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Chlorine Stable Isotopes in Sedimentary Systems: Does Size Matter?

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Chlorine Stable Isotopes in Sedimentary Systems: Does Size Matter? Chlorine stable isotopes in sedimentary systems: does size matter? Max Coleman Jet Propulsion Laboratory, Caltech, Pasadena, California, USA ABSTRACT: Stable isotope abundances vary because of size (atomic mass) differences. The chlorine stable isotope system was one of the first described theoretically, but had a slow, disappointment-strewn develop- ment, relative to other elements. Method improvement gave only small, but significant, differences (-1 %.) in compositions of geological materials. Eventually, brines and groundwater chlorides gave larger differences (-5%0). Physical processes like diffusion and adsorption, probably are the main controls of groundwater com- positions. In contrast, the processes producing brine compositions are still enigmatic and need further work for full understanding. Recent work on anthropogenic groundwater contaminants (chlorinated aliphatic hy- drocarbons and perchlorate) shows variations resulting from manufacturing processes; implying possibilities of tracing sources. However, they are subject to microbial degradation, producing much larger fractionations (115%0),but therefore offering better possibility of monitoring natural attenuation. So atomic size is important to give isotopic fractionation and the size of fractionation matters, allowing observation of otherwise unde- tectable processes. lecular and atomic weights by weighing the gases and relating them to the weight of hydrogen. All 1 INTRODUCTION atomic weights were integer multiples the weight of This paper aims to give a short and selective history hydrogen, with the exception of chlorine, 35.5. It of the frustrating development of chlorine stable was not until Aston developed what was the effec- isotope geochemistry. Readers may be depressed by tively the mass spectrograph, a modification of the apparently small return of science from the im- Thompson’s Positive Ray apparatus, that the non- mense investment of effort expended by the pioneers integer atomic weight was confirmed to be the result in this area, but please do read on because there of the abundances of two isotopes, 35Cl -75% and really is a happy ending. 37Cl- 25% (Aston, 1919). I include a brief description of the technology of analysis, which has been and still is one of the key 2.2 Later history factors both allowing and restraining progress. Some In a classic theory paper, Urey (1947) calculated the examples of its application will indicate the scope of result of the equilibrium isotopic fractionation be- the method to produce large and characteristic iso- tween perchlorate and hydrogen chloride molecules topic fractionation values. Because of its relatively to be a massive isotopic difference in 37C1P5CIof low chemical reactivity in sedimentary systems, 9.2% (more usually expressed now in parts per thou- chlorine demonstrates fewer effects of normal iso- sand as 92%0).This should have been measurable by topic fractionation but therefore has the power to re- the instruments of the day but was not found. In fact cord physical processes. the major advance in analytical capability came from the various developments from two laboratories, those of Nier and McKinney in the period 1946 to 2 HISTORY 1955. That work produced the double collector mass-spectrometer, allowing stable isotope meas- 2.1 Early history urements to be made routinely to a precision of bet- In the late 1850s Cannizzaro resurrected the idea, ter than 1%. (Nier, 1947; McKinney et al., 1950). put forward by Avogadro 50 years before, that equal However no one was able to measure any significant volumes of gas contained equal numbers of mole- natural variations in chlorine stable isotope abun- cules. This allowed him to refine estimates of mo- dance ratios. Hoering & Parker (196 1) analyzed 8 1 samples, including natural perchlorate and chloride gous with SMOW, Standard Mean Ocean Water, the from Chile, again with no detectable variation. standard for H and 0. Further development of the method by the re- 3.1 Sample preparation agonies search group in the University of Arizona, finally improved the analytical precision to about 0.25%0 For both types of measurement chlorine has to be in (Kaufmann (1984; Kaufmann et al., 1984). This a specific, chemically pure form, methyl chloride for gave hope that chlorine stable isotope geochemistry dual inlet stable isotope ratio mass spectrometry might start to follow the examples of the other light (SIRMS) and cesium chloride for thermal ionization stable isotope systems, like carbon or sulfur, and mass spectrometry (TIMS). Without going into de- show significant variations in ocean waters of differ- tails, both approaches require analyst time, skill and ent geological ages as well as characterizing geo- patience. chemical processes. In the next ten years, although There are frustrations associated with both tech- the Arizona group measured the first natural varia- niques. In the case of analysis of chlorinated ali- tions to be observed, the range of C1 isotope ratios in phatic hydrocarbons, commonly analyzed materials, more than 200 samples of all geological ages was organic chloride has to be separated as inorganic disappointingly small. Most samples were near to chloride ion and then converted into CsCl for TIMS the value for modern seawater, adopted as the refer- analysis. The situation is even worse for SIRMS, ence standard. The total range extended from 1.5%0 where the organic C1 is separated by conversion to less than seawater chloride to just over 2%0 more. an inorganic chloride, precipitated as insoluble AgCl Subtle further refinements of the technique by which is then converted back to organic methyl Eggenkamp (1994) now allow an analytical preci- chloride. The main features of each of the isotopic sion of about 0.05%0. Despite the increased data analytical techniques are described very briefly be- quality, analysis of more than 200 other geological low. samples by 1994 had produced a range from 2.5%0 3.2 less than seawater chloride to just over 1.5%0more. Comparison of SIRMS and TIMS For SIRMS the sample is in gaseous form, methyl chloride. After introduction to the mass spectrome- 3 MEASUREMENT TECHNIQUES ter, is ionized by electron bombardment. Introduc- tion of the sample gas is alternated repeatedly with The methods referred to above all measure the sam- reference gas of known isotopic composition. This ple in the form of a gas. However Xiao and Zhang allows multiple direct estimates to be made of their (1992) pioneered use of a technique, Thermal Ioni- difference in isotope ratio in about 15 minutes. zation Mass Spectrometry (TIMS), previously used Minimum sample size is 14micromole. Reproduci- for measurement of radiogenic isotope ratios. Both bility of analysis for the whole analytical procedure of these commonly used methods for measurement is 10.10%0(20). of chlorine isotope ratios require that the sample to In TIMS the CsCl solution, prepared from the be converted into an appropriate chemical form from sample C1, is evaporated on a filament to form a which it is ionized so that the different masses may solid that is ionized by resistive heating of the fila- be separated in a magnetic sector mass spectrometer. ment in the mass spectrometer. Multiple readings of Results are reported as the difference of the 37~1/35~1the sample isotope ratio are taken over a period of ratio in the sample from that ratio in a standard ref- hours. Standards can be run as samples to confirm erence material, 637Cl,in parts per thousand, %o. the calibration of the mass spectrometer. Precision for a single run is 5 0.2%0but to determine the value Rsample - Rstandard * 637~1= 1000 (1) relative to a standard, which also has the same un- Rstandard certainty, the overall uncertainty is about 0.3%0(20). where R = 37C1/35C1. To compensate for this, sample size can be as little Positive, negative and zero 6 values indicate re- as 0.3 micromole or less. spectively, greater, less and the same relative abun- Godon et al. (2004) showed that a large variety of dance of the trace isotope compared with the stan- different seawater samples all gave the same result dard. within error, thus confirming the use of SMOC as a This formulation, evolved from one of the meth- standard. They also showed that both methods give ods by which isotope measurements are made but is comparable mean values for samples near to that of used for both. In fact, there is no single standard cu- the standard but that more extreme values may not. rated and distributed by a single agency, as is the However, part of the discrepancy may be due to the case for hydrogen and oxygen. However, Kaufmann fact that in TIMS analysis the result may be changed et al. (1984) showed that all ocean water chloride by the amount of sample loaded onto the filament. had the same isotopic composition within the limits To summarize the points made above, whichever of measurement precision and defined the standard method is used, a lot of effort and time is needed to as SMOC, Standard Mean Ocean Chloride, analo- make any measurements and those made on sedi- mentary materials showed very little variation in comparison to the larger ones of other stable isotope L6.llHl systems. a 4 ISOTOPIC FRACTIONATION PROCESSES 4.1 Crystallization 5 1w -0.15 -0.10 .om 0.0 o.os 0.11) (1.15 0.20 Eggenkamp (I 994) performed careful experiments crystallizing sodium, potassium and magnesium Distance from centre, m chlorides from aqueous solutions, but only NaCl Figure 1. Laborato ?,experiments to measure the differential gave a fractionation factor significantly larger than diffusion of ’k1 and CI to produce a large change in S3’CI. the uncertainty. The solid had a value +0.24%0(0.07 corresponding, small relative enrichment in 37Cl in la). The implication of this result is that during the residual brine, from which 35Cl has diffused continued evaporation of seawater, the first-formed faster.
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