Effects of Precursor Concentration and Acidic Sulfate in Aqueous Glyoxal−OH Radical Oxidation and Implications for Secondary Organic Aerosol

Previous experiments demonstrated that aqueous OH radical oxidation of glyoxal yields low-volatility compounds. When this chemistry takes place in clouds and fogs, followed by droplet evaporation (or if it occurs in aerosol water), the products are expected to remain partially in the particle phase, forming secondary organic aerosol (SOA). Acidic sulfate exists ubiquitously in atmospheric water and has been shown to enhance SOA formation through aerosol phase reactions. In this work, we investigate how starting concentrations of glyoxal (30−3000 μM) and the presence of acidic sulfate (0−840 μM) affect product formation in the aqueous reaction between glyoxal and OH radical. The oxalic acid yield decreased with increasing precursor concentrations, and the presence of sulfuric acid did not alter oxalic acid concentrations significantly. A dilute aqueous chemistry model successfully reproduced oxalic acid concentrations, when the experiment was performed at cloud-relevant concentrations (glyoxal <300 μM), but predictions deviated from measurements at increasing concentrations. Results elucidate similarities and differences in aqueous glyoxal chemistry in clouds and in wet aerosols. They validate for the first time the accuracy of model predictions at cloud-relevant concentrations. These results suggest that cloud processing of glyoxal could be an important source of SOA

Concentrations of Pt, Pd, S, As, Se and Te in silicate melts at sulfide, arsenide, selenide and telluride saturation: evidence of PGE complexing in silicate melts?

Even though platinum group elements (PGE) solubilities are measured relative to pure metals, the PGE are assumed to dissolve as oxide complexes in silicate melts. PGE-oxide phases are, however, not known in magmatic rocks; in many cases PGE are associated with discrete magmatic phases (alloys, arsenides, bismuthotellurides, antimonides and sulfides). Here, we determine the concentrations of Pt, Pd, S, As, Se and Te in basaltic melts saturated with Fe, Pt or Pd sulfides, arsenides, selenides and tellurides and note that the solubilities of these elements are largely variable and depend on the metal-ligand reservoir in equilibrium. We equilibrated basaltic melts with immiscible Fe, Pt, and Pd sulfide, arsenide, selenide and telluride melts in a piston cylinder apparatus at 1250 degrees C, 0.5 GPa and relativefO(2)of similar to FMQ to FMQ-1.5. The concentrations of S, As, Se and Te in the basaltic melt vary considerably with the metal-ligand reservoir; the highest concentrations are recorded when the ferrous iron cation is the principal metal ligand. When instead Pt-(S/As/Se/Te) or Pd-(S/As/Se/Te) are used, the concentrations of S, As, Se and Te fall drastically. Platinum and Pd increase the activities of semimetals and chalcogenes in the silicate melt more than Fe does. Implications are that Pt and Pd can preferentially form stable associations (fundamental building blocks) with chalcogens and semimetals before the melt attains saturation in Fe-chalcogens or Fe-semimetals. Estimated concentrations of Pt-ligand and Pd-ligand required to saturate silicate melts in some Pt-ligand and Pd-ligand minerals are close to their abundances in the parent magmas of some layered intrusions

Early Moon formation inferred from hafnium-tungsten systematics

The date of the Moon-forming impact places an important constraint on Earth's origin. Lunar age estimates range from about 30 Myr to 200 Myr after Solar System formation. Central to this age debate is the greater abundance of W-182 inferred for the silicate Moon than for the bulk silicate Earth. This compositional difference has been explained as a vestige of less late accretion to the Moon than to the Earth after core formation. Here we present high-precision trace element composition data from inductively coupled plasma mass spectrometry for a wide range of lunar samples. Our measurements show that the Hf/W ratio of the silicate Moon is higher than that of the bulk silicate Earth. By combining these data with experimentally derived partition coefficients, we found that the W-182 excess in lunar samples can be explained by the decay of the now extinct Hf-182 to W-182. Hf-182 was only extant for the first 60 Myr after the Solar System formation. We conclude that the Moon formed early, approximately 50 Myr after the Solar System, and that the excess W-182 of the silicate Moon is unrelated to late accretion

The redox dependence of titanium isotope fractionation in synthetic Ti-rich lunar melts

Equilibria between Ti oxides and silicate melt lead to Ti isotope fractionation in terrestrial samples, with isotopically light Ti oxides and isotopically heavy coexisting melt. However, while Ti is mostly tetravalent in terrestrial samples, around 10% of the overall Ti is trivalent at fO2 relevant to lunar magmatism (~ IW-1). The different valences of Ti in lunar samples, could additionally influence Ti stable isotope fractionation during petrogenesis of lunar basalts to an unknown extent. We performed an experimental approach using gas mixing furnaces to investigate the effect of Ti oxide formation at different fO2 on Ti stable isotope fractionation during mare basalt petrogenesis. Two identical bulk compositions were equilibrated simultaneously during each experiment to guarantee comparability. One experiment was investigated with the EPMA to characterize the petrology of experimental run products, whereas the second experiment was crushed, and fabricated phases (i.e., oxides, silicates and glass) were handpicked, separated and digested. An aliquot of each sample was mixed with a Ti double-spike, before Ti was separated from matrix and interfering elements using a modified HFSE chemistry. Our study shows fO2-dependent fractionation within seven samples from air to IW-1, especially ∆49Ti-melt and ∆49Tiarmalcolite-orthopyroxene become more fractionated from oxidized to reduced conditions (− 0.092 ± 0.028-  − 0.200 ± 0.033 ‰ and  − 0.089 ± 0.027- − 0.250 ± 0.049 ‰, respectively), whereas ∆49Tiorthopyroxene-melt shows only a minor fractionation (− 0.002 ± 0.017-0.050 ± 0.025 ‰). The results of this study show that Ti isotope fractionation during mare basalt petrogenesis is expected to be redox dependent and mineral-melt fractionation as commonly determined for terrestrial fO2 may not be directly applied to a lunar setting. This is important for the evaluation of Ti isotope fractionation resulting from lunar magmatism, which takes place under more reducing conditions compared to the more oxidized terrestrial magmatism.Deutsche ForschungsgemeinschaftProjekt DEA

Metal Saturated Cumulates from Siberia - Lunar Basalt Analogues?

It is not well known which chemical differentiation pathways basaltic melts will take when they are iron metal saturated. Thermodynamically, the pathway seems predictable. So long as Fe metal is a stable liquidus phase and relative oxygen fugacity (fO(2)) is not subject to major fluctuations, the activity of FeO (a(FeO)(melt)) is buffered by the iron-wustite (IW) equilibrium 2Fe(metal) + O-2 -> 2FeO(melt). Metallic Fe also stabilizes olivine through the equilibrium 2Fe(metal) + O-2 + SiO2 melt -> Fe2SiO4 olivine. That equilibrium tends to suppress the enrichment in bulk SiO2 when Fe saturated basaltic melts differentiate. We document the differentiation history of tholeiitic cumulates from the Siberian craton that carry up to 30 modal % metallic Fe. Our study is complemented by differentiation experiments at two redox states, one set in Fe metal capsules at 1.6 log units below IW (IW-1.6) and a second set in graphite capsules at IW + 1.5. Iron saturated differentiation pathways do not show enrichments in FeO nor in bulk SiO2 because olivine remains stable along the entire liquid line of descent. By contrast, melts equilibrated at IW + 1.5, that is, outside metallic Fe saturation, crystallize pigeonite as first silicate and follow a normal (terrestrial) differentiation pathway involving marked SiO2 enrichment. The Fe-saturated path duplicates in detail the liquid line of descent we derive for the cumulates. Iron-saturated experiments have limited applicability to the Earth because there are so few terrestrial basalts saturated with metallic Fe; however, they might apply to the Moon. Many lunar basalts appear to have been saturated with an Fe-Ni phase during their emplacement on the lunar surface, and potentially during generation within the lunar mantle

Fractionation of rhenium from osmium during noble metal alloy formation in association with sulfides: Implications for the interpretation of model ages in alloy-bearing magmatic rocks

Although Earth's continental crust is thought to derive from melting of the Earth's mantle, how the crust has formed and the timing of its formation are not well understood. The main difficulty in understanding how the crust was extracted from the Earth's mantle is that most isotope systems recorded in mantle rocks have been disturbed by crustal recycling, metasomatic activity and dilution of the signal by mantle convection. In this regard, important age constraints can be obtained from Re-Os model ages in platinum group minerals (PGM), as Re-poor and Os-rich PGM show evidence of melting events up to 4.1 Ga. To constrain the origin of the Re-Os fractionation and Os isotope systematics of natural PGM, we have investigated the linkage between sulfide and PGM grains of variable composition via a series of high-temperature experiments carried out at 1 bar. We show that with the exception of laurite, all experimentally-produced PGM, in particular Pt3Fe (isoferroplatinum) and Pt-Ir metal grains, are systematically richer in Re than their sulfide precursors and will develop radiogenic 187Os/188Os signatures over time relative to their host base metal sulfides. Cooling of an PGM-saturated sulfide assemblage shows a tendency to amplify the extent of Re-Os fractionation between PGM and the different sulfide phases present during cooling. Conversely, laurite grains (RuS2) are shown to accept little to no Re in them and their Os isotope composition changes little over time as a result. Laurite is therefore the PGM that provides the most robust Re-depletion ages in mantle lithologies. Our results are broadly consistent with observations made on natural PGM, where laurites are systematically less radiogenic than Pt-rich PGM. These experimental results highlight the need for the acquisition of large datasets for both mantle materials and ophiolite-derived detrital grains that include measurements of the Os isotope composition of minerals rich in highly siderophile elements at the grain scale (i.e. PGM and base metal sulfides). Only with such datasets is it possible to identify past episodes of mantle melting.SCOPUS: ar.jinfo:eu-repo/semantics/publishe