Trace Element Systematics Research Articles

The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas

The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87Sr/86Sr ranging from 0.702907 to 0.703147, εNd ranging from +4.3 to +6.3, εHf ranging from +5.6 to +8.6, and 206Pb/204Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

A Temperature-Composition Framework for Crystallization of Fractionated Interstitial Melt in the Bushveld Complex from Trace Element Systematics of Zircon and Rutile

A Temperature-Composition Framework for Crystallization of Fractionated Interstitial Melt in the Bushveld Complex from Trace Element Systematics of Zircon and Rutile

Trace Element Analysis of Pyrite from the Zhengchong Gold Deposit, Northeast Hunan Province, China: Implications for the Ore-Forming Process

The Zhengchong gold deposit is located in the central segment of the Jiangnan Orogen in northeastern Hunan Province, South China. The host rocks of this deposit are the Neoproterozoic slates of the Lengjiaxi Group and granodiorite. The structures in the Zhengchong gold deposit are dominated by NE-trending reverse faults, which control the gold-bearing veins. The orebody consists of NE-trending laminated quartz veins and NW-trending quartz veins. The alteration styles include silicification, carbonatization, sulfidation, sericitization and chloritization. The Zhengchong gold mineralization can be divided into four stages: Quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV). Three generations of hydrothermal pyrite were identified: Disseminated euhedral to subhedral cubes in altered wall-rock (PyI), euhedral to subhedral cubes inter-grown with arsenopyrite and tetrahedrite in quartz veins and wall-rock (PyII), and euhedral cubes with microinclusions (native gold, galena, sphalerite, chalcopyrite, tetrahedrite, and pyrrhotite) or metasomatic textures in sulfide-rich veins or veinlets (PyIII). PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals. PyI records the lowest concentrations of Au; PyII and PyIII record similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurs by direct-ion exchange. Invisible gold is uniformly distributed within the arsenian pyrite, and visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure. There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitution that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (<1) and the decrease in the Se/Te ratio (<10) in stage III, indicating that a more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.

Provenance, depositional setting, and crustal evolution of the Cathaysia Block, South China: Insights from detrital zircon U–Pb geochronology and geochemistry of clastic rocks

We report the results of a combined study of detrital zircon U–Pb geochronology and bulk‐rock elemental geochemistry on late Neoproterozoic to Cambrian clastic sedimentary rocks from South Jiangxi within Cathaysia. These clastic rocks are characterized by moderate chemical index of alteration (CIA) values of 73 and high Th/U ratios (>3.8), indicating moderate weathering of the source area. The relatively high index of compositional variability (ICV = 0.62–1.30) values indicates a source compositionally dominated by immature material that lacks alumina‐rich minerals. Bulk‐rock major and trace element systematics on discrimination diagrams are consistent with the source provenance being felsic‐intermediate igneous rocks of ancient continental crust origin. The geochemistry is also consistent with the clastic sedimentary rocks being deposited in a setting at or in the vicinity of passive continental margins. Detrital zircon U–Pb ages of the clastic rocks record five major age populations: 2,614–2,376 Ma (peak at ca. 2,482 Ma), 1,953–1,353 Ma, 1,000–900 Ma (peak at ca. 958 Ma), 850–730 Ma (peaks at ca. 845 and 763 Ma), and 685–571 Ma (peak at ca. 635 Ma). The age data provide a record of igneous activity in the source provenance: the 2,482 Ma peak is consistent with the global Neoarchean continental crust growth; the 1,953–1,353 Ma population correlates with the period of assembly and breakup of the Columbia supercontinent; the prominent peak at ca. 958 Ma corresponds to a common thermal–tectonic event associated with the assembly of Rodinia, and the 850–730 Ma population is consistent with the breakup of the Rodinia supercontinent. The ca. 850 Ma age is indicative of initial stage of Rodinia breakup in South China. Our data also reveal a 670–530 Ma population that correlates well with the Pan‐African event associated with the formation of the Gondwana supercontinent, although no direct geological evidence for this event has been found within the SCB. Moreover, complex zircon morphology and comparisons of detrital zircon U–Pb age spectra in a global context suggest the late Neoproterozoic–Cambrian sedimentary rocks in the Cathaysia Block must have sourced from an exotic source with magmatic activities of late Archean, Grenvillian, and Pan‐African ages, which do not outcrop in the Cathaysia Block or adjacent regions and need to be further explored.

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Trace element systematics and Nd, Sr and Pb isotopes of Pliocene flood basalt magmas (Ethiopian rift): A case for Afar plume-lithosphere interaction

We present high-precision trace element and Sr-Nd-Pb isotope data for Pliocene flood basalts located at Sire (eastern rift wall) and Chefe Donsa (western rift wall) sections from the main Ethiopian rift (MER). These basalts are tholeiitic to mildly alkaline and were derived by moderate degrees of melting (~10–20%) of a spinel lherzolite containing minor amounts of amphibole within the lithospheric mantle. Strontium, Nd and Pb isotopic compositions (87Sr/86Sr: 0.7040–0.7057, 143Nd/144Nd: 0.52164–0.51287, 206Pb/204Pb: 17.75–19.20, 207Pb/204Pb: 15.53–15.64 and 208Pb/204Pb: 38.12–38.86) are between the inferred composition of the Afar mantle plume and an unspecified enriched lithospheric component, suggesting interaction of melts from asthenospheric and lithospheric mantle domains rather than melting of homogeneous mixed sources or magma mixing. It is proposed that in the Oligocene the impinging Afar plume creates zones of metasomatically enriched mantle. Later, lithospheric thinning as a result of incipient rifting leads to decompression and melting of this metasomatically enriched mantle due to its lower solidus temperature. The chemistry of the Pliocene lavas therefore reflect two associated scenarios namely (i) upwelling plume material from the advancing Afar plume may have induced metasomatism in the lithospheric mantle (ii) mobilization and melting of this enriched lithospheric mantle material resulted in melts characterized by variable mixtures of the two mantle reservoirs.

Petrogenesis of the Darvazeh mafic-intermediate intrusive bodies, Qorveh, Sanandaj-Sirjanzone, Iran

The Darvazeh intrusion in the northern Sanandaj-Sirjan zone in the Alpine-Himalayan orogenic belt is composed of olivine gabbro, gabbroic diorite and granitoid units that intruded Jurassic metamorphic rocks. Field relations, textures, and whole rock geochemistry indicate that most of the mafic to intermediate composition rocks are not cumulates and instead represent liquid compositions. The tholeiitic (or ferroan) gabbros and magnesian gabbroic diorites evolved along liquid lines of descent and have different parental magmas. This conclusion is based on separate trends, often with overlapping MgO contents, for Si, Fe, Ti, Ni, Cr, Zn, Cu and Sr. The mineralogy (late versus early magnetite and contrasting proportions of magmatic amphibole) of the two suites are also slightly different as is the trace element patterns (for example Th, U, Ti and Sr). The Mg# of mafic-intermediate rocks ranges from 46 to 73, implying that the ultimate source for the most mafic rocks was in the mantle, not the crust. Flat HREE patterns, Sm/Yb ratios (<2) show that melting occurred in the shallow mantle, above the stability field of garnet. Small negative Nb-Ta-Ti anomalies and enrichments of Cs, Rb, Pb, Pb, and Sr on mantle-normalized trace element diagrams are consistent with a subduction zone source for both the tholeiitic gabbros and the magnesian gabbroic diorites. Hence, we conclude that the parental mafic magmas were generated by partial melting of spinel peridotite in the mantle wedge above a subducting slab of oceanic lithosphere. However, the composition of plagioclase-olivine pairs suggest the gabbroic magmas were not water-rich, and their high Fe/Mg and Ti/V ratios show they were relatively reduced compared to typical arc magmas. For the gabbros, melting may have been initiated by decompression in an intra-arc extensional environment caused by slab roll back or oblique subduction that partially separated the Sanandaj-Sirjan zone from the Central Iran block. Components from the subduction fluid are smaller than in the subsequent gabbroic diorites which have more typical arc compositions. These mafic magmas rose and fractionated on different paths controlled by water content and fO2, with the gabbros following reduced Fe-enrichment trends and the magnesian gabbroic diorites following more oxidized trends. The country rocks, plate reconstructions, and trace element systematics (including high Th/La ratios) show that the arc was underlain by continental crust that was assimilated into the mafic and intermediate magmas. Thus, the Darvazeh pluton reveals the complex nature of mafic intrusive rocks, which formed in a continental arc experiencing extension as the Neo-Tethys oceanic lithosphere subducted beneath the Central Iranian plate.

The mafic alkaline volcanism of SW Madagascar (Ankililoaka, Tulear region): 40 Ar/ 39 Ar ages, geochemistry and tectonic setting

High-precision 40 Ar/ 39 Ar ages, major and trace element, and radiogenic isotope data are presented for the basanites and alkali basalts forming the southwesternmost monogenetic volcanic field in Madagascar. The volcanic rocks were erupted along fissure zones and aligned cones in a nearly flat area covered by the Cenozoic sediments of the Morondava basin. The high-precision 40 Ar/ 39 Ar ages constrain the beginning of the magmatism in the Ankililoaka area to about 12 Ma, significantly earlier than suggested by previously published K/Ar ages. The Ankililoaka basanites include primitive compositions (MgO &gt;10 wt%, Ni &gt;200 ppm and Cr &gt;400 ppm), whereas other basanites and alkali basalts experienced limited removal of olivine, chromiferous spinel and clinopyroxene. Initial Sr and Nd isotope ratios of the basanites are 0.70343–0.70445 and 0.51279–0.51282, respectively. The Pb isotope compositions are in the ranges 206 Pb/ 204 Pb = 19.08–19.38, 207 Pb/ 204 Pb = 15.61–15.64 and 208 Pb/ 204 Pb = 39.1–39.4. The alkali basalts have similar 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and 207 Pb/ 204 Pb, but slightly lower 206 Pb/ 204 Pb and 208 Pb/ 204 Pb than the basanites. The isotopic composition of the Ankililoaka rocks partially overlaps with that of the Cenozoic volcanic mafic rocks of northern Madagascar, and differs significantly from that of the mafic volcanic rocks of central Madagascar, which have lower 206 Pb/ 204 Pb and 207 Pb/ 204 Pb, and higher 87 Sr/ 86 Sr. Major and trace element systematics and geochemical modelling suggest that the Ankililoaka mafic alkaline rocks are low-degree melts of an incompatible element enriched peridotite source starting from depths where garnet is stable. Crustal contamination during ascent was insignificant. We argue that the genesis of the Ankililoaka alkaline magmas was triggered by melting an enriched, volatile-rich lithospheric mantle uplifted in the Cenozoic. Supplementary material: Analytical techniques, X-ray fluorescence whole-rock data, mineral compositions, 40 Ar/ 39 Ar dataset, mantle source modelling and figures showing volcanological features and thin sections are available at https://doi.org/10.6084/m9.figshare.c.4065743

Chemistry of quartz related to the Zinnwald/Cínovec Sn-W-Li greisen-type deposit, Eastern Erzgebirge, Germany

Magmatic and hydrothermal quartz populations related to the Late-Variscan Zinnwald/Cínovec Sn-W-Li greisen-type deposit in the Eastern Erzgebirge/Krušné Hory (German/Czech Republic) were studied in terms of trace element contents and growth structures visualized by cathodoluminescence. The sample suite represents different populations of hydrothermal greisen quartz and magmatic quartz of the granitic host collected at a 600-m long, lateral section crosscutting the northern part of the deposit at the 750-m level.The studied quartz populations are characterized by generally low concentrations and low variation of Al and Ti indicating relatively constant crystallization conditions (temperature, pressure and fluid chemistry) during ore precipitation. Magmatic and hydrothermal quartz are distinguished by their Al, Ti, Li and Ge concentrations and ratios. Magmatic quartz has commonly higher Al (>100 ppm) and Ti (>3 ppm) compared to hydrothermal quartz. The Ge/Al ratio in quartz increases systematically from high to low formation temperatures (664 to ~400 °C = temperature range including published data and our estimates) whereas Ti decreases. The Zinnwald albite granite (mineralization host), an albite-rich alkali granite, and massive quartz-zinnwaldite greisen contain both magmatic and hydrothermal quartz populations which are distinguishable by their Ge/Al weight ratio: magmatic quartz has a Ge/Al ratio of <0.008 whereas hydrothermal quartz a ratio of >0.008.Hydrothermal vein quartz related to the Sn-W mineralization (Flöze = flat veins and Morgengänge = steep veins) does not show systematic trace element variations along the sampling profile indicating that no significant chemical gradients (in lateral direction) in the ore forming fluid existed. The uptake of Li in the quartz lattice is limited by the incorporated Al although the Zinnwald/Cínovec magmatic-hydrothermal system was strongly enriched in Li. The calculated crystallization temperatures of quartz of the Sn-W mineralization applying the TitaniQ thermobarometer reveal crystallization temperatures ranging from 575 ± 48 °C (albite granite), 612 ± 12 °C (stockscheider), 610 ± 23 (massive greisen), 536 ± 77 °C (flat veins), to 516 ± 58 °C (steep veins) confirming in general early estimations based on fluid and melt inclusion studies. In economic terms, the hydrothermal quartz (flat and steep veins) with an average total trace element content of 128 ± 56 ppm has intermediate chemical quality and can serve as a by-product if colour sorting and other processing steps are applied. Rock-forming quartz of albite granite and massive greisen with an average total trace element content of 233 ± 127 ppm has medium quality and the disadvantage that quartz contains a high amount of contaminating mineral inclusion, which will be challenging to remove by processing.

A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile

Rutile occurs as an accessory mineral in many high-temperature metamorphic assemblages and has the potential to identify and investigate UHT metamorphic terranes. Whilst the use of Zr-in-rutile geothermometry to identify ultrahigh-temperature terranes is appealing, its application to granulites can be difficult owing to diffusional resetting of Zr concentrations during cooling and decompression. In order to provide constraints on P-T conditions and trace element systematics during ultrahigh-temperature metamorphism, metamorphic rutile in granulites from the Archean Napier Complex and the Palaeozoic Rauer Group, Antarctica have been investigated for trace element composition and mineral inclusions/intergrowths. Textural observations and Zr-in-rutile temperatures show that unlike the large proportion of rutiles grains in the matrix, rutile grains that are shielded by phases with low Zr-diffusivities (e.g. orthopyroxene) and/or are in chemical isolation from zircon, have the potential to retain Zr concentrations that correspond to ultrahigh-temperatures. Principal Component Analysis reveals that V, Cr, Nb, Mo, Ta and W are significantly enriched in rutile from Archean and Proterozoic ultrahigh-temperature terranes. High concentrations of elements such as Cr and V indicate a spinel-rich source, demonstrating the effect of protolith bulk chemical composition on rutile trace element signatures. Rutile is shown to contain inclusions and intergrowths of aluminium silicate, quartz, corundum and feldspar, including the first reported occurrence of prograde kyanite, which provides direct evidence that the Napier Complex experienced a typical clockwise P-T evolution.

Geochemistry of sediments of the Peninsular rivers of the Ganga basin and its implication to weathering, sedimentary processes and provenance

Abstract Geochemistry of the Himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate. Among the Himalayan rivers, the Ganga and its Himalayan catchment have been studied extensively. However, the Peninsular rivers constituting a significant part of the Ganga basin have not received much attention, heretofore. In the present geochemical study of the Peninsular river's sediments, we have found that the initial chemical weathering of rocks in the catchment produces texturally, mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine-grained suspended sediments. The overbank sediments possess an intermediate texture and geochemistry, but more towards the suspended sediments. The sediment geochemistry data indicates that the Peninsular river's sediments show an increase in the weathering intensity from the west (Banas river) to the east (Son river) along the climate gradient, i.e., from the arid-semiarid to the sub-tropical condition. The higher chemical index of alteration (CIA) values of the Peninsular river's sediments than the Yamuna and Ganga river sediments reflect the control of weatherable lithology in the tectonically stable, low relief and subtropical climate condition. The Yamuna and Ganga river sediments with lower CIA values show more physical weathering in the tectonically active Himalaya. The suspended and overbank sediments of the Yamuna and Ganga rivers show an increase in their CIA values after the confluence of the Peninsular rivers, indicating the contribution of more weathered materials by the Peninsular rivers. It has been found that there is a geochemical split between the channel and suspended sediments of the Peninsular rivers for sharing different sources, i.e., the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments. The weathering geochemistry, trace element systematics, and REE patterns suggest that the dominant sources of the channel sediments of the Peninsular rivers are the felsic crystallines and sedimentary lithologies of the Aravalli range, Bundelkhand and Chhotanagpur granite and gneisses, and Vindhyan sandstones. Whereas, the mafic lithologies such as the Deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments. Within the range of intermediate composition, the overbank sediment chemistry reflects more contribution of the mafic sources except for the Ken which shows a higher contribution of the felsic sources. The geochemical split between the channel and suspended sediments of the Peninsular river's sediments for the different provenances indicates differential weathering of the Deccan basalts, Bundelkhand crystalline, and Vindhyan sedimentary rocks; and also the hydrodynamic control of the Peninsular rivers during erosion, transport, and deposition. However, the Himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the Himalayan catchment.

Geochemical systematics of the Mauranipur-Babina greenstone belt, Bundelkhand Craton, Central India: Insights on Neoarchean mantle plume-arc accretion and crustal evolution

The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts, felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations. The intrusive and extrusive komatiitic basalts are characterized by low SiO2 (39–53 wt.%), high MgO (18–25 wt.%), moderately high Fe2O3 (7.1–11.6 wt.%), Al2O3 (4.5–12.0 wt.%), and TiO2 (0.4–1.23 wt.%) with super to subchondritic (Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO2, TiO2, MgO, Fe2O3, Al2O3 and CaO. At similar SiO2 content of type I and III basalts, the type II basalts show slightly high Al2O3 and Fe2O3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type I and II basalts. Type III basalts show high Zr/Nb ratios (9.8–10.4), TiO2 (1.97–2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO2 (55–64 wt.%), moderate TiO2 (0.4–0.7 wt.%), slightly low Al2O3 (7–11.9 wt.%), medium to high MgO (3–8 wt.%) and CaO contents (10–17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt–andesite–dacite–rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta, Nb/Th, high MgO contents (>20 wt.%), CaO/Al2O3 and (Gd/Yb)N > 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subduction-metasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.

Differentiation of the early silicate Earth as recorded by 142Nd-143Nd in 3.8–3.0 Ga rocks from the Anshan Complex, North China Craton

The Archean Anshan Complex in the northeastern part of the North China Craton is one of the few areas on Earth where rocks older than 3.8Ga have been identified, and thus it provides a rare opportunity to determine the relative roles of primordial silicate differentiation versus progressive differentiation in shaping the Earth’s chemical architecture. We undertook 142,143Nd isotopes and major and trace element analyses on well-dated 3.81–3.0Ga Anshan rocks. This study is the first to report significant positive 142Nd anomalies in the Anshan Complex of up to +9.2±2.8ppm relative to the modern accessible Earth. The most prominent feature is that the 143Nd isotopes tightly correlate with incompatible trace element ratios. Trends of ε143Nd(t) as a function of incompatible trace element ratios reveal that the Anshan source consists of high- and low-μ142Nd end-members. The high-μ142Nd end-member had μ142Nd=+15 and ε143Nd(t)=+10 and is characterized by highly enriched incompatible trace element ratios, similar to average early Archean upper continental crust, suggesting a crustal origin. The low-μ142Nd end-member had μ142Nd≤−4 and ε143Nd(t)≤0 and is characterized by depleted incompatible trace elements. Both 142,143Nd isotopes and incompatible trace element ratios of the low-μ142Nd end-member are similar to that of Barbteron komatiites derived from an early-formed and deep-seated mantle reservoir. Furthermore, the estimated incompatible trace element ratios of the low-μ142Nd end-member are similar to that of a 4.5–4.4Ga dense melt that was possibly preserved above the core-mantle boundary. The relationships between 147Sm-143Nd, 146Sm-142Nd, and incompatible trace elements provide important constraints on both primary silicate differentiation within 100–150Ma of solar system formation and late Hadean and early Archean mantle-crust differentiation events. Coupled 147Sm-143Nd and 146Sm-142Nd systematics suggest ca. 4.5–4.4Ga model ages for the precursor of the oldest components of the Anshan Complex, whereas decoupled 147Sm-143Nd, 146Sm-142Nd, and trace element systematics imply multiple mantle-crust differentiation events from 4.3 to 3.8Ga.

Coupled Pb isotopic and trace element systematics of the Tissint meteorite: Geochemical signatures of the depleted shergottite source mantle

The application of Martian meteorite U–Th–Pb isotope systematics to track the geochemical evolution of the Martian mantle has had limited success because of the difficulty in discriminating an indigenous magmatic Pb component from secondary near-surface components that have additionally been overprinted by terrestrial contamination. To mitigate this challenge, a successive acid-leaching experiment was conducted on the Tissint meteorite, the freshest, witnessed fall of a primitive, olivine-bearing Martian basalt. Trace element concentration analyses of acid leachates and residues indicate that secondary terrestrial contaminants were effectively removed by the early steps in the leaching experiments and that the acid residues contain pristine Pb from Tissint. The acid residue, which shows the most depleted REE signature, also has the least radiogenic Pb isotopic composition (206Pb/204Pb = 10.948, 207Pb/204Pb = 11.187, 208Pb/204Pb = 30.228). A two-stage mantle evolution model based on this composition indicates that the Tissint mantle has the lowest μ-value (238U/204Pb = 1.62±0.09) among the shergottite sources.

Assessing the shock state of the lunar highlands: Implications for the petrogenesis and chronology of crustal anorthosites

Our understanding of the formation and evolution of the primary lunar crust is based on geochemical systematics from the lunar ferroan anorthosite (FAN) suite. Recently, much effort has been made to understand this suite’s petrologic history to constrain the timing of crystallisation and to interpret FAN chemical diversity. We investigate the shock histories of lunar anorthosites by combining Optical Microscope (OM) ‘cold’ cathodoluminescence (CL)-imaging and Fourier Transform Infrared (FTIR) spectroscopy analyses. In the first combined study of its kind, this study demonstrates that over ~4.5 Ga of impact processing, plagioclase is on average weakly shocked (<15 GPa) and examples of high shock states (>30 GPa; maskelynite) are uncommon. To investigate how plagioclase trace-element systematics are affected by moderate to weak shock (~5 to 30 GPa) we couple REE+Y abundances with FTIR analyses for FAN clasts from lunar meteorite Northwest Africa (NWA) 2995. We observe weak correlations between plagioclase shock state and some REE+Y systematics (e.g., La/Y and Sm/Nd ratios). This observation could prove significant to our understanding of how crystallisation ages are evaluated (e.g., plagioclase-whole rock Sm-Nd isochrons) and for what trace-elements can be used to differentiate between lunar lithologies and assess magma source compositional differences.

An experimental study of trace element distribution during partial melting of mantle heterogeneities

Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0GPa), high-temperature (1230 to 1550°C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas.In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions.Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature).

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr–Nd–Hf–Pb–Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with >+5‰δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05–3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4‰δ13C for carbonatites versus −5.7 to −3.6‰δ13C for kimberlites) require open-system fractionation at magmatic temperatures.Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16–33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation – as exemplified by the Mesozoic–Cenozoic breakup of the North Atlantic craton.

In Situ Sr isotopes in Plagioclase and Trace Element Systematics in the Lowest Part of the Eastern Bushveld Complex: Dynamic Processes in an Evolving Magma Chamber

In Situ Sr isotopes in Plagioclase and Trace Element Systematics in the Lowest Part of the Eastern Bushveld Complex: Dynamic Processes in an Evolving Magma Chamber

Controls on trace-element partitioning among co-crystallizing minerals: Evidence from the Panzhihua layered intrusion, SW China

The factors and processes that control trace-element partitioning among co-crystallizing cumulus minerals in layered intrusions have long been controversial. Here we address this issue using new laser ablation ICP-MS trace element data for magnetite, ilmenite, and clinopyroxene from the Panzhihua layered intrusion in the Emeishan large igneous province, SW China. The cumulus minerals display strong Ni, Co, and Cr depletions, indicative of parental magmas low in concentration of these elements probably due to prior sulfide removal and the fractionation of chromite or Cr-magnetite in a staging magma chamber at depth. Both magnetite and clinopyroxene show cyclical variations in some transition elements (e.g., Cr, V, and Ni) along the stratigraphic section. The average concentrations of these transition elements in magnetite are positively correlated with those in clinopyroxene, likely resulting from co-crystallization of magnetite and clinopyroxene. The incompatible element (e.g., Zr, Hf, and Nb) concentrations of the cumulus minerals from the Lower Zone are highly variable compared to those of the Middle and Upper Zones. These large variations in trace element compositions are attributed to a “trapped liquid shift” in the Lower Zone. Ilmenite crystals from the Panzhihua intrusion may have undergone extensive modification of transition elements during subsolidus re-equilibration with magnetite, leading to the decoupled variations of transition elements in ilmenite across the Lower Zone stratigraphy. Our study indicates that systematic trace element variations of the main cumulus mineral assemblage, rather than a single mineral, need to be considered to better constrain the magmatic differentiation and elemental fractionation of layered intrusions.

Mineralogy and trace element geochemistry of sulfide minerals from the Wocan Hydrothermal Field on the slow-spreading Carlsberg Ridge, Indian Ocean

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990m, and Wocan-2 site, located at a water depth of 3100m, ∼1.7km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.