Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr–Nd–Hf–Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P–T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr–Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet–rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief ~ 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

High 3He/4He in central Panama reveals a distal connection to the Galápagos plume

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high 3He/4He (up to 8.9RA) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source 3He/4He >10.3RA (and potentially up to 26RA, similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1RA). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high 3He/4He values in central Panama are likely derived from the infiltration of a Galápagos plume–like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a “mantle wind” toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits.

Hemispheric geochemical dichotomy of the mantle is a legacy of austral supercontinent assembly and deep subduction of continental crust

Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including high 87Sr/86Sr and low 143Nd/144Nd indicative of ancient subduction of continental crust—are restricted to the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral domain. However, late Neoproterozoic to Paleozoic continental collisions—associated with the assembly of Gondwana and Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra-high-pressure (UHP, >2.7 gigapascals) metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted-passive margins during austral supercontinent assembly, from 650-300 Ma, resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. In contrast, EM domains are absent in boreal hotspots, for two reasons. First, continental crust subducted after 300—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes feeding northern hemisphere hotspots. Second, before the appearance of continental UHP rocks at 650 Ma, upper continental crust was not subducted to great depths, thus precluding its subduction into the northern hemisphere mantle during the Precambrian when continents may have been located in the northern hemisphere. Our model implies a recent formation of the austral EM domain, explains the geochemical dichotomy between austral and boreal hotspots, and may explain why austral hotspots outnumber boreal hotspots.

Petrographic and Geochemical Studies of the Host Rock of the Chaghoo Copper Orebody in the Southwest of Karaj

The study area is located 5 km southwest of Mahdasht city in Karaj on the Urmia-Dokhtar magmatic arc. In this area, Eocene volcanic and pyroclastic rocks are observed including basaltic andesite lavas, andesite, Trachyandesiticand trachyte lavas, tuff, and ignimbrite, along with plutonic rocks. There are two spectra of basic and acidic for the rocks in the area, of which basic rocks are chemically calc-alkaline in nature.Among the signs of subduction rocks in the area are enrichment in the Ta, Nb, and Ti lavas, as well as the anomaly of the HFSE index relative to the LILE of incompatible elements content. The geochemical and petrogenetic studies indicate the origin of the area’s plutonic rocks and the role of differential crystallization accompanied by the crustal rocks-contamination and digestion of magma in the evolution of the magma forming these rocks. This magma has been originated from the low-grade partial melting of an enriched mantle origin beneath the continental lithosphere with the lherzolite garnet composition at a depth of 100 to 110 km in a post-collision tensile environment. Investigating the fluids involved in the region, the homogenization temperature with the temperature of copper veins formation is between 120 to 306 ° C, with the salinity percentage varying between 6.45 to 15.96% of sodium chloride weight. Accordingly, this metamorphic hydrothermal orebodyis located in the mesothermal category. The presence of sub-faults, joints, and cracks in the host rock has provided a low-pressure environment for a proper place for copper mineralizationas veins.

Oxidized Mantle Sources of HIMU and EM-type Ocean Island Basalts

Oxygen fugacity (fO2) is a fundamental variable in igneous petrology with utility as a potential tracer of recycled surficial materials in the sources of mantle-derived lavas. It has been postulated that ocean island basalts (OIB) have elevated fO2 relative to mid-ocean ridge basalts (MORB) owing to more oxidized source regions. To clarify this issue, trace-element systematics of olivine grains are reported from OIB lavas with HIMU (high-; Mangaia, Canary Islands), enriched mantle (EM; Samoa; São Miguel, Azores Islands) and depleted MORB mantle (DMM; Pico, Azores) Sr-Nd-Pb-Os isotopic signatures, to constrain the fO2 of each magmatic system. Despite sampling distinct mantle reservoirs based on radiogenic isotope systematics, these OIB suites show similar fO2, ranging from +1.5 to +2.9 FMQ, with an average of 2.0 ± 0.7 FMQ, significantly higher than MORB at +0.6 ± 0.2 FMQ using the same oxybarometer. OIBs show no correlation between fO2 and bulk rock isotopic ratios or parental magma compositions. The lack of correlations with isotopic signatures likely results from radiogenic isotope signatures being hosted in volumetrically minor trace element enriched mantle lithologies, while fO2 reflects the volumetrically dominant mantle component. Higher fO2 in OIB relative to MORB implies a uniformly oxidizing plume source mantle that may be the result of either a common oxidized oceanic crust-rich reservoir parental to all modern plume lavas, or preservation of un-degassed and oxidized mantle domains formed early in Earth history.

Sr-Nd-Pb-Ca Isotopes of Holocene Basalts from Jingpohu, NE China: Implications for the Origin of Their Enriched Mantle Signatures

The geochemistry on Holocene lavas from the Jingpohu volcanic field in NE China are compared with other Cenozoic lavas from across the back-arc rift of NE China, in order to constrain their enriched mantle sources. Holocene lavas within Jingpohu volcanic field comprise two separate “Crater Forest” (CF) and “Frog Pool” (FP) volcanic areas. FP lavas have lower MgO, CaO, and heavy rare earth elements and higher Al2O3, Na2O, K2O, and large-ion lithophile elements than CF lavas. Yet, both CF and FP lavas share similar isotopic signatures, with depleted Sr and Nd isotopes (87Sr/86Sr = 0.703915–0.704556, 143Nd/144Nd = 0.512656–0.512849) and unradiogenic Pb isotopes (208Pb/204Pb = 37.79–38.06, 207Pb/204Pb = 15.45–15.54, 206Pb/204Pb = 17.49–18.15), similar to oceanic island basalts. An important new constraint for the Jingpohu lavas lies in their Ca isotopes of δ44/40Ca from 0.63 to 0.77‰, which are lower than that of the bulk silicate earth (0.94 ± 0.05‰). By comparing the isotopic signatures of sodic lavas with that of the potassic lavas across NE China, we propose a three-component mixing model as the source for the sodic lavas. In consistence with geophysical results, we propose that subducting Pacific plate induces asthenospheric mantle upwelling of an upper depleted mantle (DM), including subducted ancient sediments (EM I), which partially melted upon ascent. These primary melts further interacted with the lithospheric mantle (EM II), before differentiating within crustal magma chambers and erupting.

Ore and Geochemical Specialization and Substance Sources of the Ural and Timan Carbonatite Complexes (Russia): Insights from Trace Element, Rb–Sr, and Sm–Nd Isotope Data

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky сomplex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.

Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

The eastern Tibetan Plateau is a key part of the eastern India–Asia collisional zone, a region that records multiple overprinting tectonic and magmatic events. This study presents new geochronological, geochemical, and Sr–Nd–Hf–O isotopic data for Cenozoic potassic granitoids in eastern Tibet, southwestern China, which recorded the tectonic evolution of the eastern Tibetan Plateau. These potassic granitoids are formed between 37.6 and 32.9 Ma and are geochemically subdivided into the following: Group 1, adakite-like granites; Group 2, syenites; and Group 3, low-εNdt granitoids. The Group 1 samples are similar to high-silica adakites in that they have variable SiO2 contents (63.31–73.62 wt.%) and high Sr/Y and La/Yb ratios. These samples have εNdt and εHft values that range from −5.8 to −0.6 and from −4.3 to +5.2, respectively, with δ18O values of 6.78‰–7.36‰. The Group 2 samples are syenites, contain 56.36–63.86 wt.% SiO2 and high concentrations of Y and Yb, and have εNdt values from −8.4 to −2.4, εHft values from −6.1 to +1.1, and δ18O values of 6.37‰–6.89‰. The Group 3 samples have a narrow range of SiO2 concentrations (62.27–64.59 wt.%), high Sr/Y and La/Yb ratios, δ18O values of 6.31‰–6.82‰, and low εNdt and εHft values (−12.6 to −10.9 and −11.4 to −6.6, respectively) that are similar to the values obtained for the contemporaneous Yao’an lamprophyres. These data indicate that the Group 1 samples are formed from magmas sourced from a heterogeneous and thickened region of the lower crust containing an enriched mantle component. Group 2 magmas were most likely derived from contemporaneous mafic melts sourced from an ancient region of the lithospheric mantle previously modified by the incorporation of recycled components. The Group 3 samples have distinct Sr–Nd–Hf isotopic compositions that are indicative of derivation from magmas generated by the fractional crystallization of lower crustal melts sourced from ancient enriched mantle of the Yangtze Block. Combining these new data with the results of previous research suggests that the Cenozoic potassic igneous rocks of eastern Tibet were formed as a result of the thinning of the lithospheric mantle and an associated crustal collapse event, potentially representing a regional late Eocene to early Oligocene transition from compression to transtension in the eastern Tibetan Plateau. These potassic igneous rocks are contemporaneous with or are younger than igneous rocks in the Qiangtang Block, suggesting that the magmatic response to the India–Asia collisional event was initiated in the central Tibetan Plateau before propagating towards the eastern margin of this region.

A heterogeneous subcontinental mantle under the African–Arabian Plate boundary revealed by boron and radiogenic isotopes

The northern and northwestern margins of the Arabian Plate are a locus of a diffuse and long-lasting (early Miocene to Pleistocene) Na-alkali basaltic volcanism, sourced in the asthenosphere mantle. The upwelling asthenosphere at the Africa–Arabia margin produces very limited magma volumes in the axial zone. Therefore, portions of hot, fertile mantle continue their eastward migration and are stored at shallower depths under the 100-km thick Arabian lithosphere, which is much thinner than the African one (≈175 km): this causes the occurrence and 20-Ma persistence of magma supply under the study area. Erupted basalts sampled a continuous variation of the mantle source, with a striking correlation among temperature, pressure and isotopic composition shifting between two end members: a 100 km-deep, more depleted source, and a 60 km-deep, more enriched one. In particular, we observed an unusual variation in boron isotopes, which in the oceanic domain does not vary between more depleted and more enriched mantle sources. This study shows that, at least in the considered region, subcontinental mantle is more heterogeneous than the suboceanic one, and able to record for very long times recycling of shallow material.