Variation in Isotope Ratios

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Until only a few years ago, I would never have imagined that a volume on the stable isotope geochemistry of elements like Mg, Fe or Cu would be written. In fact, a comic book of blank pages entitled The Stable Isotope Geochemistry of Fluorine would have been a more likely prospect. In volume 16 of this series, published in 1986, I wrote Isotopic variations have been looked for but not found for heavy elements like Cu, Sn, and Fe.. .. Natural variations in isotopic ratios of terrestrial materials have been reportedfor other light elements like Mg and K, but such variations usually turn out to be laboratory artifacts. I am about ready to eat those words. 

The isotopic dilution method can be extended to non-radioactive tracers by using mass spectrometry or NMR to determine the variation in isotopic ratios. This method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultra-trace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decompose during the period of measurement, all labelled atoms are measured. Isotopic mass spectrometers are well suited for these measurements. 

Natural oxygen is 99.8% l60, 0.2% lsO, and 0.04%, 70. Small variations in isotope ratios in H20 in the ocean, the atmosphere, and rain and snow depend on local temperature. The warmer the air, the more enriched it is in H2I80. The lsO content of snow preserved in glaciers provides a record of air temperatures for 250 000 years in 2-km-long cores drilled from ice sheets in Greenland and Antarctica. Depth in the ice pack has been correlated with age in years by a variety of measurements. 

These geochemical tracers have been successfully applied to studies of the shells of a variety of marine organisms including bivalve and gastropod mollusks, ostracods, forams, brachiopods and solitary corals (47, 54, 57, 58). In the case of mollusk shells, for example, serial microsampling around the spiral whorls from earliest to oldest growth revealed sinusoidal variations in isotope ratios, which result from shell deposition in a seasonal environment (47, 50, 62-64). Our previous work showed this to be true of Olivella shells as well, where 8,sO levels fluctuate from warm summer temperatures to cold spring and winter temperatures (38). 

Variations in isotope ratios are not large so the fractionation factor a does not differ much... 

Isotope ratios of inorganic trace metals which are the topic of this chapter are not modified when incorporated into the sediment (note that some minor isotope fractionation might occur on incorporation into the sediment, but usually such shifts are either smaller than analytical precision or they are removed by the internal correction procedures of the techniques used). The variation in isotope ratios only varies... 

This book is not intended to cover important branches of mass spectrometry that provide accurate and precise quantitative measurements of relative concentrations, e.g., of variations in isotopic ratios of an element by isotope ratio mass spectrometry (IRMS) and accelerator mass spectrometry (AMS). Rather, this book is mainly concerned with determinations of absolute amount of substance (see Chapter 1 for a definition and explanation), particularly for compounds present at trace levels in complex matrices. (The only exception is the inclusion of a brief description of methods used to determine differences in levels of proteins in living cells or organisms subjected to different stimuli, e.g., disease state vs normal state). 

The slight variations in isotopic ratios have been attributed to differ-... 

Even after mass bias and possible drift corrections have been carried through and their contributions to the uncertainty budget have been estimated, comparability is not assured. Minute variations in isotope ratios caused by fractionation can in most cases only be made visible and comparable when normalized to a ratio obtained for a common reference. Depending on the magnitude of deviation from a common reference isotope amount ratio, different notations are used. 

Licensed and operated with variations in isotopic ratios among fuel rods... 

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