Geochemical characterization of rare earth elements in sediment profiles from the Oualida lagoon(Morocco)

The present work assesses the temporal distribution pattern and geochemical changes of rare earth elements and Yttrium, Scandium, Thorium, and Uranium delivery into the Oualidia lagoon. Two sediment cores were retrieved from the Oualidia lagoon and analyzed using neutron activation analysis. The results indicated that heavy rare earth elements are slightly enriched the sediment cores over light rare earth elements. The highest values of REEs were recorded in the top layers of the cores and depleted with depth, suggesting a possible change in factors controlling their accumulation, including mechanical, chemical, and environmental parameters such as weathering intensity, grain size, and Fe-Mn oxides. The sediments display positive Ce anomalies, which are probably related to the submarine weathering process and detrital input. Noting also the variation of hydrodynamics conditions and confinement of the upstream part of the lagoon played a key role in changing the sediment origins.Thus, further investigation of REEs origin in the Oualidia lagoon sediment is required to identify their sources,provenances, and the factors controlling their spatial and vertical distributions. However, these results provide baseline data of occurring changes in REEs geochemical composition and constitute a typical study case to understand the link between sedimentary and geochemistry processes in a lagoonal ecosystem.

Rb and Sr Geochemical Characterization of the Chinese Loess Stratigraphy and Its Implications for Palaeomonsoon Climate

Concentrations and distributions of Rb and Sr in loess and palaeosol samples from the Luochuan profile have been studied, in grain-size fractions and chemical forms. Results show that Rb is quite rich in the fraction of less than 2 μm, and exits only in the residual phase, while Sr is distributed in sand and silt fractions, occurring in the residual and carbonate phases. Therefore, variations of the concentrations of Rb and Sr in loess and palaeosols are closely related not only to dustfall compositions depending on winter monsoon wind strength but also to summer monsoon-induced pedogenic intensity. This indicates that variations of Rb/Sr ratio in Chinese loess sequences represent cyclic alternations of East Asian summer and winter monsoon climates on the orbital time-scale. Variations in Sr concentrations along the sequence bear a high similarity to the oxygen isotope record of the deep sea sediments over the past 2.5 Ma, whereas variations in Rb concentrations are in good agreement with the record of magnetic susceptibility determined with the duplicated samples from the same profile. This suggests that (1) the high-latitude ice volume is responsible for East Asian monsoon variations and (2) the signal of magnetic susceptibility depends dominantly on pedogenesis in Chinese loess sequences.

Geochemical Characterization of Intermediate to Silicic Rocks in the Global Ophiolite Record

Ophiolites consist predominantly of mafic-ultramafic rocks but also contain in subordinate amounts intermediate to silicic intrusive–extrusive rocks in them.These rocks,although not in large volumes in comparison to mafic–ultramafic units,reveal significant clues about melt evolution,magmatic accretion processes,and partial melting of pre-existing oceanic crust during ophiolite evolution.They also yield ample accessory minerals,such as zircon and monazite,which are widely used to date the timing of oceanic crust generation.We present a global synthesis of the occurrence of such leucocratic,intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites,and evaluate models for their genesis during the development of oceanic crust in different tectonic environments.In the ophiolite complexes we have investigated,intrusive and extrusive rocks show a wide range in composition.Intermediate rocks（with Si O2 between 52and 63 wt.%）include andesite and diorite,whereas the silicic rocks（with Si O2 more than 63 wt.%）include dacite,trondhjemite,tonalite and granite.For subduction-relatedandsubduction-unrelated MORB-type ophiolites in the Phanerozoic orogenic belts,the most commonly reported intermediate to silicic intrusive rocks,independent of their original tectonic setting,are trondhjemite,plagiogranite and tonalite,and less commonly quartz-diorite and diorite.These rock types have been documented from 80 percent of the 104ophiolites included in this study.Intermediate to silicic lavas and/or pyroclastic rocks（basaltic andesite,andesite,dacite and rhyolite）are less abundant,and have been reported from only 35 percent of the 104 ophiolites.Trachyte occurs in some of the Rift/Continental Margintype,subduction-unrelated ophiolitesEvolvedrocksinsubduction-unrelated,Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite,whereas MOR type（mid-ocean ridge）ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites.Intermediate to silicic volcanic uints in the Backarc sub-group of subduction-related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite,whereas in the Backarc to Forearc,Forearc,and Volcanic Arc sub-groups they are mainly basaltic andesite/andesite.Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE-enriched,whereas those in the MOR type vary from LREE-depleted to LREE-enriched.The Backarc and Backarc to Forearc types are similar to the MOR type;silicic rocks of the Forearc and Volcanic Arc types are generally LREE-enriched.The main process in the formation of the majority of the intermediate to silicic rocks in both subduction-unrelated and subduction-related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers,whereas a minor volume in subduction-related ophiolites are adakites that were produced from partial melting of a subducting slab.Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt.The incompatible,non-conservative elements,such as Ba and Th,are weakly to strongly enriched in subduction-related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes.The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations,providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.Fractional crystallization appears to have been an important process in the formation of the intermediate to silicic rocks in Rift/CM and P/MOR ophiolites in the subduction-unrelated class,as well as VA ophiolites in the subduction-related class.In these types of ophiolites magma chambers might have operated as closed-systems for a sufficient amount of time for fractionation and compositional zoning of the magma to take place（Fig.1C1）.Fractional crystallization was of subordinate importance during the igneous construction of the MOR,BA,FA,and BA-FA ophiolites.Instead,partial melting of hydrothermally altered crust was the most common process resulting in the majority of the intermediate to felsic rocks of the Rift/CM（estimated to 60%）,MOR（estimated to 95%）,and the BA,FA and BA-FA（estimated to 75%）ophiolites,as well as a high proportion of the VA ophiolites（;0%）(Fig.1C2).Slab-generated melts that result in the production of intermediate to silicic rocks of adakitic character represent a relatively small fraction（;5%in Phanerozoic ophiolites）of subduction-related ophiolites（Fig.1C3）.Some adakites are very low in incompatible elements（e.g.,La）,suggesting that these melts were generated from highly depleted MORB.Precambrian ophiolites contain a higher percentage of adakites（ca.35%）,indicating that physical conditions were more favorable for slab melting during the Precambrian than in the Phanerozoic.This phenomenon may have resulted from higher geothermal gradient along relatively shallow-dippingsubductionzonesduringthe Precambrian.

Geochemical Characterization of the Paleocene Ewekoro Limestone Formation, SW Nigeria: Implications for Provenance, Diagenesis and Depositional Environment

The usefulness of limestone as an industrial raw material is dependent on the level of its purity, largely controlled by diagenesis and the depositional setting. Limestone samples (83) obtained from the Ewekoro Formation exposed in quarry sections at Shagamu were analyzed using X-Ray Fluorescence spectrometry (XRF). A geochemical characterization of result was done to evaluate the purity levels and the implications on provenance, diagenesis, and depositional setting of the limestone. Five limestone beds from quarry sections (15 locations) were logged. Beds “E”, “D”, “C”, “B”, and “A” comprised sandy algal in sparry calcite cement, sandy biomicrite, algal biomicrite, sandy algal biomicrite, and the intra-sparite facies respectively. Results of major elements che- mistry indicated concentrations (in wt%) as follows: CaO (33.71 - 59.99), MgO (0.39 - 3.15), Al2O3 (0.47 - 3.23), Fe2O3 (0.51 - 3.43), SiO2 (0.47 - 45.98), SO3 (0.10 - 2.27), K2O (0.02 - 0.28), Na2O (0.00 - 10.0), TiO2 (0.00 - 0.27), P2O5 (0.02 - 0.92), MnO (0.01 - 0.06) and Loss on ignition (17.64 - 45.20). Geochemical result showed that the samples are enriched in CaO, while SiO2 varies widely, likely due to hinterland input. Plots of SiO2 versus CaO concentrations depicted a negative correlation attributable to chemical diagenetic processes. Diagenesis of the carbonate in form of reversible replacement of SiO2 with CaO and vice versa occurred under a shallow marine condition. This negative correlation between these oxides (CaO and SiO2) dictates zones of high purity limestones that are of desirable industrial applications. Relatively high silica content in the northwestern part is possible indication of outlets to the continental or inland areas where clastic sediment source may have been prominent. Ratios of Ca/Mg and Mg/Ca indicated a relatively low rate of evaporation of sea water and palaeo-salinity conditions marked by precipitation of limestone. Intermittent increase in palaeo-salinity and sea water evaporation level gave rise to the magnesian limestone.

Mobility of Major and Trace Elements during the Bauxitization Processes in Ngaoundal Area (Adamawa Cameroon): Implication on Mining Perspectives

This study was focused to assess major and trace elements in bauxitic duricrusts from Ngaoundal and its surroundings in order to establish their mining interest. To this end, fieldworks, mineralogical and geochemical analyses were carried out. Four facies of duricrust were identified and characterized from the summit to the top of the slope of the Ngaoundal mountain: scoriaceous, pisolitic, nodular and massive. Mineralogical and geochemical analyses performed on 16 samples, revealed a significant concentration of Al2O3 mainly in the scoriaceous facies (over 45% in grade), moderate in Fe2O3 (averaging 23.69%) and SiO2 (averaging 21.7%). Trace elements were generally low, excluding Cr (421 ppm on average), Zr (327 ppm on average and V (213 ppm on average). In addition, the limited quantities of alkalis (Na2O, K2O) and alkaline earths metals (MgO, CaO) coupled with the very high values of Chemical Index of Alteration (CIA) and Mineralogical Index of Alteration (MIA), (more than 99%) attest to the intense weathering of the studied materials. Allitization and monosiallitization constituted the crystallochemical phenomena that have led to the development of bauxitic minerals. More than 90% of gibbsite in scoriaceous facies, 52.21% - 76.01% of kaolinite in pisolitic facies and more than 40% of hematite in nodular facies were quantified. The relationships between the constitutive components indicated their interdependency during the bauxitization phenomenon. The mineralogical and geochemical properties highlighted the mining interest of the studied duricrusts to be valorized.

Griesbachian Cyclostratigraphy at MajiashanSection, Anhui Province, China

The Lower Triassic at Majiashan Section, deposited in the deep part of the Lower Yangtze carbonate ramp, is the only outcrop of the deep Lower Triassic preserved in the present Lower Yangtze region. Even so, the alternations of mudrock of mudrock and marl (or limestone) are still very distinct and recognizableable, though usually more or less condensed. The lithological and geochemical cycles at Majiashan Section are discussed in terms of the sedimentary characters and the contents of some elements analyzed at a 2-centimeter-interval with no hiatus of the Lower Triassic. The division resolution of the Griesbachian stratigraphy may increase to the scale of ten thousand years by the mudrock-marl (or limestone) bundles and the geochemical cycles. The analysis of the relationship between the sedimentary cycles/the geochemical cycles and the Milankovitch cycles may suggest the time limit and the sedimentary rates.

Geochemical characterization of loess-paleosol sequences:Comparison between the upper reaches of the Hanjiang and Weihe river valleys,China

This paper aims to compare the geochemical characteristics of loess-paleosol sequences in the upper reaches of the Hanjiang and Weihe river valleyswhich are located in the semi-humid temperate zone and humid subtropical zonerespectively. The Mituosi（MTS） profile in the upper reaches of the Hanjiang River valley and the Yaohecun（YHC） profile in the Weihe River valley were selected for this comparative research. The stratigraphic characteristicscompositionchemical weathering intensityleaching rates of Ca and Namobility of major elementsand transport features of Na and Fe were analyzed with respect to depth and compared between the two profiles. This study reached the following conclusions.（1） The composition of the loess-paleosol sequences in two regions are quite similar to the average composition of the upper continental crust（UCC）indicating that the loess in the two regions came from multiple sources and was mixed well. Thereforethe loess in the two regions is considered aeolian loess.（2） Compared with the loess-paleosol sequence in the Weihe River valleythe loess-paleosol sequence in the upper reaches of the Hanjiang River valley features a darker color; a higher chemical index of alteration（CIA） value; higher leaching rates of Na and Ca; higher migration ratio（relative to K） of AlSiMgand Na; and lower migration ratio of Fe and Ca. This evidence indicates that the loess-paleosol sequence in the humid subtropical environment experienced stronger chemical weathering intensity than the loess-paleosol sequence in the semi-humid temperate zone.（3） Both the YHC profile and MTS profile record a period of climate deterioration at 6000–5000 a BP. The period punctuated the mid-Holocene Climatic Optimum（8500–3100 a BP） in the study area.