Incompatible Trace Element Ratios Research Articles

Orogenic andesites and their link to the continental rock cycle

Continents are created and destroyed at convergent margins but understanding the interconnectedness of this system—a complex interplay of subaerial and tectonic erosion, volatile exchange and metamorphism, partial fusion and melt evolution—is far from complete. Here we use geochemical data from the Mexican convergent margin to reveal a strong connectivity between subducted materials and typical orogenic andesites. We find that the along-strike isotopic variations of andesitic stratovolcanoes in Mexico are mirrored by those of riverine sediments sampled along the continental margin, which presumably represent the compositions of tectonically ablated forearc crust entering the subduction zone. Isotopic modelling further indicates that the Mexican andesites represent partial melts of subduction mélanges, constituted by mixtures of altered subducted ocean crust, forearc debris and mantle peridotite in similar proportions. Remarkably, the concentrations and ratios of incompatible trace elements in the volcanoes also correlate with the major element compositions of their corresponding forearc sediments. This suggests that the modal contents of key mineral phases in the partially molten mélanges control the trace element abundances of the andesitic magmas. These findings demonstrate that a significant proportion of tectonically eroded subducted crust is not lost to the deep mantle, but effectively reincorporated into arc magmas in a closely interlaced continental rock cycle.

Mineralogy and trace elements variation of subsurface cretaceous basalts in the Eastern Tunisian margin and the Pelagic Sea: New data and geodynamic implication

Abstract- Eastern Margin of Tunisia and the Pelagic Sea were affected by four magmatic events during the Cretaceous. During these events, the products are mainly interbedded basalts, numerous mafic sills and dykes, volcanic breccias and pyroclastic products. Petrography, mineralogy and geochemistry studies prove two lineages of basalts: basalts with tholeiitic affinity and alkali basalts. Electronic microprobe studies of clinopyroxene prove an anorogenic context of this magmatism. We used variable distribution of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf. These elements are characterized by distinct mantle composition source and different partial melting trends. These data suggest that the magma of tholeiitic affinity basalt is originated within the enriched mantle (EM1), moderate impoverishment in light rare-earth elements (LREE). The mantle source of alkali basalt is compositionally transitional between the high-U/Pb mantle (HIMU) and enriched mantle (EM2). The tectonic history of the area combined to the new mineralogical and geochemical investigations of subsurface cretaceous basalts will contribute significantly to place this magmatism in its Mediterranean context. Tectonic models are proposed to explain the geodynamic and the tecto-magmatic history of the studied area during Cretaceous.

Mafic volcanic rocks of western Iron Ore Group, Singhbhum Craton, eastern India: Geochemical evidence for ocean–continent convergence

Precambrian mafic magmatism is an important global episode which played a significant role in the crustal evolution. In India, Singhbhum Craton being the oldest craton, witnessed significant occurrences of Precambrian geological activity, marked by several episodes of volcanism, plutonism, sedimentation spanning from Palaeoarchean to Mesoproterozoic age. Here we present petrological and geochemical characteristics of Precambrian mafic volcanic rocks (occurring in western Iron Ore Group (IOG), Singhbhum Craton, eastern India) to evaluate their petrogenetic aspects, tectonic setting, and magma generation. The mafic volcanic rocks are porphyritic in nature with the phenocrysts of plagioclase and groundmass composed of clinopyroxene, plagioclase, ilmenite, and volcanic glass. These rocks are mostly tholeiitic, sometimes with a transitional behaviour towards calc‐alkaline nature and display basalt‐basaltic andesite affinity. These mafic volcanic rocks also preserve geochemical signatures (high Nb/U, Nb/La, [Nb/Th]pm ratios) in support of Nb‐enriched basalts and are classified as Nb‐enriched basalts (NEB; Nb > 7 ppm) and high‐Nb basalts (HNB; Nb > 20 ppm) on the basis of Nb concentrations and mantle normalized Nb/La ratios (>0.5). The NEBs and HNBs are marked by lesser magnitude of negative Nb anomalies with high (Nb/Th)pm, (Nb/La)pm, and Nb/U ratios as compared to normal arc basalts. Several major element oxides, trace elements, and selected element ratios (like SiO2, CaO/Al2O3, Y, V/Cr, Zr/Nb, and ∑REE) show systematic variations with MgO which suggests role of magmatic fractionation. Chondrite‐normalized REE patterns for NEB and HNB rocks exhibit uniform LREE enrichment with distinct Eu negative anomalies while primitive mantle‐normalized incompatible trace element patterns reflect enrichment in LILE and LREE with prominent Nb‐Ta anomalies. Different HFSE ratios corroborate a subduction related setting for magma generation formed by ∼10%–20% melting in the domain of garnet lherzolite. Relative enrichment of LILE and LREE with depleted HFSE characteristics attest a garnet‐bearing mantle source and melt extraction with garnet in the residue. Geochemical signatures suggest that the genesis of NEB and HNB is attributable to slab‐melting and wedge hybridization processes during matured stages of subduction. Selected incompatible trace element ratios for the studied mafic volcanic rocks invoke an enriched (EM1‐EM2 type) mantle source and unequivocally suggest effects of continental crustal assimilation of the parent magma. Liquid immiscibility has played an important role as a differentiation mechanism leading to presence of high and low FeO types. The IOG mafic volcanic rocks preserve distinct geochemical signatures of matured stage of subduction, slab melting, crustal contamination and magma generation at an Archean ocean–continent convergent margin setting.

Late Jurassic Leucogranites of Macau (SE China): A Record of Crustal Recycling During the Early Yanshanian Orogeny

The Yanshanian Orogeny consists of Jurassic to Cretaceous compressional-extensional cycles related to the subduction of the Paleo-Pacific Ocean beneath East Asia. Numerous granitic plutons were emplaced across southeast China at distinct intervals over a period of ~100 million years that migrate from the interior to the coast. A key region to help constrain the secular evolution of granitic magmatism related to the Yanshanian Orogeny is the Peal River estuary as three distinct Mesozoic magmatic belts congregate. The islands of Taipa and Coloane, Macau Special Administrative Region, are located along the western bank of the Pearl River estuary and are composed of spessartine-bearing biotite leucogranite. In situ zircon geochronology yielded Late Jurassic weighted-mean 206Pb/238U ages of 160.0 ± 2.0 Ma and 161.5 ± 2.0 Ma that correlate to the late Early Yanshanian Orogeny. Inherited zircons of Middle Triassic to Middle Jurassic age were also identified. The rocks are peraluminous, ferroan, calc-alkalic to calcic and classify as post-collisional granite. Geochemical modeling indicates that the chemical variability of the rocks is related to hydrous fractional crystallization under reducing conditions (dFMQ -1) at ~7 km depth. The Sr-Nd isotopes (ISr = 0.71156 to 0.72477; eNd(t) = -7.9 to –8.6), zircon Hf isotopes (eHf(t) = -3.8 to -8.8), incompatible trace element ratios, and whole rock composition indicate that the parental magma was derived primarily from a sedimentary source. It is likely that the islands of Taipa and Coloane form a coherent plutonic complex that may be a member of larger batholith that extends across the Pearl River estuary to Hong Kong. The emplacement of the leucogranites is attributed to decompressional melting associated with a period of crustal relaxation or tensional plate stress that occurred during the transition from low angle subduction to high angle subduction of the Paleo-Pacific plate.

Unusually homogeneous helium isotope composition of the Auckland Volcanic Field and its implications for the underlying mantle

The Auckland Volcanic Field (AVF) is one of the most intensively studied monogenetic basalt fields in the world yet its origin remains enigmatic. Magmatism in the AVF occurred from ~193 ka to 500 years bp. The trace element and isotopic diversity of AVF basalts require “small-scale” compositional heterogeneity in the underlying mantle that is comparable in volume to, or slightly larger than, the scale of individual eruptions in the AVF.Olivine from tephra and lava, representing the range of AVF compositions, was crushed and analysed for 3He/4He, and He and CO2 concentrations in an attempt to further characterize and explain the significance of the AVF “end-members”. AVF basalts show a negative covariation between the amount of CO2 released by crushing of olivine and the whole rock concentrations of highly incompatible trace elements, such as Ba, Rb, Nb, Zr, Ti and K. In contrast, the amount of He released by crushing shows no simple relations with the same incompatible elements or their ratios. This leads to a significant variation in CO2/3He ratios (9.4 × 107–3.5 × 109) that may relate to differences in magma ascent dynamics, as well as to different magma sources. The measured CO2/He ratios may have been influenced by varying amounts of CO2 diffusion into vapour bubbles within melt inclusions that depend on melt composition and magma ascent rate. However, petrographically there is no evidence for systematic differences in the size or quantity of vapour bubbles in olivine-hosted melt inclusions.3He/4He ratios in 14 AVF samples studied here show a narrow range from 6.57 to 7.26 RA (mean of 7.10 ± 0.26). This may imply a dominance of the mantle helium budget by small-scale heterogeneities. Alternatively, the 3He/4He results suggest that the tectonic and magmatic history of the mantle beneath the AVF has effectively hybridized the 3He/4He ratio to a larger extent than for highly incompatible trace element ratios and Pb-Nd-Sr isotopes. The value of ~7 RA for the mantle source of AVF basalts, in light of other evidence, suggests that there is a relatively homogeneous He isotope composition for the Zealandia-Antarctic mantle domain.

Kimberlites as Geochemical Probes of Earth’s Mantle

Kimberlites are ultrabasic, Si-undersaturated, low Al, low Na rocks rich in CO2 and H2O. The distinctive geochemical character of kimberlite is strongly influenced by the nature of the local underlying lithospheric mantle. Despite this, incompatible trace element ratios and radiogenic isotope characteristics of kimberlites, filtered for the effects of crustal contamination and alteration, closely resemble rocks derived from the deeper, more primitive, convecting mantle. This suggests that the ultimate magma source is sub-lithospheric. Although the composition of primitive kimberlite melt remains unresolved, kimberlites are likely derived from the convecting mantle, with possible source regions ranging from just below the lithosphere, through the transition zone, to the core–mantle boundary.

Petrogenesis of three East Fork Member rhyolites of the Jemez volcanic field, Valles caldera, New Mexico, USA

The most recent volcanism in the Valles caldera is represented, in stratigraphic order, by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), the VC-1 rhyolite (not studied herein), and Banco Bonito Flow (BBF), which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted approximately 74 to 68 ka after an approximate 460 ka lull in ring-fracture volcanism.Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides, in order of abundance. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (≥1 mm), typically resorbed or subhedral crystals represent one population, and small (≤0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have oligoclase to andesine rim overgrowths. 40Ar/39Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 3 ka and 575 ± 2 ka and total gas ages of 125 ± 1 ka and 129 ± 1 ka, respectively. High molar Mg numbers of large euhedral biotite and 40Ar/39Ar ages older than the accepted age range indicate these crystals are xenocrystic. Radiogenic isotopes are consistent with mixing between the mantle and lower crustal reservoirs. General trends are evident between whole-rock major and trace elements with increasing SiO2 and Nb, respectively. In general, incompatible trace elements increase and compatible trace elements decrease with increasing Nb. For the major elements, MgO, P2O5, Al2O3, FeO* show decreasing trends while K2O and Na2O show increasing trends with increasing SiO2. Incompatible trace element ratios (Ta/Yb, Y/Yb, Th/Yb, Th/Nb) indicate the presence of a single magma batch.The heterogeneity in crystal morphology and chemistry can be explained by a model in which partial melting of mid- to deep continental crust occurred due to an intrusion of an intermediate composition magma. Magma mixing and an exchange of crystals took place between the partial melt and the intruding magma. The hybrid magma rose to the upper crust, where it underwent fractional crystallization prior to eruption. The geochemical and isotopic data from this study are best explained by a modified version of the rapid production and eruption model put forth by Huppert and Sparks (1988).

Geochemistry and petrogenesis of the Tsirub nephelinite intrusions, southern Namibia

Field studies and geochemical analysis of a nephelinite dyke and two plug-like bodies - Tsirub (south) and Tsirub (north), occurring in southern Namibia are reported. Bulk rock geochemical analyses show that the nephelinites have limited but significant compositional variation (MgO = 8.7–12.1 wt%; Mg# = 0.61–0.71; Zr = 415–525 ppm; Nb = 152–267 ppm; Ni = 105–305 ppm; Zr/Nb = 1.8–3.1; La/Smn = 3.8–6.1). In detail, systematic minor variations in incompatible trace element abundances and ratios and Sr and Nd isotope compositions (87Sr/86Sr = 0.70335–0.70441; 143Nd/144Nd = 0.51278–0.51272) are best accounted for by small differences in degree of partial melting and some source heterogeneity. Quantitative modelling of partial melting processes indicates that compositional variations can be accounted for by low but variable degrees of partial melting (2–6%) of a peridotitic mantle. Negative K and to a lesser extent Ti, anomalies on primitive mantle normalised diagrams require melting to have occurred in the presence of minor amphibole, constraining the depth of melting to less than ~38 kbar, i.e. within the subcontinental lithospheric mantle. Heat required to raise the ambient geotherm (to ~55 mWm−2) to allow low degrees of melting of a volatile-rich (H2O–CO2 undersaturated) peridotitic lithospheric mantle is attributed to the passage of the African plate over the Vema mantle plume at 40–50 Ma. The close spatial proximity of the Tsirub nephelinite to the Dicker Willem carbonatite, Schwarzeberg nephelinite and alkaline intrusive rocks associated with the Klinghardt phonolites is taken to indicate a similar emplacement age of ~30–50 Ma and a related cause of magmatism.

The multiple depleted mantle components in the Hawaiian-Emperor chain

Oceanic island basalts are targeted for geochemical study because they provide a direct window into mantle composition and a wealth of information on the dynamics and timescales associated with Earth mixing. Previous studies mainly focused on the shield volcanic stage of oceanic islands and the more fusible, enriched mantle components that are easily distinguished in those basalts. Mantle depleted compositions are typically more difficult to resolve unless large amounts of this material participated in mantle melting (e.g., mid-ocean ridges), or unique processes allow for their compositions to be erupted undiluted, such as very small degrees of melting of a source with minimal fusible enriched components (e.g., rejuvenated basalts) or as xenoliths (e.g., abyssal peridotites). Mantle depleted components, defined here as material with low time-integrated Rb/Sr (low 87Sr/86Sr) and high time-integrated Sm/Nd and Lu/Hf ratios (high 143Nd/144Nd and 176Hf/177Hf) relative to primitive mantle, derive from a potentially very large volume reservoir (up to 80% of the mantle), and therefore need adequate characterization in order estimate the composition of the Earth and mantle-derived melts. This review focuses on mantle depleted compositions in oceanic island basalts using the Hawaiian-Emperor chain as a case study. The Hawaiian-Emperor chain is the ∼6000km long geological record of the deeply sourced Hawaiian mantle plume, active for >81 Myr. Hawaiian volcanism evolves through four volcanic stages as a volcano traverses the Hawaiian plume: alkalic preshield, tholeiitic shield (80–90% volcano volume), alkalic postshield (∼1%), and silica undersaturated rejuvenated (<0.1%). We report Pb-Sr-Nd-Hf isotope compositions and trace element concentrations of three rejuvenated Northwest Hawaiian Ridge basalts and compare them to an exhaustive compiled dataset of basalts from the Hawaiian Islands to the Emperor Seamounts. The Northwest Hawaiian Ridge (NWHR) includes 51 volcanoes spanning ∼42m.y. between the bend in the Hawaiian-Emperor chain and the Hawaiian Islands where there is no high-precision isotopic data published on the rejuvenated-stage over ∼47% of the chain. NWHR and Hawaiian Island rejuvenated basalts are geochemically similar, indicating a consistent source for rejuvenated volcanism over ∼12.5 million years. In contrast, shield-stage basalts from the oldest Emperor Seamounts are more depleted in isotopic composition (i.e., higher 176Hf/177Hf, and 143Nd/144Nd with lower 87Sr/86Sr and 208Pb*/206Pb*) and trace element concentrations (i.e., much lower concentrations of highly incompatible elements) than all other depleted Hawaiian basalts younger than the bend, including NWHR rejuvenated basalts. The strongly depleted source for the oldest Emperor Seamounts (>70 Ma) was likely related to interaction with the Kula-Pacific-Izanagi mid-ocean ridge spreading system active near the Hawaiian plume in the Late Cretaceous. In contrast, the incompatible trace element ratios of NWHR rejuvenated basalts require a distinct source in the Hawaiian mantle plume that was imprinted by ancient (>1 Ga) partial melting, likely ancient recycled oceanic lithosphere. This review of the geochemistry of Hawaiian depleted components documents the need for the sampling of multiple distinctive depleted compositions, each preferentially melted during specific periods of Hawaiian plume activity. This suggests that the composition of depleted components can evolve during the lifetime of the mantle plume, as observed for enriched components in the Hawaiian mantle plume. Changes in the composition of depleted components are dominantly controlled by the upper mantle tectonic configurations at the time of eruption (i.e., proximity to a mid-ocean ridge), as this effect overwhelms the signal imparted by potentially sampling different lower mantle components through time.

Heterogeneous mantle melting and magmatic processes at the East Pacific Rise (2.6–3.1°S): Evidence from mid-ocean ridge basalt geochemistry and Sr–Nd–Pb isotopes

ABSTRACT We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe2O3, and 0.06–0.34 K2O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and 87Sr/86Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.

The Mount Cameroon southwest flank eruptions: Geochemical constraints on the subsurface magma plumbing system

With seven major eruptions in the 20th century, Mount Cameroon is one of the most active volcanoes in Africa. However, information on the volcano's plumbing system is still relatively scarce. In order to contribute to filling this knowledge gap, a geochemical, clinopyroxene geobarometry, and SrNd isotope study was carried out on samples collected from the 1922 and lower 1999 lava flows found on the SW flank of the volcano. Petrographic evidence shows a common crystallising mineral assemblage of olivine and clinopyroxene and/or plagioclase scattered in a microlite-rich groundmass. The total alkali-silica classification diagram shows that the lavas of 1922 exhibit basanitic compositions those of 1999 are predominantly basanites with some few hawaiites. The Sr isotope ratios for the 1999 samples in this study show values somewhat lower and higher than the limits obtained for historic Mount Cameroon samples so far. While major and trace element trends indicate fractional crystallisation processes, chemical and mineral disequilibrium textures are indicative of open-system processes such as magma mixing and magma chamber recharge. Variations in incompatible trace element ratios, which hardly change during fractional crystallisation, are also suggestive of open-system processes. Plots of K2O and Nb show a mixing trend between samples from these eruptions, implying a common magma chamber where remnant material from preceding eruptions was incorporated into succeeding eruptions. Clinopyroxene geobarometric calculations show this common dominant crystallisation depth at 23–29 km.

Across-arc geochemical variations in the Paleogene magmatic belt of Iran

Across-arc geochemical variations between igneous rocks from the magmatic front (MF) to rear-arc (RA) are commonly observed for convergent margins, but the processes that are responsible for these variations are unclear. To address these questions, we studied a well-exposed Cenozoic continental arc, the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran and its rear-arc counterpart to the north.North Iran RA magmatism is bimodal, characterized by mafic to felsic igneous rocks of calc-alkaline to shoshonitic affinity. The new zircon U -Pb ages show that plutonic rocks are ~ 42–38 Ma whereas volcanic rocks are older at ~52 Ma. 87Sr/86Sr(t) and εNd(t) values of UDMB RA igneous rocks range from 0.70483 and 0.70783 and –2.5 and + 3.6, respectively. Zircons from plutonic rocks show εHf(t) values ranging from +1.8 to +9.8, higher than volcanic rocks with εHf (t) between +2.6 and –9.2. Isotopic data agrees with inferences from incompatible trace element ratios (e.g., Th/Yb, Zr/Nb and La/Yb) that mafic plutonic rocks originated from an enriched mantle source, whereas felsic volcanic rocks show more interaction with continental crust. High magmatic fluxes did not occur at the same time in the MF and RA; UDMB MF flared-up at ~ 54–37 Ma and 20–5 Ma, overlapping RA magmatic flare-ups at 45–40 Ma and 30–25 Ma. The available major and trace element data from the UDMB show that most magmatic rocks experienced strong fractional crystallization and assimilation-fractional crystallization processes, making it difficult to identify across-arc geochemical variations. Moreover, the bulk rock εNd(t) and zircon εHf(t) values are highest at the MF, but the across-arc isotopic variability seems to be mainly controlled by crustal influence. High-flux magmatism in the UDMB was related to protracted heating of the crust as a result of subduction of Neotethys oceanic lithosphere. This weakened the continental lithosphere, leading to strong extension in Paleogene time accompanied by decompression melting and magmatic flare-ups.

Pyroxenite dykes with petrological and geochemical affinities to the Alaskan-type ultramafics at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India: Tectonomagmatic implications

Two previously reported lamprophyre dykes from the Kalwakurthy area, at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India, are reinvestigated. Petrography reveals that they have an overall cumulate texture and comprise clinopyroxene (dominant phase), amphibole (mostly secondary), magnetite, ilmenite and chromite and are reclassified as clinopyroxenites. The chemistry of clinopyroxene and chromite, bulk-rock major and trace element composition and the Sr–Nd isotopic systematics of the Kalwakurthy dykes strongly favour the involvement of subduction-related processes in their genesis and are strikingly similar to those of the continental arc-cumulates and Alaskan-type ultramafics reported from the supra-subduction type of tectonic settings. Incompatible trace element ratios, involving high field strength elements, of these clinopyroxenites are also suggestive of the fluid-related metasomatism influencing their source regions. Petrogenetic modelling reveals that 10–20% partial melting of the fertile lithospheric mantle source was involved in their genesis. The tectonomagmatic significance of the studied clinopyroxenites is evaluated in light of the existing models invoking a Neoarchaean subduction in the evolution of the Dharwar craton.

Distinguishing Plume and Metasomatized Lithospheric Mantle Contributions to Post-Flood Basalt Volcanism on the Southeastern Ethiopian Plateau

Abstract Magmatism in the East African Rift System (EARS) contains a spatial and temporal record of changing contributions from the Afar mantle plume, anciently metasomatized lithosphere, the upper mantle and the continental crust. A full understanding of this record requires characterizing volcanic products both within the rift valley and on its flanks. In this study, three suites of mafic, transitional to alkaline lavas, were collected over a northeast-southwest distance of ∼150 km along the southeastern Ethiopian Plateau, adjacent to the Main Ethiopian Rift. Specifically, late Oligocene to Quaternary mafic lavas were collected from Chiro, Debre Sahil and the Bale Mountains. New major element, trace element, 40Ar/39Ar ages and isotopic results (Sr, Nd, Pb, Hf, Os, He) show spatial and temporal variation in the lavas caused by dynamical changes in the source of volcanism during the evolution of the EARS. The trace element compositions of Oligocene and Miocene Chiro lavas indicate derivation from mildly depleted and nominally anhydrous lithospheric mantle, with variable inputs from the crust. Further south, Miocene Debre Sahil and alkaline Bale Mountains lavas have enriched incompatible trace element ratios (e.g. Ba/Nb = 12–43, La/SmN = 3·1–4·9, Tb/YbN = 1·6–2·4). Additionally, their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf and 206Pb/204Pb values trend toward a radiogenic Pb (HIMU) component. Radiogenic 187Os/188Os in these lavas correlates positively with 206Pb/204Pb and trace element indicators consistent with ancient metasomatic enrichment of their mantle source. In contrast, transitional Miocene Bale Mountains lavas have lower incompatible trace element abundances, less enriched trace element ratios (Ba/Nb ∼7, La/SmN = 2·3–2·5) and less radiogenic isotopic signatures that originate from melting garnet-bearing, anhydrous lithospheric mantle (Tb/YbN = 2·5–2·9). Pliocene and Quaternary Bale Mountains basaltic lavas are chemically and isotopically similar to Main Ethiopian Rift lavas. Trace element and isotopic indicators in both of these suites denote an amphibole-bearing source distinct from that sampled by the older Bale Mountains lavas. Isotopically, Pliocene and Quaternary Bale lavas have notably less radiogenic Sr–Nd–Pb–Hf isotopic ratios. Quaternary Bale Mountains lavas have the strongest mantle plume contribution (3He/4He = 12·1–12·5 RA), while other Bale Mountains, Debre Sahil and Chiro lavas were derived dominantly by melting of lithospheric or upper mantle sources (3He/4He = 5·1–9·1 RA). A multi-stage, regional-scale model of metasomatism and partial melting accounts for the spatial and temporal variations on the southeastern Ethiopian Plateau. Early Debre Sahil and alkaline Bale Mountains mafic lavas are melts derived from Pan-African lithosphere containing amphibole-bearing metasomes, while later transitional Bale basalts are melts of lithosphere containing anhydrous, clinopyroxene-rich veins. These ancient metasomatized domains were eventually removed through preferential melting, potentially during thermal erosion of the lithosphere or lithospheric foundering. Pliocene and Quaternary Bale Mountains lavas erupted after tectonic extension progressed throughout Ethiopia and was accompanied by increased plume influence on the volcanic products.

H2O in basaltic glasses from the slow-spreading Carlsberg Ridge: Implications for mantle source and magmatic processes

H2O in the mantle is of great interest in the fields of geochemistry and geophysics on account of its significant influence on the melting and physical properties of mantle rocks. The abundance of water in derivative mantle melts beneath mid-ocean ridges can provide important insights into the abundance and distribution of water in the mantle. For this study, we analyzed the H2O, CO2, and major- and trace-element contents in basaltic glass samples collected from the slow-spreading Carlsberg Ridge from 59°E to 66°E in the Indian Ocean. All of the studied samples are found to be normal mid-ocean ridge basalt (N-MORB) in composition. The H2O concentrations range from 0.13 to 0.29wt% with a mean value of 0.22±0.10wt%. Along the ridge from 59°E to 66°E, H2O/La, H2O/Ce, and trace-element ratios show systematic variation whereby H2O/La and H2O/Ce ratios increase with longitude, whereas K2O/Ti2O and (La/Sm)N decrease. The Nb/U (36.4–42.3) and Ce/Pb (17.6–20.8) ratios of the samples are lower in comparison with typical values reported for global MORBs and OIBs (ocean island basalts) (Nb/U=ca. 47, Ce/Pb=ca. 25). In addition, the ratios of incompatible trace elements display wide variation (e.g.,Ba/La and Nb/Zr values of 2.04–6.02 and 0.02–0.07, respectively). The limited variation in the degree of mantle melting cannot yield the observed level of variation in the basaltic glass trace-element ratios. Therefore, we suggest that the mantle source of the studied MORBs has been mixed with subducted oceanic/continental crust and that mantle heterogeneity is the main factor responsible for the chemical variation of MORB along the Carlsberg Ridge axis from 59°E to 66°E.

Zircon U–Pb geochronology, Hf isotopic compositions, and petrogenetic study of Abor volcanic rocks of Eastern Himalayan Syntaxis, Northeast India: Implications for eruption during breakup of Eastern Gondwana

This paper reports new zircon U–Pb ages and Hf isotopic compositions of felsic units of the Abor volcanic rocks (AVR) of Eastern Himalayan Syntaxis (EHS), Northeast India, and discusses their relationship to the Kerguelen plume activity. The AVR are bimodal and predominantly constituted by mafic rocks with minor felsic units. Mafic volcanics are identified as basalt and basaltic andesite with light rare earth elements (LREE) enriched and slightly depleted heavy rare earth elements (HREE) pattern without Eu anomalies. Low concentrations of LILE, high contents of Fe2O3, and other incompatible trace elements ratios reflect enriched nature of these mafic volcanics. Felsic volcanic rocks are dacitic to rhyolitic in composition, which have high REE content, high LREE/HREE, and pronounced negative Eu anomalies. Enriched LREE, high Th/Nb, Ce/Nb ratios, and variations in Rb/Zr, K/Rb, La/Sm ratios with negative anomalies of Ba, Nb, Sr, P, Ti in felsic rocks suggest substantial contribution of crustal contamination at the time of eruption. Zircons from felsic units yield an average U–Pb age of ~132 Ma and unradiogenic (ƐHf(t) &lt; 0) Hf isotope values of −7.0 to −13.3 with model ages between 1.5 and 2.1 Ga, suggesting old crustal assimilation in their genesis. The AVR were emplaced in the continental rift tectonic setting, and depth of the magma source is confirmed as near spinel stability zone. The AVR are positively comparable with other flood basalts that were formed due to the Kerguelen plume activity. Therefore, our combined new geochemical and geochronological data show that the AVR were emplaced at early stage (~132 Ma) of eastern Gondwana breakup due to outbreak of the Kerguelen plume. This study thus supports the idea of the Kerguelen plume affecting a large area of Eastern India, Western Australia, and Antarctica during early stage of Gondwana breakup.

Tracing mantle sources in the northern Lau back-arc basin by independent component analysis of basalt isotopic compositions

ABSTRACT The northern Lau back-arc basin (NLB) lavas display a diverse geochemical nature caused by complex geological processes in this region. Independent component (IC) analysis was applied to investigate the nature of mantle sources in the NLB, based on a compiled geochemical data set from the NLB, central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoan islands. We identified three ICs in the five-dimensional space of Sr-Nd-Pb isotopic ratios, which can account for 96.5% of the isotopic variance. The correlations between the ICs and the incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) separates Samoan islands (IC1 < −1) from the other groups, and shows negative correlation with (La/Sm)N ratios. The second IC (IC2) discriminates mid-ocean ridge basalts (MORBs) from Pacific (IC2 > 0) and Indian (IC2 < 0) Ocean ridges as well as the C-SLB (IC2 > 0) and NLB (IC2 < 0) lavas. IC2 correlates positively with Ba/Th ratios. The third IC (IC3) distinguishes MORBs (IC3 > 0) and back-arc basin lavas (IC3 < 0), and displays a negative correlation with Th/Nb values. On a regional scale in the NLB, there is a broad increase in IC1, IC2, and IC3 from the north to the south. Only IC3 presents obvious decrease from the west to the east, i.e. with decreasing distance from the arc. The geochemical nature and the statistical properties of these ICs suggest that IC1 is related to an enriched mantle component most likely from the nearby Samoan plume, and the IC2 corresponds to a fluid-rich component from the subducting Pacific slab. The IC3 may represent partial melt of recycled subducted sediment or recycled continental crustal materials. The geographic distribution of three ICs supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized back-arc mantle may be responsible for the observed geochemical diversity in NLB lavas.

Control of source fertility on the eruptive activity of Piton de la Fournaise volcano, La R\xe9union

The eruptive activity of basaltic hotspot volcanoes displays major fluctuations on times scales of years to decades. Theses fluctuations are thought to reflect changes in the rate of mantle melt supply. However, the crustal filter generally masks the mantle processes involved. Here, we show that the cyclic and generally increasing activity of the Piton de la Fournaise volcano (La Réunion) since the mid 20th century is tightly linked to the fertility of its source, as recorded by 87Sr/86Sr and incompatible trace elements ratios of lavas. We identify a twofold control of source fertility on eruptive activity: melt extraction from fertile, incompatible element-enriched veins initiates decadal-scale eruptive sequences, so that vein distribution in the plume source directly controls the cyclic activity. Indirectly, reactive flow of enriched melts increases mantle porosity and promotes melts extraction from the peridotite matrix. This process is thought to have caused a fourfold increase in magma supply between 1998 and 2014 at Piton de la Fournaise, and could also explain magma surges at other frequently active hotspot volcanoes, such as Kilauea, Hawaii. The short-term eruptive activity of hotspot volcanoes appears to be ultimately linked to the distribution and size of lithological heterogeneities in mantle plumes.

Mantle sources and magma evolution of the Rooiberg lavas, Bushveld Large Igneous Province, South Africa

We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with > 4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of < 4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~ 4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative eNdi (− 5.2 to − 9.4) and radiogenic eSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

Silicate melt inclusions (SMIs) in olivine, clinopyroxene, and orthopyroxene in basalt and trachyandesite of the middle Okinawa Trough (MOT) and in basaltic andesites of the southern Okinawa Trough (SOT) were investigated in detail to understand the evolution of magma and geochemical characteristics of the basaltic magma source. Both the SMIs and host bulk rocks are more heavily enriched with Pb and large ion lithophile elements than with high field strength elements and rare earth elements. The major element compositions of SMIs in the MOT trachyandesite show a scattered variation trend of medium‐K to shoshonitic series and suggest that a K‐rich magma source may exist in the Okinawa Trough. Two groups with distinct element distribution patterns observed in SMIs of the MOT trachyandesite may represent magmas with different evolutionary histories. Compared with fractional crystallization and partial melting processes, magma mixing may serve as a reasonable process for the production of trachyandesite in the MOT. Fractional crystallization has played an important role in basaltic magma evolution in the MOT and SOT. Simulations conducted using MELTS show that the fractional crystallization evolution trends observed correspond well to the presence of a magma chamber at 300 MPa with an H2O content level of 0.2 wt% in the MOT and to the presence of a magma chamber at 300 MPa with an H2O content level of 0.4 wt% in the SOT. Ratios of incompatible trace elements (e.g., Th/Ce, Th/Sm, Ba/Th, and Th/Nb); positive Pb anomalies; negative Nb, Ta, and Ti anomalies; and the existence of organic hydrocarbons (CnHm) in the SMIs can be attributed to the involvement of subducted sediment from the subducting slab. Modelling results suggest that more primitive magma represented by SMIs from the MOT has formed through the melting of less than 6% of a metasomatized depleted mantle with contributions of 1.2% sediment melt and 0.8% sediment fluid. In contrast, more primitive magma from the SOT formed through the melting of 4–10% of the depleted mantle with contributions of 3% sediment melt and 1% sediment fluid. The different features of the SMIs together with different tectonic settings of the MOT and SOT have formed in response to the westward subduction of the Philippine Sea Plate.