Basalts strontium isotopes

When ultramafic rocks were initially analyzed for strontium isotopes (Hurley et al, 1964 Roe, 1964 Stueber and Murthy, 1966 Lanphere, 1968) only whole rocks were analyzed. Such data led to numerous erroneous conclusions about the relationship between mantle samples and basalts due to the pervasive alteration suffered by many ultramafic rocks, enhancing the amount of radiogenic strontium present. Subsequent studies have shown that acid-washed mineral separates, particularly diopside and garnet, are the most reliable means of obtaining the primary, unaltered... 

The modem distribution of strontium isotopic ratios in basalts from both oceanic and continental regions is well documented (e.g., Hofmann, 1997 Chapter 2.03 Dickin, 1995), with the composition of the upper mantle today defined from analyses of N-MORBs ( Sr/ Sr = 0.7025 0.0005). Rubidium is one of the most incompatible elements during melting, and it is evident that... 

Price R. C., Gray C. M., and Frey F. A. (1997) Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia. Geochem. Cosmochim. Acta 61, 171-192. 

The duration of chemical exchange between seawater and basalt can also be determined by isotopic studies of hydrothermal minerals. A variety of techniques have been used, ranging from direct dating by K/Ar and Rb/Sr isochron techniques to comparisons of the initial strontium isotopic composition of alteration minerals with the isotopic evolution of seawater (Gallahan and Duncan, 1994 Richardson et al., 1980). 

Strontium isotope ratios and abundances in samples from the oceanic crust may be used to determine the complete chemical mass balance of strontium exchange between seawater and basalt, including the loss of basaltic strontium to hydrothermal solutions, and uptake of basaltic or seawater strontium from hydrothermal solutions. A mass balance of this exchange can be made in four steps, (i) The relative amount of basaltic and seawater strontium in altered basalt can be determined from the measured Sr/ Sr of an altered sample as a (linear) mixture of strontium from the two end members, the (contemporaneous) seawater and basalt, (ii) The inventory of the basaltic and seawater strontium in an altered sample (in mg/kg) may then be determined from the above ratio of seawater and basalt in the sample and the total strontium abundance measured for this sample, (iii) Seawater strontium addition to the basalt is given directly by the seawater strontium inventory calculated in Step (ii). (iv) The determination of flux of basaltic strontium in or out of an altered sample is more complicated because it has to be related to the original inventory of strontium. It is determined as the difference between the original basaltic inventory and the basaltic strontium present in the altered sample. 

Myers J. D., Marsh B. D., and Sinha A. K. (1986b) Geochemical and strontium isotopic characteristics of parental Aleutian Arc magmas evidence from the basaltic lavas of Atka. Contrib. Mineral. Petrol. 94, 1-11. 

Brimhall et al. (1991) used lead isotopes in zircons within a bauxite profile from Western Australia to differentiate between zircons derived from the underlying bedrock and zircons of eolian origin. Borg and Banner (1996) applied both neodymium and strontium isotopes to constrain the sources of soil developed on carbonate bedrock. Using these isotopes and Sm/Nd ratios, they were able to delineate the importance of atmospheric versus bedrock contributions in controlling the composition of the soil. Kurtz et al. (2001) used neodymium and strontium isotopes to determine the amount of Asian dust in a Hawaiian soil chronosequence. They found that the basaltic bedrock isotope signatures in soils had, in many cases, been completely overprinted by dust additions, demonstrating the profound effect of Asian dust on soil nutrient supplies. 

Muller and colleagues identified four major rock units in the potential homelands of the Iceman on the basis of strontium and lead isotope ratios (Fig. 9.9). These include Eocene basalts (ca. 0.705-0.708), Mesozoic limestones (ca. 0.710-0.714), and Permian volcanics (ca. 0.717-0.719), and a mixed unit of phyllites and gneisses (ca. 0.720-0.724). These last two geological units overlap slightly in their isotope ratios. Strontium isotope ratios in the enamel of the Iceman averaged 0.721 and values in bone were approximately 0.718. These values are most congruent with the... 

We start with another set of isotope signatures. The rate of erosion in the distant past can be estimated by measuring the ratio of strontium isotopes in marine carbonates. Two stable isotopes of strontium — strontium-86 and strontium-87 — differ in their distribution between the Earth s crust and the mantle underneath it. The mantle is rich in strontium-86, whereas the crust is more richly endowed with strontium-87. The major source of strontium-86 in the oceans is the igneous rock basalt. This rock is extruded continuously from the mantle at the mid-ocean ridges, from where it spreads slowly across the ocean floor before diving back into the mantle beneath the ocean trenches. A little strontium dissolves from the basalt into seawater. The speed of dissolution is more or less constant. The gradual build-up of dissolved strontium-86 in the oceans is balanced by a steady uptake of strontium by marine carbonates, such as limestone (calcium carbonate). This is because strontium can displace its sister element, calcium, in the crystalline structure of limestone. As each of these processes takes place at a steady rate, we would not expect the relative amount of strontium-86 in limestone to fluctuate a great deal. In fact it varies quite a lot. Strontium-87 is to blame. 

Halliday AN, Davidson JP, Holden P, DeWolf C, Lee D-C, Fitton JG (1990) Trace-element fractionation in plumes and the origin of HIMU mantle beneath the Cameroon line. Nature 347 523-528 Hamano Y, Ozima M (1978) Earth-atmosphere evolution model based on Ar isotopic data. In Alexander EC, Ozima M (eds) Terrestrial Rare Gases. Center for Academic Publications Japan, Tokyo, p 155-171 Hamelin B, Dupre B, Allegre C-J (1984) Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge a comparative study. Earth Planet Sci Lett 67 340-350 Hanan BB, Graham DW (1996) Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes. Science 272 991-995... 

The starting point for the evolution of strontium isotopes is the Sr/ Sr ratio at the formation of the Earth. This is taken to be the isotopic composition of basaltic achondrite meteorites, thought to have a composition approximating to that of the solar nebula at the time of planetary formation. It is usually referred to as BAEl (Basaltic Achondrite Best Initial) and the measured value is 0.69897 0.000003. 

Faure G (1981) Strontium isotope composition of volcanic rocks Evidence for contamination of the Kirkpatrick Basalt, Antarctica. In O Connell RJ, Fyfe WS (eds) Evolution of the earth, Geodyntunics series, vol. 5. American Geophysical Union, Washington, DC, pp 75-81... 

Faure G, Bowman JR, Elliot DH, Jones LM (1974) Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contrib Mineral... 

HUl RL (1969) Strontium isotope composition of basaltic rocks in the Transantarctic Mountains, Antarctica. MSc thesis. Department of Geology, The Ohio State University, Columbus, OH... 

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