Mantle Source Research Articles

Recycling of terrigenous sediments recorded in late Permian-early Triassic Fe-rich intrusions from Yunkai Massif, South China

The origin of Fe-rich intrusion remains debatable regarding the respective role of magmatic evolution and Fe enrichment in the source. Here, we report detailed mineral and bulk-rock geochemistry of Permian-Triassic (251–250 Ma) mafic intrusions from the Tengxian area in Yunkai Massif, South China. These rocks consist of hornblende norites, show Fe-rich affinities with high FeO*(FeO* = FeOt / (FeOt + MgO) in weight ratio, >0.8, based on total iron oxide in the rock), and exhibit significant enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletion in Sr and high field strength elements (HFSEs), resembling subduction-related magmas. In addition, they show remarkable features that include very high δ18O values in zircon (δ18O = 8.8–10.6 ‰) and apatite (δ18O = 8.7–10.8 ‰), and extremely enriched SrPb [e.g. 87Sr/86Sr(i) = 0.7181–0.7196, 207Pb/204Pb(i) = 15.77–15.78, 208Pb/204Pb(i) = 38.95–39.05] and nonradiogenic NdHf isotopic compositions [e.g. εNd(t) = −11.1 ~ − 10.1, εHf(t) = −8.8 ~ −7.3] in bulk rocks. These characteristics distinguish the Tengxian norites from modern arc basalts and subduction-related magmas in the Paleo-Pacific Tectonic Domain. Instead, they are isotopically similar to Cenozoic Tibetan and Mediterranean potassic to ultrapotassic rocks in the Tethys Tectonic Domain. Regardless of variable influence by orthopyroxene and plagioclase accumulation, the intrinsically low SiO2 and high FeOt and Fe/Mn ratios in these rocks were likely attributed to significant contribution of a Fe-rich mantle component such as Si-poor pyroxenite, which might have formed through crystal accumulation of mafic magmas at mantle conditions. The highly evolved Sr-Nd-Pb-Hf isotopic signatures and very high δ18O values in zircon and apatite required substantial (10–20 %) involvement of recycled crust in the mantle source. The combined mineral and bulk-rock geochemical data suggest that the parental magmas for the Tengxian norites originated from a metasomatized mantle wedge through addition of terrigenous sediment-derived melt following the subduction of Paleo-Tethys Ocean beneath the Yunkai Massif.

A REE Inverse Model From Bulk Distribution Coefficients and Boundary Conditions: Results for Shield and Rejuvenated Stage Hawaiian Volcanoes

AbstractA major challenge in mantle geochemistry is determining the source composition and melt fraction involved in melting. We provide a new Rare‐Earth Element (REE) inverse model that provides source concentration, source and melt mineral modes, and melt fraction based on the difference between separate determinations of bulk distribution coefficients and constrained by boundary conditions. An analytical inverse of the batch melting equation provides expressions for source, , and bulk distribution coefficient of the mantle, , with two unknowns, the initial concentration of La in the mantle, , and Pi, the bulk distribution coefficient of the melt. We traverse through a range of steps and examine thousands of melt modes, Pi, at each step. Thousands of trial melt modes fail by generating that are inconsistent with partition coefficients. Many surviving trials cannot be inverted to estimate a mantle mode. Other boundary conditions eliminate even more trials. Surviving trials are ordered by the difference between calculated from the REE data of a lava suite and calculated from partition coefficients and mantle mode. We select the solution with the closest fit that passes all the boundary conditions. We tested our new model with lava suites from Hawaii where different lines of evidence suggest that they melted from different mantle sources, Mauna Kea representing shield‐stage lava and submarine Kiekie representing rejuvenated stage lava. Our inverse determination of mantle composition and melting parameters was consistent with earlier models based on assumptions of HREE composition.

Disentangling the Roles of Subducted Volatile Contributions and Mantle Source Heterogeneity in the Production of Magmas Beneath the Washington Cascades

AbstractThe compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and the amount and composition of slab‐derived materials. This contribution presents the volatile (H2O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively spans nearly the entire range of arc magmas globally. All volcanic centers have magmas with H2O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of variably saline fluid and/or slab partial melt. Magmas from Glacier Peak in northern Washington have unusually high Th/Yb ratios that are similar to Lassen region basalts, indicating possible contributions of “subcreted” metasediments that geophysical data suggest are not present beneath central Oregon and southern Washington. This data set adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Anatomy of the Emeishan Mantle Plume Head: Insights From New Geochronologic, Geochemical, and Geologic Data

AbstractThe link between mantle plumes and the formation of large igneous provinces (LIPs) is well established although the anatomy of these remains equivocal. Recent experimental studies and geophysical data suggest that the mantle plume head is more likely to be irregular and asymmetric, rather than an axisymmetric flattened disk. The Emeishan large igneous province (ELIP) provides a unique opportunity to test this hypothesis. According to robust petrographic, geochronologic, and geochemical evidence from the late Permian basalts in the Sichuan Basin, and in conjunction with a comprehensive compilation of geologic maps and published geochemical data from the ELIP, we identified several giant radial “fingering” structures. Based on the shallow mantle source from the center to margin in the ELIP and relief of the lithosphere‐asthenosphere boundary, we propose a new mantle plume model to explain the evolution of the Emeishan plume periphery, where narrow finger‐like protrusions and plumelets developed outwards from the main body of the plume to the edges of the flattened plume head. Dragged fingers might have been torn apart into some plumelets, which dispersed and were trapped beneath the thinnest lithosphere relief, and eventually erupted to form small‐scale flood basalt in the Outer Zone of the ELIP.

Magmatic evolution of KREEP‐free lunar meteorite Asuka‐881757 inferred from sector‐zoned clinopyroxene, pyroxene symplectites, and thermodynamic modeling

AbstractLunar basaltic meteorite Asuka‐881757 (A‐881757), a member of the source crater paired YAMM meteorites (Yamato‐793169, A‐881757, Miller Range 05035 and Meteorite Hills 01210), provides information on potassium‐rare earth element‐phosphorous (KREEP)‐free magmatic sources within the Moon. Asuka‐881757 is an unbrecciated and Fe‐rich (Mg# 36) gabbro with coarse pyroxene (2–8 mm) and plagioclase (1–3 mm). The coarse pyroxene preserves mm‐scale, near‐complete hour‐glass sector zoning with strong Ca and Fe partitioning, similar to some Fe‐rich Apollo basalts. In contrast to the most Mg‐rich Apollo basalts, A‐881757 contains various types of symplectites (~8 vol%) formed by the breakdown of pyroxferroite due to slow cooling, resembling a few extreme Fe‐rich (Mg# 40) Apollo basalts. Petrographic observations and thermodynamic modeling suggest crystallizing in the order: Fe‐poor pyroxenes (Mg# 58–55) → co‐crystallized plagioclase and Fe‐rich pyroxenes (Mg# 49–20) → late‐stage assemblage including Fe‐augite, Fayalite, and Fe‐Ti oxides. Combining phase stability at variable P–T with petrographic observations, the minimum depth of formation of the A‐881757 parent magma can be constrained to between 60 and 100 km. KREEP‐free basalts (such as A‐881757 and the YAMM meteorites) originated from a relatively shallow mantle source and later underwent polybaric crystallization that occurred prior to eruption at the lunar surface. In contrast, the Apollo mare basalts mostly crystallized within lava flows from relatively deeper‐seated mantle sources. The crystallization of A‐881757 and other YAMM meteorites is unlike most Apollo basalts from the Procellarum KREEP terrane, and likely represent hidden cryptomare basalts close to lunar surface.

The magma evolutional constrains on the genesis of proximal Zn skarn mineralization: A case study from the Yaojialing deposit in Tongling district, eastern China

The major factors especially the roles of the magma evolution controlling the Zn/Cu mineralization in skarn deposits are still controversial. Yaojialing is a large-sized skarn Zn polymetallic deposit (1.74 Mt Zn at 3.6 %, 30.4 t Au at 4.2 g/t and 24.8 t Cu at 0.83 %) located in the Tongling district of the Middle–Lower Yangtze belt (MLYB), both the proximal and distal areas of the deposit exhibit high Zn/Cu mineralization. The intrusions in Yaojialing primarily consist of quartz monzonite porphyry (QMP) and granodiorite porphyry (GDP), and the QMP is related to skarn mineralization. Both the QMP and GDP display relatively high (87Sr/86Sr)i values (0.707907 to 0.709390), low εNd(t) values (−9.05 to −8.17) and negative εHf(t) values (−8.51 to −11.72), suggesting that they originated from a mixed source of enriched mantle and lower crust. Both the QMP and GDP contain type I and type II amphiboles, while type III amphibole exists only in QMP. Type I amphibole is acicular crystals shape, type II amphibole is euhedral in shape and relatively large in size (0.6–2.0 mm), while type III amphibole crystallized around the margin of type II amphibole. The type I amphibole from QMP and GDP show similar calculated temperatures (948–1010 ℃), pressures (2.9–6.5 kbar, corresponding depths at 11.0 to 24.4 km), fO2 (ΔFMQ = 0.2–1.9) and H2O content (4.2–6.8 wt%). Type II amphibole crystallized at 815–941 ℃, 1.2–2.9 kbar (corresponding to depth of 4.6–11.1 km), ΔFMQ = 0.7–2.1, and 4.4–5.7 wt% H2O. Type III amphibole have a lower temperature (679–795 ℃), pressure (<0.5 kbar) and water content (2.5–4.3 wt%) but higher fO2 value (ΔFMQ = 3.1–3.8) compared to type I and type II amphiboles. Reverse zoning of plagioclase and higher Mg# (74 to 89) of type III amphibole in QMP are resulted from injection of mafic magma at shallow depths, which provide sufficient metal and favorable conditions for the formation of QMP parental magma. Modeling of magma H2O solubility indicates that QMP begins to exsolve fluid at depth of 6.2–11.1 km. Initial low oxygen fugacity and ore-forming element differential release during fluid exsolution process resulted in the high Zn/Cu mineralization at Yaojialing.

Tourmaline composition probes serpentinite-derived fluid mobility in subduction zones

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low XMg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ11B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low XMg (<0.6), relatively high trace element concentrations and δ11B values of less than −10 ‰, whereas the overgrowths exhibit extremely high XMg values (>0.99), low trace element concentrations and high δ11B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-11B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high XMg > 0.85 and δ11B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.

Geochemical and isotopic features of the Early Cretaceous volcanism of the Torey Volcanic Field (Eastern Transbaikalia, Russia) as a record of the transition from pyroxenite to eclogite mantle source

The Mesozoic magmatic activation in Central and Northeast Asia resulted in the formation of a large volume of volcanic rocks with diverse compositions. The most dramatic compositional change occurred at the end of the Early Cretaceous, when mainly alkaline basaltic lavas began to erupt after subalkaline differentiated lavas. The nature of crustal and mantle processes that led to this change in volcanism remains unclear. The Torey Volcanic Field (TVF) of Eastern Transbaikalia (Russia) demonstrates a similar compositional change. Therefore, the TVF is crucial to studying the cause of the difference in geochemical and isotopic signatures of the Mesozoic volcanism in Central and Northeast Asia.TVF belongs to the northeastern end of the Eastern Mongolia Volcanic Area (EMVA). Like other volcanic fields of the EMVA, TVF formed in two stages: early (∼121–129 Ma) and late (∼101–119 Ma). The TVF is composed of subalkaline and alkaline basaltic trachyandesites – trachyandesites. All TVF rocks are characterized by negative Ti and Sr anomalies and positive Ba and Pb anomalies. Compared to the late TVF rocks, the early TVF rocks have distinct negative Ta and Nb anomalies and are highly enriched in light rare earth elements relative to heavy rare earth elements. TVF rocks have the following isotopic characteristics: 87Sr/86Sr(t) = 0.70477–0.70540, εNd(t) = − 0.9 – +2.4, 206Pb/204Pb(t) = 17.9–18.4, and 207Pb/204Pb(t) = 15.5–15.6. However, older rocks mainly have higher εNd(t) values and 206Pb/204Pb(t) and 207Pb/204Pb(t) ratios.Geochemical and isotopic data of samples suggest that the TVF formed by melting in the continental metasomatized lithospheric mantle (CMLM). Phlogopite-amphibole-rutile-bearing pyroxenite veins played a major role in the formation of older rocks. Eclogite, represented by the recycled oceanic crust or the buried lower continental crust, was dominant in the source of younger rocks. The common source for both groups of the TVF rocks was metasomatized hydrous peridotite.Lithospheric extension and subsequent asthenospheric upwelling led to melting in the CMLM and the formation of the TVF. At the early stage of the volcanism, melting occurred at relatively low temperatures where amphibole, phlogopite, and rutile were stable. Due to ongoing lithosphere extension, the melting of eclogite occurred at a higher temperature and/or lower pressure at the late stage of the volcanism.

Apatite and pyroxene as records of magmatic–hydrothermal processes and platinum-group mineral (PGM) formation in the Wengeqi mafic–ultramafic intrusion, Inner Mongolia, China: The role of oxidized, H2O-rich magmas

The Devonian Wengeqi mafic–ultramafic intrusion, situated on the northern margin of the North China Craton (NCC) — an Andean-style convergent margin during the Paleozoic — hosts many platinum-group minerals (PGMs), especially within the S-deficient, magnetite-rich clinopyroxenite zones. These PGMs occur in two distinct associations: Pt-rich PGMs (e.g., sperrylite) closely associated with primary magnetite and Pd-rich PGMs (e.g., sudburyite, kotuskite, arsenopalladinite) associated with secondary minerals (e.g., actinolite, pyrite). The mechanisms underlying the formation of PGMs in S-deficient rocks and the spatial decoupling of various PGM types remain elusive. The relationship of PGM formation and characteristics of magmas at convergent margins is also not well understood. In this contribution, we utilized apatite and clinopyroxene textures and chemistry alongside whole-rock SrNd isotopes to characterize i) the nature of the parental magma from which the Wengeqi intrusion crystallized, and ii) the magmatic–hydrothermal processes that operated to generate the Pt- and Pd-rich PGM. Based on the composition of clinopyroxene and clinopyroxene–melt partition coefficients, the parental magma of the Wengeqi intrusion is estimated to have an arc-like affinity on a primitive mantle-normalized trace-element diagram. This, together with the EMI-like SrNd isotope signature, implies that the Wengeqi magma was sourced from metasomatized SCLM beneath the NCC with an EMI-like composition. Three types of apatite (Ap1, Ap2, and Ap3) were identified within the intrusion. Ap1 and Ap2 are magmatic in origin, and appear as isolated grains, whereas Ap3 occurs as stringers or heterogeneous domains within Ap1 and Ap2, and likely resulted from hydrothermal modification of magmatic apatite. Notably, the abundant Ap2 exhibits higher VS contents and Eu/Eu* ratios than the less common Ap1, and contains S predominantly in the form of S6+, suggesting that the Wengeqi magma was relatively oxidized, with fO2 > FMQ + 1.2. Based on the chemistry of magmatic apatite, the Wengeqi magma had ∼5 wt% H2O, which, together with its high fO2, facilitated crystallization of magnetite, causing reduction of the magma by removal of Fe3+. This reduction process promoted PGM nucleation by decreasing PGE solubility in the magma, leading to the association of PGM with magnetite. Additionally, the oxidized, H2O-rich magma likely released oxidizing fluids, selectively mobilizing Pd rather than Pt, separating Pt-rich PGMs from Pd-rich PGMs. The budget of PGE in the parental magma could have been increased by i) metasomatism of the SCLM source (e.g., by carbonatitic fluids), which would have elevated the concentration of PGE in the mantle source, and ii) the high fO2 and H2O levels of the magma, which would have delayed sulfide saturation. Accordingly, mantle metasomatism and high fO2–H2O are deemed favorable for enrichment of PGE in magmas within convergent margins.

Alkaline Devonian magmatism of the Menorca Island: Tracking the mantle isotopic sources in the realm of the western Paleo-Tethys Ocean

Several locations with alkaline magmatism are recognised in Silurian-Devonian basins along the southern variscan autochthon units (e.g. Central Iberian Zone) of the northern Gondwana margin. The origin of the Devonian basins and their magmatism has not been studied in the context of the passive margin of Gondwana. The basement of Menorca, Balearic Islands, consists of a deep Devonian-Carboniferous basin with mafic igneous rocks, the Tramuntana Gabbros. In this study, we trace the geodynamic setting and isotopic sources of the Tramuntana Gabbros through elemental geochemistry, isotopic geochemistry (SrNd) and UPb geochronology in zircons. These gabbros are the product of an intraplate alkaline magmatism with immobile trace element and REE contents similar to those of Ocean Island Basalts. Average 87Sr/86Sr(370) of 0.708456 and εNd(370) of +4.0 indicate a source similar to a Type-2 enriched mantle with average TDM of 665 Ma, suggesting a relatively old metasomatized mantle. Concordant UPb ages of c. 597 Ma (Ediacaran, radiometric age) from a single population of 31 zircons separated from the Tramuntana Gabbros (Devonian, biostratigraphic age) reinforce the presence of older units in the corresponding lithospheric mantle. The Tramuntana Gabbros and the Devonian-Carboniferous sequences of Menorca limit the westward extension of the Paleo-Tethys Ocean, whose development never reached these westernmost regions. Assuming a common sublithospheric mantle source for the peri-Gondwanic Devonian alkaline rocks and considering the previous Cadomian (Neoproterozoic) subduction to be the most favourable origin of the separated zircons, the bulk rock TDM and zircon UPb data obtained from the Tramuntana Gabbros track the mixing, recycling, and mantle accretion in this peri-Gondwanic section from Precambrian to Devonian times.

The slab failure in Central Java (Indonesia): New insight into its tectonic setting and origin

The geochemical and tectonic characteristics of volcanic formations in Central Java, specifically the Sumbing-Slamet volcanics, were investigated to understand the processes associated with slab failure in the region. Through comprehensive geochemical analysis and comparison with other volcanic formations, insights into the magmatic evolution and tectonic settings of the Sumbing-Slamet volcanics were gained. The findings support the hypothesis of slab tearing beneath Central Java, as evidenced by distinct geochemical signatures and magmatic interactions observed in the Sumbing-Slamet volcanics. Geochemical data reveal medium to high potassium content (K2O = 0.77–2.32%), low Nb/Y (<0.6561), low TiO2 relative to Al2O3 [TiO2 < (−1.1610 + 0.1935 × Al2O3)], Th/La >0.2, as well as a wide range of Nb/La and Nb/Zr (0.14–0.89 and 0.0304–0.0744, respectively), notable depletions in high-field strength elements (HFSE; such as Nb, Ti), low to high Ta-anomaly (δTa = 0.21–1.03), and whole-rock isotopes of 87Sr/86Sr (0.704458–0.705800) and 143Nd/144Nd (0.513059–0.512766) demonstrate that they were formed from active continental margin (ACM) tectonics involving subducted sediment input. These magmatic processes likely resulted from the mixing of lithospheric and asthenospheric mantle sources due to slab failure in the northern part of Central Java. The research contributes to strengthening the geophysical view regarding the existence of slab tearing in Central Java, understanding the dynamic geological processes occurring in subduction zones, and emphasizing the importance of interdisciplinary approaches in studying such phenomena.

A depleted mantle source for Neoproterozoic continental rifting in the Seve Nappe Complex, Kebnekaise region, northern Swedish Caledonides

The Central Iapetus Magmatic Province (CIMP) is a large igneous province (LIP) emplaced in the Baltican and Laurentian paleocontinents that marks the onset of the Caledonian Wilson Cycle. Paleozoic magmatism of the CIMP is preserved in both northeastern America and northern Europe. This study investigates rocks belonging to the hyper-extended margin of Baltica currently found in the Seve Nappe Complex of the Scandinavian Caledonides. Specifically, U-Pb zircon geochronology and whole-rock geochemistry are applied to a migmatitic variety of the Vierručohkka amphibolite of the Mårma Terrane, to the Aurek gabbro, and amphibolite of the Aurek Assemblage exposed in the Seve Nappe Complex in the Kebnekaise region, northern Swedish Caledonides. U-Pb zircon geochronology yields crystallization ages of 626 ± 7 Ma for the protolith of the Vierručohkka amphibolite, and 614 ± 2 Ma and 609 ± 1 Ma for the emplacement of the Aurek gabbro and amphibolite protolith, respectively. A younger age of 599 ± 3 Ma is recorded in the Vierručohkka amphibolite and is interpreted as the age of partial melting and migmatization. The geochemical signatures of the rocks demonstrate crustal assimilation during the emplacement of their protoliths and modification due to prograde metamorphic processes during Caledonian subduction. The Vierručohkka amphibolite and the Aurek Assemblage samples display upper and lower crustal assimilation, respectively. Trace elements (Dy, Sm, Lu, and Y) record the growth of metamorphic garnet, while elevated TiO2 contents record the crystallization of metamorphic rutile. Nevertheless, high field strength elements (HSFE) and ∆Nb suggest a depleted mantle source for the magmas of the protoliths of the investigated rocks. Altogether, geochemical data indicate that the igneous activity recorded in the Vierručohkka amphibolite and the Aurek Assemblage between c. 626–609 Ma is related to continental rifting processes associated with the opening of the Iapetus Ocean.

Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sri (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sri = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

Discovery of mafic dikes in northeastern Tibet plateau belonging to the Emeishan large igneous province: Implications for paleocontinental reconstruction and mineral exploration

Due to the lack of paleomagnetic data and paleoclimatic records, the paleocontinental belonging of the Yidun terrane in northeastern Tibet plateau is debated. In this paper, we use the distribution of the Emeishan large igneous province (ELIP) to put the issue to rest. The ELIP is the product of mantle plume activity, which took place at 260 ± 3 Ma in the western margin of the Yangtze craton. The Yangtze craton is the northern part of the South China paleocontinental block. Our new discovery of four mafic dikes in the Yidun terrane with ages and chemistry like the ELIP strongly supports a new interpretation that this terrane was an integral part of the Yangtze craton during the ELIP magmatic event. Baddeleyites from these mafic dikes yield the U–Pb ages of 257.1 ± 4.8, 259.8 ± 8.5, 260.2 ± 4.5 and 261.5 ± 6.8 Ma for each of these dikes, which are similar to the zircon U-Pb ages of the mafic–ultramafic intrusions of the ELIP within the Yangtze craton. The Yidun mafic dikes and some high-Ti basalts of the ELIP within the Yangtze craton have similar Sr-Nd isotope compositions and mantle-normalized incompatible trace element distribution patterns, indicating similar mantle source compositions. Mixing calculation using Sr-Nd isotope compositions shows that the parental magmas for the Yidun mafic dikes experienced only 2–3 % crustal contamination. Partial melting modelling shows that the primary magmas for the Yidun mafic dikes were produced by 5–6 % mantle partial melting at the depths > 85 km below surface, consistent with mantle plume-related magmatism in a continental setting. The Yidun terrane is a new exploration frontier for magmatic Fe-Ti-V oxide deposits that are present elsewhere in the ELIP.

The helium and carbon isotopic signature of Ocean island basalts: Insights from Fogo volcano (Cape Verde archipelago)

Fluid inclusions (FI) entrapped in phenocrysts carried by Ocean Island Basalts (OIB) contain key information on volatiles’ abundance and origin in their mantle sources. Here, we add new piece of knowledge to our understanding of volatile geochemistry in global OIB magmas, by presenting new noble gas (He-Ne-Ar) and carbon (C) isotope results for olivine- and clinopyroxene-hosted FI from enclaves, lavas, tephra and volcanic gas samples from Fogo, the only frequently active volcano at the Cape Verde archipelago (eastern Atlantic Ocean). FI, together with crater fumaroles, constrain the Fogo 3He/4He signature at 7.14–8.44 Rc/Ra (where RC is the air-corrected 3He/4He isotope ratio, and Ra is the same ratio in air), which is within the typical MORB (Mid-Ocean Ridge Basalt) mantle. The carbon isotopic ratio (δ13C vs. Pee Dee Belemnite) of CO2 in FI and fumaroles range from -6.04 to -4.41 ‰. We identify systematic variations of δ13C and He/Ar* with FI entrapment pressure (estimated from a combination of host mineral barometry and FI microthermometry), from which we develop a model for volatile degassing in the mantle-to-crustal magma storage system. The model predicts a crustal-like signature for carbon (δ13C of -0.4 ± 1.0 ‰) in primary melts formed by mantle melting at ∼2200 MPa (∼77 km) and a source He/Ar* ratio of 0.90–0.24, which are indicative of variably depleted mantle metasomatized by carbon enriched melts/fluids from a crustal component. We also use our results to characterise regional (in the Cape Verde and Canary archipelagos) and global trends in C and He isotope composition from OIB. From a comparison with the few other OIB localities for which δ13C are available, we propose that a carbon enriched crustal component could be recurrent at a global scale in OIB magmatism, although often masked by isotope fractionation during magmatic degassing. We additionally find that, at regional scale, He isotopes in OIB scale inversely correlate with the degree of partial melting of the mantle beneath individual islands’ (inferred from the La/Yb ratio of erupted basalts). More widely, our results corroborate previously established global relationships between OIB He isotopic signature, plume buoyancy flux and overlying plate velocity. In this interpretation, the MORB-like 3He/4He (8 ± 1 Ra) at Fogo reflects a combination of (i) low to medium magma productivity, (ii) relatively low plume buoyancy flux (∼1.1 Mg/s), and (iii) slow average velocity (∼3 cm/yr) of the overlying plate.

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable εHf(t) values from -2.8 to 11.0 and δ18O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling.

Multiple mantle metasomatism recorded by Triassic post-collisional ultrapotassic and tholeiitic magmatism in West Qinling, NW China

Mantle properties and metasomatic processes, together with interlayer interactions at orogenic belts are still under debate. The post-collisional magmatic rocks could provide secondhand constraints on these issues. Hereby, an integrated geochronologic, geochemical, and isotopic study was conducted on post-collisional Zhanwa dolerites, Dashui granodiorite–monzonite-monzogabbro complex, and Jiuzigou pyroxenite–syenite complex at West Qinling. The Zhanwa dolerites, being tholeiitic with flat trace elemental pattern, were mixtures of melts from asthenospheric and oceanic slab-fluid-metasomatized lithospheric mantles. The Dashui monzonite-monzogabbro suite is ultrapotassic with strongly right-inclined trace elemental pattern and evolved isotopic composition (( 87 Sr/ 86 Sr) i = 0.7066–0.7068, ε Nd ( t ) = -4.56 to -4.35, ε Hf ( t ) = -3.14 to -2.35, and ( 206 Pb/ 204 Pb) i = 17.99–18.12)). It derived from melting of continent-crustal materials-metasomatized mantle. In contrast, the Dashui granodiorites are shoshonitic–calc-alkaline with more evolved isotopic compositions with ( 87 Sr/ 86 Sr) i , ε Nd ( t ), ε Hf ( t ), and ( 206 Pb/2 04 Pb) i of 0.7074–0.7091, -7.56 to -5.51, -5.61 to -3.53, and 18.18–18.46, respectively. They represent hybrids of mantle-derived K-rich and crustal-derived felsic melts. The Jiuzigou complex holds extremely high trace elemental contents. Its mantle source was metasomatized by both oceanic crustal fluids and sedimentary melts. Generation of these rocks (ca. 235–223 Ma) not only recorded multiple mantle metasomatism, but also captured collective asthenospheric and lithospheric magmatic responses at a post-collisional setting.

Configuration of the early-mid Cenozoic extensional arc volcanism of the North Patagonian and the Southern Central Andes (33–44°S)

The late Eocene to earliest Miocene volcanic arc of the Southern Central Andes (33–40°S) and North Patagonian Andes (40–44°S) exhibits distinctive features that shed light on the dynamics of subduction-related volcanic activity in extensional tectonic settings. This period is particularly significant in the Andes, as it records the opening of forearc, intra-arc, and retroarc extensional basins, accompanied by high-volume volcanism, preceding the middle-late Miocene final major uplift phase of the Andes. This extensional event occurred in two phases, associated with changes in the geometry and dynamics of the subduction system: from oblique convergence at low rates during the late Eocene-early Oligocene to an orthogonal convergence at high rates during the late Oligocene-early Miocene. This study presents a compilation of field, petrographic, geochemical, and isotopic data from volcanic sequences in the North Patagonian Andes and the Southern Central Andes, aiming to analyze petrogenetic processes at different stages of arc evolution.During the late Eocene to early Oligocene, subduction-related volcanism occurred in a wide area from 100 to 300 km from the trench. In this period, magmatic arc rocks exhibit intermediate composition and a relatively homogeneous geochemical signature, transitional between tholeiitic and calc-alkaline series, with a marked subduction-related signature. These magmas originated from depleted mantle sources with high slab-derived fluid and sediment contributions and limited crustal interactions. In contrast, the late Oligocene to earliest Miocene volcanic activity, at the peak of the extensional regime, displays significant geochemical differences along the Andes. In the Southern Central Andes (33–40°S), volcanism displays compositions similar to those of the previous stage with varying influence from the subducting slab. In some sectors, magmas register the climax of the extensional conditions, whereas the youngest volcanic rocks towards the north (33–34°S) present evidence of the onset of contractional tectonics, demonstrated by the increased interaction of the magmas with a progressively thickened crust. In the North Patagonian Andes (40–44°S), subaerial and subaqueous arc-related lava flows are interbedded with marine deposits, indicating a rapid subsidence regime. These rocks exhibit geochemical features transitional from arc to E-MORB and even OIB compositions, which suggest a genesis primarily dominated by decompression melting of a heterogeneous mantle source. Above all, the contrasting nature of late Eocene-early Miocene arc products in the Southern Central and Patagonian Andes highlights that no single process can explain the changes in composition, distribution, and evolution of arc magmatism; rather, it is a combination of factors, including variations in subduction zone configuration and mantle dynamics and composition.

Zircon U-Pb geochronology, major, trace elemental, and Sr-Nd-Pb isotopic geochemistry: constraints on the age and magmatic origin of Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region, South China

ABSTRACT As a direct product of lithospheric extension, the mafic dyke group have become an ideal object for studying the formation, evolution and dynamics of both the crust and continental orogenic belts. Given the scientific significance of mafic dykes and their utility in addressing outstanding questions concerning the origin and development mafic dyke swarms Mesozoic age with-in the South China Plate, in this study, 54 representative mafic dykes within the Guangxi Zhuang Autonomous Region, South China, were examined. Research methods include laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age-dating, whole rock geochemistry, and the Sr-Nd-Pb isotope geochemistry of the studied mafic dykes. The results indicate that these rocks formed between 139 Ma and 113 Ma and have typical doleritic and lamellar textures. They fall into the alkaline and shoshonitic series, are characterized by enrichment of light rare earth elements, some large ion lithophile elements (Rb, Ba, and Sr), Th, U, and Pb, and have depleted Nb, Ta, Hf, and Ti. Moreover, the mafic dykes studied have high initial87Sr/86Sr ratios (0.7055–0.7057), negative εNd(t) values (from −15.2 to −11.4), and relatively constant initial Pb isotopic ratios (that are Enriched Mantle-1 (EM I)-like; 206Pb/204Pb = 16.765–16.816, 207Pb/204Pb = 15.429–15.476, and208Pb/204Pb = 36.882–37.144). The above results indicate that the group of dolerites and lamprophyres studied herein were likely derived from magma generated via low-degree partial melting (1.0%–10%) of an EM1-like garnet-lherzolite mantle source. The parental magmas to these fractionated olivine-, clinopyroxene-, plagioclase, and Ti-bearing phases, with negligible crustal contamination during ascent and dyke emplacement. On the basis of existing research and evidence from this study, we propose a reasonable model for the origin of the mafic dykes: the ancient Pacific Plate subducting below the South China plate led to extension of the lithosphere, an upsurge of the asthenosphere, which triggered partial melting of the lower mantle lithosphere in the Early Cretaceous, and the formation of the parent magma to the Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region study area. The involvement of slab-derived fluids from the subduction of the ancient Pacific Plate was responsible for metasomatism of the mantle source to these mafic magmas.

Petrology, phase equilibria modelling, and fluid inclusion study of mafic granulites from Bhavani Suture Zone, Southern India

We report new petrology, mineral chemistry, P–T conditions, and fluid inclusion data on mafic granulites from the Mettupalayam region along the Bhavani Suture Zone, Southern Granulite Terrane, India. Phase equilibria modelling of mafic granulites yielded peak P–T conditions of 780–860 °C and 7.6–10.1 kbar followed by a near isothermal decompression along a clockwise P–T path. The trapped fluid inclusions in the peak metamorphic minerals display a melting temperature range from −57.4 °C to −56.6 °C, close to the triple-point temperature of pure CO2. The primary inclusions homogenized at −18.9 °C to +0.2 °C, corresponding to density values of 0.93–1.03 g/cm3. Homogenization of the secondary inclusions occurred within the range from −6.3 to +18.1 °C, corresponding to low CO2 densities of 0.79–0.96 g/cm3. From the textural characteristics of the high-density primary carbonic fluid inclusions, we interpret these inclusions as the CO2-rich syn-metamorphic fluid present during the high-grade metamorphism. The secondary fluids characterised by lower densities have undergone re-equilibration during the exhumation stage (decompression) from the peak granulite-facies metamorphism along a clockwise P–T trajectory. This interpretation is consistent with the occurrence of hornblende + plagioclase symplectite around the porphyroblastic garnet, suggesting decompression. We infer that the high-density CO2 was the dominant syn-metamorphic fluid components present during the granulite-facies metamorphism in the Mettupalayam region. Such carbonic fluids, possibly derived by degassing from carbonates or mantle sources, probably played a significant role in stabilizing high-grade mineral assemblages along this collisional suture zone.