Contents Of Transition Elements Research Articles

Geochemical and Nd isotopic compositions of the Palaeoproterozoic metasedimentary rocks in the Kongling complex, nucleus of Yangtze craton, South China block: implications for provenance and tectonic evolution

AbstractPalaeoproterozic metasedimentary rocks, also referred to as khondalites, characterized by Al-rich minerals, are extensively exposed in the nucleus of the Yangtze craton, South China block. Samples of garnet–sillimanite gneiss in the khondalite suite were collected from the Kongling complex for Nd isotopic and elemental geochemical study. These rocks are characterized by variable SiO2 contents ranging from 35.71 to 58.07 wt%, and have low CaO (0.45–0.84 wt%) but high Al2O3 (18.56–29.04 wt%), Cr (174–334 ppm) and Ni (42.5–153 ppm) contents. They have high CIW (Chemical Index of Weathering) values (90.4–94.7), indicating intense chemical weathering of the source material. The samples display light rare earth elements (LREE) enrichment with negative Eu anomalies (Eu/Eu*=0.40–0.68), and have flat heavy rare earth elements (HREE) patterns. The high contents of transition elements (e.g. Cr, Ni, Sc, V) and moderately radiogenic Nd isotopic compositions suggest that the paragneisses might be those of first-cycle erosion products of predominantly mafic rocks mixing with small amounts of felsic moderately evolved Archaean crustal source. Geochemical and Nd isotopic compositions reveal that at least some of the protoliths of Kongling khondalite were sourced from local pre-existing mafic igneous rocks in a continental arc tectonic setting. Combined with documented zircon U–Pb geochronological data, we propose that the Palaeoproterozoic high-pressure granulite-facies metamorphism, rapid weathering, erosion and deposition of the khondalites in the interior of the Yangtze craton might be related to a Palaeoproterozoic collisional orogenic event during 2.1–1.9 Ga, consistent with the worldwide contemporary orogeny, implying that the Yangtze craton may have been an important component of the Palaeoprotorozoic Columbia supercontinent.

Influence of contamination on banded iron formations in the Isua supracrustal belt, West Greenland: Reevaluation of the Eoarchean seawater compositions

Banded Iron Formations (BIFs) are chemical sediments, ubiquitously distributed in the Precambrian supracrustal belts; thus their trace element compositions are helpful for deciphering geochemical evolution on the Earth through time. However, it is necessary to elucidate factors controlling the whole-rock compositions in order to decode the ancient seawater compositions because their compositions are highly variable. We analyzed major and trace element contents of the BIFs in the 3.8–3.7 Ga Isua supracrustal belt (ISB), southern West Greenland. The BIFs are petrographically classified into four types: Black-, Gray-, Green- and White-types, respectively. The Green-type BIFs contain more amphiboles, and are significantly enriched in Co, Ni, Cu, Zn, Y, heavy rare earth element (HREE) and U contents. However, their bulk compositions are not suitable for estimate of seawater composition because the enrichment was caused by secondary mobility of metamorphic Mg, Ca and Si-rich fluid, involvement of carbonate minerals and silicate minerals of olivine and pyroxene and/or later silicification or contamination of volcanic and clastic materials. The White-type BIFs are predominant in quartz, and have lower transition element and REE contents. The Gray-type BIFs contain both quartz and magnetite. The Black-type BIFs are dominated by magnetite, and contain moderate to high transition element and REE contents. But, positive correlations of V, Ni, Zn and U contents with Zr contents suggest that involvement of detrital, volcanic and exhalative materials influences on their contents. The evidence for significant influence of the materials on the transition element contents such as Ni in the BIFs indicates the transition element contents in the Archean ocean were much lower than previously estimated. We reconstructed secular variations of V, Co, Zn and U contents of BIFs through time, which show Ni and Co contents decreased whereas V, Zn and U contents increased through time. Especially, the Ni and Co contents drastically decreased in the Mesoarchean rather than around the Great Oxidation Event. On the other hand, the V, Zn and U contents progressively increased from the Mesoarchean to the Proterozoic. Stratigraphical trends of the BIFs show increase in Y/Ho ratios and decrease in positive Eu anomaly upwards, respectively. The stratigraphic changes indicate that a ratio of hydrothermal fluid to seawater component gradually decrease through the deposition, and support the Eoarchean plate tectonics, analogous to the their stratigraphic variations of seafloor metalliferous sediments at present and in the Mesoarchean.

Can Fractional Crystallization, Mixing and Assimilation Processes be Responsible for Jamaican-type Adakites? Implications for Generating Eoarchaean Continental Crust

Understanding how the Earth’s first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet’s primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite–tonalite–granodiorite (TTG) suite. These can be divided into (1) a mid- to late Archaean (∼3·5–2·5 Ga) suite with low SiO2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3·5 Ga) suite with higher SiO2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid- to late Archaean (∼3·5–2·5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd–Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase- and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge; (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times.

Geochemistry of metamorphic rocks of the Kurai block (Gorny Altai)

We consider the primary nature and sources of the protoliths of metamorphic rocks of the Kurai block located in the large Teletskoe-Kurai system of deep faults separating the Gorny Altai and West Sayan structures. It has been established that the protoliths of the Kurai block metapelites were deposits of transitional crust: They lack typical rocks of mature continental crust (arkoses, litharenites) and have reduced (relative to the upper continental crust) contents of lithophile elements and elevated contents of transition elements. The average Nd model age of the protoliths of the metapelites of the Kurai complex corresponds to the Middle Riphean (1.4-1.6Ga). The metabasites of the Kurai complex are similar in petrologic composition to metamorphic products of oceanic basalts. It is shown that the formation of metamorphic associations of the Kurai block was caused by tectonometamorphic transformations of a compositionally heterogeneous rock unit (basalts, aluminous and volcanomict sediments), which were, most likely, a fragment of Middle Cambrian-Early Ordovician turbidite basin with an oceanic basement.

Factors controlling the trace-element distribution in fine-grained rocks: the Albian kaolinite-rich deposits of the Oliete Basin (NE Spain)

This paper combines mineralogical data (X-ray diffraction, petrographic and scanning electron microscopy techniques) and geochemical analyses in order to investigate the composition of a set of samples (claystones, siltstones and sandstones) which belong to the Albian deposits of the Oliete Basin in Spain. The aims of the study are to characterize the distribution of major and trace elements in kaolinite-rich rocks; to infer the role played by accessory phases and clay minerals on the trace-element distribution; and to assess the source-area composition of these materials. Mineralogical analyses show that these rocks are mainly composed of different proportions of quartz and clay minerals (kaolinite and illite) with minor feldspar. Micron accessory minerals such as monazite, zircon, pyrite, marcasite, Fe oxides and Ti oxides have also been detected. The sandstone framework implies that the source area of these sediments was probably a granitoid source which underwent low rank metamorphism. The major element distribution reflects the mineralogy of the rocks. Plots in the A-CN-K diagram, as well as their CIA values, indicate that these sediments were generated from an intensely weathered source area. This produced the development of relatively cation-poor clays such as kaolinite, and claystones with large rare-earth elements (REE) contents. The fact that transition element contents in claystones are greater than in siltstones and sandstones suggests that they are preferentially linked to clay minerals, probably by surficial sorption. In comparison with other lithologies, the high abundances of high field strength elements in siltstones imply that accessory minerals, such as zircon and Nb-bearing phases, are mainly concentrated in siltstones. It should be noted that siltstones are also the lithology which displays the highest REE abundances. The statistical analysis of the geochemical data suggests that REE are mainly controlled by Ti- and Nb-bearing phases, such as titanite and Nb- and Ti-oxides. Although clay minerals are not the main phases in the REE control, they probably also have some influence, as the analyses show. In addition, other accessory phases such as monazite, secondary phosphates, and zircon also contribute to the REE contents, although much less than amorphous Fe oxides in some samples. According to the REE patterns and elemental ratios such as La/Sc, Th/Sc, Th/Co, etc., the source area of the studied outcrops was probably felsic and with intermediate composition between granite and granodiorite.

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ 18O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al 2O 3 contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet–pyroxene cumulates or low-pressure (<1 GPa) plagioclase–pyroxene–olivine cumulates. Trace element modeling suggests a low-P origin for eclogites with flat heavy REE patterns and a high-P origin for eclogites with fractionated heavy REE. Flat heavy REE patterns, the presence of Sr anomalies, and low to moderate transition element contents in the low-P group are consistent with a low-pressure origin as metamorphosed olivine gabbros and troctolites. These metagabbroic high MgO eclogites either could represent the basal section of subducted oceanic crust or foundered mafic lower continental crust. In the former case, the metagabbroic high MgO eclogites may be genetically related to the Koidu low MgO suite. Crystal fractionation trends suggest that the metapyroxenitic high MgO eclogites formed at lower pressures than their current estimated equilibrium pressures (>4 GPa).

Hydrothermal iron-rich deposits from the Teahitia-Mehitia and Macdonald hot spot areas, Southwest Pacific

During cruise SO-47 of R.V. Sonne, iron oxyhydroxide crusts of restricted extent were recovered from Teahitia and Moua Pihaa seamounts in the Teahitia-Mehetia hot spot area and from Macdonald Seamount in the Macdonald hot spot area. The crusts show complex layering and incorporate volcanic ash layers. They consist of goethite, feldspar and pyroxene and display low contents of transition elements apart from Fe. It is believed that the crusts were deposited from relatively low temperature hydrothermal fluids. These low temperatures preclude the formation of sulphide deposits. The type of hydrothermal mineral recovered may be a function of the thermal regime of the system and the elevation, which controls the pressure of the venting fluids. The shallower depths of these seamounts compared to the axes of mid-ocean ridges may therefore be responsible for the style of mineralization. Initially, magmatic vapours migrating upwards through the hot spot volcano are thought to be absorbed into seawater at comparatively shallow levels to deposit sulphide minerals within the volcanic pile. The Fe oxyhydroxide deposits are the residual low-temperature deposits formed when the spent hydrothermal fluid reached the crest of the seamount. These deposits therefore differ fundamentally from those found at the crest of mid-ocean ridges where sulphide mounds are deposited directly upon interaction of the boiling hydrothermal fluid with seawater. The discovery of hydrothermal iron oxyhydroxide crusts in these two hot spot areas is only the second such finding after Loihi Seamount, southeast of Hawaii.

Mineralogy and geochemistry of sediments in the Southwestern Pacific Basin: Tahiti – East Pacific Rise – New Zealand

Abstract Considerable changes in colour, grain size, mineralogy, and chemical composition of marine sediments were observed on a Tahiti – East Pacific Rise (EPR) – New Zealand transect. Sediments collected on the traverse Tahiti – EPR flank are carbonate–rich silty clays dominated by smectite. The sediments have high transition element contents which suggests contributions from hydrothermal sources. EPR crest sediments are foraminiferal oozes with varying amounts of amorphous ferromanganese hydroxides. No clay minerals were detected. The sediments are extremely enriched in Fe, Mn, Cu, and Zn, a consequence ofstrong hydrothermal input at the rise crest. The dispersal of hydrothermal exhalates to the west can be traced as far as 1450 km away. On the transect from the EPR flank to New Zealand at 42°S, sediments change from calcareous silty clays to pelagic clays, according to water depth and distance from New Zealand. The Carbonate Compensation Depth lies at about 5050 m water depth. From east to west, the a...