Basalt experimental systems

A point that often receives insufficient appreciation is that a system in the laboratory often behaves differently in sometimes critical ways from what is nominally the same system in the field. If one is studying rocks that are the products of natural hydrothermal alteration, the partial equilibrium assumption is more likely to be valid than it is in a laboratory hydrothermal apparatus in which one attempts to recreate such alteration. Such differences are commonly manifested in the appearance of different mineral assemblages, though changes in fluid chemistry may be very similar. An example is hydrothermal reaction of seawater and basalt, a process which occurs naturally at midocean ridges (sec refs, i, M, and many sources cited therein). The naturally altered basalts become rich in chlorite or chlorite plus epidote. In experimental systems, smectite clays appear instead. Time appears to be the limiting factor. 

Cole DR, Mottl MJ, Ohmoto H (1987) Isotopic exchange in mineral-flttid systems. II. Oxygen and hydrogen isotopic investigation of the experimental basalt-seawater system. Geochim Cosmochim Acta 51 1523-1538... 

Poll S, Schmidt MW (1995) H2O transport and release in subduction zones experimental constraints on basaltic and andesitic systems. J Geophys Res 100 22,299-22,314 Pyle DM, Ivanovich M, Sparks RSJ (1988) Magma-cumulate mixing identified by U-Th disequilibrium dating. Nature 331 157-159... 

This form of the partition coefficient, analogous to that used for Fe-Mg fractionation between olivine and melt (see Chapter 1), is necessary only for the rare cases where trace substitution affects Cj and Cp substantially. A number of reviews (O Nions and Powell, 1977 Michard, 1989) describe the various sorts of partition coefficients expressed either in mass-fractions, atom fractions, or normalized to a major element and their respective merits. If the discussion is restricted to a narrow range of chemical compositions (e.g., basaltic systems, Irving, 1978, Irving and Frey, 1984), enough experimental information exists on trace-element partitioning to resort to the wonderfully simple equation (9.1.1). 

Final composition of the aqueous phase. The final compositions of the waters resulting from the three dissolution experiments have been summarized and listed together with compositions of waters from natural systems (Table VI). The experimental and natural basalt waters have very similar compositions. However, the experimental quartz monzonite water has a higher than natural K content while the shale water has higher than natural K and Na contents. The HCO3 content of each of the experimental waters is higher than the content of its natural counterpart while the oposite is true for 04. 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

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... 

As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

CMAS diagrams The components of the CMAS system (CaO-MgO-Al203-Si02) comprise aibout 70-85 wt % of most basalts and more than 90 wt % of most mantle peridodtes. For this reason the CMAS system is used by experimental petrologists as a simplified analogue of more complex basalt and mantle systems. The CMAS projecdon... 

Yoder H.S. and Tilley C.E., 1962, Origin of basalt magmas an experimental study of natural and synthetic rock systems, f. Petrol., 3, 342-532. 

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