Feldspar trace elements

ABSTRACT A geochemical analysis of major, trace and rare earth elements was carried out in beach sands collected from the Northeastern coast of Mexico in order to observe the spatial trends along three different beaches. Results show that major elements patterns along the beaches are controlled by heavy minerals and plutonic and sedimentary input towards the coast. In addition, trace elements tendencies indicate that the beach sands are influenced by the presence of magnetite. Finally, the differences in Eu anomalies indicate a mix of felsic to mafic and intermediate rocks and feldspar weathering. 

Trace elements are useful tracers of geochemical processes mostly because they are dilute their behavior depends primarily on the trace element-matrix interaction (e.g., Rb-host feldspar, Sr-calcite) and very little on the trace-trace interaction (e.g., Rb-Rb, Sr-Sr). Consequently, the distribution of trace elements among natural phases largely obeys the linear Henry s law. The modeling of trace elements in various geological environments (magmas, hydrothermal fluids, seawater,...) relies on three different aspects... 

Table 5.61 summarizes synthetically the observed occurrences of various trace elements in feldspars, based mainly on the indications of Smith (1983). 

Mason R. A. (1982). Trace element distributions between the perthite phases of alkali feldspars from pegmatites. Min. Mag., 45 101-106. 

Steele I. M., Hutcheon I. D. and Smith J. V. (1980b). Ion microprobe analysis of plagioclase feldspar (Cai xNaxAl2 xSi2+x08)- for major, minor and trace elements. VIII Int. Congr. X-ray Optics Micro analysis, Pendell Pub. Co, Midland, Michigan. 

Fractional crystallisation has been dominated by separation of various proportions of clinopyroxene and olivine in the mafic magmas, and of cli-nopyroxene and feldspars in the felsic melts. These generated decrease in ferromagnesian elements (FeO, MgO, Ni, Co, Cr, etc.) and increase in incompatible elements (e.g. Th, Ta, Nb, REE), with ongoing evolution. In contrast, ratios of incompatible trace elements were not affected by fractionation processes, and can be used to infer compositions of mantle-equilibrated melts. 

Variation of many major and trace elements in the Somma-Vesuvio rocks suggest that fractional crystallisation was a main mechanism in magma evolution. Different trends shown by the various rock series for both major and trace elements have been interpreted either to reveal the presence of different types of parental magma and/or to indicate fractionation dominated by different relative amounts of clinopyroxene and feldspar, possibly as a consequence of variable pressure during fractionation (Joron et al. 1987 Trigila and De Benedetti 1993). 

Suspended solid surfaces (particles or colloids) in waters play a prominent role in controlling the concentration of dissolved trace elements. Most of these elements are eliminated by sedimentation after incorporation on to or into particles, generally by complexation with the surface sites. The most common inorganic particles and colloids are non-clay silicates (quartz, potash feldspar, plagioclase, opaline silica (diatoms)) clays (illite, smectite) carbonates (calcite, dolomite) Fe-Mn oxides (goethite, magnetite) phosphates (apatite) sulfides (mackinawite). Particles and colloids in a water body may be classified as a function of their origin ... 

In a number of cases, geochemically similar elements are well correlated to each other in the sand fractions from each unit (e.g., Sm and Eu in Figure 7). This correlation is not surprising if the trace elements are associated with the heavy minerals. Small variations in the amount of a particular heavy mineral will cause large differences in the trace element content measured, if the sand fraction is mostly quartz and feldspar, which contain so few trace impurities. Figure 7 shows the relationship between Sm and Eu. There is a correlation between these elements in the samples from each of the two levels. Figure 7 suggests that there is a different Sm/Eu ratio for the sands from these two Nile sediment deposits. 

Drake M. J. (1972) The distribution of major and trace elements between plagioclase feldspar and magmatic silicate liquid an experimental study. PhD Thesis, University of Oregon Eugene, Oregon, USA (unpublished). 

White J. C. (2003) Trace element partitioning between alkali feldspar and peraUcalic quartz trachyte to rhyolite magma Part 11. Empirical equations for calculating trace element partition coefficients of large-ion lithophile, high field strength and rare-earth elements. Am. Mineral. 88, 330-337. 

Release of trace elements such as strontium from feldspar is also observed to be nonstoichio-metric (Brantley et al, 1998). At pH 3, bytownite, microcline, and albite aU release strontium at an initially fast rate that slows to near stoichiometric values at steady state. In addition, aqueous strontium is enriched in Sr compared to the bulk mineral early in dissolution. All feldspars smdied evenmally released strontium in isotopic abundance roughly equal to that of the bulk mineral. Nonstoichiometric release of strontium was explained by the presence of defects or accessory phases in the minerals. Taylor et al. (2000) also reported that the initial dissolution of labradorite was nonstoichiometric during dissolution in column reactors with inlet solution pH 3, but that the mineral dissolved and released strontium stoichiometrically at steady state. In contrast to the earlier work, however, Sr/ Sr in solution did not differ from that of the bulk labradorite during dissolution in the column experiments. 

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