, Putana, Ollague, and San Pedro—obtained during field campaigns in 2012 and 2013. The inferred gas plume compositions for Lascar and Lastarria (CO2/Stot = 0.9–2.2; Stot/HCl = 1.4–3.4) are similar to those obtained in the Southern Volcanic Zone of Chile, suggesting uniform magmatic gas fingerprint throughout the Chilean arc. Combining these compositions with our own UV spectroscopy measurements of the SO2 output (summing to ~1800 t d−1 for the CVZ), we calculate a cumulative CO2 output of 1743–1988 t d−1 and a total volatiles output of >20,200 t d−1.

Century-scale trends and seasonality in pH and temperature for shallow zones of the Bering Sea

Significance Increasing atmospheric CO2 concentrations are potentially affecting marine ecosystems twofold, by warming and acidification. The rising amount of CO2 taken up by the ocean lowers the saturation state of calcium carbonate, complicating the formation of this key biomineral used by many marine organisms to build hard parts like skeletons or shells. Reliable time-series data of seawater pH are needed to evaluate the ongoing change and compare long-term trends and natural variability. For the high-latitude ocean, the region facing the strongest CO2 uptake, such time-series data are so far entirely lacking. Our study provides, to our knowledge, the first reconstruction of seasonal cycle and long-term trend in pH for a high-latitude ocean obtained from 2D images of stable boron isotopes from a coralline alga. No records exist to evaluate long-term pH dynamics in high-latitude oceans, which have the greatest probability of rapid acidification from anthropogenic CO2 emissions. We reconstructed both seasonal variability and anthropogenic change in seawater pH and temperature by using laser ablation high-resolution 2D images of stable boron isotopes (δ11B) on a long-lived coralline alga that grew continuously through the 20th century. Analyses focused on four multiannual growth segments. We show a long-term decline of 0.08 ± 0.01 pH units between the end of the 19th and 20th century, which is consistent with atmospheric CO2 records. Additionally, a strong seasonal cycle (∼0.22 pH units) is observed and interpreted as episodic annual pH increases caused by the consumption of CO2 during strong algal (kelp) growth in spring and summer. The rate of acidification intensifies from –0.006 ± 0.007 pH units per decade (between 1920s and 1960s) to –0.019 ± 0.009 pH units per decade (between 1960s and 1990s), and the episodic pH increases show a continuous shift to earlier times of the year throughout the centennial record. This is indicative of ecosystem shifts in shallow water algal productivity in this high-latitude habitat resulting from warming and acidification.

Magmatic evolution of the Cadamosto Seamount, Cape Verde: beyond the spatial extent of EM1

The Cadamosto Seamount is an unusual volcanic centre from Cape Verde, characterised by dominantly evolved volcanics, in contrast to the typically mafic volcanic centres at Cape Verde that exhibit only minor volumes of evolved volcanics. The magmatic evolution of Cadamosto Seamount is investigated to quantify the role of magma-crust interaction and thus provide a perspective on evolved end-member volcanism of Cape Verde. The preservation of mantle source signatures by Nd–Pb isotopes despite extensive magmatic differentiation provides new insights into the spatial distribution of mantle heterogeneity in the Cape Verde archipelago. Magmatic differentiation from nephelinite to phonolite involves fractional crystallisation of clinopyroxene, titanite, apatite, biotite and feldspathoids, with extensive feldspathoid accumulation being recorded in some evolved samples. Clinopyroxene crystallisation pressures of 0.38–0.17 GPa for the nephelinites constrain this extensive fractional crystallisation to the oceanic lithosphere, where no crustal assimilants or rafts of subcontinental lithospheric mantle are available. In turn, magma-crust interaction has influenced the Sr, O and S isotopes of the groundmass and late crystallising feldspathoids, which formed at shallow crustal depths reflecting the availability of oceanic sediments and anhydrite precipitated in the ocean crust. The Nd–Pb isotopes have not been affected by these processes of magma-crust interaction and hence preserve the mantle source signature. The Cadamosto Seamount samples have high 206Pb/204Pb (>19.5), high εNd (+6 to +7) and negative Δ8/4Pb, showing affinity with the northern Cape Verde islands as opposed to the adjacent southern islands. Hence, the Cadamosto Seamount in the west is located spatially beyond the EM1-like component found further east. This heterogeneity is not encountered in the oceanic lithosphere beneath the Cadamosto Seamount despite greater extents of fractional crystallisation at oceanic lithospheric depths than the islands of Fogo and Santiago. Our data provide new evidence for the complex geometry of the chemically zoned Cape Verde mantle source.