Tracers basalt concentrations

Boron concentrations and isotopes are also useful geochemical tracers of contamination in MORB. Boron concentrations are low (<2 ppm) in unaltered ocean floor basalt but high in altered basalts (>8 ppm B) (Spivack and Edmond 1987 Ryan and Langmuir 1993). Goldstein et al. (1989) measured B concentrations in their samples and found them to be less than 1.6 ppm, inconsistent with contamination. More recently, B isotopes have been used to assess contamination since large differences in 5 B are known to exist between seawater, sediments, and unaltered MORB. Sims et al. (2002) reported that 6 B for their 9°N EPR samples were inconsistent with incorporation of any seawater or seawater-derived material. 

If both chemical concentration gradients and isotopic ratio gradients are present (e.g., basaltic melt with Sr/ Sr ratio of 0.705 and andesitic melt with Sr/ Sr ratio of 0.720), the homogenization of isotopic ratio is referred to as isotopic diffusion (Lesher, 1990 Van Der Laan et al., 1994), although some prefer to call it isotopic homogenization. If there are concentration gradients in both major and trace elements, the diffusion of the trace elements is referred to as trace element diffusion (Baker, 1989). Isotopic diffusion and trace element diffusion are really part of multicomponent diffusion, which is complicated to handle. Isotopic diffusion should not be confused with self-diffusion, and trace element diffusion should not be confused with tracer diffusion. 

Figure 9. Percentage of initial concentrations of tracers left in solution as a function of time for the sorption experiments with the basalt samples. ( ), U Np f AX Pu fax Am (O), Cm.

Unfortunately, lead concentrations are not often analyzed in oceanic basalts, partly because lead is subject to alteration and partly because it is difficult to analyze, so a literature search tends to yield highly scattered data. Nevertheless, the average MORB value of Pb/Nd = 0.04 is lower than the average continental value of 0.63 by a factor of 15. Because of this great contrast, this ratio is potentially an even more sensitive tracer of continental contamination or continental recycling in oceanic basalts. 

Watson EB, Cherniak DJ (1997) Oxygen diffusion in zircon. Earth Planet Sci Lett 148 527-544 Wendlandt RW (1991) Oxygen diffusion in basalt and andesite melts Experimental results and discussion of chemical versus tracer diffusion. Contrib Mineral Petrol 108 463-471 West AR (1984) Solid State Chemistry and Its Applications. John Wiley and Sons, New York Whipple RTP (1954) Concentration contours in grain boundary diffusion. Phil Mag 45 1225-1236 White AF, Peterson MI (1990) Role of reactive-surface area characterization in geochemical kinetic models. In Melchior DC, Bassett RL (eds) Chemical Modeling of Aqueous Systems. II. Am Chem Soc Symp 416 461-475... 

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