Mantle Values Research Articles

Mercury abundance and isotopic composition in granitic rocks: Implications for Hg cycling in the upper continental crust

Mercury (Hg) is a highly volatile metal exhibiting both isotope mass-dependent (MDF, defined as δ202Hg) and mass-independent (MIF, expressed as Δ199Hg) fractionation. Assessing the Hg isotopic composition of the upper continental crust is crucial for the use of this new geochemical tool for petrogenetic tracing, particularly for understanding crust-mantle interaction. Here, we report Hg isotopic compositions of granitic rocks with distinct source affinity (I-, A-, and S-type) and of metasedimentary enclaves from the South China Craton. The results of more than 100 rock samples show large δ202Hg variations of −2.03 to 0.47 ‰ and small Δ199Hg variations of −0.16 to 0.09 ‰. The S-type granites studied here exhibit higher δ202Hg values (−0.58 to 0.00 ‰; n = 57) than their sedimentary protoliths (metasedimentary enclaves; −2.03 to −0.91 ‰; n = 7), suggesting significant Hg-MDF during magmatic processes. The Δ199Hg values of S-type granites (−0.08 to 0.09 ‰) are indistinguishable from their sedimentary protoliths (−0.16 to 0.02 ‰), suggestive of the absence of Hg-MIF during crustal anatexis and subsequent magmatic differentiation. The I- and A-type granites studied have smaller Δ199Hg variations of −0.11 to 0.04 ‰ (n = 28) and −0.07 to 0.04 ‰ (n = 15), respectively, but are within the range of the S-type granites. The compositional range of all granite variants may be explained by the variable intensity of large-scale MASH (melting-assimilation-storage-homogenization) processes. Our new data combined with previously published data of igneous, sedimentary, and metamorphic rocks allow us to estimate that the upper continental crust (UCC) has a Hg abundance of 13.5 ppb and a weighted average δ202Hg of −1.15 ± 1.01 ‰ (2SD), both of which are higher than the corresponding values of the primitive mantle. However, the weighted average Δ199Hg value of the UCC (0.03 ± 0.15 ‰; 2SD) is nondistinguished from the primitive mantle, suggesting no obvious Hg-MIF during the formation and differentiation of the continental crust.

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Mo isotopes record recycling of anoxic sediment in a Paleo-oceanic subduction zone

Mafic arc igneous rocks with oceanic crust-derived fluids in their mantle sources generally show heavy Mo isotope compositions. However, these rocks may exhibit variable Mo isotope compositions due to involvement of seafloor sediments under different redox conditions. Thus, it is intriguing how Mo isotope recycles with sediment subduction and how Mo isotope fractionation occurs during this process. Here we report Mo isotope data for Paleozoic mafic arc igneous rocks from the Qinling-Tongbai orogenic belt in Central China. The Qinling mafic rocks have δ98Mo values of −0.03 to 0.40‰, and the Tongbai mafic rocks have δ98Mo values of −0.26 to 0.17‰. These values are considerably higher than the normal mantle values of −0.20 ± 0.01‰. In combination with low Mo/Ce ratios, enriched SrNd isotope compositions, and high melt-mobile incompatible trace element contents for these rocks, the high δ98Mo values suggest that anoxic sediment-derived melts with heavy Mo isotopes were incorporated by oceanic subduction into the mantle sources. The high δ98Mo values of the mafic rocks indicate the limited Mo isotope fractionation during subduction of the anoxic sediment. This inference is further verified by modeling of Mo isotope fractionation under reducing conditions in the subduction zone. In this regard, Mo isotopes can be used to identify the recycling of sediments in different types and thus provide geochemical constraints on the redox conditions of oceanic subduction zones.

Geochemistry (TE, REE, Oxygen) of Zircon from Leucogranites of the Belokurikhinsky Massif, Gorny Altai, as Indicator of Formation Conditions

The present contribution deals with the isotopic-geochemical study (by Electron Probe Microanalysis (EPMA) and Method of Secondary Ion Mass Spectroscopy (SIMS) of zircons from Belokurikhinsky massif leucogranites, which resulted in the determination of two morphological varieties of zircons (zonal and porous). Zonal zircons exhibit low content of trace elements, a “magmatic” type of REE distribution spectra, and geochemical indices consistent with those of magmatic zircons. Anomalously high REE (up to 38 800 ppm), Y (up to 50 700 ppm) and U (up to 24 700 ppm) contents, as well as elevated Hf, P, F, Cl, and water contents (up to 3.5 wt %) were found in porous zircons. They display gently flattened REE distribution spectra with no Ce-anomaly and the persistence of negative Eu-anomaly, as well as geochemical parameters that differ from typical zircons of magmatic origin. The obtained geochemical characteristics of porous zircons reflect the process of enrichment of the magmatic melt with rare and rare earth elements, and indicate a high fluid saturation of the melt. The δ18О determined value for porous zircons is in the range of 9.22 to 12.54‰, which significantly exceeds a mantle value and seems to be caused by influence of external fluids interacting with sedimentary rocks surrounding leucogranites. The detailed isotopic-geochemical study of zircon samples suggests that their crystallization from leucogranites of a third (final) phase in the evolution of the Belokurikhinsky massif took place from fluid-saturated melt with elevated incompatible element concentrations.

The Roles of Various Types of Crustal Contamination in the Genesis of the Jinchuan Magmatic Ni-Cu-PGE Deposit: New Mineralogical and C-S-Sr-Nd Isotope Constraints

Abstract Addition of crustal sulfur to the Jinchuan magma or oxidation of the magma associated with carbonate assimilation has been considered to be the main cause of sulfide saturation in the magma by two opposing groups of researchers. To address this controversy, we have carried out an integrated mineralogical and C-S-Sr-Nd isotope study of the Jinchuan magmatic Ni-Cu-platinum group element (PGE) sulfide ore deposit. Pure marble, olivine marble, serpentine marble, and hybrid rocks occur in the contact zone. The δ13Ccarb values of the Jinchuan sulfide-bearing ultramafic rocks containing calcite xenocrysts are from –4.7 to –2.9‰, which are similar to or slightly lower than those of the marbles and associated hybrid rocks (–3.9 to 0.8‰) but significantly higher than those of the calcite-barren intrusive rocks (–9.3 to –8.0‰). This, together with the calcite xenocrysts and calcium-silicate minerals in hybrid rocks and some intrusive rocks, indicates that carbonate assimilation took place during magma-carbonate interaction. Only less than several hundred ppm of the CO2 gas produced in the reaction zone could be added to the Jinchuan magma. Such a trace amount of additional CO2 was rapidly swamped by the much greater mass of FeO in the magma, resulting in little change in the FeO/Fe2O3 of the magma, and thereby negligible change of magma redox state. This is evident from similar calculated fO2 values for the calcite-bearing (~QFM+0.7) and calcite-barren (~QFM+0.6) intrusive rocks. The new results rule out the possibility that sulfide saturation in the Jinchuan magma resulted from in situ carbonate assimilation. The Sr-Nd isotope data from this study and previous studies are consistent with up to 20% bulk contamination with siliceous crustal materials at depth, followed by minor amounts of in situ carbonate assimilation by the Jinchuan magma. Our new sulfur isotope data expand the range of δ34S for the Jinchuan deposit significantly. The new range is from −7.6 to 3.0‰, with an average of −1.8‰, which is generally lower than the mantle value (0 ± 2‰). The new result supports the premise that crustal sulfur was involved in the genesis of the Jinchuan deposit. Very high, crustal-like S/Se ratios (as high as 8080) for some of the samples from the deposit provide additional support for the interpretation. The country rocks in the vicinity of the Jinchuan deposit analyzed to date have δ34S values varying from −4.0 to 11.3‰, with an average of 2.9‰, which is higher than both the mantle value and the average value of the Jinchuan deposit, suggesting that the Jinchuan magma acquired some crustal sulfur at depth, likely concurrent with the siliceous assimilation. Numerical modeling of δ34S-S/Se of sulfide ores and country rocks further illustrates that the observed variations of δ34S and S/Se ratios are related to the assimilation of S-rich rocks located at depth, followed by progressive dilution of the contaminated δ34S-S/Se signature. Based on the new results, we conclude that contamination with siliceous crustal materials as well as addition of crustal sulfur at depth played a critical role in triggering sulfide saturation in the Jinchuan magma.

Different effects of serpentinite-derived and slab-derived fluids on arc magmatism in the Chinese Eastern Tianshan: evidence from Mg–B isotopic systematics

Fluids are important media for elemental cycling in subduction zones, but the compositions of such fluids remain obscure, especially those derived from dehydration of serpentinite. To better understand the role of subduction-zone fluids in arc magmatism of the Chinese Eastern Tianshan (CET), we present Mg and B isotopic data for the arc diorites (e.g. Kalatage and Nanshankou). Our results show that the Kalatage diorites have higher δ 26 Mg (−0.23 to −0.13‰) and δ 11 B (−0.04 to +1.08‰) values than the corresponding mantle values, whereas the Nanshankou diorites display relatively lower δ 26 Mg (−0.26 to −0.19‰) and δ 11 B (−10.7 to −2.97‰) values. Because high-temperature magmatic and post-magmatic processes have limited effects on Mg and B isotope fractionation of our samples, their distinct Mg–B isotopic signatures inherited from the mantle sources should be caused by subduction-related metasomatism. Because neither altered oceanic crust (AOC) directly melting nor the incorporation of subducted sediments could induce significant Mg–B isotope fractionation in these rocks, we infer that their variable δ 26 Mg and δ 11 B values were probably associated with subduction fluids that were derived from different sources. Specifically, dehydrated fluids from serpentinites possess high δ 26 Mg and δ 11 B values, which resulted in the heavy Mg–B isotopic compositions of the Kalatage diorites, whereas mantle metasomatized by fluids from AOC and subducted sediments were responsible for the Nanshankou rocks with relatively low δ 26 Mg and δ 11 B values. Taking into account the different effects of serpentinite-derived and slab-derived fluids in the formation of arc magmatism, subduction-zone fluids can be probably an important factor in changing the Mg–B isotope composition of mantle sources. Supplementary material : Supplementary data tables and parameters used in models are available at https://doi.org/10.6084/m9.figshare.c.6702686

Modern ocean island basalt–like 182W signature in Paleoarchean mafic rocks: Implications for the generation, preservation, and destruction of early mantle heterogeneities

Komatiites and picrites generated by high degrees of mantle partial melting serve as potential probes of Earth’s deep mantle. Tungsten (W) isotopes in these rocks offer a rare chance to better understand early differentiation, late accretion, core-mantle interaction, and subsequent evolution of Earth’s mantle. We present new W isotope data for Archean komatiites and basalts from the Barberton (South Africa) and Suomussalmi (Finland) Greenstone Belts and Permian picrites from the Emeishan large igneous province (China). The Paleoarchean samples from the Barberton Greenstone Belt have modern ocean island basalt (OIB)−like μ182W values ranging from −20.4 to +5.6, whereas the Mesoarchean komatiites from the Suomussalmi Greenstone Belt show μ182W values of −2.2 to +11.3. The Permian Emeishan picrites give μ182W values of −7.1 to +3.1. Our data, combined with the published global data set, show that W isotope heterogeneity in the mantle has existed throughout Earth’s history, with positive μ182W values transitioning to near-zero in the upper mantle by the end of the Archean. The negative μ182W values of Paleoarchean samples in the Barberton Greenstone Belt and modern OIBs likely result from either early differentiation or core-mantle interaction. The incorporation of a plume-delivered negative μ182W component and enhanced mantle mixing is a viable mechanism to explain the transition of μ182W values in the upper mantle from positive to near-zero, while recycling of crustal materials into the mantle would result in a shift of negative μ182W values of the lower mantle closer to zero since the onset of plate tectonics. The latter process could possibly explain the slightly negative to near-zero μ182W values of the Emeishan picrites and some kimberlites. The well-resolved negative μ182W anomalies observed in this study provide important insights into the generation, preservation, and obliteration of W isotope heterogeneities in the lower mantle.

Li isotope ratios of spring fluids as an effective tracer of slab-derived subducted sources across the Costa Rica forearc

Abstract Spring waters from across the Costa Rica margin were analyzed for their Li and He isotope compositions to determine the utility of Li isotopes as a tracer of volatile sources in subduction zones. Li isotope ratios systematically decrease with increasing depth to the subducting slab: averaging +15.0‰ ± 9.2‰ in the outer forearc (<40 km to the slab), +9.3‰ ± 4.3‰ in the forearc (40–80 km to the slab), and +5.8‰ ± 2.8‰ in the arc (>80 km to the slab). In contrast, air-corrected 3He/4He values (reported relative to the ratio in air, RA) range from 0.4 to 7.5 RA and increase from predominantly crustal values near the trench to mantle values in the arc. Together, these data support progressive devolatilization of the subducting plate with slab-derived Li components sourced from shallowly expelled pore fluids in the outer forearc, sedimentary and/or altered oceanic crust contributing to the forearc, and limited slab input beneath the arc.

Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex, Greenland

The Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, west Greenland, has been considered to contain one of the oldest chromitites on Earth based on ∼ ca. 4.1 Ga Hadean whole rock Pt–Os model ages and ca. 3.81 Ga zircon U-Pb age of the surrounding orthogneiss. This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit. In–situ U–Pb–Hf–O isotope measurements were made on the zircons. Zircons from both the chromitite and the intrusive granitoids show concordant U–Pb ages of ca. 2.97–2.95 Ga. In contrast, Hf and O isotopic analyses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids. Zircons from granitoids yielded Th/U ratios higher than 0.2, initial Hf isotope ratios of 0.2805 – 0.2807 (i.e., initial εHf value of − 11 to − 5), and δ18O values of mostly 6.0‰–7.0‰, which are typical for felsic igneous rocks in Archean continental crust. The least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078 – 0.28084 (i.e., initial εHf value of − 1.1 to − 0.4), close the chondritic value at ca. 3.0 Ga and the depleted mantle at ca. 3.2 Ga. These zircons also have δ18O values of 4.2‰–6.1‰ which correspond to typical mantle values. The other chromitite zircons yielded Th/U ratios lower than 0.1, and Hf and O isotopic compositions ranging between the least altered zircons and the intrusive granitoid zircons. These results indicate that the zircons in the chromitites crystallized before or during the 2.97–2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent metasomatism. Furthermore, the present results suggest that zircons in the chromitites originally had depleted Hf isotopic compositions at ca. 3.2–3.0 Ga. This can be explained by two different models of the evolution of the UNL unit. One is that if the UNL unit was formed at > 3.81 Ga as previously thought, with the zircons in the chromitites subsequently being precipitated by ca. 3.2–2.95 Ga during metamorphism or metasomatism. The other model is that the UNL unit itself was actually formed at ca. 3.2–3.0 Ga, with zircon in the chromitite representing the crystallisation age of the unit, which was then tectonically incorporated into the ca. 3.81 Ga orthogneiss prior to the 2.97–2.95 Ga granitoid intrusion event. In either case, our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2–2.95 Ga. Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

Geology, sulphide geochemistry and geochronology of the Romsås Ni-Cu-Co mineralisation, Norway: Implications for ore formation and regional prospectivity

The Romsås intrusion, located in the southern portion of the Idefjorden lithotectonic unit, is part of a cluster of small to medium-sized mafic intrusive bodies which host NiCu sulphide mineralisation, referred to as the Indre Østfold metallogenic area. The Romsås is an almost 1 km-long intrusion with rounded shape, comprising undifferentiated quartz-norite with local orbicular texture. The assemblage, comprising cumulus orthopyroxene and plagioclase, is partly recrystallised and altered, thus orthopyroxene is partly replaced by amphibole, whereas plagioclase is locally replaced by epidote. Locally, garnet, amphibole, plagioclase and magnetite define a granoblastic texture as product of amphibolite-facies metamorphism. Several sulphide-rich lenses, normally up to 5 m wide, occur within the Romsås intrusion and display a sharp contact with the host noritic rocks. The textures vary from disseminated, net-textured and massive sulphides. The sulphide assemblage is homogeneous within the deposit and consists mainly of pyrrhotite, followed by pentlandite, and minor chalcopyrite, with variable alteration of pentlandite.This contribution provides the first systematic characterisation of the Romsås Ni-Cu-Co mineralisation and constraints for its formation and geodynamic setting. We have measured the concentrations of S, PGE, TABS+ (Te, As, Bi, Sb and Se) and other chalcophile elements in both whole rock and sulphide minerals from different ore textures, and UPb and Hf isotope systematics in zircon. The Ni and Cu concentrations in 100% sulphides are around 2% and 1%, respectively. The PGE concentrations in all sulphide-bearing samples are low, generally below 0.3 ppm, with Pt and Pd displaying a broad positive correlation and Pt/Pd ratios varying from 1 to 2. Different ore types are modelled to have formed from a mantle-derived parental magma with higher Cu/Pd ratios and lower Pd contents than mantle values, interpreted as the result of 5% prior crystallisation under sulphide-saturated conditions. Negative mantle-normalised Nb anomalies relative to Th, As and Sb suggest around 10% of assimilation of a component similar to lower crust by the parental magmas. Moreover, S/Se ratios greater than mantle values likely reflect the combined effect of Se depletion during early sulphide removal and external S addition upon crustal assimilation. The Romsås Ni-Cu-Co mineralisation likely formed as a result of sulphide segregation in response to external S addition during several magma pulses. The composition of the sulphide ores can be modelled by an R-factor of around 500. A significant budget of the chalcophile elements is not hosted by sulphide minerals and must be thus accounted for other discrete phases, such as platinum-group minerals. Nevertheless, trace elements distribution in sulphides supports only local remobilisation of chalcophile elements and records compositions comparable to sulphides from pristine NiCu deposits worldwide. Sulphide alteration may have led to exsolution of a parcel of chalcophile elements from sulphide lattice. A UPb zircon crystallisation age of 1551 ± 10 Ma, εHft values between ca. 3.5 and 6, and regional geological considerations suggest that the intrusion formed as part of an arc or back-arc rift setting in the Idefjorden lithotectonic unit. The Romsås sulphide mineralisation represents another example of magmatic sulphide deposit formed at an arc-related geodynamic setting, normally unconventional for the formation of this type of deposit. Therefore, our findings support that arc-related settings in Idefjorden lithotectonic unit, southern Norway, should also be regarded as potential for the formation of Ni-Cu-PGE deposits.

A craton-wide geochemical study of Neoarchaean carbonate rocks of Zimbabwe

Neoarchaean greenstone belts of the Zimbabwe craton host microbialite-bearing limestone successions that range in age from ~2.82 to 2.68 Ga. The best-preserved successions, according to Raman geothermometry, are situated in the Bulawayo and Belingwe greenstone belts where they have been subjected to lower greenschist facies metamorphism with a peak temperature of ~350 °C. Elsewhere, such as in the Masvingo belt, peak temperatures reached ~540 °C, giving rise to marbles. The carbonate rocks consist mainly of calcite and, to a lesser extent, dolomite and contain variably admixtures of siliciclastic detritus. Samples with the least amount of siliciclastic detritus have shale-normalized rare earth element and Y distribution patterns that indicate open marine conditions, with positive La, Eu, and Gd anomalies, superchondritic Y/Ho ratios, and depleted light rare earth elements relative to the heavy rare earth elements. The δ13СVPDB values are −2.0 to 2.1 ‰, and δ18OVPDB values cluster in the range of −20 to −10 ‰. Strontium isotope ratios vary in concert with siliciclastic detritus. The lowest initial 87Sr/86Sr ratio of 0.70155 is close to the depleted mantle value at the time of deposition, suggesting minimal contribution from evolved felsic crust to the Neoarchean ocean. Only samples from the Masvingo belt show radiogenic Sr isotope ratios (0.70202–0.70340) indicative of an epicratonic setting with exposed felsic basement. For the rest, open marine conditions prevailed in a proto-cratonic setting characterized by mantle-derived greenstone volcanism. Carbonate sedimentation on the Zimbabwe craton thus predate the fundamental rise in the marine 87Sr/86Sr Sr isotope signal in the late Neoarchaean.

When carbonatite met granite: A carbonatite magma- wall rock reaction origin for the Paleoproterozoic pyroxenite and syenite in Fengzhen, North China

A set of carbonatite dykes intruding into Paleoproterozoic granites are exposed in Fengzhen area, where the Trans-North China Orogen joined with the Khondalite Belt. Some pyroxenites and syenites were also developed close to the carbonatite dykes. But the genetic relationship between these rocks and carbonatite dykes has not been solved. The pyroxenites show low SiO2 (40–49 wt%), high CaO (>20 wt%), and are enriched in light rare earth elements, strongly depleted in high field strength elements (Nb, Ta, Zr, Hf), similar to the characteristics of carbonatite dykes. The zircon UPb ages of pyroxenites and syenites are 1789 ± 13 Ma and 1793 ± 14 Ma, respectively. The wall-rock granite yields a metamorphic age of 1839 ± 19 Ma, which is close to the magmatic age of the pyroxenites and syenites. The whole-rock εNd(t) values (−4.4 to −5.1) and εHf(t) values (−3.8 to −6.9) of the pyroxenites are both negative, which are similar to those of the syenites (−3.7 to −4.7 and − 4.1 to −5.3) but slightly lower than those of wall-rock granites (−3.5 and 1.9). The δ18O values of clinopyroxene in the pyroxenites fall in a range of 6.6–11.3‰, which are much higher than the mantle value. The calcite of the carbonatites and pyroxenites gave overlapped δ18O value ranges of 11.8–14.8‰ and 9.3–15.0‰, respectively. They are also higher than typical primary carbonatites. These lines of geochemical evidence indicate that the pyroxenites are not mantle xenolith or derived directly from mafic magma, but closely related to the carbonatite dykes and the wall-rock granites. Since the Ba/La and Ba/Mn ratios of the syenites are much higher than those of the carbonatite, an immiscibility origin is excluded. Mineral chemistry of clinopyroxene, olivine, calcite and phlogopite from the carbonatite, pyroxenite and syenite all suggest they are formed by different degrees of reaction between carbonatite magma and felsic crustal rocks. Modeling shows that the Nd isotope compositions and trace element compositions of the pyroxenites and syenites fall on the mixing trends between the carbonatite dykes and wall-rock granites. These results demonstrate that the pyroxenites and syenites in the Fengzhen area were formed when carbonatite magma intruded and reacted with felsic crustal rocks. This provides a canonical case for the formation of pyroxenite and syenite by carbonatite magma-wall rock reaction.

Zircon U–Pb ages and Hf and O isotope systematics of crustal zircons from Mesoproterozoic kimberlites of the Dharwar craton, India: Implications for Neoarchean craton assembly

Plate tectonic models for the amalgamation of the dominantly Paleo-Mesoarchean Western Dharwar Craton (WDC) and Neoarchean Eastern Dharwar Craton (EDC) remain inconclusive on the polarity of Neoarchean subduction. Here we present insitu U-Pb age and Hf-O isotopic compositions of xenocrystic zircons in ca. 1.4-1.1 Ga kimberlites and lamproites emplaced within the ca. 2.8-2.5 Ga EDC as deep probes of the crustal architecture across the WDC-EDC boundary. The different zircon populations yield wide range of ages extending back to ca. 3.6 Ga, including distinctly ‘kimberlitic zircons’ with a mean 206Pb/238U age of 1112 ± 42 Ma, consistent with previous age estimates for kimberlite magmatism. The presence of zircons with ages >3.33 Ga is surprising as this age group of zircons is atypical of magmatic rocks in the EDC, but wide-spread in the WDC. Also, the highest concentration of zircons with ages >3.0 Ga is found in the kimberlites closest to the notional boundary shear zone between the WDC and EDC. The Hf isotope systematics of a majority of magmatic zircons with ages between 2.75-2.50 Ga require melting of older (3.6-3.3 Ga) source rocks. O-isotope data for these Neoarchean zircons show a wide scatter, lie both above (δ18O to 13.5 ‰) and well below mantle values (δ18O to <2 ‰). δ18O values outside the mantle range are closely associated with low εHf. The data are interpreted as evidence for transportation of older upper crustal rocks to lower crust and mantle depths consistent with tectonic models invoking collisional under thrusting of WDC beneath EDC.

Dehydration of the Subducting Juan de Fuca Plate and Fluid Pathways Revealed by Full Waveform Inversion of Teleseismic P and SH Waves in Central Oregon

AbstractThe dehydration of subducting slabs expels a massive amount of water into the forearc and backarc mantle which is responsible for the serpentinization of the mantle wedge, as well as the production of melt and arc magmatism. These processes are expected to have characteristic signatures in density and seismic velocities models, which remain largely elusive to date due to the limited spatial resolution of classical passive tomographic approaches. Here we present a tomographic model of density, VP, VS, and VP/VS beneath central Oregon, obtained by inverting complete teleseismic P and SH waveforms recorded by the CASC93 temporary experiment. The final model shows an east‐dipping low‐velocity layer less than 10 km thick that can be associated with the fluid‐saturated Juan de Fuca oceanic crust. The distribution of tremors at the surface closely coincides with the horizontal extent of this low‐velocity layer. Below 40 km depth, seismic velocities and density increase progressively to the values of a typical mantle. This transitional domain corresponds to the eclogitization of the oceanic crust. Silica‐saturated fluids released by pore collapse migrate upward, producing serpentinization reactions in the forearc mantle that lower the density and seismic velocities. The very low VP/VS ratio documented in the Cascadia forearc crust is evidence that these silica‐saturated fluids reach the crust, where they produce extensive quartz mineralization. At greater depth, a low seismic velocity and high VP/VS ratio anomaly provides evidence for partial melting at around 75 km depth beneath the volcanic arc.

Molybdenum isotopes record recycling of subducting sediment in active continental margin, Northeast China

Highly variable molybdenum (Mo) isotope compositions have been reported for mafic arc volcanics above oceanic subduction zones. The heavy Mo isotope signatures of mafic arc magmas are generally attributed to addition of high δ98Mo oceanic crust-derived fluids to the subarc mantle sources. However, subduction zone fluids at subarc depths may occur in the forms of aqueous solution and hydrous melts, and seafloor sediments would also be subducted to subarc depths to produce hydrous melts. If such sediment-derived melts were incorporated into the mantle wedge, it is intriguing what is their effect on the Mo isotope composition of mafic arc magmas. Here we report Mo isotope data for Early Jurassic mafic igneous rocks from Northeast China, which was an active continental margin during subduction of the Paleo-Pacific Plate in the Mesozoic. These rocks have δ98Mo values of −0.26‰ to 0.17‰, which are mostly higher than the normal mantle value of −0.20‰ ± 0.01‰. They also exhibit arc-like trace element compositions, moderate (87Sr/86Sr)i ratios of 0.7042 to 0.7047, and neutral εNd(t) values of −2.8 to 2.2. The high δ98Mo values are associated with low Mo/Ce ratios, enriched SrNd isotopes, and high melt-mobile element contents in the mafic igneous rocks. These geochemical features indicate that the heavy Mo isotope signatures in these rocks were mainly inherited from subducting sediment-derived melts. Subducting Paleo-Pacific oceanic crust-derived aqueous solutions and seafloor sediment-derived hydrous melts would metasomatize the overlying mantle wedge beneath the northeastern China continent, and the sediment-derived melts would have contributed the heavy Mo isotope signatures to the mantle sources of mafic arc magmas in the active continental margin. Therefore, the Mo isotope study of mafic igneous rocks is a powerful means to decipher the recycling of crustal components at convergent plate margins.

Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma emplacement along the Irtysh Shear Zone in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt was characterized by a long-lived accretionary history from Neoproterozoic to Paleozoic, followed by a collisional phase of orogeny in the latest Paleozoic with the closure of the Paleo-Asian Ocean. However, Ni-Cu sulfide mineralization associated with mantle-derived mafic magmas in the Central Asian Orogenic Belt was relatively short-lived. Thus, the coherence between the protracted accretionary and collisional history of the Central Asian Orogenic Belt and the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to understand. We investigated both Ni-Cu sulfide-bearing and sulfide-barren mafic intrusions along a major suture zone of the Irtysh Shear Zone within the Central Asian Orogenic Belt to understand the origin of short-lived Ni-Cu sulfide mineralization in the context of the collision of two arc systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb dating results for the Ni-Cu sulfide-bearing mafic intrusions show that they were only emplaced briefly from 290 Ma to 280 Ma after subduction terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren mafic intrusions (ca. 274−271 Ma). Zircons from all Ni-Cu sulfide-bearing/barren mafic intrusions yield arithmetic mean δ18O values ranging from 6.0 to 7.4‰, higher than the typical mantle value (∼5.3‰), which indicates their derivation from a metasomatized mantle with inherited subduction signatures. The δ11B values of the rocks from these intrusions range from −17.2‰ to −2.3‰, significantly lower than those of typical volcanic arcs (up to +18‰), which suggests that the metasomatic components in the mantle source were dominated by sediment melts rather than fluids. The pressure calculated using amphibole thermobarometry shows that the parental magmas of the Ni-Cu sulfide-bearing mafic intrusions were emplaced shallower than those of Ni-Cu sulfide-barren mafic intrusions. This is supported by the fact that the country rocks of the Ni-Cu sulfide-bearing mafic intrusions are shallow crustal lithologies of sedimentary rocks, whereas the country rocks of the barren intrusions are relatively deeper crustal lithologies of high-grade metamorphic rocks and granites. Olivine from the Ni-Cu sulfide-barren intrusions has forsterite (Fo) contents ranging from 51.1 mol% to 76.2 mol% and Ni from 196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu sulfide-bearing intrusions (Fo = 72.6−79.4 mol%; Ni = 1022−1925 ppm), which indicates that the parental magmas of the barren intrusions may have experienced extensive olivine crystallization. Notably, the emplacement of the Ni-Cu sulfide-bearing mafic intrusions (290−280 Ma) coincidently overlapped with a transient period of orogen-parallel extension (ca. 295−280 Ma) in response to the collision of the Chinese Altai and East Junggar. Syn-collisional extension may lead to the rapid ascent and emplacement of mantle-derived mafic magmas at a shallower crustal level, preventing significant loss of Ni from olivine crystallization at depths, and thus providing metal-rich mafic magmas for potential Ni-Cu sulfide mineralization. Our results highlight that syn-collisional extension is an essential geodynamic mechanism that controls the emplacement of Ni-Cu sulfide-bearing mafic intrusions in orogenic belts.

Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7–2.3, La/Gdchond. = 0.6–1.7, Sm/Luchond. = 0.9–1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5–2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 ± 62 (n = 12, MSWD = 1.5) and 3396 ± 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 ± 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and εHf values for zircon of −2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (μ142Nd = −2.3 ± 2.2 to +1.7 ± 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.

Ultramafic-Hosted Ni-Cu-Co-(As) Mineralization from an Ancient Oceanic Transform Fault Zone in the Troodos Ophiolite, Cyprus: An Analogue for Ultramafic Sea-Floor Massive Sulfide Mineralization?

Abstract Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea-floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

A peridotite source for strongly alkalic ultrabasic HIMU lavas of the Oslo Rift, Norway

High alkali (Na2O + K2O) low SiO2 lavas generally have highly fractionated trace element compositions and their petrogenesis is debated. Both pyroxene-rich (pyroxenite) and olivine-rich (peridotite) mantle sources have been proposed for such lavas. HIMU (high 238U/204Pb) lavas usually have such compositions and are often interpreted as being derived from garnet-pyroxenite (eclogite) residue of subducted recycled oceanic crust, possibly including a carbonate component. In some cases, an origin from a garnet peridotite source has also been considered.We present new Mg, Pb, Nd, and Sr isotopic data and elemental data for alkalic ultrabasic lavas (melilitites, nephelinites) from the Oslo Rift, Norway. The magmatic province of the Permo-Carboniferous Oslo Rift is interpreted as part of the much larger Skagerrak-Centered Large Igneous Province (SCLIP) which may be related to a mantle plume source originating from the African Large Low Shear Velocity Province (LLSVP) at the core-mantle boundary. The range of δ26Mg is narrow (−0.32 to −0.28) and is similar to typical mantle values, and therefore is inconsistent with a carbonated source as has been suggested for other similar lavas. The 206Pb/204Pb values of all the lavas are high, which identifies them as HIMU lavas. The εNd of +1 to +2 are substantially lower than for typical HIMU lavas, and in the εNd-εSr diagram they plot on the LoNd array between HIMU and EMI. The lavas are more like archetypal kimberlites (formerly, Group I kimberlites), as these also have εNd slightly higher than the chondritic value and Pb isotopes consistent with a HIMU source. The REE patterns of the lavas are also almost as fractionated as in Group I kimberlites.We used major and trace elements to infer the primary magma composition of these lavas. This primary magma composition is compared with experimental melts of both carbonated peridotite and carbonated eclogite source compositions and shows that their major element compositions are only consistent with either a very high degree of melting of eclogite or a very low degree melting of garnet peridotite. The strongly fractionated REE patterns of the lavas can only be made by low-degree melting of either eclogite or garnet peridotite, which therefore effectively rules out eclogite as a major component of the mantle source of these lavas. Further trace element modeling points to a mantle source composition with >97% garnet peridotite that was lightly carbonated (<0.5 wt% CO2) and melted to a very low degree (∼0.4%) to produce these lavas. The modeled source trace element pattern was constrained by the Nd, Sr, and Pb isotopic compositions of the lavas. The measured high Ti contents of these lavas (3.8–6 wt%) are quantitatively shown to be the result of enrichment by both partial melting and fractional crystallization, using experimentally determined DTi for melting of this type of source. We also show that a low degree of melting of a slightly carbonated source is sufficient to result in the likely very high CO2 content (10–15 wt%) of the primary magma.The rather uniform Pb isotope composition of HIMU magmas suggests that the HIMU source is rather uniform in composition, but the Pb isotope composition requires this source to form late in Earth's history. The similarity of these lavas and kimberlites suggests a very deep source, possibly in the LLSVP that gave origin to the SCLIP. This means that LLSVPs do not represent some primordial layer around the core that escaped homogenization of the mantle by the Moon-forming giant impact. Instead, this layer must have formed late in Earth's history. The constraint that this source contains <3% of garnet-pyroxenite (eclogite) residue of recycled basaltic oceanic crust shows that this layer is primarily not a subducted slab graveyard. The process by which the LLSVPs formed, however, remains obscure.

Absence of water content contrast between lithosphere and asthenosphere in subduction zone

The distribution of water between the rigid lithosphere and ductile asthenosphere is crucial for our understanding of the Earth's dynamics because it has been proposed that the origin of the lithosphere–asthenosphere boundary (LAB; i.e., the bottom of the tectonic plate) is related to a possible elevated water content in the asthenosphere. However, obtaining the depth profile of water content using mantle xenoliths is still challenging because of the rapid diffusive loss of hydrogen during magma ascent and eruption. In this study, we present diffusion profiles of water determined by FTIR mapping and SIMS line analysis for olivine and orthopyroxene of peridotite xenoliths from Ichinomegata maar in Northeast Japan. Extensive analysis of water contents in 17 xenolith samples revealed that these xenoliths exhibit a variety of zoning patterns, from which the timescale of diffusive loss was estimated. The xenoliths from Ichinomegata underwent only limited diffusive water loss (<1h duration), because of the low temperature of andesitic host magma and rapid quench in the maar deposit. Water contents of the mantle value are well-preserved in the homogeneous core parts of large olivine and pyroxene grains. The water contents of olivine suggest clear correlations with petrogenetic factors excepting those marking anomalous high values, implying hydrous metasomatism by a water-rich fluid. Combining the estimated water contents of the minerals and depth estimation of the xenolith samples, for the first time, we obtained the depth profile of water content of the mantle, including the LAB, down to ∼55 km. Our results show that olivine, orthopyroxene, and clinopyroxene in the lithospheric mantle (depth range in 28–38 km) contain 21 ± 2, 302 ± 64, and 616 ± 99 wt. ppm H2O, respectively, which are similar to those in the top of the asthenosphere (depth range in 39–52 km) that contain 20 ± 2, 258 ± 38, and 561 ± 80 wt. ppm H2O. In the region which has experienced local metasomatism, higher water contents are recorded (30 ± 4, 414 ± 48, and 741 ± 43 wt. ppm H2O). This study verifies that there is no water content contrast across the LAB in our studied area. Therefore, we support the ‘partial-melting model’ for the origin of the LAB rather than the ‘olivine-water model’ in the western Pacific Plate subduction zone.