Iron and lithium isotope systematics of the Hekla volcano,Iceland — Evidence for Fe isotope fractionation during magma differentiation

In this study potential iron isotope fractionation by magmatic processes in the Earth's crust was systematically investigated. High precision iron isotope analyses by MC-ICP-MS were performed on a suite of rock samples representative for the volcanic evolution of the Hekla volcano, Iceland. The whole series of Hekla's rocks results from several processes. (i) Basaltic magmas rise and induce partial melting of meta-basalts in the lower part of the Icelandic crust. The resulting dacitic magma evolves to rhyolitic composition through crystal fractionation. During this differentiation the δ^56/54Fe_IRMM-014 values increase successively from 0.051 ± 0.021‰ for the primitive dacites to 0.168 ± 0.021‰ for the rhyolites. This increase can be described by a Rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases (M) and the silicate liquid (L) of Δ^56/54Fe_M–L = ? 0.1‰. (ii) The basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition. No Fe isotope fractionation was found in this series. All basalts and basaltic andesites have an average δ^56/54Fe_IRMM-014 value of 0.062 ± 0.042‰ (2SD, n = 9), identical to mean terrestrial basaltic values reported in previous studies. This observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral-melt Fe isotope fractionation factors expected at temperatures in excess of 1050 °C. (iii) Andesites are produced by mixing of basaltic andesite with dacitic melts. The iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites, in agreement with a mixing process. In the Hekla volcanic suite Li concentrations are positively correlated with indicators of magma differentiation. All Hekla rocks have δ⁷Li values typical for the upper mantle and demonstrate the absence of resolvable Li isotope fractionation during crystal fractionation. As a fluid-mobile trace element, Li concentrations and isotopes are a potential tracer of magma/fluid interaction. At Hekla, Li concentrations and isotope compositions do not indicate any extensive fluid exsolution. Hence, the heavy Fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation. Iron isotope analyses on single samples from other Icelandic volcanoes (Torfajökull, Vestmannaeyjar) confirm heavy Fe isotope enrichment in evolving magmas. Our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes.     

Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H₄SiO₄ solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments. The results reveal that, in comparison to aqueous H₄SiO₄, heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of ²⁸Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly ²⁸Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth’s surface systems can be explained. For example, the change of bulk soil δ³⁰Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil δ³⁰Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ³⁰Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadra-coordinated organosilicon complexes and the H₄SiO₄ solution may also shed light on the Si isotope distributions in the Si-accumulating plants.     

Sr, C and O isotope systematics in the Pucará basin, central Peru

A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield ⁸⁷Sr/⁸⁶Sr ratios that fall within or slightly below the published range of seawater ⁸⁷Sr/⁸⁶Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower ⁸⁷Sr/⁸⁶Sr ratios than previously published values. An ⁸⁷Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with ⁸⁷Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic ⁸⁷Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic ⁸⁷Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic ⁸⁷Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic ⁸⁷Sr. The marked enrichment in ⁸⁷Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in ¹⁸O values and essentially no change in ¹³C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive ⁸⁷Sr-enrichment, and ¹⁸O- and ¹³C-depletion. The depletion in ¹⁸O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a ¹⁸O-enrichment of the ore-forming fluids. Progressively lower ¹³C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked ⁸⁷Sr-enrichment and the slight decrease in ¹⁸O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.     

Carbon isotope systematics of the Cambrian–Vendian aquifer system in the northern Baltic Basin: Implications to the age and evolution of groundwater

Groundwater in the Cambrian–Vendian aquifer system has a strongly depleted stable isotope composition (δ¹⁸O values of about −22‰) and a low radiocarbon concentration, which suggests that the water is of glacial origin from the last Ice Age. The aim of this paper was to elucidate the timing of infiltration of glacial waters and to understand the geochemical evolution of this groundwater. The composition of the dissolved inorganic C (DIC) in Cambrian–Vendian groundwater is influenced by complex reactions and isotope exchange processes between water, organic materials and rock matrix. The δ¹³C composition of dissolved inorganic C in Cambrian–Vendian water also indicates a bacterial modification of the isotope system. The corrected radiocarbon ages of groundwater are between 14,000 and 27,000 radiocarbon years, which is coeval with the advance of the Weichselian Glacier in the area.     

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.     

Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilímaussaq Complex,South Greenland

The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe³⁺/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe³⁺/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ^56/54Fe_IRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ^56/54Fe_IRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe³⁺/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.     

Water and sanitation in the Buenos Aires metropolitan region: Fragmented markets, splintering effects?

This paper discusses the provision of water and sanitation services and the related urban impacts in Buenos Aires circa 2005. The first part of the paper focuses on the metropolitan region (BAMR) which is larger than the area served by the Aguas Argentinas S.A. (AASA) concession (i.e. the capital and its conurbano). It highlights a form of institutional fragmentation that is reflected in the diverse management practices of the numerous service providers and results in differentiated levels of and access to services. The paper discusses the technical, economic and regulatory factors that have historically resulted in this institutional fragmentation. The second part of the article focuses on AASA’s water concession agreement. It demonstrates that, although the agreement had an in-built splintering logic, its implementation led to a more “integrated approach” based on the “reality on the ground”. On the whole, the conclusion regarding the application of the splintering urbanism concept to Buenos Aires is a mixed one and depends on the scale of observation. Although at regional level (BAMR), a form of “institutional fragmentation” would appear to be obvious, it is the result of a long and complex historical process and, while recent neo-liberal reforms in service provisions have “cemented” this fragmentation, they did not actually cause it. If we take the AASA service area in isolation, the trend in the decade following the contracting out of the service has been one of improved service provision and access—although this was not achieved through increased integration of the supply system. Finally, in the case of the Buenos Aires metropolitan region, the splintering urbanism thesis defended by Marvin and Graham needs to be nuanced in several significant ways.     

Zircon typology, geochronology and whole rock Sr–Nd isotope systematics of the Mecsek Mountain granitoids in the Tisia Terrane (Hungary)

Summary Zircon typology and ages together with petrological data suggest similarities in the origin of the K–Mg-rich granitoids of the Mecsek Mountains (Mórágy Unit, Tisia Terrane, S Hungary) and the durbachitic to K–Mg-rich rocks of the South and Central Bohemian Batholiths. Our investigations evidence a characteristic bimodal distribution of zircon types and ages. Zircons of normal magmatic habit and a typology around S24 show an age of 354±5Ma. Zircons of tabular habit of subtype S4 indicate a possibly younger age of 339±10Ma. The ages and typology of zircons from plutonic rocks in the Mecsek Mountains strongly resemble those from the Rastenberg granodiorite in the South Bohemian Batholith (Austria). This, as well as Rb/Sr and Sm/Nd isotope systematics, support the assumption of simultaneous formation and close spatial relationship between the Mecsek Mountain granitoids and the Rastenberg granodiorite. Thus, the Mecsek Mountains granitoids likely formed in a Late Palaeozoic palaeo-position S or SSW of the Rastenberg granodiorite. Having experienced a similar metamorphic and magmatic evolution during the Carboniferous, at least the northernmost part of the Tisia Terrane, the Mórágy Unit, is therefore considered to represent a former part of the Helvetic-Moldanubian zone.     

Evolution and composition of the lithospheric mantle underneath the western Arabian peninsula: constraints from Sr−Nd isotope systematics of mantle xenoliths

On the basis of their textures and mineral compositions spinel-peridotite xenoliths of the Cr-diopside group (group I) from Cenozoic volcanic fields of Arabia can be classified into different subtypes. Type IA is of lherzolitic to harzburgitic composition; mineral compositions are similar to those of group I mantle xenoliths from worldwide occurrences. Type IB xenoliths have lherzolitic to wehrlitic compositions; Mg/(Mg+Fe) ratios of the clinopyroxenes (0.862–0.916) and olivines (0.872–0.914) are similar too or slightly lower than those of typical IA minerals. Texturally, type IB xenoliths are distinguished from type IA rocks by the presence of intragranular spinel, intragranular relict Cr-pargasite, and subordinate intergranular Ba-phlogopite (11.1% BaO). The hydrous minerals in type IB xenoliths are interpreted to document an earlier metasomatism 1 which did not affect type IA lithospheric mantle. Subsequent recrystallization caused the partial replacement of Cr-pargasite in type IB materials and resulted in the formation of less hydrous mineral assemblages. Some of the type IA xenoliths are characterized by secondary intergranular amphibole which must have formed recently. The absence or presence of this intergranular amphibole is used to distinguish an anhydrous subtype IA1 from a hydrous subtype IA2. Type IB xenoliths may also contain secondary intergranular amphibole (similar to the one in subtype IA2) or they contain abundant formermelt patches now consisting of glass and phenocrysts of olivine, clinopyroxene, amphibole, and spinel. The secondary intergranular amphiboles and the former melt patches, both are interpreted as results of a second metasomatism (metasomatism 2). In their trace element and isotopic characteristics, type IA1 and type IA2 clinopyroxenes do not exhibit any systematic differences. Furthermore, type IA2 clinopyroxenes are in Sr isotopic disequilibrium with intergranular amphiboles. This suggests that type IA2 clinopyroxenes were not modified during the second metasomatism 2. All type IA clinopyroxenes have low Sr contents (100 ppm); most of them show Sm/Nd ratios higher than inferred for bulk earth. In their ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, type IA clinopyroxenes exhibit a large spread from 0.70226–0.70376 and from 0.51375–0.51251, respectively. Highly variable Sr/Nd ratios (5.0–79.3) and variable T_UR and T_CHUR model age relationships require different evolutions of the respective mantle portions. Nevertheless, all but two type IA clinopyroxenes form a linear array in a Sm–Nd isochron diagram which probably can not be explained by mixing. If taken as an isochron the slope of the array corresponds to an age of around 700 Ma. The mean initial _Nd of 5.8±1.7 (1) is similar to values for juvenile Pan-African (i.e. 850–650 Ma old) crust of the Arabian-Nubian shield. It is suggested that type IA lithospheric mantle and the juvenile Pan-African crust are two counterparts fractionated from a common source during the earlier stages of the Pan-African. Type IB clinopyroxenes have high Sr contents (200 ppm), variable Sr/Nd ratios (9–111) and Sm/Nd ratios generally below that inferred for bulk earth, and show a small spread in their Sr and Nd isotopic compositions (0.70299–0.70318 and 0.51285–0.51278, respectively). In a Sm–Nd isochron diagram the data points form a linear, horizontal array indicating a close-to-zero age for the earlier metasomatism 1 and suggesting a close genetic relationship to mantle processes related to the formation of the Red Sea.     

The genesis of the ores and granitic rocks at the Hongshi Au deposit in Eastern Tianshan,China: Constraints from zircon U–Pb geochronology,geochemistry and isotope systematics

The Hongshi gold deposit is located in the southwestern margin of the Kanggur–Huangshan ductile shear zone in Eastern Tianshan, Northwest China. The gold ore bodies are predominantly hosted in the volcanogenic metasedimentary rocks of the Lower Carboniferous Gandun Formation and the Carboniferous syenogranite and alkali-feldspar granite. The syenogranite and the alkali-feldspar granite yield SHRIMP zircon U–Pb ages of 337.6 ± 4.5 Ma (2σ, MSWD = 1.3) and 334.0 ± 3.7 Ma (2σ, MSWD = 1.1), respectively, indicating that the Hongshi gold deposit is younger than 334 Ma. The granitoids belong to shoshonitic series and are relatively enriched in large ion lithophile elements (Rb, K, Ba, and Pb) and depleted in high field-strength elements (Nb, Ta, P, and Ti). Moreover, these granitoids have high SiO₂, Al₂O₃, and K₂O contents, low Na₂O, MgO, and TiO₂ contents, low Nb/Ta ratios, and slightly positive Eu anomalies. The ε_Hf(t) values of the zircons from a syenogranite sample vary from + 1.5 to + 8.8 with an average of + 5.6; the ε_Hf(t) values of the zircons from an alkali-feldspar granite sample vary from + 5.0 and + 10.1 with an average of + 7.9. The δ³⁴S values of 10 sulfide samples ranged from − 11.5‰ to + 4.2‰, with peaks in the range of + 1‰ to + 4‰. The above-mentioned data suggest that the Hongshi granitoids were derived from the melting of juvenile lower crust mixed with mantle components formed by the southward subduction of the paleo-Tianshan ocean plate beneath the Aqishan–Yamansu island arc during the Early Carboniferous. The Hongshi gold deposit was formed by post-collisional tectonism during the Permian. The granitoids most likely acted as impermeable barriers that prevented the leakage and runoff of ore-bearing fluids. Thus, the granitoids probably played an important role in controlling gold mineralization.     

New isotope–geochemical and mineralogical data on the ultramafic xenoliths in the volcanic rocks of the Kamchatka–Koryak region: Two types of mantle protolith in the modern island-arc system

The paper presents new isotope–geochemical and mineralogical data on mantle xenoliths of the “island-arc” (Avacha, Shiveluch, and Kharchinsky volcanoes) and “within-plate” (Valovayam River, Cape Navarin, and Bakening Volcano areas) types. In terms of paragenesis and mineral composition, the “islandarc” xenoliths correspond to the olivine–plagioclase depth facies, while the “within-plate” xenoliths came from spinel lherzolite to wehrlite facies, which is transitional to the olivine–plagioclase equilibrium. The majority of the “within-plate” xenoliths are enriched in high-field-strength elements (Ti, Nb, Hf, Zr, Yb). The “island-arc” xenoliths in general are depleted in REE, while the “within-plate” xenoliths are enriched in all REE. The former have low Pb isotope ratios, being in isotope equilibrium with lower crustal basites, while most of the latter group are enriched in radiogenic Pb. The island-arc xenoliths are of magmatic origin and were derived from the sublithospheric crust–mantle mixture, while the “within-plate” xenoliths reflect the composition of the asthenospheric mantle source. The primary appearance of the xenoliths is obliterated by secondary recrystallization and metasomatic reworking.     

Suspended-sediment and fresh-water discharges in the Ob and Yenisey rivers, 1960–1988

Of the world's great rivers, the Ob and Yenisey rank among the largest suppliers of fresh water and among the smallest suppliers of suspended sediment to the coastal ocean. Sediment in the middle reaches of the rivers is mobilized from bordering terraces and exchanged between channels and flood plains. Sediment in the lower reaches of these great rivers is deposited and stored (permanently, on a millennial time scale) in flood plains. Sediment discharges, already small under natural conditions, are diminished further by large manmade reservoirs that trap significant proportions of the moving solids. The long winter freeze and sudden spring breakup impose a peakedness in seasonal water runoff and sediment discharge that contrasts markedly with that in rivers of the tropics and more temperate climates. Very little sediment from the Ob and Yenisey rivers is being transported to the open waters of the Arctic Ocean under present conditions.     

Geochronology and metallogenesis of porphyry Mo deposits in east-central Jilin province,China: Constraints from molybdenite Re–Os isotope systematics

The east-central part of Jilin Province, located on the eastern continental margin of northeast China along the eastern Xing–Meng orogenic belt, hosts more than 10 large- and medium-scale Mo deposits. The major types of mineralization include porphyry, skarn, and quartz vein. To better understand the formation and distribution of porphyry Mo deposits in this area, we investigated the geological characteristics of the deposits and applied molybdenite Re–Os isotope dating to constrain the age and source of mineralization. The results, combined with existing data, show that: (a) the Daheishan Mo deposit yields an isochron age of 168.7 ± 3.1 Ma; (b) the Shuangshan Mo deposit yields an isochron age of 171.6 ± 1.6 Ma; (c) the Liushengdian Mo deposit yields a weighted mean model age of 168.7 ± 1.4 Ma; (d) the Jiapigou Mo deposit yields a weighted mean model age of 196 ± 4 Ma; and (e) the Sancha Mo deposit yields a weighted mean model age of 183.1 ± 1.8 Ma. Therefore, the Mo mineralization occurred in the Early–Middle Jurassic (196–167 Ma), during the late stages of magmatism or during the late evolution of magma chambers. The geodynamic setting at this time was dominated by subduction of the paleo-Pacific Plate beneath the Eurasian continent. The rhenium content of molybdenite varies from 0.2 to 99.7 ppm, suggesting that the ore-forming materials may come from a crustal source or a mixed crustal and mantle source.     

The genesis of the ores and intrusions at the Yuhai Cu–Mo deposit in eastern Tianshan,NW China: Constraints from geology,geochronology, geochemistry,and Hf isotope systematics

The Yuhai porphyry Cu–Mo deposit is located in the eastern Tianshan orogenic belt of the southern Central Asian Orogen Belt, being an economically important porphyry Cu deposit in NW China. The deposit comprises sixteen buried orebodies that are predominantly associated with dioritic and granodioritic intrusions and are structurally controlled by roughly NE-trending faults. LA-ICP-MS zircon U–Pb dating yielded crystallization ages of 441.6 ± 2.5 Ma (MSWD = 0.03, n = 24) for diorite and 430.4 ± 2.9 Ma (MSWD = 0.04, n = 19) and 430.3 ± 2.6 Ma (MSWD = 0.09, n = 24) for granodiorite. In situ zircon Hf isotope data on a diorite sample show ε_Hf(t) values from + 8.7 to + 18.6, and two granodiorite samples exhibit similar ε_Hf(t) values from + 12.6 to + 19.6 and + 12.6 to + 18.9, respectively. The dioritic and granodioritic intrusions belong to a low-K tholeiite series and are relatively enriched in large ion lithophile elements (K, Ba, Pb, and Sr) and are depleted in high field strength elements (Th, Nb, Ta, and Ti). Moreover, these intrusions have high SiO₂, Al₂O₃ and MgO contents, low Na₂O, P₂O₅ and TiO₂ contents, low Nb/Ta ratios, and slightly positive Eu anomalies. Re–Os dating of molybdenite intergrowth with chalcopyrite yielded a well-constrained ¹⁸⁷Re–¹⁸⁷Os isochron age of 351.7 ± 2.9 Ma (MSWD = 1.5) with a weighted average age of 355.7 ± 2.4 Ma (MSWD = 0.69) Ma, indicating that the Yuhai Cu–Mo deposit is younger than the intrusion of the diorite and granodiorite. Combined with the regional geological history and above-mentioned data, we suggest that the Yuhai intrusions were most likely derived from the partial melting of mantle components that were previously metasomatized by slab melts formed by the northward subduction of the ancient Tianshan ocean plate beneath the Dananhu–Tousuquan island arc during the Silurian to Carboniferous. Under the subduction-related tectonic setting, the metasomatized mantle magma was emplaced into the shallow crust and induced the formation of the Early Carboniferous Yuhai Cu–Mo deposit, and the hydrothermal fluids of enriched sulfides probably played an important role in the Cu–Mo mineralization.     

Pb concentrations and isotope ratios of soil O and C horizons in Nord-Trøndelag,central Norway: Anthropogenic or natural sources?

Soil O and C horizon samples (N = 752) were collected at a sample density of 1 site/36 km² in Nord-Trøndelag and parts of Sør-Trøndelag (c. 25,000 km²), and analysed for Pb and three of the four naturally occurring Pb isotopes (²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb) in a HNO₃/HCl extraction. Soil O and C horizons are decoupled in terms of both Pb concentrations and Pb isotope ratios. In the soil C horizon the Grong-Olden Culmination, a continuous exposure of the Precambrian crystalline basement across the general grain of the Caledonian orogen, is marked by a distinct ²⁰⁶Pb/²⁰⁷Pb isotope ratio anomaly. No clear regional or even local patterns are detected when mapping the Pb isotope ratios in the soil O horizon samples. Variation in the isotope ratios declines significantly from the soil C to the O horizon. On average, Pb concentrations in the O horizon are four times higher and the ²⁰⁶Pb/²⁰⁷Pb isotope ratio is shifted towards a median of 1.15 in comparison to 1.27 in the C horizon. It is demonstrated that natural processes like weathering in combination with plant uptake need to be taken into account in order to distinguish anthropogenic input from natural influences on Pb concentration and the ²⁰⁶Pb/²⁰⁷Pb isotope ratio in the soil O horizon.     

Lattice distortion in a zircon population and its effects on trace element mobility and U–Th–Pb isotope systematics: examples from the Lewisian Gneiss Complex, northwest Scotland

Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, ²⁰⁷Pb/²⁰⁶Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent ²⁰⁷Pb/²⁰⁶Pb isotopic ages (?3.0 to +0.3 % discordance) in the range of ~2,420–2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated.