Hans-Ulrich Schmincke

The annual volcanic gas input into the atmosphere, in particular into the stratosphere: a global data set for the past 100 years

We compiled a global data set of volcanic degassing during both explosive and quiescent volcanic events. The data set comprises estimates of gas emissions of volcanoes from Europe (e.g. Etna), Asia (e.g. Merapi), the Americas (e.g. Fuego), Africa (e.g. Erta Ale) and ocean islands (e.g. Kilauea) over the past 100 yr. The set includes 50 monitored volcanoes and ∼310 extrapolated explosively erupting volcanoes. Among the ∼360 volcanoes, 75% are located in the Northern and 25% in the Southern Hemisphere. We have estimated the total annual global volcanic sulfur emission into the atmosphere to be on the order of 7.5–10.5×1012 g/yr S (here as SO2), amounting to 10–15% of the annual anthropogenic sulfur output (∼70×1012 g/yr S during the decade 1981–1990) and 7.5–10.5% of the total global sulfur emission (e.g. biomass burning, anthropogenic, dimethylsulfide) with ∼100×1012 g/yr S. The estimates of other volcanic gases emitted (e.g. H2S, HCl) are based on the assumption that the different gas components emitted by a volcano are in equilibrium with each other. Accordingly, the molar ratios of the gas species in high-temperature fumaroles are similar to molar ratios equilibrated at depth where the gas separates from the magma. Thus, we can use the directly measured SO2 fluxes and known molar ratios (e.g. H2S/SO2) for a semi-quantitative estimate of other gas components emitted (e.g. H2S). The total annual emission of HCl is 1.2–170×1012 g/yr, that of H2S 1.5–37.1×1012 g/yr, of HF 0.7–8.6×1012 g/yr, of HBr 2.6–43.2×109 g/yr, and of OCS 9.4×107–3.2×1011 g/yr. We estimate an emission of 1.3×107–4.4×1010 g/yr for CS2.

Laacher See Tephra: A widespread isochronous late Quaternary tephra layer in central and northern Europe

A late Quaternary tephra layer, widespread in central and northern Europe, resulted from explosive Plinian and phreatomagmatic eruptions of the Laacher See Volcano 11,000 yr B.P. The tephra is distinguished from other late Quaternary andesitic-rhyolitic airfall tuff layers in northern Europe and from basaltic or trachytic tuff deposits in southern Europe by its phonolitic composition and abundance of sanidine, plagioclase, clinopyroxene, amphibole, and sphene. The proximal tephia sequence at Laacher See is divided into three main deposits: the predominantly Plinian deposits of Lower and Middle Laacher See Tephra (LLST and MLST) and phreatomagmatic deposits of the Upper Laacher See Tephra (ULST). The MLST member is further subdivided into beds A, B, and C1, C2, and C3. The chemical composition of the magma is highly differentiated phonolite in the LLST to MLST B sections but mafic phonolite in the MLST C1 to ULST sections. All deposits are considered to be isochronous, the frequency maximum of 16 radiocarbon datings indicating an eruption about 11,000 ±50 yr B.P. Distal ash was deposited in three main fans directed to the northeast (LST traced up to 1,100 km distance), south (LST traced up to 600 km), and southwest (LST traced up to 100 km). Tephrostratigraphic correlation of the distal ash deposits is based on (a) the major-element composition of glass shards, (b) lithology, and (c) heavy-mineral analyses. The northeastern fan consists of deposits from LLST, MLST B, and MLST C1 eruptive phases, the southern fan comprises MLST A, MLST C2, and ULST deposits, and the southwestern fan consists exclusively of ash from the ULST eruptive phase. Northeastern transport of ash during eruptive phases, with high Plinian eruption columns, but southern and southwestern transport of ash along phases of relatively low eruption columns, are interpreted in terms of prevailing southwesterly paleowinds at high altitudes (tropopause level?) but northerly winds dominating in the lower atmosphere. The Laacher See eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions, which also explains the near-vent shifting of LLST, MLST B, and MLST C1 iso-pach axes from east-southeast to northeast within the first 20 km of transport.

Volcanic and Chemical Evolution of the Canary Islands

The Canary Islands, a group of seven major volcanic islands, extends for almost 500 km roughly east-west 100 km off Northwest Africa. The islands formed chiefly during the last 20 Ma, although volcanic activity started during the Oligocene and possibly Eocene in the eastern island of Fuerteventura. Ages of the rapidly formed sub-Canarian mantle are presently active across the entire belt. Total volumes of individual islands are about 10 to 20 x 106 km3 of which the subaerial part generally makes up less than 10%.

SrNdPb isotopic evolution of Gran Canaria: Evidence for shallow enriched mantle beneath the Canary Islands

We report the Sr, Nd and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of Gran Canaria and (2) of five Miocene through Cretaceous sediment samples from DSDP site 397, located 100 km south of Gran Canaria. The isotope ratios of the Gran Canaria samples vary for 87Sr/86Sr: 0.70302–0.70346, for 143Nd/144Nd: 0.51275–0.51298, and for 206Pb/204Pb: 18.76–20.01. The Miocene and the Pliocene-Recent volcanics form distinct trends on isotope correlation diagrams. The most SiO2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and Nd isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics. At least four components are required to explain the isotope systematics of Gran Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 238U/204Pb) HIMU-like composition. During the Pliocene-Recent epochs, the plume was located to the west of Gran Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics may reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as plume material was transported through the upper asthenosphere to the base of the lithosphere beneath Gran Canaria. The shift in isotopic composition with increasing SiO2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath Gran Canaria. This enriched mantle may have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle components (EM1 and EM2) may be stored in the shallow mantle, whereas the HIMU component may have a deeper origin.

Cretaceous ocean crust at DSDP Sites 417 and 418: Carbon uptake from weathering versus loss by magmatic outgassing

Ocean crustal carbon uptake during seafloor alteration at DSDP Sites 417A, 417D, and 418A exceeds the estimated loss of carbon during magmatic ridge outgassing. If these sites are representative for oceanic crust in general, 2.2–2.9 × 1012 moles of carbon are removed from the oceans per year as a net flux of carbon between the oceanic crust and seawater. Although most of this carbon occurs as calcium carbonate, this ocean crustal carbonate probably cannot be considered part of the marine calcium carbonate sink since much of the Ca in these carbonates must be derived from basalt alteration that is not balanced by a concomitant uptake of seawater Mg. Our present estimate cannot be satisfactorily applied to global carbon budgets, because of uncertainties in the bulk CaMg budget of ocean floor alteration and because of the uniqueness of our estimate. Yet, our data document that the formation of ocean crust provides a significant sink for carbon that should be included in models of the global cycling of carbon. Furthermore, magmatic outgassing during ocean crust emplacement and seafloor basalt alteration may provide a buffering mechanism for atmospheric carbon.

Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite

Abstract We report major elements, K, Rb, Cs, Sr, Ba, Sc, Cr, Ni, Hf, Ta, Th, and REE concentrations and isotopic compositions of Sr, Nd, Pb and O of carefully handpicked volcanic glasses from the Akaki River section of the Troodos ophiolite complex. On the basis of Sr and O isotopic composition and Fe 2 O 3 /FeO ratios, nine of the ten glasses analyzed are considered to be fresh, even with respect to those elements that are easily affected by alteration. The glasses range in composition from basaltic andesite to dacite. Incompatible element concentrations in the mafic compositions are low (10 × chondritic) and the light REE are strongly depleted. However, the ratios of low field strength elements (K, Rb, Cs, Sr, Th) to high field strength elements (Ta, Hf, Ti) and rare earth elements are higher than those in N-MORB. Nd isotopic compositions show a very small range with e Nd(T = 80Ma varying from +6.5 to +7.5. Initial 87 Sr/ 86 Sr ratios range from 0.7033 to 0.7040. Pb isotope ratios have a restricted range with 206 Pb/ 204 Pb= 18.60–18.70, 207 Pb/ 204 Pb= 15.55–15.59 , and 208 Pb/ 204 Pb= 38.08–38.51 . The oxygen isotopic composition for fresh glasses falls within the range reported for fresh, mafic mantle derived volcanics, with δ 18 O varying from 5.4 to 6.5. All geochemical characteristics of the Troodos volcanic glasses are similar to those observed in supra-subduction zone volcanics, but we cannot distinguish between an early stage island arc or back-arc origin for the Troodos ophiolite on a geochemical basis only. However, considering our data and previously published geologic and geochemical data, we suggest that the Troodos ophiolite formed in the earliest stages of island arc development.

Geochronology of Gran Canaria, Canary Islands: Age of Shield Building Volcanism and Other Magmatic Phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km3 was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (∼100 km3) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago.Following a long hiatus there was resurgence of volcanism with eruption of about 100 km3 of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km3. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria.The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.

Sr, Nd, and Pb isotope geochemistry of Tertiary and Quaternary alkaline volcanics from West Germany

Mantle-derived alkaline magmas from the Quaternary East and West Eifel volcanic fields (West Germany) and a range of basalts from various Tertiary Provinces in West Germany show considerable variation in their Nd and Sr isotopic compositions. East Eifel mafic magmas (basanites, leucitites) and derivatives thereof (tephrites, phonolites) are, with the exception of two nephelinites, very similar in87Sr/86Sr (0.70456–0.70472) but show a significant range in143Nd/144Nd (0.51263–0.51273). In contrast, West Eifel melilitite-nephelinites, leucitites, and melililites (F-group magmas), and olivine-nephelinites and basanites (ONB-group), define a steeply inclined array in a SrNd isotope variation diagram extending from 0.7039 to 0.7045 and from 0.51285 and 0.51267, for87Sr/86Sr and143Nd/144Nd respectively. ONB-magmas form a separate low87Sr/86Sr-high143Nd/144Nd group. These data combined with analyzed Tertiary lavas define an array extending from E-type MORB down to estimated bulk silicate earth values. Lead isotopic compositions of Eifel volcanics are rather radiogenic compared to other continental alkaline rocks and are positively and negatively correlated with143Nd/144Nd and87Sr/86Sr, respectively. An evaluation of possible contaminants demonstrates that crustal assimilation has not played a major role in determining the isotopic compositions of the lavas. Isotopic variations thus document heterogeneity on a variety of scales within the Eifel mantle source. Estimated source Sm/Nd ratios, in conjunction with measured isotopic compositions, suggest that mixing between enriched and depleted mantle materials, which occurred sometime during the past 1.1 Ga, resulted in the production of the heterogeneous proto-Eifel source. This “mixing” event may have occurred in conjunction with Tertiary magmatism in central Europe.

Volcanic glass compositions of the Troodos ophiolite, Cyprus

Fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed. Glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals. The basal 400–500 m of the sequence consists of an andesite-dacite-rhyolite assemblage containing abundant hyaloclastites. The remainder of the section comprises a basalt–basaltic andesite assemblage with high MgO and low TiO 2 and total iron. The lower sequence is interpreted as an evolved arc-tholeiite suite; the upper has some similarities to boninitic lavas. The close association in time and space of these two suites is similar to that observed in the Mariana and Bonin arcs and suggests that all of the Troodos lavas were erupted in a subduction-zone environment, most probably in an incipient arc or fore-arc.

Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG)

The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle. Most cores are greenish pleochroic and moderately resorbed (fassaitic augite). Some are pale green and strongly resorbed (acmitic augite). Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare. The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements. The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr. A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route. It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites. The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized. Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975).