Abundant Rocks Research Articles

Intraplate extension of the Indochina plate deduced from 26 to 24 Ma A-type granites and tectonic implications

ABSTRACT In recent years, abundant Cenozoic potassic magmatic rocks from eastern Tibet and the Indochina Block have been studied extensively; however, until now, knowledge of Cenozoic A-type granites from the interior of the Indochina Block has been limited. U–Pb zircon ages for six samples of the Salei granite pluton within the Indochina Block range from 26 to 24 Ma. In situ Lu–Hf and Sr–Nd isotope data indicate that the Salei pluton was sourced mostly from Mesoproterozoic basement rocks of the Indochina Block, mixed with a small volume of juvenile crust derived from the underplated mantle. Whole-rock major element geochemistry indicates that the six samples are peraluminous high-K calc-alkaline granites. The trace and rare earth element patterns are typical of within-plate A-type granites. In combination with previous research, the present results suggest that the late Oligocene Salei granite formed from the convective removal of thickened lower continental lithospheric mantle. Moreover, the presence of 26–24 Ma A-type granites in the Indochina Block indicates within-plate extension in the interior of the block during the late Oligocene.

Deriving Amorphous Component Abundance and Composition of Rocks and Sediments on Earth and Mars

AbstractX‐ray amorphous materials have been detected in all samples measured by the CheMin X‐ray diffractometer (XRD) on board the Mars Science Laboratory rover in Gale Crater, Mars. The origin(s) of these materials are poorly understood, and there are significant uncertainties on their estimated abundances and compositions. Three methods are used to estimate the bulk amorphous component abundance and composition of Martian samples using XRD and bulk chemical data: (1) Rietveld refinements, (2) FULLPAT analyses, and (3) mass balance calculations. We tested these methods against a quantitative XRD (internal standard) method commonly used in terrestrial laboratories. Additionally, we tested for instrumentation effects by measuring our samples on a laboratory XRD instrument (PANalytical X'Pert Pro) and the CheMin test bed instrument (CheMin IV). We used three natural samples known to contain amorphous materials: glacial sediment, Hawaiian soil, and a paleosol. Our methods resulted in nine amorphous abundances and four amorphous compositions for each sample. For a single sample, amorphous abundance estimates and amorphous compositions are relatively similar across all estimation methods. CheMin analog measurements perform well in our tests, with amorphous abundances and compositions comparable to laboratory quantitative XRD measurements, though slightly underestimated. This suggests that previous amorphous component estimates for Martian samples are relatively accurate. This study highlights the usefulness of the mass balance calculation method for characterizing amorphous materials in terrestrial samples, providing important supplemental information to destructive and time consuming size separation and dissolution procedures.

Potential Geologic Significances of Hertzsprung Basin Revealed by CE-2 CELMS Data

The Hertzsprung basin (2 °N, 128 °W) is a relatively well-preserved Nectarian-age basin on the Moon farside with a diameter of 570 km. Low FeO+TiO2 abundance (FTA) and substantial topographic elevation change make the Hertzsprung Basin particularly relevant for evaluating the microwave thermal emission (MTE) mechanism over the lunar surface. In this study, microwave sounder (CELMS) data from the Chang'E-2 satellite are employed to investigate MTE features of the Hertzsprung Basin combined with FTA, surface slope, and rock abundance data. The results are as follows. First, the influence of topography on the CELMS data is essentially negligible, at least in low-latitude regions. Second, the brightness temperature ( TB ) behavior in the basin floor is similar to that in the nearby highlands, indicating homogeneity of the regolith thermophysical parameters in the highland areas. Third, abnormally low TB is interpreted to represent the existence of large boulders, which brings about two distinct and influential mechanisms on the observed T B . Fourth, a warm interior of the shallow lunar crust is inferred because of the high TB anomaly in the basin floor, Michelson crater, and southeastern region. Finally, the Orientale impact event exerted limited influence on the thermophysical structures of the lunar regolith in the study area.

A Rock Model for the Cold and Hot Spots in the Chang’E Microwave Brightness Temperature Map

Thermal anomaly spots (both hot and cold) have been found in the global 37-GHz brightness temperature (TB) map of the moon based on the Chang’E (CE) microwave radiometer measurements. To explain their origin, a rock model is proposed to simulate the TB variation against latitude along the profile of a fresh crater in a single track way, which is selected to highlight the topographic effect and avoid any modification to the data. A mixed upper layer made up of rock and soil (regolith and dust) was employed into our previous multilayer model. The thermal properties (thermal conductivity and heat capacity) of the mixture layer are presumed to be linear with the fraction of rocks. Given that high-frequency (37 GHz) measurements are chosen, only the meter size and larger rocks of the upper mixed layer are considered to avoid scattering effects. Several fresh craters poor/rich in ilmenite are selected as testing sites. Despite uncertainties in parameters such as rock abundance (RA), and iron and titanium abundances, three conclusions can be reached from these cases: 1) RA has a significant effect on both the TB value and TB variation trend against latitude; its contribution over some craters may be as high as 15 K; 2) the simulations based on our rock model fit the CE observations better than those when rocks are not included; and 3) the rock and ilmenite contributions could be the main cause for the cold and hot spots found in the CE microwave map.

Unravelling the Mystery of Lunar Anomalous Craters Using Radar and Infrared Observations

In Miniature Radio Frequency (Mini‐RF) radar images, anomalous craters are those having a high circular polarization ratio (CPR) in their interior but not exterior to their rims. Previous studies found that most CPR‐anomalous craters contain permanently shadowed regions and that their population is overabundant in the polar regions. However, there is considerable controversy in the interpretation of these signals: Both water ice deposits and rocks/surface roughness have been proposed as the source of the elevated CPR values. To resolve this controversy, we have systematically analyzed >4,000 impact craters with diameters between 2.5 and 24 km in the Mini‐RF radar image and Diviner rock abundance (RA) map. We first constructed two controlled orthorectified global mosaics using 6,818 tracks of Mini‐RF raw data and then analyzed the correlations between radar CPR and surface slope, RA, and depth/diameter ratios of impact craters. Our results show that CPR‐anomalous craters are distributed relatively uniformly across the lunar surface, with no apparent difference in CPR between the polar, potentially icy, and nonpolar, not icy, craters. Most CPR‐anomalous craters are relatively young with a large depth/diameter ratio, and they actually represent an intermediate stage of crater evolution. Comparison with a two‐component radar scattering model suggests that rocks and surface roughness are major contributors to the observed CPR values. Using craters of 4.7–22 km in diameter with known ages, we find that craters spend up to 120 Ma with a high exterior RA, and ∼3 Ga in the CPR‐anomalous phase.

Relación longitud-peso, coloración y ubicación en el ambiente de juveniles de <i>Pinguipes chilensis</i> en Bahía La Herradura, Coquimbo, Chile

Pinguipes chilensis (Chilean sandperch) is an endemic species of the Southeast Pacific Ocean. Studies have generally included specimens of a total length (Lt) of more than 21 cm, there being no formal antecedents of specimens of a smaller Lt, weakness that motivates this study. It was determined for the Lt-Pt relation (Pt= 0.000005Lt3,1101, R2= 0.974) and Fulton’s average condition factor (0.81). Three ontogenetic color morphs are characterized, which are arranged in different environments and inhabit heterogeneous backgrounds, with sandy areas, others with abundant rocks and cracked rocks, which allow fish to take refuge

Geochemical characteristics and hydrocarbon generation potential of the first member of Shahejie Formation (E2s1) source rocks in the Dongpu Depression, East China

The Dongpu Depression, containing huge amounts of hydrocarbons, is a typical petroliferous province in the Bohai Bay Basin, east China. As one set of the main source rocks, the first member of Shahejie Formation (E2s1) shows huge exploration potential with approximately 0.83 × 108 t resources remaining to be discovered. However, the geochemical characteristics and thermal evolution history of E2s1 source rocks have still not been determined. In this study, the sedimentary features were firstly analysed based on the geological data. Then the organic matter abundance, kerogen type, and thermal maturity of E2s1 source rocks in the present day were evaluated according to the Rock‐Eval pyrolysis, micro‐components of kerogen, total organic carbon (TOC), and vitrinite reflectance analysis. Furthermore, the evolution of thermal history and hydrocarbon generation was determined using the PetroMod procedure. The results show that dark mudstones and salt rocks were well developed in E2s1, which are propitious for organic matter to grow and preserve. E2s1 source rocks contain abundant organic matter and moderate to good generative source rock potential. E2s1 source rocks mainly consist of Type II kerogen and have entered early to maturation stage for hydrocarbon generation. Due to the limited buried depth, E2s1 source rocks only attained lower thermal maturity and generated finite hydrocarbons. The initial rapid subsidence and the second rift after uplift erosion resulted in the higher geo‐temperature at the end of E3d and the present day. Corresponding to the geo‐temperature evolution, there are two stages of hydrocarbon generation for E2s1 source rocks: The first stage was in the late E3d period, and the late stage was from the late Neogene to the present day. Moreover, the hydrocarbon generation rate and quantity of the late stage are much higher than the first stage, indicating the major hydrocarbon generation stage. Due to the limited hydrocarbon generation capacity, oils generated from E2s1 source rocks are low maturity; thus, it cannot migrate for a long distance and only accumulated around the sag belts.

Brecciation among 2280 ordinary chondrites – Constraints on the evolution of their parent bodies

After accretion of meteorite parent bodies, larger and smaller collisions have led to significant modifications of these bodies. Involved processes include excavation of material, thermal metamorphism, melting, mixing of different materials, re-accretion, and re-lithification. All these processes can be repeated several times. In this study polished thin sections (PTS) of 2280 chondrites (1193 H, 947 L, and 140 LL chondrites) were investigated in order to obtain the abundance of brecciated rocks among the ordinary chondrites. In addition, we have determined the abundance and characteristics of shock vein-bearing H, L, and LL chondrites and of impact melt rock clasts. We also recognized xenolithic components based on O-isotope studies. Noble gas data were considered in order to detect regolith breccias and to discuss late impact histories. The investigation of 2280 samples shows that 23% (276 of 1193) of the H chondrites, 23% (220 of 947) of the L chondrites, and 79 % (110 of 140) of the LL chondrites are brecciated. Considering the heavily-brecciated LL chondrites in 63 of the 140 chondrites (45%) shock veins were clearly detected. 57 of these 63 chondrites are brecciated rocks. The investigation of the H and L chondrites has shown that about 26% (310 of 1193) of the H chondrites and 40% (379 of 947) of the L chondrites contain shock veins. In our data-set 20% of the H chondrites and 8.3% of the LL chondrites, but only 3.0% of the L chondrites contain solar noble gases. Remarkably, about 62% of all brecciated H chondrites (with noble gases analyzed) contain solar noble gases compared to only around 11% and 10% of the brecciated L and LL chondrites, respectively. The identification of xenolithic clasts (e.g., CI-, CM-, and ureilite-like lithologies) in primitive type 3 chondrites indicates simultaneous accretion of clasts and chondrules. These clasts must have been formed early within the first 2 Ma on subsequently-destroyed precursor, first generation parent bodies. The formation of complex breccias witnesses the collisions between asteroids of very different lithologies and heritage. Although the onion-shell configuration of primordial parent bodies is necessary in order to form the chondrites with different degrees of metamorphic overprint (petrologic types 3–6) subsequent catastrophic fragmentation and reassembly to form asteroids with a rubble-pile structure are required to explain certain features discussed in this work. However, distinct peaks in the cosmic ray exposure age distributions indicate that not too many impacts in the last 100 Ma were responsible to deliver the majority of the ordinary chondritic meteoroids to Earth. Yet, this certainly does not tell anything about the number of “last-generation” parent bodies that exist in the asteroid belt, since S-type asteroids are the most abundant type of asteroid in the inner main belt and thought to be the parent bodies of ordinary chondrites.

Reconstruction of the Neoproterozoic–Cambrian Orogenesis in Princess Elisabeth Land (East Antarctica) from a Study of Granitic Rocks

Abundant Cambrian granitic rocks in Princess Elizabeth Land and adjacent regions of Antarctica occupy various positions (from syn- to postkinematic) in the structure of the crust. Their mineral and isotopic composition reflects both the character and age of the parental substrate and geodynamic formation conditions of the region. The study of Cambrian processes is important for this region, because almost all known models of the formation of the Gondwana supercontinent suggest the Neoproterozoic–Cambrian amalgamation of two or three continental blocks, parts of which can be distinguished in this sector. The paper presents geological data on granitic rocks, as well as their mineral and Nd isotopic composition. Field observations and analytical data indicate a different structure and petrographic composition of Cambrian granitic rocks in Princess Elizabeth Land, which are related to their different formation conditions. Synkinematic biotite or garnet–biotite peraluminous S-type granites mostly occur in the eastern Princess Elizabeth Land. Mostly late and postkinematic biotite and amphibole–biotite (±orthopyroxene) granites or granodiorites are typical of its western part. Their compositions are similar to that of A-type granites. In comparison with coeval granites in adjacent areas, the character of Cambrian granites in Princess Elizabeth Land substantiates the presence of structural zones identified from geological data and indicates the presence of a Cambrian orogen probably with a collisional nature, as well as the location of its hinterland in intracontinental Antarctica.

Clay mineral diversity and abundance in sedimentary rocks of Gale crater, Mars.

Clay minerals provide indicators of the evolution of aqueous conditions and possible habitats for life on ancient Mars. Analyses by the Mars Science Laboratory rover Curiosity show that ~3.5-billion year (Ga) fluvio-lacustrine mudstones in Gale crater contain up to ~28 weight % (wt %) clay minerals. We demonstrate that the species of clay minerals deduced from x-ray diffraction and evolved gas analysis show a strong paleoenvironmental dependency. While perennial lake mudstones are characterized by Fe-saponite, we find that stratigraphic intervals associated with episodic lake drying contain Al-rich, Fe3+-bearing dioctahedral smectite, with minor (3 wt %) quantities of ferripyrophyllite, interpreted as wind-blown detritus, found in candidate aeolian deposits. Our results suggest that dioctahedral smectite formed via near-surface chemical weathering driven by fluctuations in lake level and atmospheric infiltration, a process leading to the redistribution of nutrients and potentially influencing the cycling of gases that help regulate climate.

Analysis of Rock Abundance on Lunar Surface From Orbital and Descent Images Using Automatic Rock Detection

AbstractInformation on the distribution and abundance of rocks on the lunar surface is essential for understanding the characteristics of lunar regolith and for selecting suitable landing sites for lunar exploration missions. This paper first presents a novel automatic rock detection approach based on the gradient differences in illumination direction. Multiple‐source images covering different regions of the lunar surface are then used for rock detection and rock abundance analysis, including the Chang'E‐3 descent images (0.02–0.17 m/pixel) around the landing area, the Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera images (0.3–1.1 m/pixel) at the landing sites of the Chang'E‐3, Luna 17, and Luna 23, and the LRO Narrow Angle Camera image mosaic (1.5 m/pixel) in the Oceanus Procellarum area. A rock abundance model using an exponential form is derived from the results of the abovementioned analysis to describe the overall cumulative fractional area of rocks versus their diameters. The rock detection results and the derived rock abundance estimated by the proposed model are compared with the ground measurements obtained using the rover images of the Chang'E‐3 landing site and the LRO Diviner Radiometer‐derived rock abundance. A comparison analysis indicates that the derived rock abundance model can feasibly represent the rock abundance in various situations on local and large scales. In general, a remarkable consistency is observed between our results and the results obtained using the ground measurements at the Chang'E‐3 landing site and the orbit measurements from the Diviner radiometer, while our rock detection results and rock abundance model exhibit better performance in presenting detailed information in local areas. Further analyses on rock abundance and the crater morphology and other crater characteristics in the Oceanus Procellarum area indicate that it is feasible to use rock abundance information to estimate the surface maturity.

POTENTIAL GEOLOGICAL SIGNIFICATIONS OF CRISIUM BASIN REVEALED BY CE-2 CELMS DATA

Abstract. Mare Crisium is one of the most prominent multi-ring basins on the nearside of the Moon. In this study, the regolith thermophysical features of Mare Crisium are studied with the CELMS data from CE-2 satellite. Several important results are as follows. Firstly, the current geological interpretation only by optical data is not enough, and a new geological perspective is provided. Secondly, the analysis of the low TB anomaly combined with the (FeO+TiO2) abundance and Rock abundance suggests a special unknown material in shallow layer of the Moon surface. At last, a new basaltic volcanism is presented for Crisium Basin. The study hints the potential significance of the CELMS data in understanding the geological units over the Moon surface.

Uranium enrichment in a paleo-karstic bauxite deposit, Yunfeng, SW China: Mineralogy, geochemistry, transport – deposition mechanisms and significance for uranium exploration

Elevated concentrations of uranium have been found in many bauxite deposits, but the status of uranium in the ores and the mechanisms of enrichment have not been well understood. In this paper, we report a new case of uranium enrichment in a paleo-karstic bauxite deposit at Yunfeng, southwestern China, present electron probe micro-analyzer (EPMA) and Raman spectroscopic evidence for the presence of separate U-minerals, and propose a model in which uranium was enriched through successive processes from chemical weathering through early to burial diagenesis. The Yunfeng bauxite ores, developed in Lower Carboniferous mudrocks overlying Middle to Upper Cambrian carbonate rocks, contain 18.0 to 62.4 ppm (average 35.1 ppm) U, which is much high than the abundances in average crustal rocks (1–3 ppm). Micron-sized uraninite occurs as rims of Ti-oxides, fillings of micro-fractures in kaolinite, and disseminated grains in association with sulfides in the matrix of diaspore and kaolinite. Trace and rare earth elements analysis indicates that the bauxite ores could not be produced by in-situ weathering of the carbonates underlying the orebodies. Instead, the precursor rocks of the bauxite ores are inferred to be black shales originally enriched in uranium that were exposed tens of kilometers away. Detrital grains of the black shales were transported and accumulated in depressions above karstified carbonates in the Yunfeng area, where they were subjected to intense weathering to form bauxite ores. The weathering of the shale clasts liberated much of the uranium due to oxidation of uranium from U4+ to U6+. However, the majority of the U6+ was reduced to U4+ again due to availability of reducing agents such as organic matter and sulfides originally in the shales, forming nanocrystals of UO2+x in close association with Ti-oxides disseminated in Al-hydroxides and clay minerals. These nanocrystals of UO2+x were converted to uraninite during diagenesis, together with the convention of gibbsite and boehmite to diaspore and the formation of anhedral to euhedral pyrite and various Cu-, Pb-, Ni- and Co-rich sulfides. The recognition of uraninite in the bauxite ores is important for uranium exploration and for evaluation of the economic value of uranium as a byproduct of bauxite deposits.

Volcanic ash as a driver of enhanced organic carbon burial in the Cretaceous

On greater than million year timescales, carbon in the ocean-atmosphere-biosphere system is controlled by geologic inputs of CO2 through volcanic and metamorphic degassing. High atmospheric CO2 and warm climates in the Cretaceous have been attributed to enhanced volcanic emissions of CO2 through more rapid spreading at mid-ocean ridges and, in particular, to a global flare-up in continental arc volcanism. Here, we show that global flare-ups in continental arc magmatism also enhance the global flux of nutrients into the ocean through production of windblown ash. We show that up to 75% of Si, Fe and P is leached from windblown ash during and shortly after deposition, with soluble Si, Fe and P inputs from ash alone in the Cretaceous being higher than the combined input of dust and rivers today. Ash-derived nutrient inputs may have increased the efficiency of biological productivity and organic carbon preservation in the Cretaceous, possibly explaining why the carbon isotopic signature of Cretaceous seawater was high. Variations in volcanic activity, particularly continental arcs, have the potential of profoundly altering carbon cycling at the Earth’s surface by increasing inputs of CO2 and ash-borne nutrients, which together enhance biological productivity and burial of organic carbon, generating an abundance of hydrocarbon source rocks.

Hydrocarbon potential of the Lower Cretaceous mudstones in coastal southeastern China

The Cretaceous hosts the highest hydrocarbon potential among all geological stratigraphic sequences worldwide, and the Lower Cretaceous in coastal southeastern China, with the development of black mudstones and shales, has potential for hydrocarbon generation. This potential, therefore, is of significance to regional hydrocarbon exploration both onshore and offshore; however, it has not been investigated in detail. Here, we address this issue based on the results of integrated organic geochemistry and petrology of four representative outcrop sections in the region. Results show that there are two possible source rock sequences in the Lower Cretaceous of coastal southeastern China, that is, a lower sequence represented by the Bantou Formation black mudstones in the Fujian Province and an upper interval represented by the Guantou Formation black mudstones or calcareous mudstones in the Zhejiang Province. The organic matter abundance of these two formations reaches levels of poor to good source rocks. The kerogen is dominated by type III, whereas type II can also be found locally in the northwestern Fujian Province. Most samples are highly mature to overmature, except for the samples with less influence of volcanism in the northern Guangdong Province. Comparatively, the organic matter abundance of the early-stage source rocks is higher than for the late-stage rocks, and the kerogen type is less favorable; however, the maturity shows little correlation with age and is largely related to volcanism. Therefore, the Lower Cretaceous in coastal southeastern China seems to have hydrocarbon potential (gas in particular) in local areas, including the northern Guangdong Province and its analog in offshore basins for the early stage and the northwestern Fujian Province and its analog in offshore basins for the late stage. These results and understanding provide insights for expanding new hydrocarbon exploration targets in southeastern China both onshore and offshore and may also have implications for understanding the regional Cretaceous geology.

Ti–Zr–Nb-bearing accessory minerals in high-K trachyandesitic rocks from the Western Outer Carpathians, Moravia, Czech Republic

The occurrence of chevkinite-group minerals (CGM), zirconolite and Nb-rich ilmenite is reported in Miocene high-K trachyandesites from the Uherský Brod area, Czech Republic, their occurrence reflecting the high Nb and Zr abundances in the host rocks. All three were late-magmatic phases, crystallizing from silicic residual melts. Strong zonation in some crystals is ascribed mainly to kinetic factors and in the case of the CGM by the replacement of less Fe-rich, more calcic (“perrieritic”) compositions by compositions richer in Fe and poorer in Ca (“chevkinitic”) as temperatures fell during crystallization. Zirconolite has high ThO 2 (≤7.75 wt%) and UO 2 (≤3.78 wt%) abundances and is commonly zoned, with significant fractionation of Th from U and within the LREE. The minerals may have crystallized at relatively high f o 2 , low p H 2 O, and at temperatures around 840°C. Zirconolite was stabilized relative to zircon by the Ti–Ca-rich nature of the melts and to baddeleyite by the high SiO 2 activity. Formation of Zr-bearing titanite, as recorded in other potassic suites, was precluded by the preferential entry of Ti into the CGM.

Trace Elements and Minerals in Fumarolic Sulfur: The Case of Ebeko Volcano, Kuriles

Native sulfur deposits on fumarolic fields at Ebeko volcano (Northern Kuriles, Russia) are enriched in chalcophile elements (As-Sb-Se-Te-Hg-Cu) and contain rare heavy metal sulfides (Ag2S, HgS, and CuS), native metal alloys (Au2Pd), and some other low-solubility minerals (CaWO4, BaSO4). Sulfur incrustations are impregnated with numerous particles of fresh and altered andesite groundmass and phenocrysts (pyroxene, magnetite) as well as secondary minerals, such as opal, alunite, and abundant octahedral pyrite crystals. The comparison of elemental abundances in sulfur and unaltered rocks (andesite) demonstrated that rock-forming elements (Ca, K, Fe, Mn, and Ti) and other lithophile and chalcophile elements are mainly transported by fumarolic gas as aerosol particles, whereas semimetals (As, Sb, Se, and Te), halogens (Br and I), and Hg are likely transported as volatile species, even at temperatures slightly above 100°C. The presence of rare sulfides (Ag2S, CuS, and HgS) together with abundant FeS2 in low-temperature fumarolic environments can be explained by the hydrochloric leaching of rock particles followed by the precipitation of low-solubility sulfides induced by the reaction of acid solutions with H2S at ambient temperatures. The elemental composition of native sulfur can be used to qualitatively estimate elemental abundances in low-temperature fumarolic gases.

Mineralogy and chemical composition of lithic artefacts for characterization of raw materials and provenance in archaeological sites of Salobo, Carajás, Pará, Brazil

The Carajás Mineral Province is home to several archaeological sites, each with numerous stone artefacts elaborated in diferente materials. We studied the morphology, mineralogy, and chemistry of lithic artefacts recovered at the archaeological sites in the impacted area of the Solobo copper and gold mine in the Carajás Mineral Province. The results obtained from XRD, XRF and SEM/EDS analyses show that the starting material used was a semi-hard kaolin (semi-flint) that consisted of kaolinite, cryptocrystalline quartz, florencite, sericite, and hematite. This suggests that the same source of raw materials was involved and that this possibly reflected preference for this material. Additionally, the hardness of this raw material probably allowed the artefacts to be elaborated with ease. The mineralogical and chemical results, combined with characteristics of the artefacts' morphology and texture, indicate a relation among the archaeological sites. Similar material to that used in the production of the Solobo artefacts was found in the Alto Bonito amethyst mines that are located 40 km to the north. This site is therefore considered as the source of the raw material used for these artefacts. Finally, this indication is supported by the association of the abundant rock crystal chips and amethyst with the semi-hard kaolin present at the Alto Bonito mine.

Modeling incompatible trace-element abundances in plagioclase in the Skaergaard intrusion using the trapped liquid shift effect

Incompatible trace-element abundances in minerals and whole rocks from layered intrusions have been used to model the fractionation processes and evolving liquid compositions. Many such models assume that the analyzed concentration in a mineral represents that of the mineral when it first crystallized. However, overgrowth from residual liquid and subsequent diffusive equilibration can result in significant changes to the bulk mineral compositions (the more incompatible the element the more dramatic the subsequent changes). The proportion of that residual liquid relative to the cumulus minerals is the most important parameter in determining the magnitude of this effect (trapped liquid shift effect). Calculations involving Ba and La contents in plagioclase quantitatively demonstrate this effect. For Ba and La (partition coefficients of 0.4 and 0.04), 50% trapped liquid in a sample can result in two and sevenfold increases, respectively, in concentration between original and final bulk mineral compositions. Different cumulus assemblages also have a major effect on final compositions. We use examples of the concentrations of Ba and La in plagioclase from the Skaergaard intrusion from previous publications to illustrate the importance of this effect. Specifically, the La content of bulk plagioclase steadily decreases upward from the Lower Zone to Upper Zone c, and Ba in plagioclase shows no increase from the Lower Zone to the top of the Middle Zone. Such results are not explicable by fractionation processes, but can be modeled by the trapped liquid shift effect, assuming the well-established evidence for upward decrease in trapped liquid proportion through these zones.

Rare-earth-element geochemistry in soils developed in different geological settings of Cuba

The increased exploitation of rare-earth elements (REEs) over the past few decades and the disposal of various products containing these elements have prompted numerous environmental studies and the need to establish regulatory guidelines. We determined the concentration of REEs in topsoil samples of 33 soil profiles collected from areas of different geological settings in Cuba. The average REE concentrations revealed the following chemical abundances in decreasing order: Ce>Nd>La>Pr>Sm>Er>Yb>Dy>Tb>Lu>Ho>Gd>Eu>Tm. The significant variation in REE levels reflects the geological diversity of Cuba. The highest REE concentrations were observed in Typic Rhodudalf and Rhodic Eutrodox, which are derived from hard limestone. In contrast, the lowest REE concentrations were detected in the Typic Kandiustalf, which originated from mica schist. The lower light REE (LREE) contents in Cuban soils compared with soils from other countries is related to the low abundance of felsic rocks on the island. We also observed a strong positive correlation between REEs and some major elements in soils. This finding implies that the distribution of REEs in soils is partly governed by phosphates and aluminosilicates. The low fractionation between LREEs and heavy REEs is likely related to the predominance of soils developed from carbonates. This finding was also supported by the low LaN/SmN and GdN/YbN ratios and principal component analysis. The spatial distribution of REEs normalized to Upper Continental Crust coupled with univariate and multivariate statistical techniques indicated that parent materials control the prevalence of REEs in Cuban soils. The data presented here represent the first national-wide survey of REEs in Cuban soils.