Volcanic emanations from cooling basaltic lava represent a diffuse and relatively poorly constrained source of metallic and non-metallic compounds to the Earth's surface. These compounds become ...incorporated in fumarolic minerals and redeposit at the surface of lava flows before entering the environmental cycle. The semi-volatile and chalcophile element selenium (Se) can be either vital or toxic to animals and humans. Thus, understanding the pathways of Se capture and relative concentrations in fumaroles is imperative for estimating their contributions to soils and aquifers in volcanically active regions, with implications for animal and human health. In this study, we report Se concentrations and Se stable isotope composition in a sample suite comprising degassed and undegassed basaltic rocks and various fumarolic deposits (thenardite, Na-K sulfate, gypsum, fluoride, and native sulfur) fed from degassing lava flows at Piton de la Fournaise volcano, Réunion Island. Erupted basaltic lavas (136–58 ng·g−1 Se) lost up to more than half of their pre-eruptive Se due to subaerial degassing and retained a heavier isotope composition (δ82/76Se = 0.11 ± 0.17‰, 2 s.d.) compared to less undegassed volcanic glass (138 ng·g−1 Se and δ82/76Se = −0.19 ± 0.04‰, 2 s.e.). Fumarolic deposits that formed over a temperature range of ~800–100 °C and captured Se from the degassing lava show higher to very high Se concentrations ranging from 0.54 μg·g−1 to 1578 μg·g−1 and significant Se isotope fractionation (δ82/76Se = +0.6 to −2.08‰). We propose two separate models that can explain the relative concentration and Se isotope composition of the deposits: 1) A compound oxidation state-dependent Se incorporation into the various fumarolic minerals, or 2) Temperature-dependent Rayleigh condensation from a cooling gas triggered by compound saturation. The Rayleigh condensation model can entirely explain the Se concentration and isotope composition of the fumaroles and predicts that up to 80% of the Se released from the lava is likely to be captured by precipitation to form solid phases within the lava pile, most dramatically in the coldest deposits below the sublimation temperature of SeO2 (< 315 °C). In contrast, mineral-dependent isotope pathways cannot fully explain our data, including the lighter Se isotope compositions in the more oxidized compounds compared to more reduced ones. Such a mineralogical effect cannot be excluded but further investigations and experimental studies are required in order to scrutinize and invoke its role. Finally, the environmental impact of these degassing-induced secondary products will be dependent on the relative compound water solubilities resulting in either Se mobilization towards aquifers or accumulation onto developing soils and plants. Such studies could prove useful for developing risk assessments in volcanically active regions on our planet, and for reaching a better understanding of the global continent-ocean Se isotope budget and signature.
Réunion Island is the present surface expression of a major mantle plume whose homogeneity and isotopic signature, near the convergence point of many hotspot isotopic arrays, have long puzzled ...geochemists. This signature could, in part, reflect oversampling of the most recent (<0.53 Ma) Piton de la Fournaise volcano. To resolve this issue, we studied the older Piton des Neiges volcano and made a synthesis of the Sr-Nd-Pb isotope compositions of lavas produced during the early stage of La Réunion and contemporaneously at Mauritius, the second youngest island of the hotspot track.
New samples from Piton des Neiges have 87Sr/86Sr from 0.70429 to 0.70441, 143Nd/144Nd from 0.51282 to 0.51290, 206Pb/204Pb from 18.765 to 19.004, 207Pb/204Pb from 15.558 to 15.605 and 208Pb/204Pb from 38.850 to 39.082. On a SrNd isotope plot, the extended isotope field defined by the Réunion and Mauritius volcanoes overlaps with the OIB field of the Society islands, where an EM-2 signature has been recognized. Réunion also has higher 207Pb/204Pb and 208Pb/204Pb ratios for a given 206Pb/204Pb than expected from a binary mixing between depleted and enriched mantle components. Thus, the overall Sr-Nd-Pb isotope variations shown by Réunion and Mauritius volcanoes require a third source component. In addition to the already known depleted and enriched mantle components, the third component involved in the mixing is interpreted as reflecting a small contribution (≤ 8%) of Seychelles/Madagascar-like continental crust, which is incorporated into the plume before it rises into the depleted mantle. This new model challenges the proposed existence of a hidden continent below the island of Mauritius, by suggesting that the continental component is located in the asthenosphere.
Previous studies of the Ecuadorian arc (1°N–2°S) have revealed across‐arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim ...of this work is to evaluate how these processes vary along the arc in response to small‐scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1437 samples containing 71 new analyses, of major and trace elements as well as Sr‐Nd‐Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206Pb/204Pb and 87Sr/86Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207Pb/204Pb and decrease of 143Nd/144Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206Pb/204Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the subarc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc.
Key Points
Subduction of a younger oceanic crust in the northern arc promotes slab melting
Southward increase of crustal contamination along the front arc (from 3% to 14%)
The slab structure (Grijalva Fracture Zone, age) and shape (flexure along the Grijalva Fracture Zone) influences magmatism
This work characterises the volatilisation of trace elements during evaporation to dryness of HF‐dissolved silicate rock reference materials (BHVO‐2, AGV‐1, BIR‐1, UB‐N). In open‐system conditions, ...sublimation at 80 °C remained small (≤ 3%) for most elements with the exception of boron. Conversely, during closed‐system evaporation in a PTFE elbow, volatilisation loss could exceed 3% for Te, Au, Se, Ru, B, Re, As, V and Ge; 100 μg g−1 for Pt, Cd, Ag, Mo and Ti; and 10 μg g−1 for many refractory elements including Nb, Hf, U and Yb. Higher volatilisation loss in the closed‐system results from the higher vapour pressure of HF that allows for the formation of highly fluorinated species with lower sublimation temperature, or unstable species such as rare earth tetrafluorides for which sublimation competes with thermal decomposition. Increasing the temperature from 50 to 100 °C promotes the formation and sublimation of highly fluorinated species (VF5, GeF4, SeF6). Conversely, some refractory elements (Hf, Zr, Yb, U, Cu, Zn, Rb, Ba and Sr) seem to preferentially sublimate at 50 °C possibly through the formation of hydrated fluorides. Our results indicate that closed‐system evaporation must be used with caution for quantitative analyses.
Key Points
Trace element volatilisation is higher during closed‐system evaporation in a PTFE elbow than in open‐system conditions.
Average volatility sequence during evaporation of HF‐dissolved silicates is: Te > Au > Se > Ru > B > V > Re > As > Ge > Pt > Cd > Ag > Sb > Mo > Ti > Nb > Sn > Yb.
Online analysis of vapours provides evidence for sublimation of refractory elements.
This work proposes a new method to probe the hidden magmatic evolution of quiescent Andean volcanoes from the Pb isotope composition of gases. The method is based on an assimilation-fractional ...crystallisation-degassing model linking the Pb isotope composition of gases with the SiO
2
content of their magmatic source. The model is applied to El Misti volcano that threatens Arequipa, the second most densely populated city of Peru. Gas condensates and Pb-rich solid deposits (PbS, PbCl
2
, PbSO
4
) collected in 2018 in the bottom of El Misti crater at 260–150°C fumarole vents were used to reconstruct the mean composition of degassing magmas (60.8–61.8 wt% SiO
2
). These compositions are slightly more evolved than the lavas from the last AD 1440–1470 eruption, suggesting either the secular differentiation of the main magma reservoir, or the contribution of more evolved magmas to volcanic gases. On the other hand, the slight but significant difference between the instantaneous composition recorded in gas condensates and the time-integrated composition recorded in solid deposits points to the degassing of less evolved magmas over the last decades. This trend is ascribed to a recent recharge of El Misti reservoir with hot mafic magmas, in agreement with the evolution of fumarolic deposit mineralogy in the last half a century. The Pb isotope composition of gas appears to be a promising tool for probing the hidden magmatic evolution of quiescent volcanoes where assimilation-fractional crystallisation operates.
This study presents new Re–Os isotope and elemental data in gas condensates and corresponding lavas in order to examine the geochemical behavior of these two elements during magma degassing at Piton ...de la Fournaise, Réunion Island.
Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100°C include Na–K sulfate (aphthitalite), Na sulfate (thenardite), Ca–Cu sulfate (e.g. gypsum), Ca–Mg–Al–Fe fluoride (e.g. ralstonite) and native sulfur. The high temperature deposits show trace element typical of volcanic gas with high enrichment in Re (24 to 79ppb), almost two order of magnitude higher than the corresponding lavas but with Os abundances similar to those of the lavas (14–132 ppt). In contrast the Os contents of the low temperature fluoride deposits (13–77ppb) are higher than any of the other condensates. The fluorides are also enriched in Re, albeit to lesser extent than Os (2.9–15.3ppb). Based on high-temperature samples, the fluid/melt partition coefficients estimated for Re and Os are 100±80 and 1±2, respectively. Considering 1% of fluid loss, these partition coefficients translate into emanation coefficients of 0.50 (0.17–0.65) for Re and 0.01 (0–0.03) for Os. These results indicate that Re, unlike Os, is highly volatile at Piton de la Fournaise.
Osmium isotopic compositions of samples collected at medium and low temperature (<350°C) are very uniform and unfractionated (187Os/188Os between 0.130–0.135) and plot within the range of the April 2007 lava flow and the historical lavas of Piton de la Fournaise (i.e. 0.130–0.137). However the highest temperature condensates (Na–K sulfates with T of 384–400°C) yield lower 187Os/188Os ratios (i.e. 0.124–0.129) within the field of mantle signal. Such unradiogenic compositions are best explained if old mantle sulfides occur in lavas and contribute to volcanic gases. Within the general frame of osmium mantle geochemistry, loss of unradiogenic Os during magmas degassing could help to explain osmium isotope disequilibrium between lavas and melting residues.
In the Southern Volcanic Zone (Chile-Argentina), the active Caviahue-Copahue Volcanic Complex (CCVC) is approximately 30km east of the main N-S trending volcanic front, where the Nazca Plate subducts ...under the South American Plate. CCVC activity includes three major stages: the 5–4Ma old Ante-Caviahue series, the 2.6Ma old Caviahue series, and Copahue volcano that has been active since 1.2Ma ago. CCVC volcanism results from extension and slab steepening since 2.6Ma ago that produces the asthenospheric influx under Copahue. Here we investigate the link between the geochemical and multi-element (He, Li, N, Sr, Nd, Hf, and Pb) isotopic composition of CCVC magmas and evolution of the subduction regime since 5Ma ago. The CCVC magmatic source is characterized by a mid-ocean ridge basalt-like mantle signature (high 3He/4He=8 Ra) and a high δ15N (+5.8‰) related to subducted sediments. These data suggest a significant degree of N recycling, but low 4He recycling over time. Trace element and isotope modeling indicates that the influence of sediments is strongest in Copahue magmatism, whereas the mantle wedge contribution was strongest in Caviahue and Ante-Caviahue magmatism. The light rare earth element-enriched nature of Copahue rocks (compared to Caviahue and Ante-Caviahue rocks) is more likely due to incorporation of sediments from the slab rather than a very low degree of partial melting. The low δ7Li (0.44±0.31‰ to 1.42±0.17‰) of Copahue and Caviahue magmas indicates the contribution of a mature slab depleted in 7Li during previous dehydration events. Conversely, Ante-Caviahue rocks have a moderate δ7Li value (2.63‰), suggesting input from a less-dehydrated slab. This is consistent with trace element data, showing that the Copahue (and Caviahue) source is less enriched in fluid-mobile elements (low Ba/Th and U/Th ratios). We propose that the geochemical and isotopic evolution of CCVC magmas records the evolution of the subduction regime under the CCVC. Ante-Caviahue magmas were produced in an arc front extensional regime in which the shallow, extensively dehydrating slab delivered large amounts of fluids to the melting zone. Later slab steepening caused the volcanic front to migrate west of the CCVC, probably due to the beginning of subduction in the Mocha fault zone (south of CCVC) 2.5–5Ma ago. Crustal attenuation and asthenospheric influx under the CCVC produced Caviahue magmatism. Further progressive slab steepening and successive dehydration events produced a mature slab and dry mantle wedge that explain the particular signature of Copahue magmas.
Identifying the sources of distal tephra in marine sediments or polar ice provides clues on the dynamic and large-scale impact of major volcanic eruptions. However, determination of the volcanic ...source of distal tephra is challenging due to size-dependent fractionation during atmospheric transport that modifies the mineral, chemical and even isotope composition of the transported and settled tephra. The composition of distal fine ash may thus be different from the coarser proximal products of the same eruption. Identifying the volcanic source of distal ash using the compositional data may therefore prove difficult. To get around this modification of distal composition, we propose here a new isotopic method to identify the source of distal tephra that is not based on raw transport-dependent isotope ratios but on two-dimension Pb isotope mixing lines, which account for atmospheric fractionation processes. To demonstrate the robustness of our method, we used the extensive database of Pb isotope compositions of volcanic products from the Northern Andean arc, to which we append 68 new Pb isotopic analyses obtained on proximal and distal eruptive tephra. We show for the first time that proximal products define a straight line in the 208Pb/206Pb-207Pb/206Pb space, whose equation is specific to each volcanic source. We then show that distal co-genetic tephra plot on the same line as proximal products, implying that the isotopic lines are robust fingerprints of volcanic sources that do not depend on the age, mineral assemblage, and nature of the emitted products. This new method uses bulk rock analyses and therefore provides a new perspective on distal tephra correlation and source identification in the Andes and probably other volcanic arcs with significant Pb isotopic variations.
•Pb isotope ratios are efficient tools to identify the volcanic source of distal tephra.•Each volcanic system defines a single line in the 208Pb/206Pb-207Pb/206Pb space.•Equation parameters of lines can be used to correlate distal and proximal tephra.•This new method applied on the Andean arc can be extended to other subduction zones.
Continental arc lavas display geochemical signatures that reflect both mantle metasomatism by slab fluids or melts and extensive differentiation of magmas within crustal reservoirs. The relative ...effect of source and crustal processes are difficult to disentangle based on whole-rock compositions. This issue is critical in Ecuador where volcanism occurs through a thick continental crust (>50 km). This study reconstructs the history of melts feeding the Guagua Pichincha volcano, Western Cordillera, by analysing the Pb isotope composition and major-trace element content of individual minerals (33 amphiboles, 4 orthopyroxenes and 18 plagioclases) hosted in two dacite samples. It uses a low-blank wet-chemistry method for precise analysis of Pb amounts as low as 150 pg.
Early crystallized, high-Al amphiboles with Al2O3 ≥ 9.8 wt% and Eu/Eu* > 0.7 have the lowest and most heterogeneous 206Pb/204Pb (18.816–18.999), whereas plagioclases have the highest and most homogeneous 206Pb/204Pb (19.003–19.023). Low-Al amphiboles and orthopyroxenes display intermediate compositions and variability (18.934–19.007). The 206Pb/204Pb ratio correlates negatively with Eu/Eu* in amphiboles and orthopyroxenes, which indicates that the Guagua Pichincha magmas assimilate radiogenic Pb within the stability field of plagioclase (i.e. in the upper crust). The radiogenic ankaramites of the Guaranda unit, an accreted ocean terrain making the basement of the Western Cordillera, are the most suitable contaminant. If this is correct, the 206Pb/204Pb increase from the two most primitive amphiboles to their respective host rocks requires ca. 20% crustal assimilation, which is higher than previous estimates in the Northern Volcanic Zone but similar to those inferred for Central Andean mafic lavas. The two most primitive amphiboles with no significant Eu anomaly record the composition of melts before plagioclase crystallization. These deep melts have contrasted 206Pb/204Pb ratios (18.816–18.879) and contents of fluid mobile elements (Li, Cu, Rb, Pb) that probably reflect the input of different slab components to the mantle wedge. Melts in equilibrium with the two most primitive amphiboles of the Guagua Pichincha volcano are enriched in incompatible elements, but depleted in fluid mobile elements compared to the olivine-hosted melt inclusions of the older Rucu Pichincha volcano. This supports previous inferences based on whole-rock data that the mantle source of the Pichincha Volcanic Complex has changed through time.
•High-precision analysis of Pb-isotope of single minerals (amphibole, plagioclase, pyroxene)•Minerals hosted in hand-size dacite show >1% variation in 206Pb/204Pb.•Magmas' crustal evolution traced through minerals geochemical evolution.•Pb rock budget controlled by late equilibrated melts and late-forming crystals•The mantle source of the Pichincha volcanic complex has changed through time.
Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These ...represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr-Nd isotope data are presented for Mashhad granitoids and MMEs. LA-ICP-MS U-Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO
2
= 67.5-69.4 wt%, high K
2
O (2.4-4.2 wt%), and low Mg# (42.5-50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd
(t)
(−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO
2
(53.4-58.2 wt%) is lower and Ni (3.9-49.7 ppm), Cr (0.8-93.9 ppm), Mg# (42.81-62.84), and εNd
(t)
(−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.
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