Radiogenic Sr Isotopic Compositions Research Articles

Geochemical signatures of forearc serpentinites from oceanic to continental subduction

In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts

Controls on the stable Mo isotopic composition of inner-continental precipitation

The molybdenum (Mo) isotope budget of the surface environment has been well characterized in recent decades, facilitating accurate mass balance modeling of the ancient redox-driven Mo cycle. One yet unresolved component is the range of possible processes and sources that result in isotopically heavy river waters relative to continental sources. Following a recent hypothesis that isotopically heavy δ98Mo of precipitation may control the final δ98Mo of river water relative to its continental source bedrock, we investigated the δ98Mo composition of 19 snow samples from three locations in Central Europe: the Swiss Alps (“Alpine” samples) from the Hochalpine Forschungsstation Jungfraujoch (HFSJ) in both summer and winter, the Swiss Jura Mountains in winter and the French Vosges Mountains in winter. Stream waters from two snowmelt-fed streams were additionally collected from the Alpine site. Snow sample δ98Mo compositions were highly variable, ranging from −0.03 to +1.93 ‰, with no clear mixing trends indicating complex sources, source pathways, or post-depositional processing. Only the winter snow samples from the high-altitude HFSJ site had δ98Mo values consistently heavier than typical continental crust. The δ98Mo results were coupled with radiogenic Sr isotopic, major ion, and trace element compositions as tracers for three major sources of airborne ion inputs: sea salt, mineral dust, and anthropogenic aerosols. We found that the most likely source of Mo to precipitation in the lower elevation Vosges and Jura Mountains samples was isotopically light soil mineral dust, which reflected the underlying bedrock sources, with a likely additional anthropogenic component. The isotopically heavy winter snow samples at the elevated HFSJ site were attributed to a higher input of carbonate mineral dust from long-distance transport of carbonate-rich Saharan dust, which was overwritten in the summer by an influx of low elevation sources transported upwards during higher vertical thermal mixing. Finally, we concluded that precipitation has a negligible direct effect on the overall Mo content and isotopic composition of inner continental European streams and rivers relative to other sources, such as bedrock weathering. Future time-series studies may augment these conclusions to show possible heterogeneity of precipitation as well as the addition of sample sites closer to marine sources, where a larger flux of isotopically heavy marine aerosols to streams might be expected.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Genesis of tungsten mineralization in the Yangla copper-polymetallic deposit, northwest Yunnan, China: Evidence from in situ U-Pb dating, trace elements, Sr isotopes, fluid inclusions, and H-O isotopes of scheelite

Scheelite is one of the main ore minerals in tungsten deposits, and its geochemical features can be used to examine and constrain tungsten mineralization. The Yangla deposit is one of the most important Cu-polymetallic skarn-porphyry deposits in the “Sanjiang” polymetallic metallogenic domain, Southwest China, hosting 150 Mt of Cu ores with economic concentrations of Pb, Zn, and Sb. The newly discovered tungsten mineralization in the Yangla deposit remains poorly understood. Hence, in order to ascertain the timing of tungsten mineralization, investigate its genetic relationship with magmatism and ore genesis, and elucidate the source, nature, and evolution of the ore-forming fluids, we performed a combined study of geochronology, in-situ trace elements, fluid inclusions, H-O isotopes, and in-situ Sr isotopic compositions of scheelite. In-situ U-Pb dating of scheelite showed that the timing of tungsten mineralization was 30 Ma. This mineralization age is significantly later than the published Cu mineralization age (∼230 Ma), skarn (∼231 Ma), and granitoid emplacement age (∼230 Ma) in the Yangla ore district. In-situ trace element analyses revealed that scheelite is depleted in Mo, Mn, and Ta and slight enriched in Sr, Y, REEs, and Au. It is also characterized by HREE-depleted chondrite-normalized patterns with strongly positive Eu anomalies. Our data reveal that 3Ca2+ = (2REE)3++□Ca (site vacancy) is the main substitution mechanism that controls REEs in scheelite. The low concentrations of Mo, negative Ce anomalies, and positive Eu anomalies of scheelite indicate a reduced condition. Preliminary REEs features indicate that the genesis of scheelite may be related to vein-type W (Au) deposits. The homogenization temperatures for fluid inclusions present a strongly decreasing trend (328 to 173 ℃), indicate that cooling is the predominant mechanism for the formation of scheelite. The δD and δ18Ofluid values from fluid inclusions of scheelite vary broadly from −161.70 ‰ to −133.30 ‰ and from 6.32 ‰ to 9.80 ‰, respectively, indicating a mixture of magmatic and meteoric water, with possible involvement of magma degassing and fluid–rock interaction. The initial 87Sr/86Sr ratios of scheelites (0.71931– 0.72117) are significantly higher than those of the granodiorite (0.71149 – 0.71990) and basalt (0.70562 – 0.70995) in the Yangla ore district, indicating that the Oligocene magmatism and Devonian marble with radiogenic Sr isotopic compositions contributed to tungsten mineralization. The fluid-rock interaction between the ore-forming fluids derived from the concealed granitoids and the Devonian wall rocks may play an important role in tungsten mineralization and provide significant Ca for scheelite precipitation. Thus, the tungsten mineralization should be associated with Oligocene magmatism, ruling out the genetic relationship with Late Triassic magmatism and being significantly different from the skarn-porphyry Cu-Pb-Zn mineralization at Yangla.

Processes controlling rare earth element distribution in sedimentary apatite: Insights from spectroscopy, in situ geochemistry and O and Sr isotope composition

ABSTRACTIn phosphorites, the content and distribution of rare earth elements are linked to the environment of phosphogenesis. This paper focuses on the question of sources and processes controlling the rare earth element content of apatite from Belgian phosphorites formed during three major phosphogenic events in the Lower Palaeozoic, Late Cretaceous and Cenozoic. To constrain sources and processes, new data include petrological, mineralogical (including cathodoluminescence and Raman spectroscopy) and in situ trace element and Sr and O isotope analyses of apatite. Fluorapatite from Lower Palaeozoic P‐rich conglomerates has the greatest rare earth element enrichment. It is affected by metamorphism that led to deformation of apatite nodules and formation of garnet porphyroblast inclusions. The role of Fe‐oxyhydroxides in element scavenging is highlighted by some apatite nodules that maintain their primary middle rare earth element enrichment, while others are characterized by altered rare earth element patterns resulting from competition for these elements between co‐crystallizing minerals during deformation. A systematic shift towards lower δ18O and radiogenic Sr isotopic composition compared to contemporaneous seawater indicate interaction with 18O‐depleted meteoric fluids and a crustal component. By contrast, carbonate‐rich fluorapatite from the Late Cretaceous phosphatic chalk mostly keeps its primary trace element and isotopic signatures (close to seawater), although an external rare earth element addition is noted, as well as rare earth element redistribution induced by diagenetic alteration. Cenozoic carbonate fluorapatite nodules mostly present flat rare earth element patterns that are indicative of a detrital influence. Slight changes in rare earth element distribution are assigned to post‐depositional alteration, which also led to an increase in radiogenic Sr, with unchanged δ18O compared to seawater. The methodology followed here efficiently helps in deciphering the processes that modified the chemistry of apatite in the frame of major phosphogenic events.

Mantle Metasomatism and REE Enrichment in the Genetic Source of the Dalucao Carbonatite Complex (Sichuan, China): Insights from Elemental Geochemistry and In-Situ Sr Isotopes of Two Calcite Types

Abstract Carbonatites possess the highest rare earth element (REE) concentrations among all magmatic rocks, yet the mechanisms governing the enrichment of REEs in carbonatites remain enigmatic. Carbonatite-hosted calcites provide crucial mineralogical and geochemical insights into addressing this matter. In this study, we present a dataset comprising major and trace elemental geochemistry, as well as in-situ Sr isotopic data, for two types of calcites from the Dalucao carbonatite complex in Sichuan Province, China. Our primary objective is to shed light on the origin of fertile carbonatites. These calcites are believed to have originated at different stages of carbonatitic evolution. Type I calcites, characterized by a euhedral granular texture and a homogeneous surface, crystallized at the early stage of carbonatite magmatism. In contrast, type II calcites, closely associated with bastnäsite in a paragenesis, formed within a fluid setting of hydrothermal overprinting. Both calcite types exhibit highly radiogenic (87Sr/86Sr)i ratios, ranging from 0.7059 to 0.7060 for type I calcites and 0.7059 to 0.7068 for type II calcites. The high Sr concentrations (3646–7315 µg/g for type I calcites and 6566–13,427 µg/g for type II calcites) and uniform Pb isotopic compositions (206Pb/204Pb ratios of 18.23–18.27) in the Dalucao calcites refute the hypothesis of crustal assimilation as their genetic source. Instead, the radiogenic Sr isotopic compositions suggest that subducted marine sediments have contaminated the lithospheric mantle that generated the carbonatitic magma. Mineralogical evidence indicating the presence of abundant silicate minerals in the Dalucao carbonatites, combined with a compilation of published C-O isotopic data, suggests that these carbonatites may have resulted from low-degree partial melting (melting proportion <1%) of a metasomatized lithospheric mantle. Finally, we propose a comprehensive model for REE enrichment in the Dalucao carbonatite complex and explore the significance of tectonism in the genesis of fertile carbonatites.

Middle-Late Miocene to Pleistocene Post-Collisional Magmatism in the Arabia-Eurasia Collision Zone, an Example from Northwest Iran

Abstract Post-collisional volcanism contains important clues for understanding the processes that prevail in orogenic belts, including those in the mantle and the uplift and collapse of continents. Here we report new geochronological and geochemical data for a suite of post-collisional Miocene to Pleistocene volcanic rocks from northwest Iran. Four groups of volcanic rocks can be distinguished according to their geochemical and isotopic signatures, including: (1) Miocene depleted lavas with high Nd and Hf but low Pb and Sr isotopic ratios, (2) less depleted lavas with quite variable Pb isotopic composition, (3) lavas with non-radiogenic Nd and Hf isotopic values, but highly radiogenic Sr and Pb isotopic composition, and (4) Pleistocene adakitic rocks with depleted isotopic signatures. The isotopic data reveal that the Miocene rocks are derived from asthenospheric and highly heterogeneous sub-continental lithospheric mantle sources. Evidence suggests that the lithospheric mantle contains recycled upper continental material and is isotopically similar to the enriched mantle two (EMII) end-member. Analysis of Sr-Nd-Pb-Hf-O isotopes in both mineral and rock groundmass, in conjunction with energy-constrained assimilation and fractional crystallization (EC-AFC) numerical modeling, demonstrates that the incorporation of continental crust during magma fractionation via AFC had an insignificant impact on the isotopic composition of the Miocene lavas. Moreover, adakites are the youngest rocks and show a geochemical signature consistent with the partial melting of a young and mafic continental lower crust. Both seismological data and geochemical signatures on these Miocene to Pleistocene volcanic rocks indicate the initiation of asthenospheric upwelling and orogen uplift in the Arabia-Eurasia collision zone, which occurred after slab break-off, following the Neotethyan closure.

Petrogenesis of Syn-Collisional Adakitic Granitoids and Their Copper Mineralization Potential in the North Qilian Suture Zone

The petrogenesis of late Ordovician–early Silurian adakitic plutons in the North Qilian suture zone (NQSZ) and their copper mineralization potential remain poorly understood. Here we present a detailed study of the Heishishan (HSS) granodiorite–granite pluton, spatially associated with Cu–Au mineralization in the eastern section of the NQSZ. Zircon U–Pb dating confirms that the granodiorite–granite were formed at ca. 438–435 Ma, in association with a continental collision. Geochemically, the granitoids resemble low-Mg adakitic rocks featured by elevated Sr/Y and (La/Yb)N ratios with depleted MgO, Cr, and Ni concentrations, suggesting minimal mantle contribution. They are sodium rich with K2O/Na2O < 1, and have higher and more varied Sr/Y, but lower La/Yb than those from the continental lower crust. The εHf(t) values of zircon grains are positive and vary in a wide range of +2.0–12.7, indicating a heterogeneous source rather than a single arc basaltic source. They show moderately radiogenic Sr and Nd isotope compositions with initial 87Sr/86Sr ratios of 0.705101–0.706312 and εNd(t) values of +0.5–1.0, most likely a mixed source of the oceanic basaltic crust plus ca. 15–20% overlying sediments. The magmatic oxygen fugacity was relatively low as indicated by zircon Ce(IV)/Ce(III) ratios of 32–156, which is unfavorable for a large copper mineralization.

Lithium enrichment in the Salar de Diablillos, Argentina, and the influence of Cenozoic volcanism in a basin dominated by Paleozoic basement

Salars with lithium-rich brines are a characteristic feature of the Central Andes, but knowledge about the main sources of lithium and the mobilization processes of lithium in the salar deposits is still incomplete. This work focuses especially on the Salar de Diablillos (southern Puna) as part of a larger area that includes the neighboring Salar Centenario and Salar de Ratones. Building on the ability of Li as a tracer of silicate weathering, we investigate the Li content and isotope composition of samples from the depocenter and catchment of the Diablillos basin (3-D) and conduct a surface reconnaissance in the Centenario and Ratones depocenters to identify the key metallogenic processes. Radiogenic Sr and Nd isotope compositions are also provided to discriminate the main local Li sources. The isotope data in all three depocenters show that most of the Li in the brines and evaporite deposits are derived from Cenozoic volcanic rocks, despite the dominance of the Paleozoic basement in the catchment. In the Centenario and Ratones depocenters, near-surface chemical weathering appears to be the dominant Li mobilization process. In contrast, hydrothermal mobilization of Li also plays a role in the Salar de Diablillos, possibly related to the presence of a fractured basement with pressure zones and artesian conditions in the aquifer at depth. These fluids also show a larger element contribution from the basement.

Subducted carbonates not required: Deep mantle melting explains stable Ca isotopes in kimberlite magmas

Ca isotope geochemistry has great potential for improving our understanding of magmatic systems and for tracing the deep Earth carbon cycle. There are still many open questions, however, regarding the proper application of this relatively novel proxy to the study of mantle-derived magmas, including (i) the possible effects of pressure on mineral-melt Ca isotope fractionation factors, and (ii) the potential for Ca isotopes to be used as tracers of recycled marine carbonates in mantle-derived magmas. Kimberlites are mantle-derived melts that are highly enriched in CO2 and are the deepest-sourced magmas (>200 km depth) known to erupt at Earth’s surface, providing an excellent opportunity to explore these questions. We present Ca isotope data combined with detailed petrographic observations, bulk-carbonate C-O isotope data, and bulk-rock major element analyses, for a suite of 23 well-characterized kimberlite samples from their type-locality (Kimberley, South Africa). These kimberlites have abundant previous evidence for recycled surface materials in their mantle source, including low S isotope and moderately radiogenic Sr isotope compositions, yet display only limited variations in their Ca isotope compositions (δ44CaBSE of −0.08‰ to −0.27‰), with an average of −0.17 ± 0.02‰ (2SE, n = 21). This composition is indistinguishable from average carbonatites [−0.19 ± 0.03‰ (2SE, n = 106)] and OIB from recent studies [−0.16 ± 0.01‰ (2SE, n = 41)], and slightly lower than average MORB [−0.11 ± 0.02‰ (2SE, n = 31)]. Although our samples display a wide range of emplacement styles, alteration conditions, extents of magmatic differentiation, and degrees of mantle-cargo entrainment (i.e., xenocryst accumulation), we find no correlations between Ca isotopes and any of these factors. Instead, we find that low-degree partial melting of the likely kimberlite source lithology (i.e., carbon-bearing garnet lherzolite) yields modelled melt δ44CaBSE values ranging between −0.12‰ and −0.16‰ (at 1400–1500 °C), in agreement with the measured Ca isotope compositions of the Kimberley kimberlites. This observation, and the lack of heavy carbon isotope signatures in the examined samples, indicates that kimberlites do not require subducted carbonates in their mantle sources, despite their very high CO2 contents. Although several recent studies have suggested that equilibrium mineral-melt Ca isotope fractionation factors (e.g., 1000lnαgrt-melt) could be significantly different at higher pressures (i.e., due to pressure-induced changes in CaO bond lengths and coordinations), our models successfully reproduce the kimberlite data using pressure-independent predictions for mineral-melt fractionations. It remains possible, however, that differences in isotopic fractionation due to the peculiar composition of kimberlite melts (e.g., high CO2, low SiO2) are effectively cancelled out by competing pressure effects, and future work independently targeting these factors will be especially important for our understanding of Ca isotope fractionation in mantle-derived melts and the Ca isotope systematics of Earth’s mantle.

Calcium isotopic geochemistry of geothermal systems in the tectonically active southeastern Tibetan Plateau

The global Calcium (Ca) cycle is closely coupled to the carbon cycle, and Ca isotopes have potential in tracing it. Even though groundwater is one of the main reservoirs of Ca at the Earth's surface, few data are available for groundwater, and the behavior of Ca and its isotopes in geothermal systems remains unknown. Here we analysed the stable Ca and radiogenic Sr isotope compositions of thermal waters distributed along the Jinsha and Yalong river valleys in the southeastern Tibetan Plateau. The Ca isotopic composition of the thermal water ranges from 0.45 to 2.16 ‰ (δ44/40Ca values relative to SRM 915a). The thermal waters collected from carbonate aquifers have higher δ44/40Ca values than bedrocks, which was attributed to secondary carbonate precipitation accompanied by CO2 degassing. In contrast, δ44/40Ca values in thermal waters collected from clastic and igneous rocks are similar to bedrock. Despite some thermal waters undergoing secondary silicates formation and CaNa ion exchange, such processes maybe not play a significant role in governing the Ca isotopic composition of these thermal waters. This suggests that Ca isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing (e.g. travertine) in geothermal systems located in tectonically active areas.

Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge

The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.

Mixed carbonate-siliceous hydrothermal chimneys ahead of the Asal propagating rift (SE Afar Rift, Republic of Djibouti)

Numerous active and fossil hydrothermal carbonate chimneys have been reported at Lake Asal and Lake Abhe, in the southeastern part of the Afar Rift (Republic of Djibouti): a tectonically and volcanically active region for the last 30 million years. In this study, we report on a detailed mineralogical and geochemical investigation of unusual hydrothermal mixed carbonate-siliceous chimneys recently discovered in the Allols area (Harralol, Haralé Awda and Sakallol), northwest of the Asal Rift. These inactive chimneys, up to 20 m high, consist of tall spires partly stained by Fe-oxides. They are located along major faults and were formed sub-aqueously when the area was occupied by a lake. Hydrothermal carbonate structures were also found in the same area. The oxygen (δ18O) and carbon (δ13C) isotopic compositions of the chimneys range from −4.5 to 25.3‰ and from −9.44 to 8.92‰, respectively, indicating mixing between magmatic and surficial waters. Their radiogenic Sr isotopic compositions (87Sr/86Sr = 0.7049–0.7056) show values typical of altered basaltic rocks. Chimneys also exhibit negative Ce anomaly suggestive of a genetic link with basalt alteration. Overall, this study suggests that lake water chemistry in the Allols area experienced major changes in the past, reflecting various contributions of mixing fluids (lake water and hydrothermal fluids), leading alternatively to deposition of carbonate chimneys and precipitation of (and/or replacement with) silica.

Tracking crustal assimilation processes in kimberlites from the Alto Paranaíba Igneous Province, Brazil: Petrographic and geochemical controls and the role of perovskites

Anomalous kimberlite varieties from the Cretaceous Alto Paranaíba Igneous Province (APIP, Brazil) do not follow simple classification charts, showing abundant evidence of interaction with crustal xenoliths. In this paper, we track the potential effects of crustal assimilation processes on textural, mineralogical, geochemical and isotopic signatures. Formation of reaction diopside and poikilitic phlogopite, as well as Fe/Nb variations in perovskites, records progressive increases of not only alumina and K2O contents, but also of silica activity and fO2, compatible with the assimilation of and/or interaction with the Si-rich crust. High bulk rock “contamination indexes”, decreasing Ce/Pb and Gd/Lu and slightly increasing Rb/Sr ratios are observed in the APIP kimberlite varieties and/or facies showing progressively more remarkable petrographic evidence of crustal contamination. Significant variability of 87Sr/86Sri ratios detected in whole rock, bulk perovskite concentrates (TIMS, MC-ICP-MS) and in-situ perovskite (multi-grain approach, LA-MC-ICP-MS) of the Indaiá-I/Limeira-I intrusions (0.70507–0.70539) and their satellites Indaiá-II/Limeira-II/III (0.70508–0.70592), of the different Pântano facies (0.70513–0.70551), and of the Catalão 1b (0.70495–0.706546) indicates a tendency to more radiogenic Sr isotope compositions during kimberlite open-system crystallization. Geochemical modeling indicates the important crustal contribution to the genesis of these anomalous occurrences, with an extensive crust input for the Indaiá-II and Limeira-II/III occurrences, consistent with their petrographic evidence. The potassic to ultrapotassic geochemical affinity and the similarities between contaminated APIP kimberlite and bulk kamafugite compositions can be related to the incorporation/digestion of crustal components to the parental kimberlite magma.

Spatiotemporal evolution of late Neoproterozoic marine environments on the Yangtze Platform (South China): inking continental weathering and marine C-P cycles

Abstract The geodynamic reorganization of major cratonic blocks during the assembly of Gondwana in the late Neoproterozoic caused a (bio)geochemical evolution of marine habitats that witnessed fluctuating seawater oxygenation and nutrient fluxes. Previous studies of carbonates deposited on the Yangtze Platform (South China) have shown their use as reliable archives of paleo-redox and bio-productivity changes intrinsically linked to continental weathering fluxes and water mass cycling. Despite its complex submarine morphology, only a few carbonate-bearing sections comprising deeper depositional environments of the Yangtze Platform have been evaluated. Here we report temporal, lateral, and vertical variations of stable C and radiogenic Sr, Nd, and Os isotopic compositions together with trace element and rare earth and yttrium (REY) systematics in carbonates from the Doushantuo Formation representing different paleo-marine environments of the Yangtze Platform, including I) inner shelf, II) restricted intra-shelf basin, III) slope and IV) deep-water basin sections to trace the extent and evolution of paleo water masses. Variations in shale-normalized (subscript SN) Ce anomalies argue for variably oxidizing atmosphere/ocean conditions. The combination of REY SN systematics with carbonate associated P enrichments defines three distinct local water masses from which the carbonates precipitated: i) fully oxidized shallow-water, ii) nutrient-poor intra-shelf basin waters, and iii) organic carbon-rich slope and deep-waters with temporal evolution to higher dissolved p O 2 . Negative carbon isotope excursions in the carbonates such as during the putative Shuram equivalent Doushantuo Negative Carbon Excursion (DOUNCE) correlate with less negative Ce SN anomalies and ambient seawater shifts to more radiogenic initial 87 Sr/ 86 Sr and 187 Os/ 188 Os, but unradiogenic 143 Nd/ 144 Nd compositions, arguing for coupling of redox and weathering trends. Our comprehensive geochemical study highlights the vertical and temporal variations of physicochemical water mass properties recorded in late Neoproterozoic Yangtze Platform carbonates until the emergence of the first metazoans. Highlights • Intensive geochemical study spanning four paleo-depths on the Yangtze Platform • First carbonate Osmium isotopic compositions from the Late Neoproterozoic • Os, Nd and Sr isotope variations in concert with influx of terrigenous solutions towards the end of the Doushantuo Fm • REY systematics and P enrichments reveal significantly different water masses along the passive margin of the Yangtze craton • Changes in C-P cycles during Shuram anomaly linked to intensified terrigenous solution flux

A Showcase of Igneous Processes in the Urumieh-Dokhtar Magmatic Arc: the Miocene-Quaternary Collisional Magmatism of the Bijar-Qorveh Area, Northwest Iran

Abstract The Cenozoic Arabia–Iran continental collision was associated with emplacement of a large variety of magmatic rock types. This aspect is particularly evident in the Bijar-Qorveh area of NW Iran, where Miocene andesitic to rhyolitic rocks and Quaternary basic alkaline rocks crop out. The Miocene intermediate to acid products show radiogenic Sr and Pb isotopic compositions (87Sr/86Sri 0.70531–0.71109, 206Pb/204Pb 18.71–19.01, 207Pb/204Pb 15.66–15.73, 208Pb/204Pb 38.76–39.14), coupled with unradiogenic Nd isotopic ratios (143Nd/144Ndi 0.51223–0.51265). These characteristics, together with primitive mantle-normalised multielemental patterns resembling ‘subduction-related’ geochemical fingerprints, are considered ultimately derived from the Iranian plate mantle wedge, metasomatised during previous NE-directed Neothetyan Ocean subduction. The alkali-rich andesitic and dacitic rocks evidence both closed- and open-system differentiation, as typically observed for collisional settings in general. Both rock types display a high Sr/Y (37–100) and La/Yb (29–74) ‘adakitic’ signature that it is interpreted here with plagioclase (± amphibole) accumulation or melting of local mafic crustal rocks. Open-system processes involve recycling of crustal cumulates for pyroxene-rich andesite and biotite-rich dacite varieties and low-degree partial melting of the local crust for rhyolites. A radical change occurred during the Quaternary, when SiO2-understaturated to SiO2-saturated poorly evolved rocks (basanites, tephrites, alkaline and subalkaline basalts) were emplaced. The complete change of mantle sources suggests a phase of local extensional tectonics related with WNW-ESE right-transcurrent faults. The major oxide, as well as incompatible trace element and Sr-Nd-Pb isotopic fingerprint of these younger rocks is more akin to that of intraplate magmas, but still bearing some evidences for a variable contribution from a ‘subduction-modified’ mantle source. The NW-trend of increasing involvement of this subduction component is indicative of the strong tectonic control on magmatism. Additional lithotypes indicate the presence of open-system differentiation and remelting processes in the youngest phase of magmatic activity.

Complex magmatic processes recorded by clinopyroxene phenocrysts in a magmatic plumbing system: A case study of mafic volcanic rocks from the Laiyang Basin, southeastern North China Craton

Understanding the evolution of magmatic plumbing systems requires an understanding of the mechanisms of magma ascent from the magma source to the surface. We undertook textural and in situ geochemical studies of clinopyroxene phenocrysts in Early Cretaceous mafic volcanic rocks from the Laiyang Basin, China. Four distinct clinopyroxene populations were recognized based on their textures and zoning patterns in the mafic volcanic rocks (types 1, 2, 3, and 4). Type 1 is reversely zoned and contains rounded, irregularly resorbed, low-Mg cores (Mg# = 60–75) surrounded by clear, high-Mg mantles (Mg# = 75–90). Type 2 phenocrysts are typically normally zoned and characterized by relatively high-Mg cores (Mg# > 85) and thin lower Mg# rims (Mg# = 70–85). Type 3 comprises strong oscillatory zoned crystals (Mg# = 72–85) that are generally normally zoned and, in some cases, exhibit substantial sector zoning. Type 4 crystals are euhedral and unzoned phenocrysts. The textural and compositional features of the clinopyroxenes record complex magmatic processes including fractional crystallization, recycling of early formed crystals, magma convection, and magma recharge and subsequent mixing. Clinopyroxene–melt equilibrium pairs in the volcanic rocks yielded a relatively wide range of temperatures (984–1264°C) and pressures (0.1–10.1 kbar). The P–T array defines two main magma storage zones: (1) a deep, high-T, magma reservoir (~22–28 km), and (2) a shallow, low-T, magma reservoir (~8–19 km). At pressures of >3 kbar, the amount of water in melts in equilibrium with the clinopyroxene is in the range 1.5–3.0 wt.%, but at pressures of <3 kbar, the melt water contents decrease to 0.1 wt.%, indicative of extensive magma degassing. The clinopyroxene phenocrysts have a wide range of 87Sr/86Sr ratios from 0.70683 to 0.71366. The broad range of 87Sr/86Sr ratios (0.70796–0.71171) for the most primitive clinopyroxene (Mg# > 89) might have been inherited from a heterogeneous mantle source. However, some type 1 crystals with more radiogenic Sr isotopic compositions may record assimilation of crustal material during evolution of the mantle-derived magma. The textural and compositional features of the studied clinopyroxene phenocrysts suggest that Laiyang volcano is fed by a vertically extensive magmatic plumbing system that extends from shallow to deep levels, in which geochemically distinct magmas crystallized and mixed under variable physicochemical conditions.

Ore genesis and fluid evolution of the Sandaozhuang supergiant W-Mo skarn deposit, southern margin of the North China Craton: Insights from scheelite, garnet and clinopyroxene geochemistry

The Sandaozhuang deposit (0.55 Mt WO3 at 0.112% and 0.75 Mt Mo at 0.109%), located in the southern margin of the North China Craton, is one of the largest W-Mo deposits in the world. This study carried out in-situ elemental analyses on skarn minerals, as well as elemental and Sr-Nd isotopic analyses on scheelites from the deposit, to constrain the metal source and fluid evolution of the deposit. Three generations of scheelite (Sch A to C) were identified based on petrographic observations, including prograde skarn and diopside-plagioclase hornfels-hosted Sch A, retrograde skarn-hosted Sch B, and quartz vein-hosted Sch C. Varieties of REE patterns in the scheelite are mainly controlled by different substitution mechanisms and crystallization of REE-enriched skarn minerals. The Mo contents decrease from early generation Sch A (1.57–24.45%) to Sch B (0.19–1.52%) and finally to Sch C (0.20–0.97%), and Eu anomalies of scheelite change from negative to positive and then back to negative values. The Sandaozhuang deposit is characterized by Mg-rich clinopyroxene (Di40-81Hd17-47), and grossularite–andradite (And22–89Gro6-77), similar to well-studied oxidized W skarn deposits worldwide, but the andradite contents of one garnet grain in the late prograde stage, decrease from the core (84.09%) to rim (75.12%). Composition changes of scheelite and garnet indicate that the ore-forming fluids changed from oxidized to reduced state in the skarn stage then to oxidized state in the quartz-sulfide stage, and the variations in redox state favoured the muti-stage Mo mineralization. The relatively uniform Y/Ho ratios of the Nannihu granite porphyry and scheelite imply that the ore-forming fluids are mainly derived from the porphyry. Scheelites from Sandaozhuang are characterized by low εNd(t) values (−14.7 to − 11.8), which are consistent with those for the Nannihu granite porphyry. Such a coincidence indicates that ore-forming fluids were mainly derived from the ore-hosting porphyry. However, the initial 87Sr/86Sr ratios of scheelites (0.7158–0.7295) are significantly higher than those of the Nannihu granite porphyry and other granites in the Luanchuan ore district (0.7047–0.7098), indicating that the Neoproterozoic Luanchuan Group with radiogenic Sr isotopic compositions contributed to tungsten mineralization, and the intense fluid-rock interaction between the ore-forming fluids derived from the granite porphyry and the Luanchuan Group country rocks may play an important role in tungsten mineralization.

Disentangling diagenetic and biogenic trace elements and Sr radiogenic isotopes in fossil dental enamel using laser ablation analysis

The reconstruction of life histories traits such as the ontogenic evolution of diet or sequences of mobility can be achieved for fossil mammals thanks to laser ablation analysis of trace elements concentration and radiogenic Sr isotope composition in dental enamel. However, a major limitation for the use of laser ablation to study fossil tooth enamel is that the analysis must be carried out on bulk sample hence prohibiting any ad hoc leaching step to remove diagenetic compounds. Biogenic and diagenetic trace elements along with the 87Sr/86Sr ratio must be monitored and post hoc correlation tests processed to isolate areas free of diagenesis. The present study combines a unique combination of biogenic (Sr and Ba), diagenetic (Mn and U) trace elements, major elements (Mg and Ca) and 87Sr/86Sr ratio time series obtained by means of laser ablation ICPMS and MC-ICPMS on a large number (n = 94) of fossil tooth enamel samples of humans and animals from a set of neighboring Neolithic sites in France. Bone fragments sampled from the same specimens are also analyzed for comparison. Trends between diagenetic and biogenic trace and major elements are first identified using inclusive data analysis. The trends are then validated at the scale of the laser ablation raster, and a protocol of data cleaning is developed to identify areas free of diagenesis. Non-significantly altered Sr/Ca, Ba/Ca and 87Sr/86Sr average values are thus gathered for each cleaned tooth enamel samples and are discussed, along with previously measured bone collagen δ13C and δ15N values, in terms of diet and mobility in the context of the Neolithic revolution.

Coexisting Early Cretaceous arc-type and OIB-type mafic magmatic rocks in the eastern Jiangnan Orogen, South China Block: Implications for paleo-Pacific plate subduction

Mafic magmatic rocks can provide important information about the nature of mantle source, material recycling, and mechanism of crust–mantle interaction. However, owing to the scarcity of such rocks, the nature and evolution of the Late Mesozoic mantle source beneath southeastern China have not been well constrained. Here, we present data on zircon UPb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopes for two types of simultaneous mafic intrusive rocks from the eastern Jiangnan Orogen (EJNO), with the aim of providing insights into their petrogenesis and the relationship of extensive Late Mesozoic magmatism in southeastern China and subduction of the paleo-Pacific plate. The Qukou diabases (~ 128 Ma) show a subalkaline affinity and have a mean Mg# value of 53.4. They exhibit arc-like trace-element distribution patterns, namely, enrichment in light rare earth elements (LREEs), Ba, Th, and U, and depletion in Nb, Ta, P, and Ti, as well as moderately radiogenic Sr and chondritic Nd isotopic compositions, with 87Sr/86Sr(i) = 0.7057–0.7063 and εNd(t) = −1.01 to 1.13. In contrast, the Yangboping gabbros (~ 125 Ma) belong to alkaline basalt and have a mean Mg# value of 56.8. They present OIB-like trace-element characteristics, show enrichment in LREEs, Sr, and Ba, and no depletion in Nb, Ta, Zr, Hf, and Ti, and possess highly radiogenic Sr and weakly nonradiogenic Nd isotopes of 87Sr/86Sr(i) = 0.7096–0.7114 and εNd(t) = −0.97 to −0.11. These geochemical features suggest that the Qukou diabases were generated by partial melting of a pyroxenitic source region, and this pyroxenitic mantle source was formed via the metasomatism by HFSE-depleted hydrous melts of subducted slab-derived terrigenous sediments, whereas the Yangboping gabbros were generated from a metasomatite that formed by metasomatic reaction between depleted MORB mantle and subducted slab-derived HFSE-enriched hydrous melts, in which the ratios of altered oceanic crust-derived melts to sediment-derived melts were 90:10–60:40. The differences in trace-element and isotopic compositions of these two types of mafic magmatic rocks suggest that their mantle sources were metasomatized by hydrous melts from different depths: the HFSE-depleted hydrous melts were from the shallow depths (<80 km), whereas the HFSE-enriched melts were from the greater depths (>200 km), indicating that the subducted oceanic slab would have founded/broken off to provide enough heat to melt the subducted oceanic crust. Consequently, we propose that the flat-slab subduction and slab break-off model of the paleo-Pacific plate was the most likely geodynamic mechanism for Late Mesozoic magmatism in the South China Block.