Ppm In Rocks Research Articles

Germanium Isotope Geochemistry

Germanium (Ge) is a trace element in the Earth’s crust and natural waters, averaging about 1.6 ppm in rocks and minerals (El Wardani 1957; Bernstein 1985) and 75 picomol/L in seawater (Froelich and Andreae 1981). The naturally occurring oxidation states of Ge are +2 and +4, with the +4 state forming the principal common and stable compounds. Germanium has outer electronic structure 3 d 10 4 s 2 4 p 2 and mainly occurs in the quadrivalent state, although in some minerals it is octahedrally coordinated. Germanium is chemically similar to silicon (Si), both belonging to the IVA group in the periodic table, with Ge immediately above Si. Germanium is classified as a semimetal, whereas Si is a nonmetal element. Because of nearly identical ionic radii and electron configurations for Ge and Si, the crustal geochemistry of Ge is dominated by a tendency to replace Si in the lattice sites of minerals (Goldschmidt 1958; De Argollo and Schilling 1978b). These two elements exist in seawater as similar hydroxyacids, i.e., Ge(OH)4 and Si(OH)4 (Pokrovski and Schott 1998a) and the concentration profile of Ge is similar to that of Si (Froelich and Andreae 1981), thus making Ge/Si ratio an interesting tracer for biogenic silica cycling in the ocean. Although Ge and Si are geochemically similar, their behavior is different enough so that decoupling of Ge and Si can occur. Germanium commonly occurs in 4-fold (tetrahedral) coordination but in contrast to Si, Ge has a stronger tendency for the 6-fold coordination. Unlike Si, Ge also forms methylated compounds, and high concentrations of monomethyl- and dimethyl-germanium have been detected in ocean waters, accounting for > 70% of the total Ge (Lewis et al. 1985). Germanium is a particularly interesting element for geochemists since it exhibits siderophile, lithophile, chalcophile and …

Detection of ammonium in geological materials by evolved gas analysis

The association of anomalously high levels of ammonium with both metallic and energy mineral deposits is of potential use in mineral exploration. Conventional geochemical methods for detecting NH4 often provide only whole-rock NH4 values and do not identify the specific minerals hosting NH4. They may also lack sensitivity or be prone to interference. Evolved NH3 analysis is shown to be capable of distinguishing between different NH4-bearing minerals and can detect NH4 values down to 120 ppm in rocks hosting silver-gold vein mineralization. Fully quantitative determination of NH4 by this method is not possible as the NH3 evolved from minerals during heating partially oxidised; however, amounts of evolved NH3 do show a moderate positive linear correlation with NH4 content determined by a modified Kjeldahl method.

Juvenile helium in ancient rocks: II. U-He, K-Ar, Sm-Nd, and Rb-Sr systematics in the Monche Pluton. [formula omitted] ratios frozen in uranium-free ultramafic rocks

The important geodynamic parameter, the 3He 4He ratio in rocks and fluids of the continental crust, is generally decreasing from the mantle values (≅10 −5) to the radiogenic ratio (≅10 −8) on the time scale of about 1 Ga or less. However, the ratios, observed in some ancient rocks and minerals, are much higher than the radiogenic value due to a preferential retention of trapped He, when compared with radiogenic helium and / or a low U 3He ratio in a sample. The distribution of He, Ar, Nd, and Sr isotopes, K, Rb, Sm, and U in ultrabasic rocks, in rock-forming minerals, in ores from the 2.49 Ga Monche Pluton, and in basic rocks of the Main Range (the Kola Peninsula) enables us to distinguish sources of the rocks and trapped fluids and outline some peculiarities of petrogenetic and fluid processes. The initial values of ϵ Nd ( T) = −0.9 ± 0.5, 87Sr 86Sr (T) = 0.7021 ± 0.0002 , for the 2.49 Ga Monche Pluton are rather similar to these for other layered intrusions of the Baltic Shield. They differ considerably from the model values for the depleted 2.5 Ga old mantle. Probably the melts originated in the specific enriched subcontinental mantle. [ 4He] concentrations of ( 10–480)∗ 10 −6 cm 3/ g ) and [U] concentrations of 0.01–0.001 ppm in rocks of the Pluton are translated into atomic ratio of 4He U up to ≅500 and U-He “age” up to ≅10 Ga (!). These rocks contain dominantly trapped He. Despite the fact that [U] in gabbros from the Main Range is about two orders of magnitude higher, this conclusion appears to be valid because of better retention of trapped He as compared with radiogenic one. The 3He 4He ratios, (0.16–0.74)∗ 10 −6, exceed considerably the radiogenic value but are much lower than those in the mantle due to a contribution of crustal He. Isotopic and geochemical evidence implies that a melt from which the ultrabasic rocks were formed originated from the mantle; thus crust-mantle fluid occurred in the mantle melt. Geological observations and the relationship between 3He 4He and Mg (Fe + Mg) values show that the high-magnesium divines formed during melt ascent i.e., under higher temperatures and, probably, at slightly greater depth as compared with those parameters for the magma chamber. The observed and rather different sources of 3 He and 4He as well as the con siderable constancy of their ratio in different minerals, separated from both the ultramafic rocks and gabbros, implies: 1. (1) an intensive process of mixing between mantle and crustal components: a melt convection in the chamber may have occurred; 2. (2) the two types of rocks could originate by crystallization differentiation of one and the same melt. Moreover, similar ratios of 3He 4He ≅ 0.2 ∗ 10 −6 are observed in the enclosing gneisses of the Kola-Belomorian series. Practically all 3He and 4He are concentrated in secondary amphiboles; hence the fluid which stimulated the metamorphic process was probably released from the ultramafite-bearing melt. The source of trapped 4He and 40Ar ∗ (corrected for air) appears to be the continental crust, but their ratio varies over a wide range, in contrast to approximately constant 3He 4He ratio. Degassing of the melt is probably responsible for extremely high 4He Ar ∗ values in some minerals.

Determination of lead in geological and biological materials by graphite furnace atomic absorption spectrometry

Serious systematic errors inherent in the determination of lead in geological and biological samples by flameless atomic-absorption spectrometry are demonstrated. The reduced absorption of lead is due to partial interaction with alkaline, alkaline-earth and iron chlorides during the atomization stage. Incomplete dissociation of the volatile monochloride of lead in the gaseous phase reduces the absorption signal. An addition of 20% hydrogen to nitrogen (purge gas) diminishes signal suppression by removing the chlorine in form of volatile HCl. The detection limit is about 0.6 ppm in rocks and about 0.3 ppm in plant materials. Depending on the content in the samples the relative standard deviation is between 1 and 12%. The accuracy of the method was tested on 39 international standard reference samples.