MORB Source Research Articles

Mesozoic crustal growth and recycling along the Southern margin of Eurasia: Magmatic rocks from the Sanandaj-Sirjan Zone of Iran

The Sanandaj-Sirjan Zone of western Iran is characterized by significant Mesozoic magmatic activity (from the early Jurassic to the late Cretaceous), with Cenozoic rocks being less common. Granitoids are prevalent in this zone, displaying A-, I-, and S-type geochemical characteristics. Despite several studies on these granitoids, there are still some questions regarding their sources– especially the S- and A-type granitoids – and the crustal growth and recycling in this zone, which require further studies using isotopes. This study focuses on the Aligoodarz, Kolah-Ghazi, and Golpayegan granitoids from the central SaSZ to better understand crustal growth and recycling in the Sanandaj-Sirjan Zone. The Aligoodarz and Kolah-Ghazi granitoids are identified as S-type granites, showing strong peraluminous signatures. Geochemical evidence suggests a mixture of melts from sub-crustal metasedimentary and meta-igneous sources, with factors such as source diversity and disequilibrium melting contributing to their formation. in contrast, the Golpayegan granitoids exhibit high total alkali (Na2O + K2O) contents and high field strength elements, indicating alkaline magmatic affinity. Their geochemical signatures suggest derivation from an enriched portion of an Indian MORB-like protolith. Golpayegan syenites share similarities with within-plate A1 granites, suggesting they are related to anorogenic silica-oversaturated granitoids. Golpayegan A-type granites probably originated from high-degree fractionation of alkali basic magmas. Golpayegan gabbros may be derived from a MORB or OIB-like mantle source, with characteristics indicating the presence of garnet in the mantle source and potentially deep melting from a mantle plume. Newly compiled zircon UPb data from the Sanandaj-Sirjan Zone show a complex magmatic chronology spanning from approximately 180 million years ago to the Eocene epoch, revealing peaks of magmatic activity at various intervals. Distinct phases of A-, I-, and S-type magmatism are identified, and synthesis of zircon UPb ages and HfO isotope data reveals the simultaneous occurrence of S-type and I-type magmas during specific periods. Furthermore, our study explores the tectonic settings underlying these magmatic processes and proposes scenarios involving subduction, continental rifting, and asthenospheric upwelling to elucidate the geological dynamics of the SaSZ.

Geochemical dichotomy of Tethyan oceanic mantle and the supercontinent connection: Insights from Os isotopic signatures of mantle peridotites from Naga Hills ophiolite

Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ∼ 5–20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218–0.1266) and γOs values of −3.73 to −0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative γOs values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.

Cenozoic mantle plume activity in East Asia: Constraints from geochemistry of olivine, spinel, and melt inclusions

It is generally believed that the Cenozoic basalts distributed across eastern China, east of the Daxinganlin-Taihangshan gravity lineament (DTGL), are related to the big mantle wedge (BMW) beneath East Asia. Basalts that are widely distributed across central and eastern Mongolia form a contemporaneous ‘diffuse’ igneous province with similar major element, trace element and Sr-Nd-Hf-Pb isotopic characteristics to those in eastern China. However, they are located to the west of the DTGL, far from the influence of the BMW system. To explain the consistency of geochemistry and eruption ages of Cenozoic basalts on both sides of the DTGL, we have analyzed the compositions of Cenozoic basalts, olivines, spinels, and olivine-hosted melt inclusions from central-eastern Mongolia. We report olivine crystallization temperatures (Tc, up to 1404 and 1343 °C in central and eastern Mongolia, respectively) and mantle potential temperatures (Tp: 1456–1519 °C in eastern Mongolia) that are much higher than those in the MORB source (Tc 1230–1300 °C and Tp ∼ 1350 °C), indicating that there are thermal anomalies in the mantle sources feeding the central-eastern Mongolia volcanism. The Pb isotope ratios of basalts and olivine-hosted melt inclusions on both sides of the gravity zone show a linear array between EM1 and FOZO mantle components, which can be explained by the mixing of ancient recycled ocean crust (EM1-like) with lower mantle peridotite (FOZO-like) in different proportions. Combined with reported high Tp, 3He/4He, and a low-velocity anomaly extending to the lower mantle in the source of the Cenozoic basalts in East Asia, we propose a branched plume model that could explain the similarity of geochemistry and eruption ages for Cenozoic basalts on both sides of the DTGL.

Multiple Sulfur Isotope Geochemistry of the Precambrian Mafic Dykes and Komatiites in the Dharwar Craton, Southern India: Evidence for Crustal Recycling and Enrichment in the Subcontinental Lithospheric Mantle

Abstract Komatiites, greenstone volcanic rocks, and mafic dyke swarms are constituents of early earth magmatic activity, crucial for understanding the chemical evolution of the Archean mantle. The composition of the subcontinental lithospheric mantle (SCLM) is systematically modified throughout the Earth’s history by the addition of geochemically diverse oceanic and continental crustal materials through subduction and can be sampled through intraplate mafic/ultramafic volcanic activities. Here, we present a first report on the multiple sulfur isotope characteristics of the mafic dyke swarms and komatiites from the Dharwar craton in southern India and discuss the geochemical modifications of SCLM through crustal recycling. δ34SV-CDT values of the samples are all negative ranging from -0.15 to -2.91‰. Δ33S values for all the samples are close to 0 with the lowest value of -0.060‰ and highest of 0.146‰. Δ36S values are mostly negative with very few exceptions, ranging from -1.184 to 1.111‰. Near zero values of Δ33S and negative values for δ34S indicate an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. Together with trace element geochemistry, we suggest a depleted MORB source mantle (DMM) modified by oceanic crustal components and a depleted mantle (DM) modified by recycled continental crustal sediments as the two end members of the mantle source that produced mafic dyke swarms in the Late Archean to Proterozoic Dharwar craton.

Geochemistry of Mafic-Intermediate Intrusive Rocks from the Twangiza-Namoya Gold Belt, Eastern Democratic Republic of Congo: Trace Element Constraints on their Origin, Petrogenesis and Tectonic Setting

Bulk geochemical data for the Twangiza-Namoya Gold Belt, Democratic Republic of Congo, are presented in order to classify the rock assemblages, elucidate their petrogenesis and tectonic settings. Nb/Yb and Zr/Ti constraints reveal two suites for the rocks: sub-alkaline to andesitic basalts (Suite-1); and alkaline basalts (Suite-2). Ratios of Ti/Zr = 22–70; Zr/Hf = 35–42; Nb/Ta = 12–13; Nb/Th = 1–2 and La/Nb = 2.4–3.8 suggest arc-generated mantle melts with crustal inputs and arc-fluids metasomatism for Suite-1. The samples have positive LILE and negative Nb, Ta, Ti anomalies, enrichments in the LREE over middle and HREE (La/Yb)CN = 3.6–8.9 with negative Eu anomalies (0.7–0.9) in Chondrite-normalized diagrams. Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest active continental margin settings, whereas Ta/Hf-Th/Ta and Nb/Yb-Th/Yb diagrams suggest a depleted MORB source. Ti/Zr, Zr/Hf, Nb/Ta, Nb/Th and La/Nb constraints for Suite-2 suggest MORB and oceanic plateau basaltic sources. REE patterns show enrichments in the LREE with negative to positive Eu anomalies (0.7– 1.3). Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest a within plate tectonic setting. It is suggested that Suite-2 rocks were sourced from mantle-derived melts with a composition between an enriched MORB to OIB.
 Keywords: Twangiza-Namoya; Petrogenesis; Tectonic setting; Trace element ratios

Ancient and recycled sulfur sampled by the Iceland mantle plume

Stable sulfur isotope ratios of mid-ocean ridge and ocean island basalts (MORBs and OIBs) preserve unique information about early Earth processes and the long-term volatile cycles between Earth's mantle and the surface. Icelandic basalts present ideal material to examine the oldest known terrestrial mantle reservoir, accessed through a deep-rooted mantle plume, but their multiple sulfur isotope systematics have not been explored previously. Here, we present new sulfur concentration (30–1570 ppm) and isotope data (δ34S = −2.5 to +3.8‰ and Δ33S = −0.045 to +0.016‰; vs. Canyon Diablo Troilite) from a sample suite (n = 62) focused on subglacially erupted basaltic glasses obtained from Iceland's neovolcanic zones. Using these data along with trace element systematics to account for the effects of magmatic processes (degassing and immiscible sulfide melt formation) on δ34S, we show that primitive (MgO > 6 wt.%), least degassed glasses accurately record the δ34S signatures of their mantle sources. Compared to the depleted MORB source mantle (DMM; δ34S = −1.3±0.3‰), the Iceland mantle is shown to have a greater range of δ34S values between −2.5 and −0.1%. Similarly, Icelandic basalts are characterized by more variable and negatively shifted Δ33S values (−0.035 to +0.013‰) relative to DMM (0.004±006‰). Negative δ34S-Δ33S signatures are most prominent in basalts from the Snæfellsnes Volcanic Zone and the Kverkfjöll volcanic system, which also have the lowest, most MORB-like 3He/4He (8–9 R/RA, where RA is the 3He/4He of air) and the highest Ba/La (up to 12) in Iceland. We propose that subduction fluid-enriched, mantle wedge type material, possibly present in the North Atlantic upper mantle, constitutes a low-δ34S-Δ33S component in the Icelandic mantle. This suggests that volatile heterogeneity in Iceland, and potentially at other OIBs, may originate not only from diverse plume-associated mantle components, but also from a heterogeneous ambient upper mantle. By contrast, a set of samples with high 3He/4He (up to 25.9 R/RA) and negative μ182W anomalies define an ancient lower mantle reservoir with a near-chondritic Δ33S and δ34S signature of ∼0‰. The difference between DMM and the high high-3He/4He mantle may reflect separate conditions during core-mantle differentiation, or a previously unidentified flux of sulfur from the core to the high-3He/4He reservoir.

Hydrous silicate melts and the deep mantle H2O cycle

We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing.

Cambrian-Ordovician mid-ocean ridge magmatism in the Kyrgyz Middle Tianshan and origin of the Karaterek ophiolite

The Karaterek ophiolite represents a remnant of a previously unknown early Paleozoic ocean basin in the Kyrgyz Middle Tianshan (MTS) and sheds new light on the accretionary history of the western Central Asian Orogenic Belt (CAOB). Late Cambrian (~495–485 Ma, this study) and Middle Ordovician (~470–460 Ma) conodont ages support more than 30 Ma of seafloor spreading. The ophiolite comprises predominantly LREE-depleted mid-ocean ridge basalts (N-MORB) with chemically evolved compositions (Mg# 57–45). La-Th-Nb systematics and Ti/Zr and Ti/V ratios corroborate MORB sources. Al2O3 and FeO variations support plagioclase-controlled and tholeiitic magma fractionation processes, respectively. Initial εNd (i) values of +6 to +8 overlap the range of the contemporaneous global sub-ridge mantle. A section of OIB and N-MORB pillows is interpreted as near-ridge seamount tapping small-scale little (εNd (i) + 1) and highly depleted (εNd (i) + 8) mantle, respectively. Coinciding MORB and arc-features in subordinate basaltic dike samples suggest melting of hydrous residual peridotite (εNd (i) ~ 9) during subduction initiation. We propose a geodynamic model of rifting of the MTS prior to the late Cambrian (~490 Ma), open-ocean seafloor spreading during the late Cambrian to Middle Ordovician, and basin closure and plate collision in the Late Ordovician at ~450 Ma.

The mantle source of basalts from Reunion Island is not more oxidized than the MORB source mantle

Glasses quenched from relatively undegassed ocean island magmas erupted from volcanoes at Iceland, Hawaii, the Canary Islands, and Erebus have elevated Fe3+/∑Fe ratios compared to glasses quenched from mid-ocean ridge basalts. This has been ascribed to elevated fO2 of their mantle sources, plausibly due to subducted, oxidized near-surface-derived components in their mantle sources. The basaltic magmas from Reunion Island in the Indian ocean have Sr–Nd-Hf-Pb-Os isotopic compositions suggesting that their mantle sources contain little or no subducted near-surface materials and contain the C/FOZO/PREMA mantle component. To constrain the fO2 of the C/FOZO/PREMA mantle component and test the link between oxidized OIB and recycled surface-derived materials in their sources, we measured major and volatile element abundances and Fe3+/∑Fe ratios of naturally glassy, olivine-hosted melt inclusions from Piton de La Fournaise volcano, La Reunion. We conclude that the fO2 of the mantle source of these Reunion lavas is lower than of the mantle sources of primitive, undegassed magmas from Hawaii, Iceland, the Canary Islands, and Mt. Erebus, and indistinguishable from that of the Indian-ocean upper mantle. This finding is consistent with previous suggestions that the source of Reunion lavas (and the C/FOZO/PREMA mantle component) contains little or no recycled materials and with the suggestion that recycled oxidized materials contribute to the high fO2 of some other OIBs, especially those from incompatible-element-enriched mantle sources. Simple mixing models between oxidized melts of EM1 and HIMU components and relatively reduced melts of DMM can explain the isotopic compositions and Fe3+/∑Fe ratios of lavas from Hawaii, Iceland, the Canary Islands, and Mount Erebus; this model can be tested by study of additional OIB magmas, including those rich in the EM2 component.

Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ({D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}}) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} = 0.63 ± 0.10 and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{cpx}/mathrm{melt}} = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}} function alone allows ~ 40% of the variation in MORB compositions. If we allow {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

Recycling of nitrogen and light noble gases in the Central American subduction zone: Constraints from 15N15N

How much nitrogen and light noble gases are recycled in modern subduction zones is unclear. Fumaroles act as a means for passive degassing in arcs. They receive variable contributions of volatiles from arc magmas, themselves sourced from the mantle wedge. The gas compositions reflect the extent of volatile enrichment in sub-arc mantle sources and constrain slab dehydration. However, contributions from atmospheric components in fumaroles are unavoidable. For N2, neon and argon, the atmospheric components are challenging to discern from slab-derived components. Here, we report 15N15N measurements from eight fumaroles and seven bubbling springs, along the Central American arc. Our new 15N15N data are coupled with noble gases measurements and show that air-derived components in volcanic gas discharges can easily be underestimated, in both fumaroles and springs, using conventional stable isotope or noble gases methods. We show that, in the absence of 15N15N data, previously used tracers for air (e.g., δ15N, N2/Ar, N2/He, among others) may lead to erroneous conclusions regarding the origin of volatiles in mixed gases. In contrast, 15N15N data provide quantitative constraints on the nature and contributions of both atmospheric and magmatic components. Most springs are heavily dominated by air-derived N2, while fumaroles show substantial contributions of volcanic endmembers. Based on the fumarole data, we show that magma sources beneath the central American arc are enriched in all volatiles relative to 3He, by two to three orders of magnitude compared to the MORB source. We use new 15N15N data to obtain source N2/3He, 3He/36Ar and 3He/22Ne ratios which we then use to compute volcanic N2, Ar and Ne degassing fluxes. Using this approach, we show that outgassing fluxes appear to match subduction fluxes in the Central America subduction zone. We determine an N2 outgassing flux of between 4.0×108 and 1.0×109 mol N2/y, comparable to the subduction flux of 5.7×108 mol N2/yr determined previously. We obtain a similar conclusion for 22Ne and 36Ar. Overall, the volatile fluxes in the central American subduction zone no longer seem to require net transfer of N2, Ar, and Ne, to the deep mantle.

High-precision Pb and Hf isotope and highly siderophile element abundance systematics of high-MgO Icelandic lavas

Ocean island basalts (OIB) exhibit significant variations in their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 186,187Os/188Os, and 206,207,208Pb/204Pb compositions that indicate contributions from multiple mantle sources. To further constrain the relationships between diverse long-lived isotopic systems in OIB, new double-spike Pb and Hf isotope data, as well as isotope dilution highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances are reported for nineteen Icelandic picrites and basalts for which 3He/4He, 87Sr/86Sr, 143Nd/144Nd, and 186,187Os/188Os data have been reported previously (Brandon et al., 2007. GCA). Hafnium-Nd-He isotope systematics for the sample set reveal the presence of an ancient refractory source with low-3He/4He, while He-Pb isotope correlations indicate that recycled oceanic crustal components with high-3He/4He added from a less degassed mantle source are present in the Iceland plume. The highly depleted, low-3He/4He signature is sampled in the Northern and Western Volcanic Zones, likely due to higher degrees of partial melting at those localities enabling sampling of this refractory endmember. The 187Os/188Os ratios show correlations with Pb, Hf, and Nd isotope ratios indicating binary mixing between depleted and enriched sources, with no evidence for decoupling due to sulfide metasomatism. The spread of 187Os/188Os- 206,207,208Pb/204Pb data can be explained by addition of 1–11% young recycled oceanic crust to a depleted mantle source distinct from the source of Atlantic MORB. Abundances of the HSE in Northern and Western Volcanic Zone lavas overlap with those from other OIB with similar MgO contents, showing fractionated patterns that are enriched in the more incompatible Re, Pd, and Pt over the compatible Ru, Ir, and Os. Calculated parental melt HSE abundances of those lavas also overlap with those of other OIB. Eastern Volcanic Zone lavas are highly depleted in Pt and Pd relative to other Icelandic lavas. Abundances of Pt and Pd correlate with isotopic parameters, such as 206Pb/204Pb, in Icelandic lavas, indicating that the isotopically enriched endmember sampled in the Eastern Volcanic Zone is depleted in the HSE, consistent with it being mafic in nature. The abundances of MgO in the lavas also roughly correlate with isotopic parameters, supporting the identity of the depleted endmember as ultramafic and the enriched endmember as mafic. New and previously published double-spike Pb isotope data show chiefly binary mixing with an additional component present in the vicinity of the volcano Öræfajökull. Icelandic 207Pb/204Pb and 208Pb/204Pb data plot below and above the majority of Mid-Atlantic Ridge basalt data, respectively, indicating that both depleted and enriched end-members in the plume have a lower U depletion age and a higher Th/U than ambient mantle. This is consistent with the enriched and depleted end-members within the Iceland plume being <2 Ga recycled oceanic crust and lithospheric mantle, respectively.

The Boninite‐like Dolerites in the Xigaze Ophiolites, Tibet: Similar to the MORB‐like Dolerites

AbstractThe Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (&gt; 52 wt%), high MgO (&gt;8 wt%) and low‐TiO2 (&lt;0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation.

Mesozoic lithospheric modification and replacement beneath the Cathaysia Block: Mineral chemistry and water contents of the Daoxian peridotite xenoliths

Water contents (a key parameter controlling the stability of a continent) of the Mesozoic sub-continental lithospheric mantle (SCLM) beneath the Cathaysia Block (South China) are still poorly constrained. Here, we report mineral chemistry and water contents of peridotite xenoliths entrained by Mesozoic basalts from the Daoxian area in the Cathaysia Block. The peridotite xenoliths are spinel-facies lherzolites with porphyroclastic textures and have olivine Mg# (Mg2+/(Mg2++Fe2+)) of 88.0–89.3, suggesting an origin from a fertile lithosphere mantle. Clinopyroxenes in the lherzolites show LREE-depleted patterns and negative anomalies in Ba, Nb, Pb, Sr, Zr and Ti. The contents of HREE and Y in clinopyroxenes suggest low degrees of melt extraction (e.g., 1–5% fractional melting) from a primitive mantle source. The weak enrichments in Th, U show later weak metasomatism by silicate melts. The water contents of clinopyroxenes and orthopyroxenes in the xenoliths are 182–417 ppm and 53–123 ppm, respectively. Calculated whole-rock water contents, based on the mineral modes and partition coefficients, are 35–82 ppm. The water contents are similar to those estimated for the MORB source mantle and the newly accreted fertile SCLM beneath the Cathaysia Block. Because the Daoxian area is located at the Ningyuan-Jianghua fault region, the translithospheric weak zone can provide a channel for the upwelling of asthenosphere and the subsequent accretion of fertile SCLM. The Mesozoic replacement of the ancient SCLM by fertile one therefore re-shaped the lithospheric architecture of the Cathaysia Block.

Novel insights into the degassing history of Earth's mantle from high precision noble gas analysis of magmatic gas

The noble gas isotope composition of the mantle can provide unique insights into the origin and evolution of volatile elements on Earth. Xenon isotopes combine primordial signatures with contributions from extinct and extant radionuclides, therefore offering the potential to set constraints on both the nature of Earth's planetary precursor(s) and the timing of their contributions. However, measuring the Xe isotope composition of mantle-derived samples to sufficiently high-precision has proven difficult due to (i) large occurrence of a modern-like atmospheric component in the mantle, and (ii) contribution from shallow and post-eruptive atmospheric contamination. Mantle-derived samples therefore exhibit only small deviations from the modern atmospheric composition, making the identification and deconvolution of mantle-derived Xe signals challenging. Here, we use the Giggenbach sampling method to concentrate magmatic noble gases from the Eifel volcanic area (Germany) into glass bottles in order to conduct high-precision analyses of Ne, Ar and Xe isotopes. The three samples collected from Victoriaquelle and Schwefelquelle wells (South East Eifel) show variable contributions from atmospheric contamination, with the least contaminated sample reaching 40Ar/36Ar ∼8,300. Our data indicate that the mantle beneath the Eifel volcanic area, and by extension the Central European Volcanic Province, resembles the convective upper mantle reservoir with limited evidence for an OIB-like deep plume source contribution. It has a geochemical signature that is similar (e.g. in Ne isotopic composition, 40Ar/36Ar, 129Xe/130Xe and 129Xe/136Xe) to the mantle source of the so-called popping rocks (thought to best represent the upper mantle), with an additional source of 238U-derived Xe and low 3He/4He that we attribute to the influence of an ancient subducted component (HIMU). A dichotomy exists between the main sources of fissiogenic xenon isotopes measured in popping rocks and Eifel gas, which appear to be mainly derived from 244Pu and 238U, respectively. According to their respective ratios of 244Pu- to 238U-derived Xe, the mantle sources for Eifel volcanism and popping rocks would have experienced extensive and limited degassing, respectively. In this regard, high Pu–Xe/(Pu+U)–Xe may no longer be considered as being indicative of a mantle deep origin, therefore calling for the geochemical differences between plume and MORB sources to be redefined, with the possibility that volatile signatures within the solid Earth may be more heterogeneously distributed than previously thought.

Variation of melting processes and magma sources of the early Deccan flood basalts, Malwa Plateau, India

The Deccan flood basalts cover large parts of India and represent one of the largest volcanic provinces on Earth. We present geochemical and isotopic data on a magnetically defined stratigraphic section of lavas from the northern portion of the Deccan, the Malwa Plateau, which are dated at ∼67.5 Ma. These are therefore some of the oldest Deccan lavas, erupted before the Main Deccan units further to the south. Although the younger Malwa Plateau lavas have undergone significant crustal contamination by at least two distinct crustal components, the older, more alkalic lavas are less contaminated and have similar isotope and trace element compositions to Réunion oceanic intraplate basalts. Our data therefore provide geochemical evidence for a contribution of the Réunion hotspot to the generation of the early Deccan flood basalts. Most lavas reflect fractional crystallization and assimilation of less than 10% of a crustal melt but samples with the highest SiO2 and Sr isotope compositions underwent more extreme crustal contamination. The composition of the least contaminated basalts from the Malwa Plateau suggests that the degree of partial melting increased significantly from the base to the top of the section. Most Malwa Plateau basaltic magmas formed by <5% partial melting of garnet peridotite in the Réunion mantle plume whereas the degree of melting in the Main Deccan Volcanic Province increased threefold and included MORB sources. Together, these geochemical variations indicate increasing degrees of melting between 68.5 and 65.4 Ma and a variation of the mantle sources from Réunion plume mantle to an increasing contribution from depleted upper mantle. These changes probably reflect lithospheric erosion by the mantle plume within ∼2 million years that allows melting of entrained upper mantle material. The geochemical data thus agree with geophysical models of significant thinning of cratonic lithosphere during the passage across mantle plumes. The magma evolution from alkaline to tholeiitic basalts observed in the Deccan flood basalts resembles that observed in early oceanic plume-related volcanism like Hawaii.

Recycling of heavy noble gases by subduction of serpentinite

Isotopic analyses of natural samples suggest that the heavy noble gases (HNG; Ar, Kr, and Xe) in the mantle are dominated by components recycled by subduction (Holland and Ballentine, 2006; Kendrick et al., 2011; Parai and Mukhopadhyay, 2018; Smye et al., 2017). However, the mechanism of recycling is not clear: are they primarily trapped in pore spaces and inclusions, are the gases dissolved in the lattices of mineral phases, or are they located along grain boundaries? Serpentinite is a dominant volatile-bearing lithology in subducting plates, and experimental studies suggest that HNG may be highly soluble in serpentinite (Jackson et al., 2015; Zaikowski and Schaeffer, 1979). To understand its role in HNG recycling, the solubility of noble gases in natural serpentinite has been experimentally measured at pressures from 0.15 to 1.13 kbar. Measured solubilities for He, Ne, Ar, Kr, and Xe are 1.78 ⋅ 10−9, 1.35 ⋅ 10−10, 3.03 ⋅ 10−10, 1.25 ⋅ 10−9, and 3.29 ⋅ 10−9 mol ⋅ g−1 ⋅ bar−1 with an average relative error of 9%. The solubilities are relatively high, indicating that even if less than 1% of the initial HNG estimated to be in subducting serpentinite survives slab dehydration, there would be enough remaining to dominate the mantle HNG budget. That the MORB and OIB mantle sources retain any primordial Kr or Xe in the face of such a large potential slab flux suggests subduction zones extract most (but not all) HNG from the slab at shallow depths. Serpentinite fractionates the HNG, with solubilities increasing from Ar to Kr to Xe. Kr/Ar and Xe/Ar ratios in MORB are quite similar to those predicted for subducted serpentinite, consistent with most HNG in the MORB-source coming from subducted serpentinite, possibly in combination with marine sediments. HNG ratios in most EM1- and EM2-type OIBs can also be explained primarily by serpentinite recycling. A prominent exception is Iceland, which appears to require dominantly sediment recycling, with little or no serpentinite. HIMU OIBs have highly fractionated HNG ratios that cannot be explained by serpentinite recycling but are consistent with subduction of HNG in amphibole (Chavrit et al., 2016).

Compositions and formation conditions of primitive magmas of the Karymsky volcanic center, Kamchatka: evidence from melt inclusions and trace-element thermobarometry

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H2O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H2O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of DQFM = 1.5 ± 0.4. The obtained estimates of H2O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H2O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably to decrease the existing temperature estimates for the mantle wedge above subduction zones.

New insights into the plumbing system of Santorini using helium and carbon isotopes

We have analysed He, Ne and CO 2 concentrations and isotopic ratios in seawater samples collected using a ROV above hydrothermal fields and in CO 2-accumulating pools within Santorini caldera (Greece). Our results provide new insights on the plumbing and magmatic sources of this volcano, which is critical to understand and better predict its volcanic eruptions, and its relationships to nearby volcanic systems. The 3 He/ 4 He ratio is as high as 6.54 times the atmospheric ratio, and significantly higher than any other samples from Santorini, including the active eruptive centre of Nea Kameni (~3.6 Ra). The 3 He/ 4 He ratio, corrected for atmospheric helium dissolved in seawater is ~7 Ra, a value similar to that measured at the nearby Kolumbo submarine volcano crater. The fluids display typical mantle signatures never recorded before in the active centres of the island of Santorini, in which fluids and gases are contaminated at shallow depths. The helium isotopic signature suggests that the source of volatiles for these two volcanoes is similar to the MORB source instead of being the subcontinental lithospheric mantle as suggested previously.

Compositions and Formation Conditions of Primitive Magmas of the Karymsky Volcanic Center, Kamchatka: Evidence from Melt Inclusions and Trace-Element Thermobarometry

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H2O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H2O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of ΔQFM = 1.5 ± 0.4. The obtained estimates of H2O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H2O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably decrease the existing temperature estimates for the mantle wedge above subduction zones.