Ca: isotopic composition

Using the three measured ratios, Ca/ Ca, Ca/ " Ca and Ca/ " Ca, three unknowns can be solved for the tracer/sample ratio, the mass discrimination, and the sample Ca/ Ca ratio (see also Johnson and Beard 1999 Heuser et al. 2002). Solution of the equations is done iteratively. It is assumed that the isotopic composition of the Ca- Ca tracer is known perfectly, based on a separate measurement of the pure spike solution. Initially it is also assumed that the sample calcium has a normal Ca isotopic composition (equivalent to the isotope ratios listed in Table 1). The Ca/ Ca ratio of the tracer is determined based on the results of the mass spectrometry on the tracer-sample mixture, by calculating the effect of removing the sample Ca. This yields a Ca/ Ca ratio for the tracer, which is in general different from that previously determined for the tracer. This difference is attributed to mass discrimination in the spectrometer ion source and is used to calculate a first approximation to the parameter p which describes the instrumental mass discrimination (see below). The first-approximation p is used to correct the measured isotope ratios for mass discrimination, and then a first-approximation tracer/sample ratio and a first-approximation sample CeJ Ca... 

New insights on biomineralization may be revealed by measuring Ca isotope variations in shell secreting organisms (e.g. Griffith et al. 2008). Two factors influence the Ca isotope composition of shells (1) the chemistry of the solution, in which the organisms live and (2) the process by which Ca is precipitated. 

Sahijpal S., Goswami 1. N., and Davis A. M. (2000) K, Mg, Ti, and Ca isotopic compositions and refractory trace element abundances in hibonites from CM and CV meteorites implications for early solar system processes. Geochim. Cosmochim. Acta 64, 1989-2005. 

The most pronounced fractionations of Mg isotopes between carbonate minerals and dissolved Mg can be attributed to mineralogy and rate of precipitation, although there are some significant exceptions (Figure 12.7). In contrast to Ca isotopic compositions wherein values tend to be higher in calcite and... 

Almost any type of analyzer could be used to separate isotopes, so their ratios of abundances can be measured. In practice, the type of analyzer employed will depend on the resolution needed to differentiate among a range of isotopes. When the isotopes are locked into multielement ions, it becomes difficult to separate all of the possible isotopes. For example, an ion of composition CgHijOj will actually consist of many compositions if all of the isotopes ( C, C, H, H, 0, O, and 0) are considered. To resolve all of these isotopic compositions before measurement of their abundances is difficult. For low-molecular-mass ions (HjO, COj) or for atomic ions (Ca, Cl), the problems are not so severe. Therefore, most accurate isotope ratio measurements are made on low-molecular-mass species, and resolution of these even with simple analyzers is not difficult. The most widely used analyzers are based on magnets, quadrupoles, ion traps, and time-of-flight instruments. 

The reaction of seawater with country rocks is also a possible but unlikely explanation. Tertiary volcanic sediments in the vicinity of Kuroko deposits are altered and tend to have lost both Ca and Sr (Farrell and Holland, 1983). The ratio of Sr loss to Ca loss is roughly equal to the Sr/Ca ratio in seawater. If seawater was the altering medium, its Sr/Ca ratio was probably not strongly affected by the alteration process. The 87sr/86si- ratio would be intermediate between an initial value of 0.7088 and ca. 0.740 — the Sr/ Sr ratio of unaltered Tertiary volcanics of the Hokuroku basin. It is unlikely, therefore, that this type of alteration can account for the Sr content and for the isotopic composition of Sr in the anhydrites at the upper end of the trend line in Fig. 1.49. On the other hand, mixing of seawater with solutions which have a Sr/Ca ratio much smaller than that of seawater could have led to the deposition of Kuroko anhydrites. 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

The presence of 26Mg excesses correlated with Al/Mg ratios in fifteen Ca-Al-rich inclusions from the Allende and Leoville carbonaceous chrondrites has provided additional strong evidence for the in situ decay of 26A1 (see [9] for a recent review of isotopic anomalies). There are also, however, several examples of minerals whose isotopic compositions depart substantially from a unique Al-Mg isochron, even within a single inclusion [10,11]. Since deviations from the isochron may reflect either differences in the formation age of individual minerals or intrinsic heterogeneities in the initial 26A1/27A1 ratio, the value of the Al-Mg system as a chronometer for early solar system events remains unclear. 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

The nucleosynthetic sources for Ti isotopes are very similar to those of the isotopes of Ca, and Ti requires a neutron-rich zone to be produced in significant amoimts. In addition to the nonlinear effects, absolute isotopic compositions have been measured in a number of samples using double spike techniques (Niederer et al. 1985). Mass dependent fractionation effects are rarely resolved and are small, below 1 %o/amu except in one sample, where it reaches 1.3 %o/amu. In general the fractionation is in favor of the heavy isotopes partial condensation or evaporation may explain of this observation. 

Iron. Fe has 4 isotopes of which the heaviest Fe has a very small abimdance of about 0.3%. The precision of thermal ionization mass spectrometers is around 10 s on this isotope and there is only a hint in some normal inclusions for an excess in 5 Fe (VoUcening and Papanastassiou 1989). Recent ICPMS measurements at the 2 s precision level display normal isotopic compositions for Fe in planetary materials but no Allende inclusion was reported in this study (Kehm et al. 2003). If excesses of similar magnitude to Ca, Ti, Cr were present they would not be clearly resolved in agreement with the observations. When Fe and Fe are used to correct for instrumental mass fractionation, Fe exhibits normal abundances, suggesting all three isotopes are present in solar relative abundances. 

Niederer PR, Papanastassiou DA, Wasserburg GJ (1981) The isotopic composition of titanium in the Allende and Leoville meteorites. Geochim Cosmochim Acta 45 1017-1031 Niederer PR, Papanastassiou DA (1984) Ca isotopes in refractory inclusions. Geochim Cosmochim Acta 48 1279-1293... 

Prombo CA, Podosek FA, Amari S, Lewis RS (1993) s-Process Ba isotopic compositions in presolar SiC from the Murchison meteorite. Astrophys J 410 393-399... 

Magna T, Wiechert UH, Grove TL, Halliday AN (2003) Lithium isotope composition of arc volcanics from the Mt. Shasta region, N California. Geochim Cosmochim Acta 67 A267 Marriott CS, Henderson GM, Belshaw NS, Tudhope AW (2004) Temperature dependence of 6 Li, 6 Ca and Li/Ca incorporation into calcium carbonate. Earth Planet Sci Lett (in press)... 

The Ca isotope ratios of meteoritic samples are of interest because they can give information on early solar system processes and because meteorites represent the materials from which the Earth accreted and hence relate to the expected values for the bulk Earth. Russell et al. (1978b) made the first measurements of stable Ca isotope variations in meteorites. They formd variations of about +l%o for the Ca/ Ca ratio in samples from six different meteorites. Although some of these samples were spiked after having separated the Ca with an ion exchange column and hence may contain artifacts, it is clear from their data that bulk meteorites have some variability in 8 Ca and that the average value is quite close to the terrestrial standard. No data on bulk meteorites have been reported since the Russell et al. (1978b) measurements, and since their one measurement of an ordinary chondrite had a poor Ca column yield, there exist no reliable measurements that can be used to verify the composition of typical chondritic meteorites. 

To relate the isotopic composition of marine calcium to variations in the calcium cycle requires characterization of the 5 Ca values of the sources and sinks of Ca to the oceans, and estimates of the Ca fluxes. Neither is well documented presently. Estimates of the calcium fluxes from Milliman (1993) are shown in Table 3. There are six analyses of 5 Ca from mid-ocean ridge hydrothermal vents, and these average -1-0.2 + 0.2 (Zhu and MacDougall... 

Figure 16. (a) Ca isotope record from marine carbonates (De La Rocha and DePaolo 2000). The variations are inferred to reflect variations in the isotopic composition of seawater (which is heavier by about 1,4%o). The small excursions of S Ca reflect changes in the global weathering cycle they are recast in (b) in terms of the ratio of the flux of calcium being delivered to the ocean by weathering (Fw) to the flux of Ca being removed from the ocean by carbonate sedimentation (c) Smoothed record of benthic foraminiferal 5 0 for the Cenozoic time period from Zachos et al. (2001). 

Schmitt AD, Bracke G, Stille P, Kiefel B (2001) The calcium isotope composition of modem seawater determined hy thermal ionisation mass spectrometry. Geostandard Newsletter 25 267-275 Schmitt A-D, Stille P, Venneman T (2003a) Variations of the " Ca/ Ca ratio in seawater during the past 24 million years evidence from 5 Ca and values of Miocene phosphates. Geochim Cosmochim Acta 67 2607-2614... 

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