Early Cretaceous (∼129 Ma) silicic rocks crop out in SE Uruguay between the Laguna Merín and Santa Lucía basins in the Lascano, Sierra São Miguel, Salamanca and Minas areas. They are mostly rhyolites with minor quartz-trachytes and are nearly contemporaneous with the Paraná-Etendeka igneous province and with the first stages of South Atlantic Ocean opening. A strong geochemical variability (particularly evident from Rb/Nb, Nb/Y trace element ratios) and a wide range of Sr-Nd isotopic ratios (143Nd/144Nd(129) = 0.51178-0.51209; 87Sr/86Sr(129) = 0.70840-0.72417) characterize these rocks. Geochemistry allows to distinguish two compositional groups, corresponding to the north-eastern (Lascano and Sierra São Miguel, emplaced on the Neo-Proterozoic southern sector of the Dom Feliciano mobile belt) and south-eastern localities (Salamanca, Minas, emplaced on the much older (Archean) Nico Perez terrane or on the boundary between the Dom Feliciano and Nico Perez terranes). These compositional differences between the two groups are explained by variable mantle source and crust contributions. The origin of the silicic magmas is best explained by complex processes involving assimilation and fractional crystallization and mixing of a basaltic magma with upper crustal lithologies, for Lascano and Sierra São Miguel rhyolites. In the Salamanca and Minas rocks genesis, a stronger contribution from lower crust is indicated.
The element chemistry of biogenic carbonates can provide important data on past environments. However, the Sr/Ca and Mg/Ca ratios as well as the Mg and Sr concentrations of biological carbonates, especially aragonitic bivalves often depart from apparent thermodynamic equilibrium. When measured in situ by means of LA-ICP-MS, the Mg concentration is often substantially enriched (two- to threefold) near the organic-rich, annual growth lines. To test the hypothesis that some organic components exert a major influence on the skeletal metal content, the element chemistry of different shell components (insoluble organic matrix, IOM; dissolved CaCO3 and soluble organics, SOM) of Arctica islandica was measured by means of ICP-OES and LA-ICP-MS. The ICP-OES data indicate that the IOM is strongly enriched in Mg (130 ppm) and depleted in Sr and Ca (10 ppm and 0.22 wt%, respectively) when compared to the whole biomineral (Mg: 68 to 99 ppm, Sr: 860 to 1,060 ppm, Ca: ∼35.72 wt%). Although the average relative abundance of the IOM barely exceeds 0.46 wt%, its chemical composition in combination with its heterogeneous distribution across the shell can significantly increase estimates of the Mg concentration if measured in situ by LA-ICP-MS. Depending on the distribution of the IOM, the Ca concentration may be significantly lower locally than the average Ca concentration of the whole shell (35.72 wt%). If this remains undetected, the Mg concentration of shell portions with higher than average IOM content is overestimated by LA-ICP-MS and, conversely, the Mg concentration is underestimated in shell portions with lower than average IOM content. Removal of the IOM prior to the chemical analysis by LA-ICP-MS or mathematical correction for the IOM-derived magnesium concentrations is therefore strongly advised. The different chemistry of the IOM may also exert a major control on the trace element to calcium ratios. Shell portions enriched in IOM will show up to 200 times higher Mg/Ca and up to two times higher Sr/Ca ratios than the average shell of A. islandica. Without removal of the IOM prior to the analysis, Mg/Ca and Sr/Ca ratios of shell portions with higher IOM content cannot be used as paleothermometers. Because it is currently impossible to remove the IOM prior to chemical analyses by LA-ICP-MS, we recommend the use of wet chemical techniques (= possibility to separate and measure individual shell components) such as ICP-OES at the expense of lower sampling resolution. The results of this study will significantly improve our understanding of shell-based climate and environmental proxies.
Complexation of rare earth elements (REEs) with humic substances (HSs) at various REE loading levels was studied by solvent extraction and equilibrium dialysis methods coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Apparent stability constants (βM-HS) of REE ions with standard Suwannee River humic acid (SRHA) were determined simultaneously for all REEs except for Pm. βM-HS depends on the REE loading level to HSs, where βM-HS decreases with an increase in the REE loading level at a lower REE loading region, whereas βM-HS is almost constant beyond a certain REE loading level. The variation in βM-HS with the REE loading level suggests the existence of several types of binding sites of REEs in HSs. Next, βM-HS was converted into KMA (which is the REE-HS interaction parameter for Tipping's “Humic Ion-Binding Model VI”) to be compared with the data by previous studies on REE-HS complexation at various REE loading levels in terms of relative differences among REEs (i.e., REE pattern). In the case of solvent extraction, the REE pattern of logKMA changes with variation in the REE loading level; a peak around Sm and Eu was observed in the REE pattern of logKMA at a higher REE loading region, whereas logKMA increases with an increase in the atomic number of REEs at a lower REE loading region. On the other hand, such variation in the REE pattern of logKMA was not observed in the results of equilibrium dialysis. This difference in the REE pattern of logKMA can be explained by the total metal loading level, including the contributions of Fe3+ and Al3+ originally contained in HS. The main binding sites of REEs in HSs were determined by comparing the REE pattern of logKMA experimentally obtained for HSs with those estimated based on the linear free-energy relationship (LFER) for carboxylates. Similarity of the REE pattern suggested that simple carboxylates are the main binding sites of REEs at a higher metal loading region, whereas chelating sites play a key role at a lower metal loading region. Our results show that relative stabilities of HS complexes among various REEs can change as a function of the metal loading level.
We present a statistical analysis of 2201 electron microprobe determinations of Hf contents and Zr/Hf ratios in zircon from 65 samples of granitic rocks. The modal Hf content and modal Zr/Hf ratio of zircon taken from the data are 1.43 wt% HfO2 and 38.5 respectively. Estimated partition coefficients are 2520 for DZr and 2420 for DHf between zircon and granitic magma. The large and similar Zr and Hf partition coefficients indicate that the crystallization of zircon would deplete the melt in both Zr and Hf, but would not cause significant fractionation of Zr from Hf. On the basis of the geochemical behaviour of Hf, granitic zircons can be divided into three types (early, late and hydrothermal zircon). Compared to the early zircon in granitic rocks, late-crystallizing zircon and especially hydrothermal zircon are characterized by much higher Hf contents and lower Zr/Hf, which can be explained by a change in DHf with decreasing crystallization temperature. The behavior of Hf in granitic zircon may be used as geochemical indicator to track the origins and crystallization environments of the host magma.
A method for the rapid determination of Re-Os isotopes by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was developed. For Os measurements, samples were introduced into the plasma through conventional solution mode. The main Os memory occurs in the nebulizer; therefore, the use of a glass nebulizer rather than a Teflon nebulizer leads to a significant reduction in the Os memory effect. The Os memory is quickly and easily eliminated by washing the inlet system with 5% HCl-EtOH. The external reproducibility of the 187Os/188Os ratio of the Johnson-Matthey Chemical ICP (JMC) Os standard solution, as measured using the multi-ion counter system, is 0.07% (2σ, n = 12) for analyte amounts ranging from 0.1 to 1 ng total Os. Using MC-ICP-MS, the mean values of 187Os/188Os obtained for standards were found to be within analytical uncertainty of values obtained by negative thermal ionization mass spectrometry (N-TIMS). An improved mass-fractionation correction technique using Ir was applied to the isotopic analysis of Re. The present methods demonstrate the feasibility of MC-ICP-MS for precision measurements of the 187Re-187Os isotope system.
Helium isotopes have been measured in fluorite and sulphides from fluorite-rich Mississippi Valley-type (MVT) deposits from Asturias, Spain, in order to assess the involvement of mantle and crust-derived volatiles in the genesis of the deposits. The 3He/4He of the ore fluids are ≤0.1 Ra ruling out a contribution of mantle-derived magmatic helium in the inclusion fluids. Data support a crustal source for the volatiles and is consistent with the existing basinal model for the genesis of the Asturias MVT mineralization by mixing of sedimentary brines with infiltrating superficial fluids.