Vincent Busigny

Massive recycling of nitrogen and other fluid-mobile elements (K, Rb, Cs, H) in a cold slab environment: evidence from HP to UHP oceanic metasediments of the Schistes Lustrés nappe (western Alps, Europe)

Abstract Nitrogen and hydrogen isotopic compositions together with N, K, Rb, Cs and H 2 O contents were measured on several high-pressure (HP) to ultrahigh-pressure (UHP) metasediments from the Schistes Lustres nappe (western Alps) and on unmetamorphosed sedimentary protoliths from the Apennines (Italy). These samples represent a sequence of pelagic sediments subducted to different depths down to 90 km along a ‘cold’ geothermal gradient (∼8°C/km). Nitrogen isotopic composition (δ 15 N between +2.6 and +4.8‰) does not show any specific evolution with increasing metamorphic conditions and can be considered constant during subduction. Large variations of the N content (between 169 and 1721 ppm N) together with K, Rb and Cs content are observed but the constancy of K/N (14), K/Rb (385) and K/Cs (10 190) molar ratios in protoliths and metamorphic rocks indicates that none of these fluid-mobile elements was lost through devolatilization processes. This suggests that fluid circulation was limited to sample scale and that the rocks behaved as closed systems during subduction. This interpretation is also supported by the small range of δD values (from −54.1 to −78.0‰). The present results indicate that N, K, Rb and Cs were retained in the subducted sedimentary veneer at least down to the depth locus of island arc magmatism. Based on the correlation between N and K contents, the flux of sedimentary N recycled in subduction zones is estimated at 7.6×10 11 g/yr. Mass balance calculations strongly support the fact that nitrogen is efficiently recycled to the deeper mantle.

Ammonium quantification in muscovite by infrared spectroscopy

Abstract Single muscovite grains of different chemical composition (in particular Si-content from 3.05 to 3.69 per formulae unit (pfu)) and origin (metamorphic and granitic rocks) were analysed by infrared (IR) spectroscopy. Their ammonium concentrations were subsequently measured by capacitance manometry after extraction as N2 using a sealed-tube combustion technique. The values range from 0 to 2203 ppm NH4+. Correlation between ammonium concentration and ammonium IR absorbance corresponding to NH4+ bending permits an assessment of the ammonium molecular absorptivity at 1430 cm−1 (eN–H1430=462±32 l mol−1 cm−1). In order to avoid uncertainties on thickness estimates using an optical technique, a relationship between sample thickness and IR absorbance was determined. Ammonium concentration of muscovite can be directly derived from its IR spectrum using the relationship [NH4+] (ppm)=1142.5×[(AN–H1430−A2514)/(A1282−A2514)]−606 where A1282, AN–H1430 and A2514 are absorbances corresponding to wavenumbers 1282 (Si–O vibration peak), 1430 (NH4+ bending) and 2514 cm−1 (silicate network vibration), respectively. The precision of ammonium concentration estimates using this method is better than 20% (2σ) for muscovites thicker than 30 μm. This is independent of mineral composition, thus implying that the ammonium concentration vs. IR absorbance relationship is independent of the muscovite–celadonite solid solution.

Nitrogen content and isotopic composition of oceanic crust at a superfast spreading ridge: A profile in altered basalts from ODP Site 1256, Leg 206

The present paper provides the first measurements of both nitrogen content and isotopic composition of altered oceanic basalts. Samples were collected from Ocean Drilling Program Site 1256 located at the eastern flank of the East Pacific Rise. Twenty-five samples affected by low temperature alteration were analyzed. They include moderately altered basalts together with veins and related alteration halos and host rocks, as well as unique local intensely altered basalts showing green (celadonite-rich) and red (iron oxyhydroxide-rich) facies. Nitrogen contents of moderately altered basalts range from 1.4 to 4.3 ppm and are higher than in fresh MORB. Their d 15 N values vary in a large range from +1.6 to +5.8%. Veins, halos, and host rocks are all enriched in N relative to moderately altered basalts. Notably, veins show particularly high N contents (354 and 491 ppm) associated with slightly low d 15 N values (+0.4 and A2.1%). The intensely altered red and green facies samples display high N contents of 8.6 and 9.7 ppm, respectively, associated with negative d 15 N values of A3.8 and A2.7%. Detailed petrological examination coupled with N content suggests that N of altered basalts occurs as ammonium ion (NH 4 +) fixed in various secondary minerals (celadonite, K-and Na-feldspars, smectite). A body of evidence indicates that N is enriched during alteration of oceanic basalts from ODP Site 1256, contrasting with previous results obtained on basalts from DSDP/ODP Hole 504B (Erzinger and Bach, 1996). Nitrogen isotope data support the interpretation that N in metasomatizing fluid occurred as N 2 , derived from deep seawater and likely mixed with magmatic N 2 contained in basalt vesicles.

Quantitative analysis of ammonium in biotite using infrared spectroscopy

Abstract The present paper provides a calibration of the Beer-Lambert law allowing the determination of the ammonium (NH4) content of biotite using infrared (IR) spectroscopy. Single biotite crystals were analyzed by Fourier Transform Infrared spectroscopy. Using a linear correlation between the NH4 infrared absorption band intensity and the NH4 content as determined by vacuum techniques, the NH4 molar absorption coefficient at 1430 cm-1 was found to be 441 ± 31 L/mol·cm. After having calibrated the biotite thickness to Si-O absorption band, the NH4 content of biotite can be calculated directly from its IR spectrum by the relation: where A1249, A1430, and A2395 are absorbances corresponding to wavenumbers 1249 cm-1 (Si-O vibration peak), 1430 cm-1 (NH4 bending), and 2395 cm-1 (spectrum baseline), respectively. The analysis of biotites having different chemical compositions suggests that, to a first approximation, the calibration is independent of biotite chemical composition. An infrared determination of NH4 partitioning between muscovite and biotite coexisting in the same rocks shows good agreement with results of previous studies and further validates the method.

Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria.

An isotope record of magnetic bacteria Microorganisms have shaped Earths oceans and atmosphere over billions of years. Ancient microbes left very little direct morphological evidence of their existence in the rock record, thereby requiring geochemical clues for evidence of their activity. Amor et al. show that magnetotactic bacteria impart a distinct isotopic signature to their internal iron nanoparticles. Cultures of a modern magnetic bacterium fractionated 57Fe isotopes independent of their mass, in contrast to fractionation patterns often observed for other isotopes. Because this signature is not produced abiotically or by other iron-metabolizing bacteria, it could serve as a reliable biomarker of this ancient magnetic microbial lifestyle. Science, this issue p. 705 The iron isotope fractionation patterns of magnetotactic bacteria hint at a reliable biomarker of ancient microbes. Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe3O4) nanoparticles. Although they may be among the earliest microorganisms capable of biomineralization on Earth, identifying their activity in ancient sedimentary rocks remains challenging because of the lack of a reliable biosignature. We determined Fe isotope fractionations by the magnetotactic bacterium Magnetospirillum magneticum AMB-1. The AMB-1 strain produced magnetite strongly depleted in heavy Fe isotopes, by 1.5 to 2.5 per mil relative to the initial growth medium. Moreover, we observed mass-independent isotope fractionations in 57Fe during magnetite biomineralization but not in even Fe isotopes (54Fe, 56Fe, and 58Fe), highlighting a magnetic isotope effect. This Fe isotope anomaly provides a potential biosignature for the identification of magnetite produced by magnetotactic bacteria in the geological record.

Chemical signature of magnetotactic bacteria

Significance Magnetite precipitates through either abiotic or biotic processes. Magnetotactic bacteria synthesize nanosized magnetite intracellularly and may represent one of the most ancient biomineralizing organisms. Thus, identifying bacterial magnetofossils in ancient sediments remains a key point to constrain life evolution over geological times. Although electron microscopy and magnetic characterizations allow identification of recent bacterial magnetofossils, sediment aging leads to variable dissolution or alteration of magnetite, potentially yielding crystals that barely preserve their structural integrity. Thus, reliable biosignatures surviving such modifications are still needed for distinguishing biogenic from abiotic magnetite. Here, we performed magnetotactic bacteria cultures and laboratory syntheses of abiotic magnetites. We quantified trace element incorporation into both types of magnetite, which allowed us to establish criteria for biomagnetite identification. There are longstanding and ongoing controversies about the abiotic or biological origin of nanocrystals of magnetite. On Earth, magnetotactic bacteria perform biomineralization of intracellular magnetite nanoparticles under a controlled pathway. These bacteria are ubiquitous in modern natural environments. However, their identification in ancient geological material remains challenging. Together with physical and mineralogical properties, the chemical composition of magnetite was proposed as a promising tracer for bacterial magnetofossil identification, but this had never been explored quantitatively and systematically for many trace elements. Here, we determine the incorporation of 34 trace elements in magnetite in both cases of abiotic aqueous precipitation and of production by the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1. We show that, in biomagnetite, most elements are at least 100 times less concentrated than in abiotic magnetite and we provide a quantitative pattern of this depletion. Furthermore, we propose a previously unidentified method based on strontium and calcium incorporation to identify magnetite produced by magnetotactic bacteria in the geological record.

Globally asynchronous sulphur isotope signals require re-definition of the Great Oxidation Event

The Great Oxidation Event (GOE) has been defined as the time interval when sufficient atmospheric oxygen accumulated to prevent the generation and preservation of mass-independent fractionation of sulphur isotopes (MIF-S) in sedimentary rocks. Existing correlations suggest that the GOE was rapid and globally synchronous. Here we apply sulphur isotope analysis of diagenetic sulphides combined with U-Pb and Re-Os geochronology to document the sulphur cycle evolution in Western Australia spanning the GOE. Our data indicate that, from ~2.45 Gyr to beyond 2.31 Gyr, MIF-S was preserved in sulphides punctuated by several episodes of MIF-S disappearance. These results establish the MIF-S record as asynchronous between South Africa, North America and Australia, argue for regional-scale modulation of MIF-S memory effects due to oxidative weathering after the onset of the GOE, and suggest that the current paradigm of placing the GOE at 2.33–2.32 Ga based on the last occurrence of MIF-S in South Africa should be re-evaluated.The Great Oxidation Event (GOE) is considered to have occurred at 2.33–2.32 Ga based on the last occurrence of MIF-S in South Africa. Here, based on sulphur isotope analysis of samples from Western Australia, the authors show preservation of MIF-S beyond 2.31 Ga and call for a re-evaluation of GOE timing.

The Iron Wheel in Lac Pavin: Interaction with Phosphorus Cycle

Lac Pavin is a crater lake, characterized by water column stratification, with oxygenated shallow waters lying above anoxic and ferruginous deep waters. In the deep waters, ferrous iron, Fe(II)aq, is the main dissolved cation, with concentrations up to 1 mM. Iron is efficiently confined below the oxic-anoxic boundary due to the formation of insoluble ferric iron species, Fe(III)s, by oxidation with O2 and other oxidants (e.g., NO3−, Mn(IV)). The Fe(III)s particles settle down and are reduced in the anoxic waters and at the lake bottom by reaction with organic matter to soluble Fe(II)aq. It then diffuses upward in the water column and finally is re-oxidized to Fe(III) at the redox boundary. This process, known as the “iron wheel”, is described in the present paper that reviews available data for dissolved and particulate matter in the water column, settling particles collected by sediment traps and sediment cores. Detailed analyses for some major and trace element concentrations, along with iron speciation and isotope composition, high-resolution microscopy, and geochemical modeling provide a picture of biogeochemical cycling in this Fe-rich aqueous system. At Lac Pavin the P and Fe cycles are tightly coupled. Orthophosphate is sorbed onto Fe oxyhydroxides and/or precipitated as Fe(II)-Fe(III)-phosphates at the redox interface, confining P ions in the deep anoxic waters. Deeper in the water column, particulate Fe concentrations progressively increase due to Fe(II) phosphate (vivianite) formation. In the sediment, Fe is buried as various ferrous minerals, such as vivianite, pyrite and siderite.

The fate of ammonium in phengite at high temperature

Abstract Nitrogen (N) is the main component of the atmosphere and is largely considered as a volatile element. However, most researchers now agree that a significant amount of N, in the form of ammonium ( NH4+

Identification of chemical sedimentary protoliths using iron isotopes in the > 3750 Ma Nuvvuagittuq supracrustal belt, Canada

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