Composition Of Upper Continental Crust Research Articles

Tin isotope heterogeneity and baseline of the upper continental crust

Stable Sn isotope ratios are emerging as a novel tracer for a wide range of geological processes; however, the Sn isotopic baseline of the upper continental crust (UCC) is not yet well-constrained. Here, we report high-precision Sn isotope data of a wide range of UCC samples, including granites, pegmatites, and sediments, to document the Sn isotopic composition of UCC. Significant variations in δ122/118Sn3161a (per mil deviation in 122Sn/118Sn relative to NIST 3161a) values are revealed for I-type (δ122/118Sn3161a = 0.025 ± 0.026‰ to 0.495 ± 0.046‰, n = 20) and S-type (δ122/118Sn3161a = 0.156 ± 0.018‰ to 0.501 ± 0.075‰, n = 22) granites. More extreme Sn isotope variability is observed for pegmatites, which have δ122/118Sn3161a from 0.256 ± 0.047‰ to 0.930 ± 0.049‰ (n = 13). The δ122/118Sn3161a of I-type granites decrease with decreasing TFe2O3 (total Fe as Fe2O3) and MgO contents and are attributed to the segregation of Fe-bearing minerals. Conversely, the Sn isotope variation of S-type granites appears to reflect source heterogeneity, whereas the Sn isotope variability of pegmatites may reflect fluid activity. In contrast, the loess samples display homogeneous δ122/118Sn3161a values (0.132 ± 0.034‰ to 0.239 ± 0.020‰, n = 20) that show no correlation with the degree of chemical weathering, suggesting that loess is representative of the average Sn isotope composition of UCC. The δ122/118Sn3161a values of modern sediments and sedimentary rocks range from 0.080‰ to 0.490‰ (n = 25) and the Sn isotope variations may be related to chemical weathering or sediment provenance. Based on the lithology-weighted average δ122/118Sn3161a of UCC samples (41 granites and 45 sediments) in this study, the δ122/118Sn3161a value of UCC is estimated to be 0.233 ± 0.099‰, providing a reference point for further investigations. Tin is isotopically lighter in UCC compared to the mantle, owing to magmatic differentiation and crustal evolution processes.

Geochemistry of Triassic metasediments from easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria

Triassic metasediments from the easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria, show a high variety in their mineral and chemical composition from shales, wakes, litharenites to arkoses. Geochemical signatures suggest low to moderate weathering degree at their source and upper continental crust composition. The Triassic metasedimentary rocks were derived from felsic to intermediate igneous rocks with some recycled sediment input. On tectonic setting discrimination diagrams, based on immobile trace elements, the studied metasediments fall in the fields of continental island arc and active continental margin.

Rubidium isotope compositions of the average upper continental crust and the Himalayan leucogranites: Implications for magmatic-fluid interaction

Rubidium is a highly fluid-mobile and incompatible element. Rb isotopes have great potential to trace the migration and enrichment of Rb in magmatic and fluid-related processes. In order to decipher the behavior of Rb isotopes during magmatic-fluid interaction, this study presents an estimate of average δ87Rb of the upper continental crust (UCC) and Rb isotope data for highly evolved leucogranites from Kampa, Himalayan orogen. The samples used to estimate the average UCC include 21 granites, 42 loess, and 10 sediments, which show small variations from −0.27‰ to 0.02‰, −0.17‰ to −0.07‰, and −0.31‰ to 0.07‰, respectively. Based on the weighted average δ87Rb of these samples, the estimate of average Rb isotope composition of UCC is −0.14 ± 0.01‰ (2SD/√n, n = 73), which is consistent with the δ87Rb of the bulk BSE (−0.13 ± 0.01‰).A comparison of the Kampa leucogranites with the UCC indicates that the δ87Rb of the Kampa leucogranites (−0.29‰ to 1.20‰) is significantly higher than the average value of the UCC. Such heavy Rb isotope composition cannot be due to chemical weathering, source heterogeneity, fractional crystallization, or partial melting. Instead, because the Kampa leucogranites have been modified by magmatic fluids during magma evolution, the high δ87Rb most likely reflects the signature of magmatic fluids derived from deeper magma. Our study for the first time indicates that the heavy Rb isotope characteristics of highly fractionated leucogranites provide a novel tracer for processes involving fluid activity. This could be useful for understanding the enrichment and mineralization of rare metals related to magmatic fluids.

Geological Controls on Geochemical Anomaly of the Carbonaceous Mudstones in Xian’an Coalfield, Guangxi Province, China

The anomalous enrichment of the rare earth elements and yttrium (REY), U, Mo, As, Se, and V in the coal-bearing intervals intercalated within the carbonate successions in South China has attracted much attention due to the highly promising recovery potential for these elements. This study investigates the mineralogical and geochemical characteristics of the late Permian coal-bearing intervals (layers A–F) intercalated in marine carbonate strata in the Xian’an Coalfield in Guangxi Province to elucidate the mode of occurrence and enrichment process of highly elevated elements. There are two mineralogical assemblages, including quartz-albite-kaolinite-carbonates assemblage in layers D–F and quartz-illite-kaolinite-carbonates assemblage in layers A–C. Compared to the upper continental crust composition (UCC), the REY, U, Mo, As, Se, and V are predominantly enriched in layers A and B, of which layer A displays the REY–V–Se–As assemblage while layer B shows the Mo–U–V assemblage. The elevated REY contents in layer B are primarily hosted by clay minerals, zircon, and monazite; Mo, U, and V show organic association; and As and Se primarily display Fe-sulfide association. Three geological factors are most likely responsible for geochemical anomaly: (1) the more intensive seawater invasion gives rise to higher sulfur, Co, Ni, As, and Se contents, as well as higher Sr/Ba ratio in layers A–C than in layers D–F; (2) both the input of alkaline pyroclastic materials and the solution/rock interaction jointly govern the anomalous enrichment of REY; and (3) the influx of syngenetic or early diagenetic hydrothermal fluids is the predominant source of U, Mo, V, Se, and As.

Decoupling of dissolved and particulate Li isotopes during estuarine processes

Lithium isotopes in marine authigenic or detrital sedimentary archives have been recently used to trace continental weathering over geologic timescales. However, interpretations are predominantly based on the assumption that riverine Li isotopic signals can be propagated through estuaries without modification. Here, we verify this hypothesis by investigating the behaviour of Li isotopes in the Changjiang (Yangtze) River estuary. We observe a conservative mixing of dissolved Li and its isotopes between the Changjiang River water and seawater. The dissolved δ 7 Li yields a non-linear increase with salinity, and a significant increase occurs during the initial water mixing. Through the studied transect, estuarine flocculation and resuspension processes cause the homogenisation of offshore particulate δ 7 Li values, which are close to the average composition of upper continental crust. This study provides clear and direct evidence that the riverine dissolved Li isotopic signal is not modified during estuarine processes in large rivers, but caution should be exercised when using detrital δ 7 Li in marginal seas to investigate past continental weathering.

Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought, having greater proportions of basalt and komatiite than modern-day continents. Here, we show via petrological modelling that secular change in the petrology and bulk composition of upper continental crust would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 km deep, corroborating independent isotopic evidence for large-scale emergence of the continents no earlier than c. 3 Ga. Our findings thus weaken arguments that early life on Earth likely emerged in shallow subaerial ponds, and instead support hypotheses involving development at hydrothermal vents in the deep ocean.

Late Summer Peak and Scavenging-Dominant Metal Fluxes in Particulate Export Near a Seamount in the Western North Pacific Subtropical Gyre

We measured sedimentation fluxes around JA06 Seamount (Xufu Guyot; 19°30′N, 158°00′E) as part of an environmental baseline survey in Japan’s exploration area for cobalt-rich crusts in the subtropical Northwest Pacific. Sinking particles were collected at the flat top (sediment trap depths: 900 and 1000 m) and northeastern base (sediment trap depths: 1000 and 4720 m) of the seamount from June 2016 to April 2017. Total mass fluxes were very low, with average values of 4.3–4.9 mg m–2 d–1 and 9.3 mg m–2 d–1 in the shallow traps at the flat top and base, respectively, which is consistent with an oligotrophic system. The lower fluxes at the flat top probably reflect lower productivity of siliceous microplankton, such as diatoms. However, we were unable to substantiate any potential mechanisms for this difference in productivity and cannot evaluate whether this is representative of typical conditions. When combined with previous observations at two adjacent seamounts, our results indicate widespread seasonality in sediment fluxes with a peak in late summer (August–September). However, satellite data indicate that summer is the season with the lowest primary production. This discrepancy could be explained either by phytoplankton blooms fueled by symbiotic nitrogen fixation that only cause minor increases in surface-ocean chlorophyll or short-lived blooms induced by passing typhoons under thick cloud cover. At the base site, we also analyzed material and element transport rates from shallow to deep waters. Half of the organic matter and >80% of the carbonate in sinking particles was not degraded in the water column, suggesting that most of the regeneration of these materials occurs near or on the sediment surface. Furthermore, four major processes appeared to control elemental fluxes in the area: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, rare earth elements) processes. The estimation of excess flux based on the composition of upper continental crust demonstrated that >85% of total Mn, Co, Ni, Cu, Zn, Cd, and Pb fluxes were attributable to scavenging (+biogenic uptake). Scavenging-dominant metal fluxes are likely ubiquitous in the oligotrophic open ocean.

Petrography and whole-rock geochemistry of Oligocene Barail Sandstones of Surma basin: Implications for tectono-provenance and paleoclimatic condition

The Oligocene Barail Sandstones of Surma basin from parts of Champhai District of Mizoram, northeast India has been studied for their tectono-provenance setting using petrography and geochemistry. The studied sandstones are poor to moderately sorted, subarkosic to sub-litharenite and show dominance of quartz (avg. 54.46%) followed by feldspars (avg. 7.22%), rock fragments (avg. 4.98%), mica (avg. 5.89%), matrix (avg. 14.47%) and cement (avg. 12.98%). Dominance of polycrystalline and undulatory monocrystalline quartz indicates contribution from medium grade metamorphic sources, primarily of granite gneisses. High concentration of SiO2 with moderate to low concentration of Al2O3, Fe2O3, MgO, Na2O and K2O resembles the composition of upper continental crust. Moreover, moderate to high concentration of Th and V, depleted value of Ni along with TiO2/Zr suggests derivation of sediments from felsic sources. The ratios of Eu/Eu* (avg. 0.63) and LaN/LuN bears resemblance with the upper continental crust. Overall analyses show that the sediments were derived from the felsic terrain of neighbouring orogens probably represented by granite-gneisses and have undergone a moderate degree of weathering (avg. values of CIA: 70.20, CIW: 81.03, PIA: 77.63, WIP: 38.45, ICV: 1.06, Iw = 2) in a semi-humid to humid climatic conditions [Qp/(F + RF):Qt/(F + RF) = 0.20].

Geochemistry of Asara Shale member of Karaj Formation, Central Alborz, Iran: Provenance, source weathering and tectonic setting

The Asara Shale Member of the Karaj Formation, Middle to Upper Eocene in age, includes mixed volcaniclastic, siliciclastic and carbonate strata, in the Alborz Mountains, northern Iran. These strata have been geochemically analyzed to constrain their source area provenance. Most of the major and trace element contents of Asara Shale samples are generally similar to the upper continental crust (UCC) values and their chemical index of alteration (CIA) values (31.7–50.9), suggesting a semi-arid climate in a weathering-limited source area. Moreover, correlation of major element oxides suggests the controlling effect of plagioclase and clay minerals on these data when XRD analysis and SEM images indicate clastic sourced illite and chlorite in the samples, probably due to the weathering of volcanic ash and glass in an aqueous transport and depositional environment. However, the chondrite-normalized REE plot of mudrocks of the Asara Shale show a similarity with the UCC composition. Mineral composition and geochemistry of the Asara Shale mudrocks reveal the rapid reworking of penecontemporaneous volcaniclastic deposits, which had covered the surrounding areas, instead of their epiclastic genesis related to erosion and transport. Major, trace, and rare earth element (REE) data, imply a felsic composition of the volcanic source, related to an active continental margin/continental island arc and most probably back-arc tectonic setting for the depositional basin, inferring a mid-upper Eocene extensional volcanism in a subduction complex.

Genesis of a florencite-bearing kaolin deposit on ordovician schists at Saint-Aubin-des-Châteaux, Armorican Massif, France

Several kaolinite occurrences of economic interest have been reported in Europe and are usually interpreted as resulting from weathering during the Paleogene tropical climate. However, a hydrothermal contribution is also recognized in some kaolinite occurrences. Such occurrences are common throughout the Hercynian Armorican Massif, where they are developed mostly on granitoids. The kaolin deposit of the Tertre Rouge pit, South Brittany (France), has been mined for the past 70 years. It is the only deposit developed from carbon-rich (~1% C) Ordovician schists affected by a low-grade metamorphic event during the late Hercynian orogeny. Herein, we report the results of a detailed field and petrological study combining optical microscopy with SEM investigation, XRD analyses, and ICP-OES and ICPMS geochemical data. A typical weathering profile was identified in the pit. It is marked from the bottom to the top by an increasing kaolinite volume (~45%) at the expense of muscovite/illite, a decreasing volume of chlorite and graphite, and numerous occurrences of Fe oxides/oxyhydroxides that replaced former pyrites or sulfates in veins. Despite this intense weathering, the analyzed Tertre Rouge schists preserved a black shale bulk chemical composition characterized by lithophile trace elements contents at 2× to 5× the Upper Continental Crust composition. This was made possible by accessory minerals inherited from the parent rock, especially Al phosphates, namely florencite (Ce), which were systematically identified as the main concentrators of LREE and Sr. Their occurrence in the Tertre Rouge kaolinized schist confirms the regional Sr and P geochemical anomalies previously reported in the Paleozoic sedimentary formations in this area of the Armorican Massif. High temperature clay minerals, pyrophyllite (>270 °C) and nacrite/dickite (generally considered to be high-T kaolinite polymorphs), were also identified inside the weathered zone, filling millimeter-thick veinlets cutting across or lying parallel to the cleavage plane S of the schist. We ascribe this high-temperature clay mineral assemblage to a hydrothermal event related to the numerous documented evidences of Late Hercynian fluid circulations in Central/South Armorican tectonometamorphic domains. Hence, we strongly favor a two-stage formation for the Tertre Rouge kaolin deposit, including a minor hydrothermal contribution to weathering-related kaolinite.

Geological cycling of potassium and the K isotopic response: insights from loess and shales

Shales are a major sink for K into seawater delivered from continental weathering, and are potential recorders of K cycling. High precision K isotope analyses reveal a > 0.6 ‰ variation in δ41K values (41K/39K relative to NIST SRM 3141a) from a set of well characterized post-Archean Australian shale (PAAS) samples. By contrast, loess samples have relatively homogenous δ41K values (− 0.5 ± 0.1 ‰), which may represent the average K composition of upper continental crust. Most of the shales analyzed in this study have experienced K enrichment relative to average continental crust, and the majority of them define a trend of decreasing δ41K value (from − 0.5 to − 0.7 ‰) with increasing K content and K/Na ratio, indicating cation exchange in clays minerals is accompanied by K isotope fractionation. Several shale samples do not follow the trend and have elevated δ41K values up to − 0.1 ‰, and these samples are characterized by variable Fe isotope compositions, which reflect post-depositional processes. The K isotope variability observed in shales, in combination with recent findings about K isotope fractionation during continental weathering, indicates that K isotopes fractionate during cycling of K between different reservoirs, and K isotopes in sediments may be used to trace geological cycling of K.

Lithium isotopic composition and concentration of Himalayan leucogranites and the Indian lower continental crust

The lithium isotopic compositions of adakitic rocks and K-rich volcanic rocks in southern Tibet range from +1.3‰ to +7.5‰ and −4.9‰ to +3.5‰, respectively. The subduction of the Indian Plate beneath the Lhasa Terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned δ7Li data. Therefore, it is necessary to determine the Li isotopic compositions of the different geological endmembers underneath southern Tibet. Here, we report the lithium isotopic composition and concentration of Indian upper and lower continental crust. On the basis of whole-rock major and trace element data and Sr–Nd–Pb isotope data, leucogranites from Luozha and Longzi are considered as representative of Indian upper crust, whereas two-mica granites from Quedang and Dala and granulites and gneisses from Nyalam are considered as representative of Indian lower crust. The Li concentration of Indian upper crust varies from 23 to 45ppm with a mean of 34ppm, consistent with a weighted mean Li concentration for the upper crust of 35±11ppm. In contrast, the Li abundance of Indian lower crust is estimated to be 33–84ppm with a mean of 58ppm, much higher than the average Li concentration for the lower crust of ~8ppm because of the high modal abundance of biotite and muscovite. The Li isotopic compositions (+0.9‰ to +5.6‰) of Indian upper continental crust are relatively heavy compared with the average Li isotopic composition of upper continental crust. On the other hand, the Li isotopic compositions of Indian lower continental crust (−4.4‰ to −0.1‰) are lighter than those of Indian upper continental crust. The isotopically heavy signature of Indian upper crust is produced by high-δ7Li fluids released from the Indian lower crust slab, whereas the lighter signature of Indian lower crust is generated by the partial melting of residual Indian lower crust slab during metamorphic dehydration of Indian lower crust.

Late Triassic sedimentary records in the northern Tethyan Himalaya: Tectonic link with Greater India

The Upper Triassic flysch sediments (Nieru Formation and Langjiexue Group) exposed in the Eastern Tethyan Himalayan Sequence are crucial for unraveling the controversial paleogeography and paleotectonics of the Himalayan orogen. This work reports new detrital zircon U-Pb ages and whole-rock geochemical data for clastic rocks from flysch strata in the Shannan area. The mineral modal composition data suggest that these units were mainly sourced from recycled orogen provenances. The chemical compositions of the sandstones in the strata are similar to the chemical composition of upper continental crust. These rocks have relatively low Chemical Index of Alteration values (with an average of 62) and Index of Compositional Variability values (0.69), indicating that they experienced weak weathering and were mainly derived from a mature source. The geochemical compositions of the Upper Triassic strata are similar to those of graywackes from continental island arcs and are indicative of an acidic-intermediate igneous source. Furthermore, hornblende and feldspar experienced decomposition in the provenance, and the sediment became enriched in zircon and monazite during sediment transport. The detrital zircons in the strata feature two main age peaks at 225–275 Ma and 500–600 Ma, nearly continuous Paleoproterozoic to Neoproterozoic ages, and a broad inconspicuous cluster in the Tonian–Stenian (800–1200 Ma). The detrital zircons from the Upper Triassic sandstones in the study area lack peaks at 300–325 Ma (characteristic of the Lhasa block) and 1150–1200 Ma (characteristic of the Lhasa and West Australia blocks). Therefore, neither the Lhasa block nor the West Australia blocks likely acted as the main provenance of the Upper Triassic strata. Newly discovered Permian–Triassic basalt and mafic dikes in the Himalayas could have provided the 225–275 Ma detrital zircons. Therefore, Indian and Himalayan units were the main provenances of the flysch strata. The Tethyan Himalaya was part of the northern passive margin and was not an exotic terrane separated from India during the Permian to Early Cretaceous. This evidence suggests that the Neo-Tethyan ocean opened prior to the Late Triassic and that the Upper Triassic deposits were derived from continental crustal fragments adjacent to the northern passive continental margin of Greater India.

Average iron isotopic compositions of the upper continental crust: constrained by loess from the Chinese Loess Plateau

Iron isotopic composition of the upper continental crust (UCC) is critical for understanding Fe mobilization and migration through the Earth. Because rocks exposed at Earth’s surface have heterogeneous δ56Fe, fine-grained clastic sediments can be used to estimate the average composition of UCC. In this study, we report δ56Fe of loess-paleosol sequences from Yimaguan, Chinese Loess Plateau (CLP), to constrain the average Fe isotopic composition of UCC. The loess-paleosol sequences in this area formed in glacial-interglacial cycles and are characterized by varying degrees of weathering. Our data show that the loess-paleosol layers have extremely homogeneous Fe isotopic compositions with δ56Fe ranging from +0.06‰ to +0.12‰, regardless of variations in the major element composition and weathering intensity. Our study indicates that since Fe isotopes are not significantly fractionated during loess deposition, the loess can be regarded as representative of UCC. It follows that the average δ56Fe of UCC is 0.09‰ ± 0.03‰ (2SD), consistent with previous estimates based on igneous rock data.

Late Neoarchean crustal evolution of the eastern North China Craton: A study on the provenance and metamorphism of paragneiss from the Western Shandong Province

Metamorphosed clastic rocks provide important constrains on the composition of upper continental crust. Here we report and characterize a sequence of Neoarchean paragneiss in association with migmatite from the Qixingtai area in Western Shandong Province (WSP) in the North China Craton (NCC), that represent metamorphosed moderate- to fine-grained clayey clastic rocks.The garnet from the paragneiss is characterized by nil or weakly compositional zoning. The biotite shows low TiO2 content (<2wt.%) and high Fe/[Fe+Mg] molar ratios (0.45–0.48). The P–T estimates suggest that the paragneiss sequence was metamorphosed at 5.5–5.7kbar and 678–679°C. The detrital zircons from the sillimanite garnet gneiss yield concordant 207Pb/206Pb ages from 2.78 to 2.53Ga (discordance ≤15%), suggesting that the protolith sediments were deposited after 2.53Ga. The overgrowth rims on the detrital zircons of the paragneiss and those from the leucosome of the migmatite show similar low Th/U values, moderate Th and high U and common lead contents, indicating that the metamorphic and anatectic zircons formed under fluid-rich conditions. The intrusive contact between the massive monzogranite and the banded migmatite suggests that the regional metamorphic/anatectic event occurred before the emplacement of the monzogranite. However, the upper intercept ages, defined by the overgrowth rims of the pre-existing zircons from the leucosome, are broadly similar to those of the crystallization of the magmatic zircons from the monzogranite within errors at ∼2.50Ga.The paragneiss exhibits SiO2 contents in the range of ∼58wt.% (garnet gneiss)–∼70wt.% (sillimanite gneiss), and negative correlations with Al2O3 (23.4–15.4wt.%), TiO2 (0.90–0.35wt.%), MgO (4.5–1.1wt.%), FeOt (8.3–2.6wt.%) and Cr (382–131ppm). The REE patterns also show a systematic change from garnet gneiss to sillimanite gneiss. In trace element discrimination diagrams, the paragneiss plots between the end-members of komatiite/basalt and TTG/granite, suggesting that the sediments sourced from a mixture of ultramafic–mafic and felsic protoliths. Combined with the geochronological data, we suggest that the 2.75–2.71Ga komatiite–tholeiite sequence of the Taishan association and TTGs in the central terrane and the 2.56–2.53Ga TTGs in the southwestern part of the WSP as the potential provenance for the clastics.The protolith of the paragneiss in the Qixingtai area belong to the sedimentary sequence of the Taishan association. Spatially associated coeval arc-like assemblage and other geological evidence suggest that, (1) the metasediments were likely deposited in a back arc basin, (2) the upper amphibolites-facies metamorphism, constrained by the mineral assemblages in the paragneiss, might be related to the closure of the basin, and (3) the generation of large volumes of matured K-rick continental crust occurred not earlier than 2.53Ga in the WSP granite–greenstone belt.

Physicochemical variations in atmospheric aerosols recorded at sea onboard the Atlantic–Mediterranean 2008 Scholar Ship cruise (Part II): Natural versus anthropogenic influences revealed by PM 10 trace element geochemistry

The geochemistry of PM 10 filter samples collected at sea during the Scholar Ship Atlantic–Mediterranean 2008 research cruise reveals a constantly changing compositional mix of pollutants into the marine atmosphere. Source apportionment modelling using Positive Matrix Factorization identifies North African desert dust, sea spray, secondary inorganic aerosols, metalliferous carbon, and V–Ni-bearing combustion particles as the main PM 10 factors/sources. The least contaminated samples show an upper continental crust composition (UCC)-normalised geochemistry influenced by seawater chemistry, with marked depletions in Rb, Th and the lighter lanthanoid elements, whereas the arrival of desert dust intrusions imposes a more upper crustal signature enriched in “geological” elements such as Si, Al, Ti, Rb, Li and Sc. Superimposed on these natural background aerosol loadings are anthropogenic metal aerosols (e.g. Cu, Zn, Pb, V, and Mn) which allow identification of pollution sources such as fossil fuel combustion, biomass burning, metalliferous industries, and urban–industrial ports. A particularly sensitive tracer is La/Ce, which rises in response to contamination from coastal FCC oil refineries. The Scholar Ship database allows us to recognise seaborne pollution sourced from NW Africa, the Cape Verde and Canary islands, and European cities and industrial complexes, plumes which in extreme cases can produce a downwind deterioration in marine air quality comparable to that seen in many cities, and can persist hundreds of kilometres from land.

Concentration and distribution of trace elements in lignite from the Shengli Coalfield, Inner Mongolia, China: Implications on origin of the associated Wulantuga Germanium Deposit

Abstract The Wulantuga Germanium Deposit (WGD), hosted in coal seams with Ge resources up to 1600 Mt, is located in the Shengli Coalfield in Xilingol, Inner Mongolia, China. Forty-two channel samples of Ge-bearing lignites of the No.6-1 coal seam in Lower Cretaceous Bayanhua Formation were collected and analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The mode of occurrence of elements in selected lignite samples were studied by cluster analysis and scanning electron microscope with energy-dispersive X-ray spectrometer (SEM-EDX). The lignite samples of WGD are distinctly enriched in Be, Ge, Sb, W, and U and depleted in Rb, Nb, Sn, and Ta, compared with the average composition of upper continental crust. The average concentrations of above enriched elements also are higher than those of overlying sandstone of WGD and contemporary Ge-barren lignite of Hongqi Coal Mine in Shenli Coalfield, as well as the average concentration of elements in the USA coals and worldwide coals. With an exception of Sr, most trace elements and ash yields obey the log normal distribution in all 42 lignite samples collected from WGD. The elements in these lignites may be classified into four groups: The first group (Ge–Mo association) contains elements with negative correlation coefficients with ash yields, and they show mainly organic affinity. The elements in the second group (Tl–Ga–Zn–Co association) show negative to moderate correlation coefficients with ash yields, and may also have an organic affinity, but most of them are mainly related to mineral matter (such as sphalerite). The last two groups (Rb–Cs and W–U–Cd–Y–Pb–Cu–Hf–Zr–Th–Sn–Nb–Ta–TiO 2 –Sb–Ba–Sr–MnO–Be associations) contain elements with moderate to high correlation coefficients with ash yields, much of them associated mainly with mineral matter (such as anatase, manganite, zircon, and barite) as identified by SEM-EDX analysis, and show mainly aluminosilicate affinity. Silver-bearing particles or native silver were identified by SEM-EDX in the lignites from WGD and Hongqi Coal Mine. Germanium and Mo can concentrate in different proportion of coal seam in different sections, while other elements more or less follow the distribution of ash yield. The TiO 2 -normalized elemental profiles reveal that Be/TiO 2 , Ge/TiO 2 , W/TiO 2 , U/TiO 2 , Mo/TiO 2 , Sb/TiO 2 , Tl/TiO 2 , and Sr/TiO 2 ratios of the lignite samples in three sections of WGD are generally one or two order of magnitude higher than the reference ratios of sandstone from WGD and lignite from Hongqi Coal Mine, much (more than 90%) of these elements (Be, Ge, W, U, Mo, Sb, Tl, and Sr) may be transported into coal seam by solution. The Ge-bearing lignites from WGD are distinctly characterized by lower Rb/Cs ratios, higher U/Th ratios, different from those of the overlying sandstone from WGD and contemporary lignite from Hongqi Coal Mine. Most trace elements in the Ge-bearing lignite of WGD may have been derived from a granitic source, and the enrichment of Be, Ge, Sb, W, and U may be attributed to an epigenetic lateral transferred Ge-bearing solution that leached these elements from the granitic source and transported them into the lignite.

Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS”, by F. Poitrasson and R. Freydier [Chem. Geol. 222 132–147

Scrutiny of published Fe isotope data for igneous rocks that have > 71 wt.% SiO 2 reveals that most samples have the same Fe isotope composition as mafic igneous rocks, contrary to the conclusions of Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]. The only silicic igneous rocks that have Fe isotope compositions that are significantly different than mafic igneous rocks are 4 granitic samples that have SiO 2 > 75 wt.% analyzed by Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]; three silicic igneous rocks analyzed by Beard et al. [Beard, B.L, Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003a. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol. 195 87–117] (SiO 2 > 75 wt.%) have Fe isotope compositions that are identical to mafic igneous rocks, but these samples were ignored. Moreover, the Fe isotope composition of upper continental crust as deduced from the 58 sedimentary clastic rocks (shales, loess, modern marine sediments, and suspended load of rivers), reported by Beard et al. [Beard, B.L., Johnson, C.M., von Damm, K.L. and Poulson, R.L, 2003b. Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans. Geology 31 629–632], is the same as the average of igneous rocks. These data suggest that granitic rocks which deviate from the igneous average may be rare.

Evidence for a ridge subduction event in the Ordovician rocks of north-central Maine

Modern and past ridge subduction events are characterized by the intrusion of midocean-ridge basalt (MORB) magmas into an overlying accretionary prism. The fi eld relationships and trace-element geochemistry of Ordovician mafi c igneous rocks of the Weeksboro-Lunksoos and Munsungun anticlinoria of north-central Maine indicate that they resulted from such an event. The Bean Brook gabbro intrudes the Hurricane Mountain melange and other related sedimentary strata of continental derivation. The gabbro and associated Dry Way volcanics have MORB trace-element chemistries, while the Bluffer Pond and Stacyville volcanics are more enriched (E-MORB), all of which indicate derivation from a mid-ocean ridge. On petrogenetic diagrams, mafi c samples plot in MORB fi elds, or when Th is used as a discriminator, in arc fi elds along trends that originate from MORB fi elds and extend toward the composition of upper continental crust. These trends are consistent with the presence of silicic and metasedimentary xenoliths in the Dry Way volcanics and Bean Brook gabbro and indicate the magmas were not subduction products, but were contaminated by Th-rich upper continental crust. The nearby Chain Lakes Massif likely represents the basement to the “Chain Lakes microcontinent,” and the geographic relationship and ages of the massif, Hurricane Mountain melange, and Dry Way‐Bean Brook magmatic rocks indicate northwestdirected subduction (modern coordinates) on the southeastern margin of the Chain Lakes microcontinent, within the Early to Middle Ordovician Taconic ocean. Subduction at this boundary probably terminated because of the ridge subduction, analogous to Neogene California.

Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina

Abstract A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Q t 60–80, F15–35, L5–20, P/F 0.2–0.4, L v /L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.