Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could ...involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32 S, 33 S, 34 S, 36 S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ~2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ~ 2,500 mbsl) and 600 ppm S (~2,000 mbsl). The d 34 S values can be accounted for under equilibrium isotope fractionation during degassing, with agas-melt between 1.0020 and 1.0001 and starting d 34 S values between-0.9‰ and +0.6‰. The d 34 S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ~ 1‰ 34 S enrichment compared to pacific upper mantle. The D 33 S and D 36 S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1s). Only D 33 S is statistically different from MORBs, by +0.02‰. The D 33 S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33 S-36 S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn.
Modern anthropogenic aerosols usually exhibit low but significant Δ33S signatures (−0.6 to 0.5‰) whose origin still remains unclear. While isotope fractionation factors associated with the oxidation ...of SO2 by O2+TMI (Transition Metal Ion), H2O2 or OH cannot lead to such extreme Δ33S-values, an increasing number of studies points to the significant role of NO2 as a contributing oxidant, especially in the urban environment.
To address the possible relation between atmospheric NO2 and observed Δ33S-values in aerosols, we carried out laboratory experiments oxidizing SO2 by NO2 at temperatures ranging between −7 and 52 °C. Our results show that at temperatures ≥10 °C SO2 oxidation by NO2 is characterized by 1) a 34α-value whose temperature dependence (0.2437/T+0.0457) is distinct from those related to oxidation by O2+TMI, H2O2 and OH oxidation pathways and 2) 33β (0.514 ± 0.0003) and 36β (1.90 ± 0.002) values that are closer to the mass dependent values (0.515 and 1.89 respectively) than those reported for the other oxidation pathways. This implies that the NO2 oxidation pathway cannot explain the extreme Δ33S-values measured in urban aerosols.
Our data show that if atmospheric SO2 oxidation by NO2 is neglected, both the O2+TMI and OH oxidation pathways would be overestimated in urban areas. Finally, we conclude that another oxidation reaction is responsible for the high Δ33S-values measured in urban aerosol samples.
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•Atmospheric SO2 oxidation by NO2 cannot explain mass-independent fractionation of sulfur isotopes measured in urban aerosols.•Neglecting the NO2 oxidation pathway leads to overestimate SO2 oxidation by O2+TMI and OH.•SO2 oxidation by NO2 presents a cross-over.
In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium ...shows radiogenic isotopic ratios of R/Ra
=
0.14–1.29, while the neon ratios (
21Ne/
22Ne of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources.
40Ar/
36Ar ratios of between 477 and 6056 are consistent with this interpretation. The (
129Xe/
130Xe) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The δ
13C and δ
15N isotopic values of −
1.04 to −
9.79‰ and +
4.5 to +
15.5‰ respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1–3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle – no fissiogenic Xe from fission of
238U being detectable – imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger.
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.
In order to better understand the formation and evolution of their parent bodies, the three isotope ratios of sulfur were analyzed in 33 enstatite meteorites (24 enstatite chondrites and 9 aubrites). ...The results show that on average all enstatite chondrite groups are enriched in the lightest isotopes compared to other chondrite groups, with means of δ34S of −0.28±0.22‰ for EH3/4, −0.16±0.16‰ for EH5, −0.32±0.15‰ for EL3, −0.67±0.16‰ for EL6 and −0.64±0.00‰ for EL7 (all 1σ). Aubrites show a larger isotope variability in their composition, with a δ34S varying from −1.350‰ to +0.154‰. Contrary to previously published results, our data show a distinct composition for EL6 compared to other enstatite chondrites. This could be related to an impact-induced loss of isotopically heavy oldhamite (δ34S=by 3.62±3.02‰ (1σ)) on the EL parent body. Although the bulk sulfur in both enstatite meteorites and aubrites does not show any significant Δ33S and Δ36S, the oldhamite fraction shows clear evidence of mass independent fractionation on the 36S/32S ratio (in 3 out of 9 analyzes, Δ36S up to +2.2‰), a signal that is not correlated to any 33S/32S anomaly (in 1 out of 9 analyzes, Δ33S down to −0.085‰). Though a nebular or photochemical origin cannot be ruled out, the most plausible mechanism to produce such isolated non-mass dependent 36S/32S anomalies would be a contribution of FeCl2 containing excesses of 36S due to the decay of 36Cl to the leached oldhamite fraction. Even though the sulfur isotopic composition measured in enstatite meteorites is distinct from the Bulk Silicate Earth (BSE), the isotopically lightest samples of EL6, EL7 and aubrites are approaching the isotopic composition of the BSE and enstatite meteorites remain the meteorites with the sulfur isotopic composition the closest to the terrestrial one.
Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic ...characteristics directly reflect their growth environment within the mantle as far back as ∼3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ∼3.5 to 1.4 Ga for P peridotitic-type diamonds and 3.0 to 1.0 Ga for E eclogitic-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total δ13C data range from −17.1 to −1.9 ‰ (P = −8.4 to −1.9 ‰; E = −17.1 to −2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in δ13C of peridotitic diamonds has been at −5 (±2) ‰ since the earliest diamond growth ∼3.5 Ga, and this mode is also observed in the eclogitic diamond record since ∼3 Ga. The skewness of eclogitic diamonds’ δ13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.
No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal δ13C mode of −5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and δ18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of ...the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026–0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy δ18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu∗, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like δ18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome.
The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ∼3.2 to 4.9GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the – possibly oxidising – metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0Ga Bushveld event.
We revisit the optimality of the most rapid approach paths for problems of the calculus of variation in infinite horizon, which are scalar and linear w.r.t. the derivative. Our approach is based on ...the characterization of the value function in terms of the viscosity solutions of a Hamilton-Jacobi equation. We obtain a new necessary and sufficient condition when there is only one turnpike, but characterize also the optimality of several turnpikes in competition. In this last case, nonsmooth analysis is used. Finally, we illustrate the results on a fishery management problem, for which the growth function has a depensation and the price is variable.
Triple oxygen isotope (∆17O with δ18O) signals of H2O and O2 found in sulfate of oxidative weathering origin offer promising constraints on modern and ancient weathering, hydrology, atmospheric gas ...concentrations, and bioproductivity. However, interpretations of the sulfate-water-O2 system rely on assuming fixed oxygen-isotope fractionations between sulfate and water, which, contrastingly, are shown to vary widely in sign and amplitude. Instead, here we anchor sulfate-water-O2 triple oxygen isotope systematics on the homogeneous composition of atmospheric O2 with empirical constraints and modeling. Our resulting framework does not require a priori assumptions of the O2- versus H2O‑oxygen ratio in sulfate and accounts for the signals of mass-dependent and mass-independent fractionation in the ∆17O and δ18O of sulfate's O2‑oxygen source. Within this framework, new ∆17O measurements of sulfate constrain ~2.3 Ga Paleoproterozoic gross primary productivity to between 6 and 160 times present-day levels, with important implications for the biological carbon cycle response to high CO2 concentrations prevalent on the early Earth.
The origin(s) of the chassignites and nakhlites, closely related martian olivine and augite cumulates, respectively, are much debated. Northwest Africa (NWA) 8694 is the third chassignite to be ...discovered and the most ferroan, containing 85% olivine (Fo54). Its O-isotope compositions (δ18O ∼ 4.4‰, Δ17O ∼ 0.30‰) are typical of other martian meteorites. It has adcumulate texture and contains cumulus chromite, poikilitic pigeonite (En56Fs37Wo7) and mesostasis (trapped interstitial liquid). The latter contains pyroxene and plagioclase (An23 Ab70 Or8) plus rare K- feldspar (Or74), and has a trachyandesitic to trachytic bulk composition. Melt inclusions in olivine contain a variety of phases including biotite and rare amphibole. Olivine, chromite, and pigeonite compositions are intermediate between those of the other chassignites and those of the nakhlites. Augite, which appears to mantle pigeonite, has a composition overlapping that in nakhlite NWA 998 and some other nakhlites at (En41-40Wo38-39). The augite lamellae in pigeonite 1–2 μm in apparent width, and the survival of Ca zoning in olivine, suggest a near-surface cooling environment. The bulk-rock REE concentrations in the three chassignites do not correlate with Mg# but depend on the abundance of trapped liquid. The form of REE patterns calculated for olivine subtraction is very like those of nakhlite mesostases, but the observed concentrations of LREE in NWA 8694 trapped liquid have a very steep slope. This is explained by undersampling of baddeleyite and zirconolite that occur near olivine contacts with mesostasis. Though pyroxene is unzoned, its trace element variations indicate fractional crystallization. The range of olivine compositions in the three chassignites (Fo79-54) is too large to result from the crystallization sequential growth of olivine from a single magma undergoing fractional crystallization. The Ge/Si ratios show degassing of NWA 8694 which sets this chassignite apart from other chassignites and nakhlites, implying a unique batch of magma for its genesis. Many potential parent liquids are capable of generating the NWA 8694 olivine composition, though not its alkaline mesostasis. We calculated that Nakhla parent liquid NA01a (Stockstill et al., 2005) with 10% Nakhla core olivine added would produce both olivine crystals and alkaline daughter liquids with compositions matching those of NWA 8694. This meteorite is a chassignite cumulate containing nakhlitic mesostasis, a direct link between the chassignites and the nakhlites and the association of dunitic to trachytic compositions is reminiscent of terrestrial shield volcanoes. Chassignites and nakhlites were possibly formed when solidification fronts on chamber walls were disrupted, mainly as side eruptions of olivine-charged magmas from the deeper zones, and augite-charged fractionated magmas from nearer the summit of a volcano resembling Piton de la Fournaise on Earth.
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