A correção do drift instrumental em ICP-AES com espectrômetro seqüencial e a análise de elementos maiores, menores e traços em rochas

Analytical results of major, minor and trace elements are presented, which were obtained on international rock reference materiais during the development of work routines in the ICP-AES Laboratory, Mineralogy and Petrology Department, Instituto de Geociências, São Paulo University. Analyses were performed on an ARL-3410 model with sequential spectrometer using solutions obtained by alkaline fiision of 0.25 g of rock powder with 0.75 g of lithium tetra and metaborate (eutectic mixture), with a 1:1000 sample dilution (method used at the Imperial College, London). In order to minimize the effects of intensity drift, caused mostly by variations in the uptake conditions of the solution, strict procedures of drift controi and off-line correction were adopted. The results show good repeatability and maintain long-term precision, demonstrating that reliable analyses can be obtained by careful correction procedures. However, as it is not possible to controi short-term drift in sequential spectrometers, most of the analyses add up to between 98 and 101%; key major element ratios are nevertheless maintained, ensuring the quality of the analysis. The accuracy is reliable for the analyzed elements, which, besides major and minor elements, also include Zn, Ni, Cr, V, Zr, Sc, Y, La, Ba and Sr, and even for those notoriously difficult to analyze by ICP-AES, such as K, P and La.Apresentam-se resultados de análises químicas de elementos maiores e traços em materiais de referência de rochas, obtidos durante o desenvolvimento de rotina analítica no Laboratório de Química e ICP-AES do Departamento de Mineralogia e Petrologia do Instituto de Geociências da USP. As análises, realizadas em equipamento ARL-3410 dotado de espectrômetro seqüencial, foram efetuadas em soluções obtidas por fusão alcalina, com misturas de 0,75 g de tetra e metaborato de lítio e de 0,25 g de pó de rocha, e diluição final dos analitos de 1:1000 (metodologia do Imperial College, Londres). Para mmimizar os efeitos de drift, provenientes especialmente de variações nas condições de introdução da amostra no plasma, adotam-se procedimentos rígidos de controle e correção off-line da flutuação do sinal. Os resultados apresentam boa repetibilidade e precisão a longo intervalo, refletindo a eficiência usualmente alcançada pelo procedimento adotado. Em vista da dificuldade de corrigir o drift de pequeno intervalo em plasmas dotados de espectrômetro seqüencial, os fechamentos das análises são variáveis entre 98 e 101%, mas as relações entre os elementos maiores se mantêm constantes, garantindo a qualidade dos resultados. A exatidão é satisfatória para todo o conjunto de elementos por ora estudado (além dos maiores e menores, ainda Zn, Ni, Cr, V, Zr, Sc, Y, La, Sr e Ba), mesmo para aqueles de determinação mais difícil no ICP-AES, como K, P e La

Crustal reworking in a shear zone: Transformation of metagranite to migmatite

This study uses field, microstructural and geochemical data to investigate the processes contributing to the petrological diversity that arises when granitic continental crust is reworked. The Kinawa migmatite formed when Archean TTG crust in the São Francisco Craton, Brazil was reworked by partial melting at ~730 °C and 5-6 kbar in a regional-scale shear zone. As a result, a relatively uniform leucogranodiorite protolith produced compositionally and microstructurally diverse diatexites and leucosomes. All outcrops of migmatite display either a magmatic foliation, flow banding or transposed leucosomes and indicate strong, melt-present shearing. There are three types of diatexite. Grey diatexites are interpreted to be residuum, although melt segregation was incomplete in some samples. Biotite stable, H2O-fluxed melting is inferred via the reaction Pl + Kfs + Qz + H2O = melt and geochemical modelling indicates 0.35-0.40 partial melting. Schlieren diatexites are extremely heterogeneous; residuum-rich domains alternate with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO2 and K2O contents and are coarse-grained, leucocratic rocks. Homogeneous diatexites, quartzofeldspathic domains from the schlieren diatexites and the leucosomes contain both plagioclase-dominated and K-feldspar-dominated feldspar framework microstructures and hence were melt-derived rocks. Both types of feldspar frameworks show evidence of tectonic compaction. Modelling the crystallization of an initial anatectic melt shows plagioclase appears first; K-feldspar appears after ~40% crystallization. In the active shear zone setting, shear-enhanced compaction provided an essentially continuous driving force for segregation. Thus, Kinawa migmatites with plagioclase frameworks are interpreted to have formed by shear-enhanced compaction early in the crystallization of anatectic melt, whereas those with K-feldspar frameworks formed later from the expelled fractionated melt. Trace element abundances in some biotite and plagioclase from the fractionated melt-derived rocks indicate that these entrained minerals were derived from the wall rocks. Results from the Kinawa migmatites indicate that the key factor in generating petrological diversity during crustal reworking is that shear-enhanced compaction drove melt segregation throughout the period that melt was present in the rocks. Segregation of melt during melting produced residuum and anatectic melt and their mixtures, whereas segregation during crystallization resulted in crystal fractionation and generated diverse plagioclase-rich rocks and fractionated melts

Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the S\ue3o Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite

New SHRIMP U-Pb zircon dating indicates that, contrary to previous thought, part of the Archean crust in the southern S\ue3o Francisco Craton (SFC), NW of the Jeceaba Bom Sucesso Lineament was affected by the Paleoproterozoic Transamazonian orogeny. The Kinawa migmatite is a reworked part of the Archean crust in the southern SFC, and occurs within the Cl\ue1udio Shear Zone, a NE-NW crustal-scale structure. The protolith of the migmatite is a metagranodiorite that has an U-Pb zircon crystallization age of 2704\ua0\ub1\ua03\ua0Ma coeval with the voluminous potassic magmatism in the region. Moreover, the \u3b5Hf(t=2704) data suggests a slightly evolved crust ( 121.5 to 126.1) and thus a relatively long crustal residence ( 3c700\ua0Ma). Surprisingly, the migmatite itself yielded much younger ages of 2034\ua0\ub1\ua032\ua0Ma and 2048\ua0\ub1\ua025\ua0Ma, which coincide with the ages of the Mineiro Belt and Mantiqueira Complex. The distribution of ages using data from the literature and this study show that within the Archean domain the occurrence of Paleoproterozoic ages is restricted to the Cl\ue1udio Shear Zone. Inherited zircon crystals from all samples yield Neoarchean ages ( 3c2.7\ua0Ga), and similar Hf signatures to the metagranodiorite protolith (\u3b5Hf(t=2704)\ua0=\ua0 125.5 to 120.5). The \u3b5Hf(t) signatures of the Kinawa migmatite are heterogeneous and record two events, one is interpreted to be the formation of the protolith with lower 176Hf/177Hf (at 3c2.7\ua0Ga) and the other with higher 176Hf/177Hf (at 3c2.05\ua0Ga) as the anatexis that produced the migmatite. An important result is that the migmatite had two sources. Two samples are consistent with derivation from an evolved crust comparable with the metagranodiorite protolith (e.g. 176Lu/177Hf 3c0.01 and \u3b5Hf(t=2034) 1213.4 to 1217), but the other sample has much less negative values (\u3b5Hf(t=2048)\ua0=\ua0 124.6 to 128.7), which requires a different source with much lower 176Lu/177Hf, and therefore younger, possibly Post-Archean material

Enhancing maficity of granitic magma during anatexis: Entrainment of infertile mafic lithologies

Most studies of migmatites examine how anatexis occurred in the most fertile units and what happened to that melt, whereas the associated minor lithologies are typically ignored. The Kinawa migmatite in the southern São Francisco Craton of Brazil is the product of water-fluxed melting of a leucogranodiorite that contained dykes of amphibolite. The migmatite consists mostly of pink diatexites, metatexites and leucosomes, but it also contains schollen of amphibolite. This study examines the behaviour of these minor mafic rocks during anatexis to determine what role they play in the formation of migmatites and development of granitic magmas in their source region.The amphibolites are massive or banded Hbl+Pl, and rarely Hbl+Pl+Cpx, schollen in the diatexite migmatite. The amphibolite schollen melted very little, and show a complex morphology suggesting mechanical and chemical interaction with the enclosing leucocratic pink diatexite migmatite. Diatexites and leucosomes immediately adjacent to the schollen have a considerably higher proportion of amphibole (up to 12%) and/or biotite (up to 10%) compared with the diatexite a few tens of centimeters farther away. Six stages of disaggregation and interaction of mafic schollen with the enclosing diatexite magma are recognized: (1) amphibolite layers break up to form schollen, but are mineralogically and texturally unchanged; (2) melt infiltrates into fractures and foliation in the schollen; (3) schollen disaggregate into swarms of single amphibole crystals within the diatexites; (4) amphibole is partially replaced by biotite; (5) flow of the enclosing diatexite magma arranges the detached amphibole crystals into schlieren and aggregates of biotite; (6) detached crystals are completely replaced by biotite and dispersed by magmatic flow to produce a mesocratic to melanocratic homogeneous diatexite. Geochemical modelling indicates that the composition of the diatexites and leucosomes is changed by the wholesale entrainment of the disaggregated mafic schollen or in some cases by the preferential entrainment of detached hornblende or plagioclase crystals. This contamination increases the maficity of initially felsic, leucodiatexite magma, by the addition of FeO+MgO, CaO and TiO2 (which results in a concomitant decrease in SiO2), to become a mesocratic to melanocratic diatexite magma that is comparable with typical I-type granites found around the world. Entrainment of mafic material and hornblende in particular strongly influences the behaviour of the rare earth elements, lowering LaN/YbN ratios. Thus, non-protolith mafic lithologies within migmatites represent a source of contamination for anatectic melts that results in a significant increase in maficity

Removal of Pb2+ from aqueous solutions using two Brazilian rocks containing zeolites

The removal of Pb2+ from aqueous solution by two Brazilian rocks that contain zeolites-amygdaloidal dacite (ZD) and sandstone (ZS)-was examined by batch experiments. ZD contains mordenite and ZS, stilbite. the effects of contact time, concentration of metal in solution and capacity of Na+ to recover the adsorbed metals were evaluated at room temperature (20A degrees C). the sorption equilibrium was reached in the 30 min of agitation time. Both materials removed 100% of Pb2+ from solutions at concentrations up to 50 mg/L, and at concentrations larger than 100 mg/L of Pb2+, the adsorption capacity of sandstone was more efficient than that of amygdaloidal dacite due to the larger quantities and the type of zeolites (stilbite) in the cement of this rock. All adsorbed Pb2+ was easily replaced by Na+ in both samples. the analysis of the adsorption models using nonlinear regression revealed that the Sips and the Freundlich isotherms provided the best fit for the ZS and ZD experimental data, respectively, indicating the heterogeneous adsorption surfaces of these zeolites.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Universidade Federal de São Paulo, BR-09972270 Diadema, SP, BrazilInst Geol, BR-04301903 São Paulo, BrazilUniv São Paulo, Inst Geosci, BR-05508080 São Paulo, BrazilUniversidade Federal de São Paulo, BR-09972270 Diadema, SP, BrazilFAPESP: 03/06259-4Web of Scienc