Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania

International audienceOldoinyo Lengai is the only volcano on Earth currently erupting natrocarbonatites, of which the source and genesis remain controversial. Cognate xenoliths and fumaroles were sampled at the summit of Oldoinyo Lengai, and deep crustal xenoliths from Oltatwa maar, in 2010 and 2014, after the 2007-2008 sub-Plinian eruption. The summit cognate xenoliths provide direct information on the isotopic composition of the mid-crustal magma chamber that was active during the 2007-2008 explosive eruption. Cognate xenolith-hosted pyroxenes from Oldoinyo Lengai have an average 3 He/ 4 He = 6.58 ± 0.46 R A , similar to values from nearby silicate volcanoes (4.95-7.30 R A), and reflecting a sub-continental lithospheric mantle (SCLM) signature. This similarity implies that Oldoinyo Lengai carbonatites form from a similar mantle reservoir as the nearby silicate volcanoes. We identify SCLM, metasomatized by fluids/melts derived from the depleted convective mantle, as the common source of magmas in the Arusha volcanic province. Fumarole measurements highlight that fumarolic 3 He/ 4 He values have been relatively constant since at least 1988, indicating that dramatic changes to the crater region morphology during the 2007-2008 eruption did not affect the architecture of the hydrothermal system, which is probably connected to the crustal magma chamber(s). Moreover, the similarity between 3 He/ 4 He values from the mid-crustal magma chamber (6.58 ± 0.46 R A) and fumaroles (7.31 ± 0.24 R A) of Oldoinyo Lengai attests that helium is not subjected to atmospheric contamination or crustal assimilation during transport to the surface

Isotopic and geochemical constraints on lead and fluid sources of the Pb-Zn-Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

International audienceThe Wsingle bondAu, Pbsingle bondZnsingle bondAg, and Sbsingle bondBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The Pbsingle bondZnsingle bondAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan Wsingle bondAu deposit (295-280 Ma). The Pbsingle bondZnsingle bondAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the Pbsingle bondZnsingle bondAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust.Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (Wsingle bondAu, 286 Ma) followed by a Pbsingle bondZnsingle bondAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events.The proposed metallogenic model for the Pbsingle bondZnsingle bondAg Tighza-Jbel Aouam deposit provides new constraints for the Pbsingle bondZnsingle bondAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies)

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems

Argon isotope constraints on modification of oxygen isotopes in Iceland Basalts by surficial processes

New data from the Reykjanes Peninsula, Iceland, show that ^3He/^4He ratios of subglacial basaltic glasses are well correlated with CaO/Al_2O_3. High CaO/Al_2O_3 ratios are likely due to assimilation of lower crustal clinopyroxene and the large variations in ^3He/^4He likely result from addition of crustal ^4He to the magma during this assimilation. Low ^(40)Ar/^(36)Ar ratios occur only in samples with low δ^(18)O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ^(18)O and low ^(40)Ar/^(36)Ar in most samples. However, high ^(40)Ar/^(36)Ar ratios (up to 6500) measured in some samples—the samples that have the highest δ^(18)O—is consistent with little or no alteration of these samples. These high ^(40)Ar/^(36)Ar samples have δ^(18)O ±4.6‰, significantly lower than that of the MORB source region (5.2‰). It seems likely that, while alteration by meteoric fluids does influence oxygen isotopic composition, δ^(18)O of the mantle source to these basalts is ±4.6‰. Low δ^(18)O in the Icelandic mantle source could be the result of recycling lithospheric mantle, although this is not likely the source of high ^3He/^4He ratios in Iceland

Argon isotope constraints on modification of oxygen isotopes in Iceland Basalts by surficial processes

New data from the Reykjanes Peninsula, Iceland, show that ^3He/^4He ratios of subglacial basaltic glasses are well correlated with CaO/Al_2O_3. High CaO/Al_2O_3 ratios are likely due to assimilation of lower crustal clinopyroxene and the large variations in ^3He/^4He likely result from addition of crustal ^4He to the magma during this assimilation. Low ^(40)Ar/^(36)Ar ratios occur only in samples with low δ^(18)O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ^(18)O and low ^(40)Ar/^(36)Ar in most samples. However, high ^(40)Ar/^(36)Ar ratios (up to 6500) measured in some samples—the samples that have the highest δ^(18)O—is consistent with little or no alteration of these samples. These high ^(40)Ar/^(36)Ar samples have δ^(18)O ±4.6‰, significantly lower than that of the MORB source region (5.2‰). It seems likely that, while alteration by meteoric fluids does influence oxygen isotopic composition, δ^(18)O of the mantle source to these basalts is ±4.6‰. Low δ^(18)O in the Icelandic mantle source could be the result of recycling lithospheric mantle, although this is not likely the source of high ^3He/^4He ratios in Iceland

Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge

New measurements of the He, Ar and CO_2 abundances trapped in basaltic glasses from the Southeast Indian Ridge (SEIR) show that volatile concentrations in the SEIR magmas were controlled by fractional degassing. Fractionation between volatile species is consistent with their solubilities in silicate melts. As a result, there are linear relationships between (for example) ln(^4He/^(40)Ar^*) vs. ln[^(40)Ar^*] and between ln(^4He/^(40)Ar^*) vs. ln(^(40)Ar^*/CO_2) (where ^(40)Ar^* is the ^(40)Ar corrected for atmospheric contributions). The slopes of these correlations permit the relative He/Ar and Ar/CO_2 solubilities to be estimated; these are generally consistent with experimentally determined noble gas solubilities in basaltic melts. However, there are systematic differences in the degassing trajectories. For example, in a plot of ln(^4He/^(40)Ar^*) vs. ln(^(40)Ar^*/CO_2), samples from the deepest portions of the ridge consistently plot at lower ^4He/^(40)Ar^* for a given ^(40)Ar^*/CO_2, compared to shallower sections of ridge. These variations in ^4He/^(40)Ar^* likely reflect variations in He/Ar in the primary melt, i.e. their relative abundances prior to degassing. We estimated the variation in ^4He/^(40)Ar^* in the initial melts (i.e. the ^4He/^(40)Ar^* prior to degassing) by extrapolating the degassing trend to a constant mantle-like ^(40)Ar^*/CO_2 ratio and assuming that the relative He–Ar–CO_2 solubilities do not vary between samples. The ^4He/^(40)Ar^* corrected for degassing in this manner varies by a factor ≈10 and correlates positively with the ^3He/^4He ratio. It is possible that the correlation between “degassing corrected” ^4He/^(40)Ar^* ratios and the ^3He/^4He ratio results from preferential diffusion of ^3He relative to ^4He and of 4He relative to ^(40)Ar from the solid mantle into primary melts during melting. However, modeling this diffusive process fails to reproduce the comparatively large variations in ^3He/^4He found in the basalts; therefore, it seems likely that mantle heterogeneities, in combination with diffusive fractionation, resulted in coupled He isotope and He/Ar variations

An observed link between lithophile compositions and degassing of volatiles (He, Ar, CO2) in MORBs with implications for Re volatility and the mantle C/Nb ratio

International audienceThere are systematic variations between relative noble gas abundances and lithophile tracers such as 87Sr/86Sr, εNd and La/Sm in a suite of basaltic glasses from the South East Indian Ridge (SEIR). 4He/40Ar⁎ (where 40Ar⁎ is 40Ar corrected for atmospheric contamination) correlates positively with 87Sr/86Sr and La/Sm but anticorrelates with εNd. The large range in 4He/40Ar⁎ observed in the glasses is due to fractionation during magmatic degassing caused by the very different solubilities of He and Ar in silicate liquids, whereas 87Sr/86Sr, εNd, La/Sm, etc. are insensitive to magmatic processes but rather reflect mantle heterogeneity. Thus, there is a curious situation in this suite of basalts where tracers of mantle heterogeneity (87Sr/86Sr, εNd, La/Sm, etc.) correlate with a tracer of magmatic volatile processes (4He/40Ar⁎).Here, we propose that “enriched” mantle (with high La/Sm and 87Sr/86Sr, low εNd) also has a higher C concentration than “depleted” mantle. Magmas derived from enriched mantle will therefore have higher initial C concentrations, leading to a greater fraction of CO2 degassed and thus a higher 4He/40Ar⁎ ratio on eruption. Simple solubility-determined fractional degassing models show that the range in 4He/40Ar⁎ observed in SEIR basaltic glasses can be generated if the mantle C concentration varies by a factor of 2 over the length of the ridge, consistent with independent estimates of C concentration heterogeneity in the MORB mantle. The correlations between lithophile tracers and 4He/40Ar⁎ can be reproduced by mixing between a depleted endmember with 87Sr/86Sr = 0.70275, εNd = 8.2 and [C] = 12 ppm and an enriched endmember with 87Sr/86Sr = 0.70360, εNd = 5 and [C] = 24 ppm, followed by degassing.The proposed degassing model allows us to estimate the initial C concentration (i.e. prior to degassing) of each SEIR basalt (for which Sr or Nd isotopes are available); using independent Nb concentration data (Mahoney et al., 2002), we show that C/Nb ratios prior to degassing along the SEIR are relatively constant, probably with a C/Nb ratio of 200±100. However, although the constancy of C/Nb in these samples is a robust conclusion, the estimated C/Nb ratio itself is model dependent.We also use these data to evaluate volatility of Re during degassing of MORBs; Re is known to be moderately volatile during subaerial and shallow marine volcanism, although it is not known if this element is also volatile at conditions appropriate to MORB emplacement. Although there is a (poor) correlation between Re/Yb (Yb being a non-volatile element of similar apparent bulk compatibility to Re) and 4He/40Ar⁎ in these samples, it is more likely that this correlation results from Re/Yb variation in the mantle source and is not due to loss of Re during magmatic degassing

Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations

International audienceClosed hopper and complex swallowtail morphologies of olivine microcrysts have been described in the past in both mid-oceanic ridge basalts and subaerial tholeitic volcanoes and indicate fluctuations in magma undercooling. We describe similar morphologies in a Mid-Atlantic ridge pillow basalt (sample RD87DR10), and in addition we estimate the duration of temperature fluctuations required to produce these textures as follows: (1) Pairs of melt inclusions are arranged symmetrically around the centre of hopper crystals and each pair represents a heating-cooling cycle. Using the literature olivine growth rates relevant to the observed morphologies, and measuring the distance between two successive inclusions, we estimate the minimum time elapsed during one convection cycle. (2) The major element composition of melt inclusions (analysed by electron microprobe) was found to be in the range of the boundary layer measured in the glass surrounding the olivines, irrespective of their size. Several major elements demonstrate that this boundary layer results from rapid quenching on the seafloor, and not from crystal growth at depth, implying the inclusions had the same composition as the surrounding magma when they were sealed. Using diffusivity of slow diffusing elements such as Al2O3, we estimate the minimum time required for inclusion formation. These two independent approaches give concordant results: each cooling-heating cycle lasted between a few minutes and 1 h minimum. Thus, these crystals probably recorded thermal convection in small magmatic bodies (a dyke or shallow magma chamber) during the last hour or hours before eruption

Noble Gases in the Atmosphere

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