Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a well-established technique for elemental and isotopic analyses. However, one of its major developments involving ultrafast ...lasers delivering pulse widths of tens to a few hundred femtoseconds has yet to fully emerge, fifteen years after its introduction. The lack of a widespread use of femtosecond (fs) lasers in LA-ICP-MS analysis appears surprising since this technology nearly eliminates several of the drawbacks of using nanosecond (ns) lasers: (1) at appropriate fluences, fs laser ablation is nearly athermal; (2) it ablates materials irrespective of their optical properties and (3) it produces particles with a modal distribution close to ∼0.1-0.2 micrometres, easily decomposed in the ICP torch. These properties result in significantly reduced chemical fractionation, a process through which evolving compositions are detected relative to that of the target sample. Hence, the use of fs lasers alleviates the need for matrix-matched calibration for elemental concentration and isotope ratio analyses. This is a decisive advantage for natural materials, often chemically complex and heterogeneous. It is indeed difficult to manufacture or find in nature equivalent homogeneous solids for calibration purposes using ns LA-ICP-MS. Easy ablation of optically transparent materials makes analysis much easier using a fs laser. Furthermore, the ablation yield is higher in the fs regime than in the ns regime, thereby increasing the sensitivity of the LA-ICP-MS technique. However, strategies have been developed to obtain reliable data using ns LA-ICP-MS. This makes the advanced properties of the more expensive and, until recently, more complicated to operate fs laser ablation systems less interesting for several applications. These include elemental analyses of solids used in the form of ratios instead of absolute concentration values, or radiogenic isotope ratio determinations easily corrected through internal normalization using two non-radiogenic isotopes. Lastly, mainstream U-Pb geochronology in zircon and monazite does not benefit much from femtosecond lasers since enough homogeneous standards exist for calibration purposes. Yet, for absolute element concentration determinations or for stable metal and metalloid isotope studies, recent publications demonstrate the decisive benefit of fs LA-ICP-MS analysis in terms of versatility, accuracy, precision and for improved spatial resolution.
The figures of merit of fs laser ablation for LA-ICP-MS analysis are reviewed.
As an essential micronutrient, iron plays a key role in oceanic biogeochemistry. It is therefore linked to the global carbon cycle and climate. Here, we report a dissolved iron (DFe) isotope section ...in the South Atlantic and Southern Ocean. Throughout the section, a striking DFe isotope minimum (light iron) is observed at intermediate depths (200–1,300 m), contrasting with heavier isotopic composition in deep waters. This unambiguously demonstrates distinct DFe sources and processes dominating the iron cycle in the intermediate and deep layers, a feature impossible to see with only iron concentration data largely used thus far in chemical oceanography. At intermediate depths, the data suggest that the dominant DFe sources are linked to organic matter remineralization, either in the water column or at continental margins. In deeper layers, however, abiotic non-reductive release of Fe (desorption, dissolution) from particulate iron—notably lithogenic—likely dominates. These results go against the common but oversimplified view that remineralization of organic matter is the major pathway releasing DFe throughout the water column in the open ocean. They suggest that the oceanic iron cycle, and therefore oceanic primary production and climate, could be more sensitive than previously thought to continental erosion (providing lithogenic particles to the ocean), particle transport within the ocean, dissolved/particle interactions, and deep water upwelling. These processes could also impact the cycles of other elements, including nutrients.
High mass resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) was assessed for iron isotope measurement of natural samples after matrix separation by anion exchange ...chromatography. No remaining interferences were observed on the plateaus used for the mass spectrometric measurements. The approach developed and the instrument used permitted analyses in the static mode. Various mass bias corrections using Ni doping were tested, and even the assumption of similar fractionation factors for Fe and Ni did not produce significantly inaccurate data. However, the daily regression method between ln
57Fe
/
54Fe and ln
61Ni
/
60Ni on the standard reference material IRMM-14 to characterize the instrumental mass bias appeared to give the best precision. The reproducibility observed over four months is about 0.013‰/amu, 2 SD, on both
δ
57Fe
/
54Fe and
δ
56Fe
/
54Fe values, provided that each sample is analyzed at least six times. Accuracy, as estimated on interlaboratory comparison of natural samples that included geostandards, lies within this uncertainty. Among the bulk granitic rocks analysed, those with MgO below 0.6 wt.% and SiO
2 above 71 wt.% have
δ
57Fe
/
54Fe values significantly heavier than the bulk mafic Earth. This shows that the iron isotope composition of terrestrial igneous rocks is more scattered than previously thought. There are good correlations between the Fe isotope composition and the MgO and SiO
2 contents of the granitoids. These correlations are interpreted as reflecting the exsolution of late magmatic aqueous fluids from the granitic melt that preferentially removed the lighter isotopes of iron and enriched the residual magma in the heavier isotopes.
•Assessment and calculation of magmas parental to Martian meteorites.•Iron isotopes become heavier in more differentiated Martian igneous rocks.•Martian basalts and calculated mantle are ...chondritic•Heavy isotope enrichment in planetary basalts correlates with Fe/Mn and Si isotopes.•Low-T volatile depletion (≈1300 K) produces heavy δ57Fe in the terrestrial planets.
Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δFe57≈0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δFe57≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth's mantle is δFe57=+0.04±0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δFe57(+0.05±0.03‰), whereas MgO-rich rocks are lighter (δFe57≈−0.01±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δFe57 SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.
Accurate and precise Si isotope measurements were obtained using magnesium doping and high-resolution plasma source mass spectrometry for samples representative of the Earth, as well as lunar ...samples, meteorites from Mars (SNC), eucrites, a howardite, carbonaceous chondrites (CC), ordinary chondrites (OC) and enstatite chondrites (EC). Our data confirm that significant Si isotope fractionations exist among the inner solar system planetary bodies. They show that the Earth and the Moon share the same Si isotopic composition, which is heavier than all other measured bodies, in agreement with most of previous studies. At the other end of the spectrum, enstatite chondrites have the lightest Si isotope compositions. In order to precisely estimate the amount of Si that may have entered the Earth’s core, we developed a refined model of Si partitioning based on continuous planetary accretion that takes into account the likely variations in T, P and fO2 during the Earth’s accretion, as well as isotopic constraints involving metal–silicate partitioning derived from both experimental and natural sample data sets.
Assuming that the difference between the isotopic signature of the bulk silicate Earth (BSE) and chondrites solely results from Si isotope fractionation during core formation, our model implies that at least ∼12wt% Si has entered the Earth’s core, which is greater than most of the estimates based on physical constraints on core density or geochemical mass balance calculations.
This result leads us to propose two hypotheses to explain this apparent contradiction: (1) At least part of the Earth’s building blocks had a Si isotope composition heavier than that observed in chondrites (i.e., δ30Si>−0.39‰). (2) If on the contrary the Earth accreted only from material having chondritic δ30Si, then an additional process besides mantle–core differentiation is required to generate a stronger isotope fractionation and lead to the observed heavy isotope composition of the bulk silicate Earth. It may be the loss of light Si isotopes during partial planetary vaporization in the aftermath of the Moon-forming giant impact. This process, which may have affected metallic cores, required a thorough isotopic re-equilibration between core and silicate to explain the similar heavy isotope composition of the silicate portions of the Earth and the Moon.
We have measured the iron isotope compositions and trace element concentrations of a suite of iron formation (IF) samples from the Neoproterozoic Rapitan Group, which was deposited during the older ...of two glacial episodes recorded in the Windermere Supergroup of the northern Canadian Cordillera. Like most other Neoproterozoic examples, iron in the Rapitan IF resides almost exclusively as hematite. This mineralogical simplicity compared to Archean and Paleoproterozoic banded iron formations is attributed to a limited supply of organic carbon to the Rapitan glacial ocean that inhibited diagenetic production of reduced iron phases. Sedimentological considerations indicate that the Rapitan IF was deposited during a rise in relative sea level related to a period of glacial advance and isostatic subsidence. Trace element data, including rare earth element plus yttrium (REE
+
Y) patterns, suggest an anoxic deep ocean dominated by low-temperature hydrothermal input and capped by a weakly oxic surface ocean. The iron isotope data show a trend of increasing δ
57Fe (versus IRMM-14) up-section from ~−0.7‰ to 1.2‰, corresponding to a shift from a muddy IF facies to a dominantly jaspilitic IF facies. This distinct isotopic pattern likely records a steep isotopic gradient across the iron chemocline in Rapitan seawater.
► We present trace element and iron isotope data on the Rapitan iron formation. ► The iron isotope data show a large rise in iron isotope values up-section. ► This isotopic trend is coupled to increasing water depth. ► The trend likely records a vertical iron isotope gradient in Rapitan seawater. ► We propose a new model for iron isotopic variability in ancient iron formations.
Significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in (1) subarctic ...organic-rich boreal river and stream, mire, lake and soil solutions in northern taiga zone (Karelia, NW Russia) and (2) temperate river and lake waters of the southern boreal zone (Central Russia). Water samples were filtered in the field employing progressively decreasing pore size from 100μm to 1kDa followed by iron isotope analysis. In all river samples, there was a gradual increase of δ57Fe relative to IRMM-14 with decreasing pore size, from +0.4±0.1‰ at 100μm up to +4.2±0.1‰ at 10kDa fraction in the subarctic zone and from −0.024±0.2‰ at 100μm up to +1.2±0.2‰ at 10kDa in the temperate zone. In the series of filtrates/ultrafiltrates of subarctic and temperate streams and rivers, the δ57Fe value decreases with increasing molar Fe/Corg ratio. Therefore, small-size, Fe-poor, C-rich colloids (1–10kDa) and Low Molecular Weight (LMW) fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas High Molecular Weight Fe-rich colloids (100kDa–0.22μm) and particles (1–100μm) are isotopically lighter and closer to the continental crust Fe isotope composition.
The relative enrichment of 1–10kDa ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind Fe more strongly (Fe–O–C bonds) than Fe(III)oxy(hydr)oxides (Fe–O–Fe bonds), in accord with quantum mechanics principles. Highly positive δ57Fe of the LMW fraction of labile and potentially bioavailable Fe in small subarctic rivers may turn out to be a very important source of isotopically heavy Fe in the Arctic Ocean. The mechanisms involved in the production of this isotopically heavy Fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone.
Steady state dissolution rates of Manangotry monazite ((Ca
0.04La
0.21Ce
0.43Pr
0.05Nd
0.15Sm
0.02Gd
0.01Th
0.13)P
0.90Si
0.09O
4) were determined in open system titanium mixed flow reactors at pH=2 ...at temperatures from 50 to 229 °C, and at pH 1.6, 2.6, and 10 at 70 °C. Dissolution rates at 70 °C and pH 2, 6, and 10 were determined from closed system experiments. All dissolution rates are calculated from release rates of Ce into solution as measured by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).
Measured pH=2 monazite dissolution rates increase from 4.3×10
−17 to 1.2×10
−14 mol/cm
2/s with increasing temperature from 50 to 229 °C. This is consistent with an apparent activation energy of 10.3 kcal/mol. Measured and estimated 70 °C monazite dissolution rates reach a minimum at near neutral pH; dissolution rates were found to be 1×10
−16, 4×10
−18, and 4×10
−17 mol/cm
2/s at pH=2, 6, and 10, respectively.
The light rare earth elements (REE) (La, Ce, Pr, and Nd) and uranium were found to be released in approximately stoichiometric quantities in all experiments. Sm/Ce and Gd/Ce ratios in the outlet solutions were found to be slightly higher than those of the dissolving monazite at all pH and temperatures. In contrast, thorium release stoichiometry depended on pH and temperature. Th was released at close to stoichiometric proportions at basic conditions, but was released to solution in far lower proportions at pH 2, likely due to the precipitation of a Th-rich secondary phase.
Biodegradation and photolysis of dissolved organic matter (DOM) in boreal high-latitude waters are the two main factors controlling not only the aquatic fluxes and residence time of carbon but also ...metal nutrients associated with DOM such as Fe. The DOM is usually present in the form of organic and organomineral colloids, which also account for the majority of dissolved Fe. Here, we use the stable Fe isotope approach to unravel the processes controlling Fe behavior during bio- and photodegradation of colloids in boreal Fe- and DOM-rich humic waters (a stream and a fen). The adsorption of Fe colloids onto heterotrophic bacteria Pseudomonas aureofaciens produced enrichment in +0.4‰ (δ57Fe) in the heavier isotopes of the cell surface relative to the remaining solution. In contrast, long-term assimilation of Fe by live cells yielded preferential incorporation of lighter isotopes into the cells (−0.7‰ relative to aqueous solution). The sunlight-induced oxidation of Fe(II) in fen water led to the removal of heavier Fe isotopes (+1.5 to +2.5‰) from solution, consistent with Fe(III) hydroxide precipitation from Fe(II)-bearing solution. Altogether, bio- and photodegradation of organoferric colloids, occurring within a few days of exposure time, can produce several per mil isotopic excursions in shallow lentic and lothic inland waters of high-latitude boreal regions. Considerable daily scale variations of Fe isotopic composition should therefore be taken into account during the interpretation of the riverine flux of Fe isotopes to the ocean or tracing weathering processes using Fe isotopes in surface waters at high latitudes.
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