In situ carbon and oxygen isotopes measurements in carbonates by fiber coupled laser diode-induced calcination

. Stable isotopic compositions of carbon and oxygen (δ13C et δ18O ) measured from carbonates are used in geology to reconstruct paleotemperatures and to learn about the evolution of the biogeochemical carbon cycle. The standard technique used since the middle of the XXth century [1] to measure isotopic ratios is based on a wet chemical protocol which CO2 is evolved from the acidic dissolution of carbonates followed by quantification of CO2 molecules isotopologues using mass spectrometer or infrared spectroscopy. This is a lengthy protocol that necessitate to manipulate acid solution and numerous gas phases purification steps before isotopic measurements. Our new preparation technique aims at offering an alternative to the wet chemical preparation of the samples by using a direct extraction of CO2 via a laser-induced calcination process. In addition to save time, this method allows to consider spatially resolved and automated in-situ measurements and does not necessitate further purification steps of the evolved CO2 during calcination.


Background & principle
The first experiment to prepare carbonates for δ 13 C[‰] & δ 18 O [‰] isotopic analysis using a laser dates back to the 90s [2] with a CO2 laser of about 20W average power as laser source.Although experimentally validated, these first systems were not widely used for routine analysis in geochemical laboratories due to the cost of the laser of source and the rather complex scheme of analyses that includes multiple gas purifications steps and intercalibrations.Moreover, the laser source wavelength of 10.6µm does not allow to carry the light by optical fiber making such carbonate preparation system cumbersome and difficult to install close to a mass spectrometer.The benefit of such laser preparation device for carbonate isotopic analyses was hence not convincing enough especially since the development of secondary ion mass spectrometry, for instance.
The originality of the presented work is to use as laser source a compact, robust and low-cost fiber laser diode module.The choice of such module allows sensible simplification of the experimental assemblies together with a strong gain of space, an improved energy efficiency and an increased accessibility.This evolution thus makes it possible to carry out punctual analyses with high spatial resolution (punctual laser spot) compared to the methods of chemical preparation, to perform field analyses with a portable system (compact laser source), and to realize sample isotopic mapping (fibered source, therefore simple scanning).
Laser-induced calcination is based on a light-matter interaction of essentially thermic nature.A laser beam is focused on a carbonate sample (with a typical spot diameter less than 500µm).The laser energy is absorbed by the carbonate and relaxed essentially as heat for a short time (typically <10s).This local heating allows the sample to reach the calcination threshold temperature (around 900°C for most carbonates) and to release CO2 according to equation ( 1) Accordingly, there is a complete transfer of carbon between the carbonate and the evolved CO2 gas.It is therefore expected that the carbon element will not fractionate during this process.On the other hand, the yield of the reaction is incomplete for oxygen and a transfer function is necessary to scale the oxygen isotope values of the carbonate.

Experimental setup
Several prototypes have been developed in the laboratory, which allowed to engineered an integrated and transportable prototype.The apparatus described here (fig.1a) is the latest version developed.It could be use both in the field and in the laboratory depending on the type of spectrometer coupled to it, a Delta Ray™ infrared spectrometer (Thermo Scientific™) or a Delta V™ mass spectrometer (Thermo Scientific™).This system allows to fill standard gas tubes (e.g.Exetainers™), with CO2 gas diluted in nitrogen for a final pressure of 1bar.This device, includes a collection head (Fig. 1b), integrating the optical functions of laser beam shaping, XY scanning of the laser spot, collection and control of the measurement and gas sensors.The transportable integrated assembly integrates all the electronics control including two HMI driving the laser and gas modules and controlling the scanning and the measurement area.The fiber laser source used in this system delivers a maximum power of 90W@915nm at the output of a silica fiber with a core diameter of 100µm and 0.22 NA.The laser beam enters a hermetically sealed gas collection vessel placed in tight contact with the sample to be analyzed.Several sensors allow to finely control the calcination process and the gas circulation.The CO2 produced during calcination is diluted with nitrogen (or other vector gas) and then transferred to 12 ml Exetainers™ (Labco™) sample tubes.In addition to this setup, whose performance will be described and discussed, a new fully portable setup that can be used in difficult conditions (for example, at a site with poor access such as cliff) will be presented.This prototype will allow to produce and collect CO2 tubes for analysis either in the laboratory or for example during a field trip.

Validation of the method
We first intercalibrate the laser calcination technique with the reference method based on preparation by acid digestion.In geology, there are no international solid standards for carbon and oxygen isotopes available over a wide range of isotopic variations, so we used necessary to define "internal standards" to infer on the intercalibration accuracy between the two preparation methods.These samples have been chosen to be locally homogeneous and representative of the natural isotopic and mineralogical compositions diversity (e.g.calcites, aragonites, stromatolites, malachite, siderite, dolomite, rhodochrosite, marbles, etc) [3].The figure 2 shows an example of intercalibration obtained for carbon between the laser method and the acid method for two isotopic spectrometers (optical and magnetic mass spectrometer).These cross measurements show the absence of instrumental isotopic fractionation for carbon isotope (slope of 1 and ordinate close to 0).Intercalibration results for oxygen will also be presented as well as a detailed metrological analysis.The effects of geological parameters (homogeneity of the sample, humidity, weathering layer, etc) and experimental conditions (laser exposure, calcination reaction environment, amount of gas produced) on the measurement and its accuracy will be discussed.

Application examples
An example of analyses using the laser calcination method applied to the analysis of a stratified sample is shown in figure 3. The stratification of this sample resulted from time successive precipitation events controlled by several environmental factors such as saturation with respect to calcium and carbonate ions and temperature.Finally, the figure 3b shows the possibilities of coupling spatially resolved isotopic analyses with image processing to feed a predictive model.

Fig. 2 .
Fig. 2. Comparison of the "standard" δ 13 C measurements with the new laser preparation method