Strontium ratio

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

Another precise radiochronometric tool is initial Sr/ Sr, especially in objects with low bulk rubidium. Gray et al. (1973) found one Allende CAl to have the lowest initial ratio yet seen in any solar system material ( Sr/ Sr)o = 0.69877. This value has endured as a solar system benchmark for strontium isotopes, and was given the name ALL. Later work by Podosek et al. (1991) on a different set of Allende CAIs also found very low initial ratios 0.698793-0.698865, which are, however, not quite as low as ALL. An interesting result of the study of Podosek et al. (1991) is that they reanalyzed the same CAI (D7 USNM 3898 of Podosek et al.) on which ALL is based but obtained a significantly higher initial ratio than did the Caltech group. This discrepancy remains unresolved. Nevertheless, the low initial strontium ratios of CAIs are consistent with their being the oldest objects formed in the solar system. 

Fig. 6. Isotopic composition of calcite cements from North Coles Levee, South Coles Levee and Canal fields. North Coles Levee dolomite cements shown for comparison. North Coles Levee data from Schultz et al. (1989) South Coles Levee and Canal data from Table 2. Increasingly negative 5 OpDB and lower Sr ratios are correlated with higher temperatures of crystallization. Strontium ratio decrease is attributed to Sr from plagioclase alteration. Shaded box shows calculated composition of calcite in equilibrium with present pore water at Coles Levee fields based on fluid temperature and composition in Fisher Boles (1990) and Sr isotopic data in Feldman et al.( 993).

Strontium ratios were analysed on a multicollector Finnigan MAT262 mass spectrometer in static mode using W filaments. Analytical uncertainty (2ct), based on the reproducibility of analyses of the international standard NBS987, is 0.00002. Pure sqiarates of siderite and ankerite were totally dissolved in 1 N HCl. Calcite was dissolved from whole-rock powders by leaching in 0.2 N HCl. This procedure was verified to not leach strontium from silicates. Pure calcite extracted (by heavy liquids)... 

Rubidium-87 emits beta-particles and decomposes to strontium. The age of some rocks and minerals can be measured by the determination of the ratio of the mbidium isotope to the strontium isotope (see Radioisotopes). The technique has also been studied in dating human artifacts. Rubidium has also been used in photoelectric cells. Rubidium compounds act as catalysts in some organic reactions, although the use is mainly restricted to that of a cocatalyst. 

Bismuth trioxide forms numerous, complex, mixed oxides of varying composition when fused with CaO, SrO, BaO, and PbO. If high purity bismuth, lead, and copper oxides and strontium and calcium carbonates are mixed together with metal ratios Bi Pb Sn Ca Cu = 1.9 0.4 2 2 3 or 1.95 0.6 2 2 3 and calcined at 800—835°C, the resulting materials have the nominal composition Bi PbQ4Sr2Ca2Cu20 and Bi 25PbQgSr2Ca2Cu20 and become superconducting at about 110 K (25). 

Goldschmidt predicted from his empirical rule that calcium chloride would not have the fluorite structure, and he states that on investigation he has actually found it not to crystallize in the cubic system. Our theoretical deduction of the transition radius ratio allows us to predict that of the halides of magnesium, calcium, strontium and barium only calcium fluoride, strontium fluoride and chloride, and barium fluoride, chloride,... 

In this discussion, two mutually canceling simplifications have been made. For the transition value of the radius ratio the phenomenon of double repulsion causes the inter-atomic distances in fluorite type crystals to be increased somewhat, so that R is equal to /3Rx-5, where i has a value of about 1.05 (found experimentally in strontium chloride). Double repulsion is not operative in rutile type crystals, for which R = i M + Rx- From these equations the transition ratio is found to be (4.80/5.04)- /3i — 1 = 0.73, for t = 1.05 that is, it is increased 12%. But Ru and Rx in these equations are not the crystal radii, which we have used above, but are the univalent crystal radii multiplied by the constant of Equation 13 with z placed equal to /2, for M++X2. Hence the univalent crystal radius ratio should be used instead of the crystal radius ratio, which is about 17% smaller (for strontium chloride). Because of its simpler nature the treatment in the text has been presented it is to be emphasized that the complete agreement with the theoretical transition ratio found in Table XVII is possibly to some extent accidental, for perturbing influences might cause the transition to occur for values a few per cent, higher or lower. 

In Table XVIII are given values of the radius ratio for the salts of beryllium, magnesium and calcium (those of barium and strontium, with the sodium chloride structure, also obviously satisfy the radius ratio criterion). It is seen that all of the sodium chloride type crystals containing eight-shell cations have radius ratios greater than the limit 0.33, and the beryl-... 

The elements Na, K, Cl, SO, Br, B, and F are the most conservative major elements. No significant variations in the ratios of these elements to chlorine have been demonstrated. Strontium has a small (< 0.5%) depletion in the euphotic zone (Brass and Turekian, 1974) possibly due to the plankton Acantharia, which makes its shell from SrS04 (celestite). Calcium has been known since the 19th century to be about 0.5% enriched in the deep sea relative to surface waters. Alkalinity (HCOf") also shows a deep enrichment. These elements are controlled by the formation... 

Recognition among bone-chemistry researchers that strontium enters bone in proportion to dietary levels has resulted in widely accepted yet erroneous inferences about the relationships among various elements in bone and past diet. One such inference is that more of any element in the diet translates directly to more of that element in bone. If an element is not biogenically incorporated within bone, or if biological levels are metabolically controlled, then that element will not reflect diet. A second erroneous inference is that strontium can be used to measure the dietary plant/meat ratio. Sr/Ca ratios in meat are generally lower than those of plants, but meat is also low in calcium and hence has little effect on the composition of bone. Plants, on the other hand, contribute substantially to bone composition. Variations in the strontium levels of bone thus more likely reflect differential consumption of plants rather than trophic position. Although efforts to determine plant/meat ratios from strontium and to draw dietary inferences from elements other than strontium and barium have not been successful, this failure has been due to inappropriate expectations, not to a failure of bone strontium to reflect diet. 

Strontium, that of Toots and Voorhies (1965), used these compositional differences among plants to assess not the dietary plant/meat ratio but the degree of browsing versus grazing. It is this phenomenon due to shifting plants that we are likely to observe in comparative Sr/Ca studies of bone, rather than the amount of meat in the diet, simply because plants have a much greater impact on diet composition. 

Although there are many failed attempts to determine even relative measures of meat eonsumption, the disappointment is not due to any failure of strontium to reflect diet but to an unwarranted expectation that bone strontium should necessarily reflect meat consumption. The frustration with efforts to draw paleodietary inferences stems from simplistically equating Sr/Ca ratios with plant/meat ratios and to the inappropriate use of elements not... 

Burton, J.H. and Price, T.D. 1990 The ratio of barium to strontium as a paleodietary indicator of consumption of marine resources. Journal of Archaeological Science 17 547-557. 

Schoeninger, M.J. (1985). Trophic level effects on 15N/14N and 13G/12C ratios in bone collagen and strontium levels in bone mineral. Journal of Human Evolution, Vol.l4, pp. 515-525. 

Strontium was from igneous rocks. Sr/ Sr ratios of barite are in a range from 0.706 to 0.708, suggesting smaller contribution of seawater strontium than anhydrite and gypsum. 

Figure 1.49. Change of the strontium content and Sr/ Sr ratio of Kuroko anhydrite during the deposition and dissolution due to the mixing of hot ascending solution and cold solution (normal seawater) (Shikazono et al., 1983). R mixing ratio (in weight) = S.W./(S.W.+H.S.) in which S.W. and H.S. are seawater and hydrothermal solution, respectively. Open triangle Fukazawa deposits. Solid triangle Hanawa deposits. Open square Wanibuchi deposits. Solid square Shakanai deposits. Concentration of Ca, Sr " " and SO of H.S. are assumed to be 1,(XX) ppm, 1 ppm, and 10 mol/kg H2O, respectively. Concentrations of Ca, Sr " and SO of S.W. are taken to be 412 ppm, 8 ppm, and 2,712 ppm. Temperatures of H.S. and S.W. are assumed to be 350°C and 5°C (Shikazono et al., 1983).

Graham, D.D., Bender, M.L., Williams, D.F. and Keigwin, L.D. Jr. (1982) Strontium-calcium ratios in Cenozoic planktonic foraminifera. Geochim. Cosmochim. Acta, 46, 1281-1292. 

Honma, H. and Shuto, K. (1979) On strontium isotope ratio of barite from Kuroko-type deposits, Japan. 

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