Low Sr Contents Research Articles

Dolomite geochemistry of the Cambrian Longwangmiao Formation, eastern Sichuan Basin: Implication for dolomitization and reservoir prediction

Based on petrology and mineralogy, the geochemical characteristics of dolomite in the Cambrian Longwangmiao Formation in the eastern Sichuan Basin, were carried out. Results showed that dolomites mainly occurred in the middle and upper parts of Longwangmiao Formation, with most commonly as a crystalline dolomite and less commonly as a gain dolomite; the percentage content of dolostone in dolomite ranged from 48.6% to 75%, ratios of Mg/Ca were generally small, and positive and negative relationships between content of MgO and content of CaO were identified, respectively; the dolostones were generally characterized by high contents of Fe and Na while low contents of Sr and Mn, and only a part of dolostones had high content of Mn; compared with composition of contemporaneous seawater, most of dolostones were characterized by positive δ13C values and negative δ18O values. Based on the analyses of regional geological setting and geochemical characteristics, the dolomite in Longwangmiao Formation was mainly from the seepage-refluxion dolomitization, while evaporative-concentration dolomitization and burial dolomitization were not developed. In general, the dolomitization of Longwangmiao Formation was not complete. Furthermore, through the dolomitization model of Longwangmiao Formation, the distribution of sedimentary facies as well as the controlling effect of dolomitization on reservoir, the favorable reservoir was predicted, and was mainly developed around Qiyueshan fault zone.

Fluid origin and migration of the Huangshaping W–Mo polymetallic deposit, South China: Geochemistry and 40Ar/39Ar geochronology of hydrothermal K-feldspars

The Huangshaping deposit is a world-class W–Mo–Pb–Zn–Cu polymetallic deposit that formed during the Jurassic magmatic event in the central Nanling region, South China. In order to reveal the three-dimensional development of mineralization and alteration in this complicated fluid-rock system, four typical hydrothermal K-feldspar samples were collected from potassic alteration zones at different elevation levels around main W–Mo ore bodies, followed by precise trace element analysis and 40Ar/39Ar dating. The results show that these K-feldspars have a pronounced lanthanide tetrad effect with high contents of Ga, Ta, Nb, U, Y and HREE but low concentrations of Ba, Sr, Eu and Zr, suggesting a hydrothermal origin. Additionally, correlation between the Ca/Cl/K ratios and the apparent ages suggest that metasomatic fluids played an important role in the formation of the hydrothermal K-feldspars. The plateau 40Ar/39Ar ages which represent the timing of potassic alteration vary from 152.9±0.5 to 154.3±0.6Ma, spatially showing younger trends from north to south and from shallow to deep. These dates correspond to the previous published molybdenite Re–Os data variations in different elevation levels, which record the mineralization event with relatively older ages ranging from 153.8 to 159.4Ma. Combined with the spatial relation analysis among samples, the fluid migration rates associated with the mineralization and alteration are calculated at 10–100mMa−1. Thus, we propose a fluid downward migration model for the Huangshaping polymetallic deposit.

Paleoproterozoic granulite-facies metamorphism and anatexis in the Oulongbuluke Block, NW China: Respond to assembly of the Columbia supercontinent

The Oulongbuluke Block, which is located in the northeastern margin of the Tibet Plateau, has traditionally been considered to be a fragment of the Tarim Craton. Here we present a systematic petrologic, geochemical, and zircon U-Pb and Hf isotopic investigation on mafic granulite and migmatite in the Oulongbuluke Block. The mafic granulite is mainly composed of clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz, with peak metamorphic P-T contions of 6.5–8.8kbar, 745–770°C. Macroscopic and microscopic observations provide strong evidence for in situ partial melting of the felsic gneiss involving breakdown of biotite within the Oulongbuluke Block. The Pl-rich leucosomes with positive Eu anomalies and higher Sr contents were generated as the early-formed feldspar cumulate, and the Kfs-rich pegmatite with negative Eu anomalies and lower Sr contents may represent percolating fractionated melt that was trapped during cooling. Zircon U–Pb dating and Hf isotopic analyses on the mafic granulite and migmatite of the Oulongbuluke Block reveal two distinct age populations: the early Paleoproterozoic (∼2.37Ga) and late Paleoproterozoic (1.93–1.92Ga). The ∼2.37Ga magmatic zircon cores of the migmatite have εHf(t) values between −4.3 and 0.4, with two-stage Hf model ages (TDMC) mainly between 2.82Ga and 3.05Ga. The age of 1.93–1.92Ga obtained from the mafic granulite and migmatite is interpreted as the age of Late Paleoproterozoic metamorphism and anatexis. Most of the 1.93–1.92Ga metamorphic and anatectic zircons have significantly lower 176Lu/177Hf ratios but higher 176Hf/177Hf (t) values than the inherited magmatic zircon cores, which demonstrates that both the zircon U–Pb and Lu–Hf isotope compositions were significantly reset during metamorphism and anatexis. The late Paleoproterozoic metamorphic and anatectic event coincided with global orogenic events that are recorded in many continental fragments, which suggests their link to the Columbia supercontinent.

A new method for TIMS high precision analysis of Ba and Sr isotopes for cosmochemical studies

A new protocol using Eichron™ Sr-resin for high-resolution Sr and Ba isotope measurements using thermal ionization mass spectrometry for cosmochemical samples.

Zircon U–Pb ages and geochemistry of newly discovered Neoproterozoic orthogneisses in the Mishan region, NE China: Constraints on the high-grade metamorphism and tectonic affinity of the Jiamusi–Khanka Block

The Jiamusi–Khanka Block, located in the easternmost segment of the Central Asian Orogenic Belt (CAOB), is one of the least understood blocks in northeastern China. The age of its basement, the timing of the high-grade metamorphism, and the tectonic affinity of this block have all been subjects of controversy. Here we describe newly discovered Neoproterozoic orthogneisses from the Mishan region in the central Jiamusi–Khanka Block. SIMS and LA–ICP–MS U–Pb dating of the magmatic cores of zircons from three of these high-grade orthogneisses yielded weighted mean ages that range from 898±4 to 891±13Ma, indicating that early Neoproterozoic magmatism did occur in the Jiamusi–Khanka Block. Twenty-two analyses of metamorphic zircons (rims as well as some grains) gave two groups of concordant ages, one at ca. 563Ma, interpreted as the timing of the high-grade metamorphism, and the other around 518–496Ma, interpreted as representing a subsequent retrograde metamorphism. The orthogneisses represent metamorphosed peraluminous syn-collisional syenogranites with SiO2=71.29–78.08wt%, A/CNK=1.06–1.99, and Na2O/K2O=0.03–1.49, and they belong chemically to the high-potassium calc-alkaline series. They have low Sr (32–134ppm) contents and Sr/Y ratios (1.6–16.9), and display strongly fractionated to nearly flat REE patterns with negative Eu anomalies (δEu=0.22–0.95) and depletions in Nb–Ta. These geochemical characteristics suggest that the syenogranitic rocks were derived under low pressures from a crustal source with a high proportion of sedimentary rocks, perhaps in a subduction-related setting that was undergoing transition from compression to extension. In the context of the reconstruction of Gondwana, the ca. 898–891Ma magmatic activity and the ca. 563Ma high-grade metamorphism in the Mishan region provide constraints on the possible linkages between the Jiamusi–Khanka Block and East Gondwana during the Neoproterozoic to early Paleozoic.

Combined effects of Sr substitution and pressure on the ground states in CaFe2As2

We present a detailed study of the combined effects of Sr substitution and hydrostatic pressure on the ground-state properties of CaFe$_2$As$_2$. Measurements of the electrical resistance and magnetic susceptibility, both at ambient and finite pressure $P$ $\leq$ 2$\,$GPa, were performed on Ca$_{1-x}$Sr$_x$Fe$_2$As$_2$ single crystals grown out of Sn flux. We find that upon Sr substitution the ranges of stability of both the structural-magnetic transition and the transition into the non-magnetic collapsed tetragonal phase are shifted to higher pressure levels with the latter moving at a higher rate. This suggests the possibility of separating the two phase lines, which intersect already at elevated temperatures for $x$ = 0 and low Sr concentration levels. For $x$ = 0.177 we find strong evidence that both phases remain separated down to lowest temperature and that a zero-resistance state emerges in this intermediate pressure window. This observation indicates that Sr-substitution combined with hydrostatic pressure provides another route for stabilizing superconductivity in CaFe$_2$As$_2$. Our results are consistent with the notion that (i) preserving the fluctuations associated with the structural-magnetic transition to low temperatures is vital for superconductivity to form in this material and that (ii) the non-magnetic collapsed tetragonal phase is detrimental for superconductivity.

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

ABSTRACT: The Monte de Trigo alkaline suite is a SiO2-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs) occur in one nepheline microsyenite dyke, associated with aegirine-augite, wöhlerite, låvenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu ), Nb (0.31- 0.76 apfu ), Fe (1.40-2.13 apfu ), and Mn (1.36- 2.08 apfu ). The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu ), moderate enrichment of rare earth elements (0.38-0.67 apfu ), and a relatively low Na content (11.02-12.28 apfu ), which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce), georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm), Mg (1,179-3,582 ppm), and Zn (732- 852 ppm) than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange.

Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model.Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite–trondhjemite–granodiorite (TTG) suites, with slightly magnesian character (average Mg#=33), moderate LREE (average LaN=154), low HREE (average YbN=6) and Y (1–53ppm; average 11ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu*=0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg#=22), low to moderate degrees of REE fractionation (average [La/Yb]N=34.84), high contents of Y (5–128ppm; average 68), and low Sr/Y (1–98) ratios. Significant negative Eu anomalies (Eu/Eu*=0.18–0.91; average 0.50) and low Sr (65–690ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg#=21), low to moderate degrees of REE fractionation (average [La/Yb]N=26), high contents of Y (71–99ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu*=0.03–0.31; average 0.23) and low Sr (average 160ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values >6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields.The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic (~1.5Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2Ga) implying similar episodic nature in the lower crustal evolution of the KKB.

The Kuradawe Granitic Pegmatite From the Mawat Ophiolite, Northeastern Iraq: Anatomy, Mineralogy, Geochemistry, and Petrogenesis

Abstract The highly evolved Kuradawe granitic pegmatite dike is hosted in a harzburgite-dunite succession of the Mawat ophiolite in northeastern Iraq. It consists mainly of quartz, orthoclase, albite, tourmaline, and muscovite. Accessory minerals include cordierite, andalusite, monazite-(Ce), biotite, zircon, Nb-rich rutile, chlorapatite, chlorite, Sc-bearing columbite-(Fe), uraninite, U-rich thorite, and xenotime-(Y). The internal structure of the pegmatite consists of border, wall, intermediate, and core zones. Plagioclase composition increases from Ab 92 to Ab 97 towards the core. Whole rock major oxides such as MgO, Al 2 O 3 , and CaO decrease and SiO 2 increases toward the core. Na 2 O, K 2 O, and total FeO are enriched in specific zones. Among the trace elements, incompatible elements such as Sr, Ba, and Zr decrease and the compatible elements Ni and Cr slightly increase toward the core. The pegmatite is characterized by the dominance of tourmaline; muscovite as the dominant mica; normative corundum; high contents of SiO 2 , Al 2 O 3 , A/CNK, and Rb; low contents of CaO and Sr; and a high total FeO/MgO ratio in biotite. These features suggest an affinity with calc-alkaline peraluminous S-type granite in a subduction-related, syncollisional setting. The pegmatite is classified as beryl-columbite subtype of the LCT family. The restricted occurrence of high-temperature cordierite in the border zone may suggest that the pegmatite was overprinted by modal metasomatism during intrusion into the mantle rocks. Zircon saturation geothermometry indicates inward crystallization at temperatures of 782 °C in the border zone, 717 °C in the wall zone, and 550 °C in the intermediate zones. Geochemical data suggest that inward crystallization typified the formation of the pegmatite.

Mesoproterozoic continental breakup in NW China: Evidence from gray gneisses from the North Wulan terrane

The North Wulan terrane forms part of the micro-continental blocks in the early Paleozoic Qilian and North Qaidam orogenic belts in NW China. LA-ICP-MS dating of magmatic zircons from two gray gneisses from this terrane yielded U–Pb ages of 1519±5Ma and 1497±8Ma, suggesting emplacement of their precursor magmas at ∼1.5Ga. The rocks have high SiO2 (70.6–75.6wt.%) and Na2O (3.96–4.84wt.%), but relatively low Al2O3 (mostly <15wt.%) and K2O (0.90–2.52wt.%) contents. They possess low contents of Ni, Cr, Sc, Sr, Rb and moderate Y, with slightly enriched LREE ((La/Yb)N=3.46–11.3) and flat HREE ((Gd/Yb)N=0.95–2.06) patterns and strongly negative Eu anomalies (Eu/Eu∗=0.22–0.38). Thus, these rocks have trondhjemite compositions and are comparable to the low-Al or low-pressure TTG gneisses worldwide. Zircons from these rocks yielded εHf(t) and TDM2 values of −2.2–5.1 and 1.93–2.39Ga, respectively. Our data suggest that the precursor low-Al trondhjemite magma was generated from partial melting of the early Paleoproterozoic mafic rocks at shallow crust level through possible heat input from upwelling mantle in a continental rifting setting. We suggest that these ∼1.5Ga gneissic rocks were formed in a tectonic setting identical to that of coeval mafic sills and dykes in the northern Tarim Craton and western Yangtze Craton, and can be broadly correlated with the anorogenic magmatism associated with the initial fragmentation of the supercontinent Columbia.

Generation of Palaeoproterozoic tonalites and associated high-K granites in southwestern Tanzania by partial melting of underplated mafic crust in an intracontinental setting: Constraints from geochemical and isotopic data

The southwestern part of the 2.0–1.8Ga Palaeoproterozoic Usagaran Belt in the Njombe area of SW Tanzania is intruded by two types of synchronous granitic rocks with different chemical and petrological characteristics. The first type consists of hornblende-rich tonalites that have major element compositions similar to those of Archaean TTG but differ significantly in their trace element composition. The tonalites are spatially and closely associated with felsic, high-K, I-type granites, some of which are gneissic and/or aplitic. U–Pb zircon geochronology shows that the emplacement of tonalites at 1887±11Ma was largely contemporaneous with emplacement of high-K granitic gneisses at 1877±15Ma and aplitic granites at 1857±19Ma. The data also reveal the presence of Archaean crust of 2648±25Ma in the zircon cores of some samples in the otherwise Palaeoproterozoic terrane.The tonalites are characterized by MgO contents of 1.60–4.11wt.% at a SiO2 range of 58.1–67.9wt.%, the Mg# of 34–55, lower Sr contents (220–462ppm) and less fractionated REE patterns (La/YbCN=3.55–12.9) compared to Archaean TTG (Sr>500ppm, La/YbCN>20). These features, coupled with the εNd (1887Ma) values of +0.37 to −0.66 as well as the associated mafic enclaves are suggestive of derivation of the tonalites by low pressure (below the garnet stability) partial melting of a mantle-derived mafic underplate that was subsequently contaminated with small amounts of pre-existing igneous crustal rocks. The evolved nature of the high-K granites (MgO=0.20–1.30wt.%, SiO2=65.5–73.9wt.%, Mg#=25–42, εNd=−3.20 to −4.75) coupled with old TDM ages which are 200–1000Ma older than their emplacement age requires a higher degree of assimilation of older crustal material by the magma derived from partial melting of the underplated mafic crust which was subsequently followed by crystal fractionation involving plagioclase, pyroxene and amphibole.The close spatial and temporal association of the tonalites, mafic enclaves and the high-K granites and gneisses in the Njombe area provides the first direct evidence of the role of magmatic underplating for the regional thermal anomaly that caused widespread crustal anatexis leading to the generation of the 1.8–1.9Ga granitic rocks (in Njombe area) and/or associated felsic volcanism in the Usagaran (in Ndembera) and Ubendian (in Ngualla) Belts of SW Tanzania.

Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O- and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated δ53Cr signals due to (i) detrital contamination consistent with Cr signatures of other post-Marinoan cap dolostones, (ii) diagenetic carbonate alteration and post-depositional remobilization of Cr. Al concentrations were used as indicator of detrital contamination and to calculate authigenic Cr concentration and isotope signatures when the detrital contribution of Cr was <40%, which generally resulted in slightly lower Cr concentrations and slightly higher Cr isotopic signals in these samples. Diagenetic alteration of a majority of the studied cap dolostones is evident from low Sr concentrations and light δ18Ocarb signals that correlate with low Cr concentrations and negatively fractionated δ53Cr values. Post-depositional remobilization of Cr results in the loss of heavy Cr isotopes and consequently light Cr isotopes are retained during precipitation of secondary carbonates. However, some samples might still show pristine Ediacaran seawater signals. Two samples pertaining to the deep water cap dolostone section at Yanwutan and one sample pertaining to the shallow water section at Jijiawan yield slightly positively fractionated δ53Cr signals, particularly when corrected to presumably authigenic signals (δ53Crauth). These values are consistent with those from other Neoproterozoic carbonate sections deposited in an oxygenated environment. This short sequence of positively fractionated δ53Crauth is concurrent with a cap dolostone 87Sr/86Sr signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few least-altered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in post-glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration.

New carbonatite complex in the western Baikal area, southern Siberian craton: Mineralogy, age, geochemistry, and petrogenesis

A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar–39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.

Improvement of laser ablation in situ micro-analysis to identify diagenetic alteration and measure strontium isotope ratios in fossil human teeth

Strontium isotope ratios measured in fossil human teeth are a powerful tool to investigate past mobility patterns. In order to apply this method, the sample needs to be investigated for possible diagenetic alteration and a least destructive analytical technique needs to be employed for the isotopic analysis. We tested the useability of U, Th, and Zn distribution maps to identify zones of diagenetic overprint in human teeth. Areas with elevated U concentrations in enamel were directly associated with diagenetic alterations in the Sr isotopic composition. Once suitable domains within the tooth are identified, strontium isotope ratios can be determined either with micro-drilling followed by TIMS analysis or in situ LA-MC-ICP-MS. Obtaining accurate 87Sr/86Sr isotope ratios from LA-MC-ICP-MS is complicated by the potential occurrence of a significant direct interference on mass 87 from a polyatomic compound. We found that this polyatomic compound is present in our analytical setup but is Ar rather than Ca based, as was previously suggested. The effect of this interference can be significantly reduced by tuning the instrument for reduced oxide levels. We applied this improved analytical protocol to a range of human and animal teeth and compared the results with micro-drilling strontium isotopic analysis using TIMS. Tuning for reduced oxide levels allowed the measurement of accurate strontium isotope ratios from human and animal tooth enamel and dentine, even at low Sr concentrations. The average offset between laser ablation and solution analysis using the improved analytical protocol is 38 ± 394 ppm (n = 21, 2σ). LA-MC-ICP-MS thus provides a powerful alternative to micro-drilling TIMS for the analysis of fossil human teeth. This method can be used to untangle diagenetic overprint from the intra-tooth isotopic variability, which results from genuine changes in 87Sr/86Sr isotope ratios related to changes in food source, and by extension mobility.

Geochemical and zircon U–Pb–Hf–O isotopic evidence for a coherent Paleoproterozoic basement beneath the Yangtze Block, South China

The early history of the Yangtze Block is not well constrained owing to the scarce outcrops of the Archean to Paleoproterozoic rocks. Here, we report an integrated study of zircon U–Pb and Hf–O isotopes, as well as whole rock major and trace elements and Nd isotopes for the orthogneisses of the Dongling Complex in the eastern Yangtze Block. LA-ICP-MS zircon U–Pb dating yields a 207Pb/206Pb age of ca. 1.85Ga for the protolith age of the orthogneiss, revealing a reliable Paleoproterozoic basement in the eastern Yangtze Block. Zircons from the orthogneiss have εHf(t) values ranging from −13.4 to −9.0, corresponding to THfDM2 model ages of 3.03–2.80Ga. The zircon δ18O values range between 6.5‰ and 8.8‰ with an average of 7.8‰. In addition, the whole rock εNd(t) values of the orthogneisses range from −9.3 to −8.0 and TNdDM2 model ages from 2.98Ga to 3.05Ga. The orthogneisses have relatively high SiO2, K2O+Na2O, Rb, Ga, Zr, Nb, Y and Ce contents and show low CaO, Eu, Ba and Sr concentrations. Their 10,000*Ga/Al ratios range between 3.0 and 3.5. All these geochemical and Nd–Hf–O isotopic signatures suggest A-type granite protolith for orthogneisses in the Dongling Complex. They were generated by partial melting of Archean crustal materials in a post-collisional tectonic setting. When our data is combined with the existing geochronological data in other parts of the Yangtze Block, it demonstrates a coherent Paleoproterozoic basement beneath the Yangtze Block, representing an important part of the Columbia supercontinent. The ca. 1.85Ga A-type magmatism may have led to craton stabilization over all of the Yangtze Block. In addition, a potential Precambrian dual basement structure may exist in the eastern Yangtze Block, which is supported by Paleoproterozoic metamorphic basement and overlaying Neoproterozoic sedimentary cover.

Mt. Etna plumbing system revealed by combined textural, compositional, and thermobarometric studies in clinopyroxenes

Coupled textural and in situ geochemical studies of clinopyroxene (cpx) phenocrysts, from both historical and recent eruptions of Mt. Etna volcano, provide a means to investigate the processes occurring in the deepest portion of the feeding system (>10 km depth). Five distinct textures were recognized: (1) normal oscillatory zoning, (2) normal zoning with Fe-rich rim, (3) sieve-textured core, (4) reverse oscillatory zoning, and (5) dusty rim. Electron microprobe analyses indicate an almost constant diopside–augite composition, with a slight enrichment in the enstatite for more recent erupted cpx. Core-to-rim compositional profiles, performed along the cpx, reveal distinct compositional characteristics. Normal oscillatory zoning is often characterized by a sharp increase in FeO (Δ ~ 2 wt%) accompanied by a drop in Al2O3 on the outermost 30 μm. Reverse oscillatory zoning, by contrast, exhibits a drop in FeO, Al2O3 (Δ ~ 2 wt%), and a remarkable crystal rim increase in MgO (up to 5 wt%). Similar compositional changes are evident in dusty-textured rims, which are characterized by dissolution edges and overgrowth containing glass pockets and channels. No significant compositional variations have been observed across crystals with sieve-textured cores. Trace element concentrations show enrichments in Sr, La, Zr, and REE, together with a decreasing La/Yb ratio (from ~7 to ~4) in rims of normally zoned crystals. Cpx with reverse zoning and dusty rims has low Sr, La, Zr, and REE contents toward crystal rims. Thermometers and barometers, based on equilibrium cpx-melt pairs, suggest that cpx cores start nucleating at 720 MPa, with the majority of them forming between 600 and 400 MPa but continuing to crystallize until very shallow depths (<100 MPa). Normal oscillatory-zoned phenocrysts surrounded by rims form at pressures shallower than 400 MPa, while reverse zoning and dusty rims occur between 400 and 500 MPa. Coupled petrologic and thermobarometric studies on both clinopyroxenes and plagioclases, associated with detailed textural and in situ geochemical analyses, are promising tools to reconstruct the entire magma ascent path beneath open-system volcanoes. At Mt. Etna, two distinct processes could account for the observed textures: Fe-rich rims in normal oscillatory-zoned crystals can be related to decompression-induced crystallization, while reverse zoning and dusty rims can be produced by mixing with a more basic magma at 400–500 MPa (i.e., ~10 km). Textural features are not restricted to a particular evolutionary phase of the volcano, which suggest that the deep feeding system has not changed significantly since the first alkaline volcanic phase.

Crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at Mt. Etna during the last 400 years

Trace element zoning in plagioclase of selected alkaline lavas from the historic (1607–1892AD) and recent (1983–2013AD) activity of Mt. Etna volcano has been used to explore the possible role that volcano-tectonics exert on magma transfer dynamics. The observed textural characteristics of crystals include near-equilibrium textures (i.e., oscillatory zoning) and textures with variable extent of disequilibrium (patchy zoning, coarse sieve textures and dissolved cores). Historic crystals exhibit lower K concentrations at lower anorthite contents, a feature in agreement with the general more potassic character of the recent lavas if compared to the historic products. Historic plagioclases have statistically higher Ba and lower Sr concentrations than the recent crystals, which result in different Sr/Ba ratios for the two suites of plagioclase. Variations in the anorthite content along core-to-rim profiles obtained on crystals with different types of textures for both the historic and recent eruptive periods were evaluated particularly versus Sr/Ba. At comparable average An contents, crystals characterized by oscillatory zoning, which are representative of near-equilibrium crystallization from the magma, display distinct Sr/Ba ratios. We suggest that these features are primarily related to recharge of a new, geochemically-distinct magma into the storage and transport system of the volcano. In addition to distinct trace element and textural characteristics of plagioclase, Sr diffusion modeling for plagioclase suggests that residence times are generally shorter for crystals found in recently erupted lavas (25–77 years, average 43years) compared to those of the historic products (43–163 years, average 99years). Shorter residences times correlate with gradual increases in eruption volume and eruption frequency rates through time. We attribute these features to an increasing influence, since the 17th century, of extensional tectonic structures within the upper 10km of the Etnean crust, which have promoted shorter residence times and higher eruption frequency.

Upper Neoproterozoic transitional magmatism of Bou-Gafer (Saghro, Anti-Atlas, Morocco): geochemical features and zircon typology

The Upper Neoproterozoic magmatism in Bou-Gafer hill, situated at the northeastern part of Saghro inlier, appears to be post-collisional with transitional character between high-K calc-alkaline and alkaline composition. The body belongs to the final stage of Anti-Atlas Pan-African orogeny. The Bou-Gafer body is composed of quartz monzonite as the main facies and pink granite forming the North and the South segments of the intrusion. Quartz monzonite is abundantly composed of abundant K-feldspar, zoned and twined plagioclase, quartz, green hornblende, clinopyroxene and biotite while pink granite shows abundance of alkali feldspar, quartz, plagiocalase and biotite with minor content of accessory minerals. The rocks of the body form a continuous series ranging from 57 to 76 % SiO2. Chemically, Bou-Gafer rocks are peraluminous, ferriferous and highly potassic, with agpaicity index down to 0.87. The large ion lithophile element (LILE) and high field strength element (HFSE) content of Bou-Gafer granitoids show high values in Rb, Ba, Zr, Y, Nb and Ga and slightly low contents of Sr. The spider diagrams of quartz monzonite and pink granite show enrichment in LILE characterising transitional magmatism and Nb-negative anomaly indicating the vestiges of subduction zone. Bou-Gafer rocks show similar rare earth element (REE) fractionation suggesting that fractional crystallisation can account for the continuous evolution from quartz monzonite to pink granite. The zircon typological distribution in quartz monzonite shows predominance of type S compared with types P and J with slightly represented [301] pyramid while zircon in pink granite is more represented by types S and P. A and T are higher suggesting high alkalinity (alkaline character) and high crystallisation temperature. The Bou-Gafer temperatures obtained and trace element composition (type A2: Y/Nb ratio higher than 1.2) seem to agree with liquid production in the lower crust or by interaction between magma resulting from the lithospheric-enriched mantle and magma produced in the lower continental crust. The magmatism of Bou-Gafer represents a transitional magmatic activity in Saghro hill, and part of the various post-collisional episodes that marked the final amalgamation of the Gondwana supercontinent.

Sr isotope tracing of multiple water sources in a complex river system, Noteć River, central Poland

Anthropogenic impact on surface waters and other elements in the environment was investigated in the Noteć River basin in central Poland. The approach was to trace changes in the Sr isotope composition (87Sr/86Sr) and concentration in space and time. Systematic sampling of the river water shows a very wide range of 87Sr/86Sr ratios, from 0.7089 to 0.7127. This strong variation, however, is restricted to the upper course of the river, whereas the water in the lower course typically shows 87Sr/86Sr values around 0.7104–0.7105. Variations in 87Sr/86Sr are associated with a wide range of Sr concentrations, from 0.14 to 1.32mg/L. We find that strong variations in 87Sr/86Sr and Sr concentrations can be accounted for by mixing of two end-members: 1) atmospheric waters charged with Sr from the near-surface weathering and wash-out of Quaternary glaciogenic deposits, and 2) waters introduced into the river from an open pit lignite mine. The first reservoir is characterized by a low Sr content and high 87Sr/86Sr ratios, whereas mine waters display opposite characteristics. Anthropogenic pollution is also induced by extensive use of fertilizers which constitute the third source of Sr in the environment. The study has an important implication for future archeological studies in the region. It shows that the present-day Sr isotope signatures of river water, flora and fauna cannot be used unambiguously to determine the “baseline” for bioavailable 87Sr/86Sr in the past.

Age and petrogenesis of Na-rich felsic rocks in western Iran: Evidence for closure of the southern branch of the Neo-Tethys in the Late Cretaceous

Intermediate to felsic volcanic and granitic rocks with high Na2O concentrations (5.2–9.1wt.%) are widely distributed in the Harsin area along the Zagros thrust zone in western Iran. Most of these rocks are classified as low-potassium tholeiite, display affinity with oceanic plagiogranite and contain somewhat high Na content and low concentrations of K2O, Al2O3, Rb, Sr and Ca. Thus, we prefer to apply the term Na-rich felsic rocks to this complex. U–Pb dating yielded ages of 94.6±2.7Ma (2σ) from baddeleyite and 95.0±2.4Ma (2σ) from zircon grains, indicating that the complex crystallized in the Late Cretaceous. Based on the mineral compositions, the crystallization occurred at low pressures (mostly <2kbar) and low temperatures (<750°C). High initial ratios of 143Nd/144Nd (0.51288–0.51304) and positive values of ɛNd(t) (+7.0 to +11.5) are consistent with those of mid-oceanic ridge basalt (MORB). During collision of the Arabian plate and Biston-Avoraman block in the Late Cretaceous, an increasing geothermal gradient was responsible for partial melting of altered mafic rocks and for producing the Na-rich felsic rocks in the Harsin area. The presence of these types of rocks along the main Zagros fault indicates local collisions. These collisions were caused by southwestward subduction under the Arabian plate in the southern branch of the Neo-Tethys. This event was the first stage of the Zagros collision, which was followed by collision of the Arabian and Iranian plates during the Eocene through Neogene.