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The sulfate aerosols formation through the study of black crusts
The sulfate aerosols formation through the study of black crusts
This project started during a volunteer internship that I did during my undergraduate at Sorbonne Université, with Erwan Martin, based on an idea of Marc De Rafélis. I collected the black crust samples, carried out the sulfate extraction in clean lab, and measured the oxygen multi-isotope composition at IPGP, while David Au Yang measured those of sulfur. Here is the abstract of the paper we wrote with David Au Yang, Erwan Martin, Pierre Cartigny, Erwann Legendre and Marc De Rafélis, and published in 2020 in Atmospheric Chemistry and Physics.
Oxygen and sulfur mass-independent fractionation of black crusts recording the negative ∆33S complementary reservoir of sulfate aerosols
Oxygen and sulfur mass-independent fractionation of black crusts recording the negative ∆33S complementary reservoir of sulfate aerosols
Abstract: To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts”and investigate their bearing on the whole atmospheric SO2 cycle, we measured the oxygen (d17O, d18O, and ∆17O) and sulfur (d33S, d34S, d36S, ∆33S, and ∆36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The d18O and d34S values, ranging between 7.5‰ and 16.7 ± 0,5‰ (n=27) and between -2.66‰ and 13.99 ± 0.20 ‰, respectively, show anthropogenic SO2 as the main sulfur source (from 2% to 81 %, average 30 %) with host-rock sulfates making the complement. This is supported by ∆17O values (up to 2.6 ‰, on average 0.86 ‰), requiring > 60% of atmospheric sulfates in black crusts. Negative ∆33S and ∆36S values between -0.34‰ and 0.00 ± 0.01‰ and between -0.76‰ and -0.22 ± 0.20 ‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the SO2 oxidation on the building stone, leading to ∆33S depletion in black crust sulfates and subsequent ∆33S enrichment in residual SO2 . Except for a few samples, sulfate aerosols mostly have ∆33S values > 0 ‰, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative ∆33S reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.
The time delivery of volatiles on Earth
The time delivery of volatiles on Earth
The time delivery of sulfur and water is still a matter of debate and the study of basaltic glasses sampling the mantle can bring some constraints. The depleted mantle has been shown to be 34S-depleted compared to chondrites, resulting from the early accretion of sulfur and its core-mantle fractionation footprint that led to a depleted mantle with a d34S ~ -1‰ (Labidi et al., 2013). The same year, Wang and Becker (2013) showed that S was delivered mainly during the Late Veneer based on highly siderophile elements, which suggests a chondritic d34S for the primitive mantle. However, these two extreme models are based on samples that probe the upper mantle, which is affected by continental crust extraction and recycling, processes that could alter the primitive mantle signature.
Here, I investigated high 3He/4He basaltic glasses from Fernandina volcano, Galapagos Archipelago, provided by Mark Kurz (Woods Hole oceanographic institution), reflecting a less degassed and primitive mantle. I analyzed the D/H ratio as well as the sulfur multi-isotopic composition of both sulfide and sulfate fractions and the sulfur speciation using XANES performed at synchrotron SOLEIL. We proposed in our paper under review that the primitive mantle sampled by the Galapagos hotspot is depleted in sulfur (S content close to 0 ppm), which could imply the sulfur delivery during the early accretion, followed by its segregation in the core and then, its sequestration in a sulfide-rich layer at the core-mantle boundary.
We are currently processing the D/H data, with Matthieu Clog (University of Glasgow) that could help us constrain the origin of the volatile depletion in the primitive mantle.
Chemical extraction of sulfur, purification in vacuum line and analysis using isotope-ratio mass spectrometry
Chemical extraction of sulfur, purification in vacuum line and analysis using isotope-ratio mass spectrometry
Unravelling the sulfur cycling in subduction zones
Unravelling the sulfur cycling in subduction zones
My PhD project aimed at identifying the different reservoirs involved in the recycling of sulfur in the deep mantle and in the mantle wedge and typifying its speciation in metasomatic fluids and melts by using the sulfur multi-isotopes. The study of arc lavas being relatively difficult due to sulfur degassing and crustal assimilation, I focused on three different approaches to constrain the S cycling.
1) Constraints on fluid composition from high pressure metamorphic rocks
This project was a collaboration with Samuel Angiboust (ENS Lyon) who brought us, Pierre Cartigny and I, in the Western Alps to sample exhumed metagabbros and serpentinites in eclogite facies from the Monviso, that are supposed to preserve the channelized fluid composition of the downgoing slab.
We showed in our paper published in Geochimica et Cosmochimica Acta (2024) that metasomatized rocks are 34S- and 33S-enriched compared to non-metasomatized rocks. We proposed that this enrichment results from an open-system sulfate reduction leading to elevated d34S and ∆33S values. Consequently, this mechanism implies the occurrence of a sulfate-bearing oxidizing fluids circulating within the slab, and that could oxidized the mantle wedge.
2) Arc-like signature of Chile ridge basalts
Basalts formed at the Chile ridge near the trench carry an arc-like signature with significant enrichment in fluid mobile elements, resulting from the depleted mantle metasomatism by fluids liberated from the downgoing slab through a slab window. These MORB glasses are thus a direct window into the S speciation and composition released by slab dehydration processes. I analyzed the S multi-isotopic compositions of 17 samples provided by Alberto Saal and Soumen Mallick (Brown University), which show d34S and ∆33S values more elevated than the MORB depleted mantle. Besides, these basalts contain unusual high sulfate fraction (measured by Maryjo Brounce, UC Riverside, using XANES at synchrotron APS) compared to normal MORB. We suggest that these features reflect the depleted mantle metasomatism beneath the Chile ridge by 34S-33S-enriched oxidizing fluids, implying the occurrence of sulfate-bearing fluids liberated by hot slab dehydration.
3) Back-arc basalts from Lau Basin
The last part of my PhD consisted of analyzing glasses from the Lau Basin to constrain the S speciation and isotopic composition in the sub-arc mantle. Indeed, shallow processes such as S degassing, melt differentiation, crustal assimilation, prevent from a clear characterization of S speciation and isotopic composition in arc lavas. Despite the fact that submarine back-arc basalts from the Lau Basin are less affected by crustal assimilation and melt differentiation, the basalts with the most pronounced arc-like signature, suggesting mantle metasomatism by slab fluids, are also the most S degassed samples, preventing us from constraining the S speciation and isotopic composition of slab fluids without ambiguity.
From Smith and Price, 2006
Constraints on the archean material recycling in the deep mantle using in-situ S multi-isotope analysis by laser ablation
Constraints on the archean material recycling in the deep mantle using in-situ S multi-isotope analysis by laser ablation
One of the objectives of my postdoc at IPGP within the SHRED ERC team is to set up the in-situ sulfur multi-isotope analysis in sulfides contained in oceanic island basalts. Previous studies have shown large ∆33S anomalies measured in sulfides from Mangaia and Pitcairn oceanic island basalts (French Polynesia), which would attest for archean crust recycled in the deep mantle and sampled by South Pacific hotspots. However, large uncertainties cover these values obtained by secondary ion mass spectrometry (SIMS) and no ∆33S anomaly has been measured using isotope-ratio mass spectrometry (IRMS), questioning the occurrence of archean recycled material in the mantle source.
During these two years, I will use a femtosecond laser (NWR200 ESI) coupled, first, to an ICP-MS/MS (Agilent 8900), and then to a MC-ICP-MS (Neptune) to analyze the S multi-isotope composition of sulfides in Tubuai lavas (Austral chain, French Polynesia). Tubuai lavas show the most radiogenic Pb isotope composition, which reflects the recycling of old oceanic crust, and would thus contain anomalous ∆33S values, if any. I started by characterizing the S isotopic composition of various sulfides using IRMS (Thermo Scientific MAT253)to have standard of sulfides for the laser ablation and I am now ready to analyze sulfides in basalts!
Investigating the mantle source composition variability by analyzing trace elements in pyroxenes
The other objective of this postdoc is to measure in-situ the trace element content of pyroxene using the femtosecond laser (NWR200 ESI) coupled to an ICP-MS/MS (Agilent 8900). Previous studies have shown that the variation in trace element contents between the core and the rim of pyroxenes in volcanic rocks can reflect different mantle sources, after ruling out partial melting, differentiation and contamination processes. Here, I study oceanic island basalts from Tubuai (Austral chain, French Polynesia) in which I identified pyroxenes with core and rim having different trace element patterns. I am currently processing the data, with recently measured bulk trace element contents.
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