Mid-ocean Ridge Basalt Research Articles

Iron isotope evidence in continental intraplate basalts for mantle lithosphere imprint on heterogenous asthenospheric melts

Iron isotope studies on ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) have disclosed the contribution of pyroxenite lithologies in the generation of basaltic magmas. Whether Fe isotopic compositions of continental intraplate basalts can be applied to trace lithological heterogeneity within the mantle source regions remains poorly investigated. To explore the systematics of stable Fe isotopes as a potential probe of lithological heterogeneity in the source of continental intraplate basalts, we present twenty-four Fe isotope data on a suite of well-characterized Cenozoic basalts from SE China. The samples show a large range of Fe isotope values (δ56Fe = +0.09‰ to +0.20‰), which correlate with SiO2, CaO/Al2O3, Ti/Eu, Hf/Hf*, Zr/Nb, Dy/Yb, La/Yb, Nb/Y, K/La, Sr/Ce, δ66Zn and estimated equilibrium pressures. The samples with the highest δ56Fe values represent early-stage low-silica basalts with moderately enriched Sr-Nd isotope ratios and high δ66Zn values, while the late-stage high-silica basalts display a broad δ56Fe decrease with increasing 87Sr/86Sr and with decreasing εNd and δ66Zn. We demonstrate that the heaviest Fe isotope signatures cannot be derived from a pure peridotite source and require a pyroxenite component in the source. Combined with other geochemical proxies, we show that the heaviest basalt compositions are consistent with low-degree melts produced by the adiabatic decompression of a carbonated pyroxenite-bearing asthenospheric mantle. Mixing of these hybrid melts with melt produced from in-situ melting of the subduction-modified sub-continental lithospheric mantle (SCLM) reproduces the Fe-isotope variability of the late-stage basalts. Our results demonstrate the significance of lithological heterogeneity in the mantle source of continental intraplate basalts and highlight the subsequent imprint of mantle lithosphere in the evolution of their Fe isotope compositions. Finally, this study exemplifies the potential of the Fe-Zn stable isotope pair for mantle geochemistry; coupled with traditional radiogenic isotopes and major and trace element concentrations, they formed an efficient tool to trace the nature and contribution of the mantle source components in the formation of intraplate basalts.

Metasomatized mantle source of nascent oceanic crust in the Guaymas Basin, Gulf of California

The geochemical characteristics of nascent oceanic crust play a crucial role in unraveling the mantle dynamics of the initial opening of an ocean. The Gulf of California, located at the East Pacific margin, is one of the best examples of an active and obliquely rifted continental margin. However, the mantle source composition of the nascent oceanic crust in the central part of the Gulf has not yet been comprehensively investigated. Here, we report in situ major and trace element contents as well as BSr isotope compositions for basaltic glass samples from off-axis sills drilled by the International Ocean Discovery Program (IODP) Expedition 385 at Sites U1547 and U1548 in the Guaymas Basin in the central part of the Gulf of California. These glassy samples represent tholeiites and predominantly show trace element patterns akin to those of enriched mid-ocean ridge basalts (E-MORBs) but with distinctive enrichments in Ba and K and depletions in Nb, Ta, and Ti. In addition, these samples have high B contents (3.07–3.67 ppm) with enriched Sr isotopes (87Sr/86Sr = 0.7032–0.7037) and heavy B isotopes (δ11B = −5.52‰ to 1.20‰). The mixing model based on B/Nb values and SrB isotopes shows that the nascent oceanic crust in the Guaymas Basin might be generated through partial melting of a depleted MORB mantle (DMM) source metasomatized by melts from subducted slab materials referring to partially dehydrated sediment and altered oceanic crust components. The magmas in the Gulf of California show a systematic decline in their enrichment in fluid-mobile elements (Ba) and depletion in fluid-immobile elements (Nb, Ta, and Ti) from the northern (e.g., Isla San Luis volcanic center) to the central part (Guaymas Basin) and southward to the mouth (e.g., Alarcón Basin) of the Gulf. This suggests that the enriched (recycled) components in their mantle source were gradually extracted and exhausted during Gulf opening and oceanic crustal accretion that advanced in a northward direction. Our results indicate that the Guaymas Basin magmas were derived from a mantle that was fertilized by subduction components. The subduction signature corroborates the Gulf opening as a process that started in response to long-term oblique convergence at the eastern Pacific plate margin without any influence from a mantle plume. This evolution is different from nascent ocean basins that evolved from intraplate rifting, such as the Red Sea, leading to vastly different mantle source characteristics.

Concordance of V-in-olivine and Fe-XANES oxybarometry methods in mid-ocean ridge basalts

Oxygen fugacity (fO2) is a fundamental parameter controlling the behavior of redox-sensitive elements, as well as the speciation of degassing volatiles in igneous systems. Therefore, the average fO2 of typical mid-ocean ridge basalts (MORB) is important to constrain for models of atmospheric and mantle chemical evolution. Although there are several independent oxybarometry methods, in recent years, the Fe X-ray absorption near edge spectroscopy (Fe-XANES) oxybarometry of volcanic glasses has become the gold standard for evaluating the fO2 of modern lavas. Other oxybarometry methods, such as the V-in-olivine method, are useful for samples in which fresh glass is not preserved, but it is not clear whether the two methods give comparable results when applied to the same samples due to the lack of inter-calibration studies. Presented here are V-in-olivine oxybarometry data for eleven MORB lavas, eight of which have been previously characterized by Fe-XANES oxybarometry. The new data show that the V-in-olivine method is not applicable to MORB with glass containing <8.3 wt. % MgO, and that MORB with glass having MgO >8.3 wt. % have an fO2 averaging -0.28 ± 0.28 ΔFMQ, overlapping with the global Fe-XANES MORB average of -0.18 ± 0.16 ΔFMQ. It is therefore recommended that this oxybarometry technique only be applied to MORB with primitive (>8.3 wt. % MgO) glass. The lack of any systematic offset between the data from the two oxybarometers supports our previous conclusion that ocean island basalt sources (OIB) are more oxidized than average MORB, and that the majority of Archean komatiite sources record significantly lower fO2 than modern MORB.

Causes of Cretaceous subduction termination below South China and Borneo: Was the Proto-South China Sea underlain by an oceanic plateau?

The South China, Indochina, and Borneo margins surrounding the South China Sea contain long-lived arcs that became inactive at approximately 85 Ma, even though an embayment of oceanic crust (the ‘Proto-South China Sea’) remained in the intervening region. This oceanic crust eventually subducted in the Cenozoic below Borneo and the Cagayan arc, while the modern South China Sea opened in its wake. To investigate the enigmatic cessation of Mesozoic subduction below South China and Borneo, we studied a fragment of oceanic crust and overlying trench-fill sediments that accreted to NW Borneo during the final stages of Paleo-Pacific subduction. Based on radiolarian biostratigraphy of cherts overlying the pillow basalts and detrital zircon geochronology of the trench-fill, we constrained the minimum age of the oceanic crust during accretion to 40 Ma. This shows that subduction cessation was not related to ridge subduction. Geochemical analysis of pillow basalts revealed an enriched mid-ocean ridge basalt signature comparable to oceanic plateaus. Using paleomagnetism, we show that this fragment of oceanic crust was not part of the Izanagi Plate but was part of a plate (the ‘Pontus’ Plate) separated from the Izanagi Plate by a subduction zone. Based on the minimum 40 Ma age of the oceanic crust and its geochemistry, we suggest that Mesozoic subduction below South China and Borneo stopped when an oceanic plateau entered the trench, while the eastern plate margin with the Izanagi Plate remained active. We show how our findings offer opportunities to restore plate configurations of the Panthalassa-Tethys junction region.

The journey of the Band-e-Zeyarat ophiolite (Makran Accretionary Wedge, SE Iran) from the mid-ocean ridge to the accretionary complex: new insights from its sedimentary cover and associated basaltic dykes and sills

The Band-e-Zeyarat ophiolite (BEZO) sedimentary cover in the Makran Accretionary Prism (SE Iran) records a complex tectono-sedimentary evolution, extending from its formation at a mid-ocean ridge (MOR) setting to deformation in an accretionary prism. Stratigraphic and biostratigraphic data indicate the occurrence of a Valanginian transition zone sequence separating the volcanic sequence and pelagic sedimentary cover. The latter consists of lower Hauterivian cherty limestone passing upwards to upper Hauterivian–Barremian marl and limestone. The pelagic sedimentary cover continues with post-Barremian–Cenomanian (?) marls. Arenites are interlayered in the sedimentary cover; they are composed of rock fragments derived from volcanic-arc and continental margin settings. The BEZO sedimentary cover is intruded by dykes and sills showing enriched MOR basalt chemical affinity. Structural analysis indicates a polyphase deformation history that involved faulting and folding. Our multidisciplinary results indicate that the BEZO formed in a MOR setting and that it was subsequently overprinted by off-axis and within-plate magmatism as it spread away from this MOR. The Band-e-Zeyarat oceanic crust was incorporated into the Makran prism in the latest Late Cretaceous–Paleocene and was further deformed via strike-slip faulting along the dextral Minab-Sabzevaran fault during the Miocene–Pliocene. Supplementary material: A geological sketch map of the Band-e-Zeyarat ophiolite, photomicrographs of arenites, microphotographs of selected calcareous nannofossil taxa, a geochemical discrimination diagram, a table with semiquantitative estimation of calcareous nannofossil abundance, and a text with detailed descriptions of analytical methods, accurancy, and detection limits for whole-rock geochemical analysis are available at https://doi.org/10.6084/m9.figshare.c.6843835 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Recognizing different types of ocean plate stratigraphy in the southeastern South Qilian Accretionary Belt, northeastern Tibet Plateau

Dissection of relicts of ocean plate stratigraphy (OPS) in orogens could provide important insights into oceanic evolution, orogenic formation and continental growth. Here we describe a study of petrology, geochronology, geochemistry and Sr–Nd isotopes for OPS in the South Qilian Accretionary Belt with the aim of providing new constraints on the evolution of the Proto-Tethys Ocean (PTO). Three types of OPS units were recognized by their differing petrological, geochemical and isotopic features. Type 1 OPS, representing relicts of ocean islands or seamounts, comprises ocean island basalt and gabbro ( t = 525 ± 4 Ma, ( 87 Sr/ 86 Sr) i = 0.7035–0.7039 and ε Nd ( t ) = +3.63 to +4.16) and (siliceous) carbonate rocks. Type 2 OPS, representing fragments of normal oceanic crust, consists of normal mid-ocean ridge basalt (N-MORB) and basaltic andesite (( 87 Sr/ 86 Sr) i = 0.7059–0.7063 and ε Nd ( t ) = +6.33 to +7.44), chert and siliceous mudstone. Type 3 OPS could be oceanic plateau-derived and comprises enriched (E)-MORB and dolerite (( 87 Sr/ 86 Sr) i = 0.7051–0.7064 and ε Nd ( t ) = +4.81 to +6.24), carbonate–siliceous mudstone and carbonate rock. These OPS units witnessed evolution of the PTO. Using available studies on regional arc-type magmatism and U–Pb geochronological data for detrital zircon in this study, a model of ‘trench jam accompanying subduction flip induced by accretion of the OPS’ is put forward. Supplementary material : Zircon U–Pb data, whole-rock major and trace element data and a supplementary figure are available at https://doi.org/10.6084/m9.figshare.c.6764637 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Mantle Oxidation Induced by Recycled Carbonate: Insights From Mg‐Zn‐Fe‐Cu Isotopic Systematics of Intraplate Basalts

AbstractMantle oxidation plays an essential role in volatile cycling between Earth’s interior and exterior. However, the conceivable oxidizer remains enigmatic and needs to be explored by geochemical tools. Here, we examine the potential of recycled carbonates in inducing redox changes of the deep mantle through combined isotopic studies of Mg (δ26Mg), Zn (δ66Zn), Fe (δ56Fe), and Cu (δ65Cu) on a suite of intraplate alkaline basalts proposed to have originated from the mantle transition zone (∼440–660 km). Compared with mid‐ocean ridge basalts (MORB), these basalts have variably lower δ26Mg and higher δ66Zn, the typical features of marine carbonates, indicating the presence of various amounts of deeply recycled carbonates in their sources. As the estimated amounts of recycled carbonates in sources increase (i.e., lower δ26Mg and higher δ66Zn), the basalts have higher δ56Fe and lower δ65Cu compared with MORB. These correlations cannot be explained alone by binary mixing between recycled carbonates and the pristine mantle because of the much lower Fe and Cu contents of marine carbonates than those of the mantle and instead require redox‐driven isotope fractionation induced by mantle carbonate metasomatism. During partial melting of an oxidized mantle source, isotopically heavy ferric iron and isotopically light sulfide were preferentially incorporated/dissolved into the melts that are enriched in heavy Fe and light Cu isotopes. These observations highlight that recycled carbonates serve as an important oxidizer for the deep mantle and facilitate the extraction of mantle sulfides, which can be effectively tracked by Fe and Cu isotopic systematics of intraplate basalts.

Mineral and whole-rock geochemistry of high-Al podiform chromitites in the Fizh Massif of the Cretaceous Oman ophiolite: origin of hydrous N-MORB melts in a nascent forearc setting

The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has &gt;450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent &gt;20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu–Bonin–Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle. Supplementary material: The whole-rock and mineral data of the Rajmi harzburgites, dunites and chromitites are available at https://doi.org/10.6084/m9.figshare.c.6795689 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Major, trace elements and Sr–Nd–Pb isotopes systematics of mafic dykes from the Figuil (northern Cameroon) and Léré (southwestern Chad) areas

In this study, field descriptions of the dolerite dykes cross‐cutting the Pan‐African basement (DPB) and the dolerite and gabbroic dykes cross‐cutting the Cretaceous sedimentary basin (DGCSB) are used together with petrographic, bulk‐rock geochemistry, as well as Sr–Nd–Pb isotopic data to constrain the geodynamic setting and origin of these mafic (basanite, basalt and basaltic andesite in composition) rocks that outcrop in the Figuil–Léré area. The DPB dykes are alkaline with SiO2 of 46.63–49.03 wt% and MgO of 3.93–7.11 wt%. In contrast, the DGCSB dykes are subalkaline with SiO2 ranging between 50.24 and 53.16 wt% and MgO fluctuating between 5.22 and 7.90 wt%, comparable to dykes described in other regions of the Cameroon Line (CL). The rocks studied belong to the tholeiitic series and were emplaced in within‐plate basalts (WPB) and mid‐ocean ridge basalts (MORBs) setting. The variations in major and trace elements from the basanite to basaltic andesite compositions are dependable with fractional crystallization (FC) of different mineral phases such as opaque minerals, olivine, clinopyroxene, amphibole and alkali feldspar. Most samples exhibit negative Nb–Ta and positive Sr anomalies, which together with the negative correlation between MgO and 87Sr/86Sr, and isotopic ratio of 206Pb/204Pb &lt;19.5 (18.27–19.49) suggests interactions of crustal components. Nevertheless, this contamination seems limited because there is a positive correlation between SiO2 and 143Nd/144Nd plot and low Ba contents (72–210 ppm) of the samples compared to the mid‐continental crust (259–628 ppm). Trace element modelling proposes a derivation of the Figuil–Léré dykes from parental melts generated by 4%–15% partial melting of a source containing garnet peridotite in the mixed lithospheric–asthenospheric mantle. The isotopic data of the dykes studied with hybrid magma composition are characterized by more radiogenic 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb isotopic ratios with a dominant mixing of depleted MORB mantle (DMM)–enriched mantle 1 (EM1)–enriched mantle 2 (EM2), like those of other mafic dykes of the CL.

Preserved Nd-Hf-Os isotope variability in replacive channels from the Lanzo ophiolite: Traces of incomplete melt aggregation in the shallow mantle

Long-lived radiogenic isotopes of abyssal peridotites, residues of MORB extraction, show that the asthenosphere is intrinsically heterogeneous, which is inherited from ancient melting events and crustal recycling during Earth's history. Yet, Mid Ocean Ridge Basalts (MORB) have a rather uniform average composition, suggesting that the variability of their mantle source is concealed during their ascent. Here we document that mantle heterogeneity is exceptionally well preserved in high permeability mantle conduits from the Lanzo South mantle massif, Western Italian Alps. Nd-Hf-Os isotopes of decametre-scale replacive bodies provide evidence for the existence of two generations of mantle channels. The first generation consists of dunites concordant to the main foliation of host peridotites. The replacive dunites include clinopyroxene with MORB-like incompatible element signature and initial (160 Ma) ƐNd and ƐHf ranging from +4 to +7 and from +10 to +15, respectively. The second generation, made up of pyroxene-poor harzburgites discordant to the main foliation, is geochemically depleted in incompatible elements and its clinopyroxene displays highly radiogenic Hf isotopes (initial ƐHf up to +202). The mantle channel heterogeneity is confirmed by ReOs isotopes and platinum-groups elements. The MORB-type dunites have high Pt, Pd and, locally, Re, and have 187Os/188Os ratios similar to the host peridotite (0.122–0.128). On the other hand, the depleted bodies have lower Pt, Pd and Re, and 187Os/188Os ratios ranging from those of host peridotites (0.124) to highly unradiogenic values (0.118) in the most refractory sample. The preserved heterogeneity in trace elements, PGE, and Nd-Hf-Os isotopes highlights infiltration of melts from a highly heterogeneous mantle, still partially preserved within these mantle bodies. If applied to present-day Mid Ocean Ridges, our model indicates that the isotopic variability of melts migrating through replacive mantle conduits is by far larger than magmas erupted on the seafloor, which implies that diverse mantle components are mainly delivered and homogenised above the crust-mantle boundary.

Platinum solubility in silicate melts: The effects of sulfur (S2–), temperature, and melt composition

Platinum (Pt) solubility in silicate melt (SolPtSil) has been determined to understand its behavior in planetary magmas. However, the effect of sulfur (S) in the silicate melt on SolPtSil was paid little attention. Here we present experiments performed at 1–1.5 GPa, 1300–1700 °C, and oxygen fugacity (fO2) from IW−1.6 to IW+2.3 (IW denotes the iron–wüstite buffer) to determine SolPtSil for silicate melts with major element compositions corresponding to those of rhyolite, mid-ocean ridge basalt, high-FeO basalt, and Martian basalt. The results show that SolPtSil increases from ∼0.1 to 10 μg/g as the S content in silicate melt increases from nominally zero to ∼4000 μg/g, which indicates that Pt dissolves predominantly as Pt-sulfide species in S2–-bearing silicate melt. Our results also show that SolPtSil increases with increasing temperature and/or the silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations). Using our newly obtained SolPtSil and previous data, we developed an empirical model that describes SolPtSil as a multi-function of temperature, fO2, the silicate melt NBO/T, and the S2– content in silicate melt in the case at reducing conditions (fO2 < IW+3.5). The application of our empirical model to mantle partial melting suggests that Pt-rich alloys form only after sulfide liquid exhaustion because of the S-enhanced SolPtSil. We also applied our empirical model to predict the behavior of Pt during arc magmatic differentiation. The results agree with the observations that Pt-rich alloys formed before the saturation of sulfides in the oxidized Pual Ridge lavas. We suggest that the observed coexistence of Pt-bearing alloys and sulfide globules in oxidized arc magmas can be explained by an early formation of Pt-bearing alloys followed by a late formation of sulfide liquids. The strong effect of S2– on SolPtSil also implies that the oxidization of magmatic melts can lead to direct precipitation of Pt-bearing alloys due to the S2– to S6+ transformation, which can also be subsequently entrapped in the sulfide liquids formed later. Finally, our results show a strong effect of S2– in the silicate melt on lowering the metal–silicate melt partition coefficients of Pt, which should be considered when using Pt to trace planetary accretion and differentiation processes. Collectively, our study demonstrates an important role for S2– in controlling the behavior of Pt during planetary core–mantle–crust differentiation.

Uranium isotopes as tracers of serpentinite weathering

The aqueous alteration of mantle rocks is considered to play a key role in Earth's global biogeochemical cycling. Uranium enrichment in fluid-altered mantle rocks is a common feature that may reflect redox state or fluid source, but it remains unclear when, why, and how uranium is mobilized within these systems. In this study we use uranium (U) isotopes (234U/238U and 238U/235U ratios) to deduce the timing, mobilization processes, and fluid sources responsible for weathering of subaerial and submarine serpentinites. We find that complex post-formation weathering processes continue to perturb serpentinite U isotope compositions and U concentrations long after initial serpentinization. Enriched 234U/238U ratios in dredged seafloor serpentinites require recent (<1.5 Myr) uptake of U, likely from derived from chemically evolved seawater that has circulated through igneous basement rocks. In contrast, depleted and variable 234U/238U ratios in a terrestrial serpentinite across <30 cm spatial scales from the Wadi Fins outcrop in the Samail Ophiolite, Oman, indicate alpha recoil effects, with differences in these ratios reflecting different grain sizes or shapes of U-hosting minerals in different weathering zones. Both seafloor and Wadi Fins serpentinites have 238U/235U ratios significantly different from seawater and mid-ocean ridge basalts, reflecting different initial protolith composition and/or overprinting of 238U/235U ratios during weathering. Using 238U/235U and U concentrations we calculate the isotopic composition of evolved fluids weathering the serpentinites at both localities. We also use 234U/238U and models of alpha recoil to determine the grain sizes of minerals hosting U from samples at Wadi Fins. Our results suggest that U isotopic compositions and concentrations are sensitive to post-serpentinization weathering processes in subaerial and submarine environments. The U isotopic composition of abyssal serpentinites is set by seawater that has chemically evolved during flow through basement rocks, questioning the utility of weathered serpentinites as an archive of paleo-seawater U isotope compositions, as well as other fluid-mobile elements modified during crustal fluid flow.

Revisiting the South China Sea MORBs: Mg isotope and whole-rock geochemical constraints

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted great attention from geologists in the past decade, but their petrogenesis still remains controversial. Here, we show that the Mg isotopes of MORBs from Integrated Ocean Drilling Program (IODP) Sites U1431, U1433 and U1434 are heterogeneous and present the mechanisms causing such heterogeneity. The MORBs from Sites U1431, U1433 and U1434 have δ26Mg values ranging from −0.35‰ to −0.21‰, from −0.34‰ to −0.13‰, and from −0.30‰ to −0.20‰, respectively. Together with all the published data, we found the MORBs at Site U1431 with high MgO contents (> 9 wt%) usually have δ26Mg values that are generally lower than the normal mantle (−0.25‰ ± 0.04‰), whereas some of the Site U1433 MORBs are higher than the normal mantle. Shallow-level geological processes like seawater alteration have negligible effects on the Mg isotope fractionation of our samples. Elemental ratio indicative of the degree of partial melting (like Sm/Nd) from the same site also shows no relationship with the δ26Mg variations. Olivine is proposed to have light Mg isotopes. Considering the high MgO (> 9 wt%) contents of low-δ26Mg samples and the petrographic evidence, we propose that their slightly light Mg isotopic compositions of U1431 MORBs may result from the enrichment of olivine, rather than reflecting the mantle source. Moreover, low Ce/Pb ratios and most variable δ26Mg values of some U1433 MORBs suggest that lower continental crustal materials have contributed to their source. As all the previously published MORBs are evolved, we use the Revpet to estimate the mantle melting conditions of SCS MORBs, including the Site U1500 samples recovered from the northern margin of SCS. Compared with the global MORBs data, there is no thermal anomaly in SCS MORBs. Together with our new Mg isotope data, therefore, we propose that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.

Refining Boron Isotopic Measurements of Silicate Samples by Multi‐Collector‐Inductively Coupled Plasma‐Mass Spectrometry (MC‐ICP‐MS)

Solution MC‐ICP‐MS is an established technique for high precision boron isotope measurement results (δ11BSRM 951) in carbonates, yet its application to silicate rocks has been limited. Impediments include volatilisation during silicate dissolution and contamination during chemical purification. To address this, we present a low‐blank sample preparation procedure that couples hydrofluoric acid‐digestion and low‐temperature evaporation (mannitol‐free), to an established MC‐ICP‐MS measurement procedure following chemical purification using B‐specific Amberlite IRA 743 resin. We obtain accurate δ11BSRM 951 values (intermediate precision ±0.2‰) for boric acid (BAM ERM‐AE121 19.65 ± 0.14‰) and carbonate (NIST RM 8301 (Coral) 24.24 ± 0.11‰) reference materials. For silicate reference materials covering mafic to felsic compositions we obtain δ11BSRM 951 with intermediate precision &lt; ±0.6‰ (2s), namely JB‐2 6.9 ± 0.4‰; IAEA‐B‐5 ‐6.0 ± 0.6‰; IAEA‐B‐6 ‐3.9 ± 0.5‰ (2s). Furthermore, splits of these same reference materials were processed by an alternative fusion and purification procedure. We find excellent agreement between δ11BSRM 951 measurement results by MC‐ICP‐MS of the reference materials using both sample processing techniques. These measurement results show that our sample processing and MC‐ICP‐MS methods provide consistent δ11BSRM 951 values for low B‐mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in δ11BSRM 951 from ‐5.6 ± 0.3‰ to ‐8.8 ± 0.5‰ (2s), implying some upper mantle δ11BSRM 951 heterogeneity.

The Late Carboniferous Mafic–Ultramafic Complex Induced by Slab Breakoff in Eastern North Tianshan, Central Asian Orogenic Belt

The Late Carboniferous to Early Permian is a critical period of the Chinese Tianshan, witnessing the tectonic transition from subduction to post-collisional extension during the final amalgamation of the Central Asian Orogenic Belt (CAOB). The late Carboniferous Mozbaysay mafic–ultramafic complex in the Qijiaojing–Balikun area, eastern North Tianshan, provides important clues for revealing the nature and timing of this tectonic transition. The Mozbaysay complex comprises mainly hornblende gabbros and lherzolites. LA-ICP-MS U-Pb zircon ages of hornblende gabbro yielded a weighted mean age of 306 ± 1.9 Ma for this complex. These mafic–ultramafic rocks have high contents of MgO (up to 30 wt.%), Cr (up to 2493 ppm), and Ni (up to 1041 ppm), but low contents of SiO2 (40.34–47.70 wt.%). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Th/Yb and Ba/Nb ratios imply the mantle sources could have been metasomatized by slab–mantle interaction with aqueous fluids from dehydration of the subducted slab. Thus, these mafic–ultramafic rocks were most likely produced by partial melting of the asthenospheric and lithospheric mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous mafic–ultramafic rocks was triggered by the decompression-induced influx of asthenospheric heat and melting through a slab window during post-collisional slab breakoff. Combined with geological data, the petrogenetic links of the Late Carboniferous mafic–ultramafic rocks in eastern North Tianshan to slab breakoff suggest that the tectonic transition from convergence to post-collision most likely initiated in situ at ca. 306 Ma and lasted to ca. 300 Ma.

Petrology and Geochemistry of Adak Island Plutonic Xenoliths: Implications for Primitive Magma Generation and Crustal Differentiation in the Aleutian Island Arc

Abstract Deep crustal cumulates in arcs offer a window into the chemistry and crystallization conditions (P–T–H2O–fO2) of primitive basalts in the upper mantle and lower crust and can be studied in ancient exhumed terranes or in xenoliths erupted in young arc lavas. Here, we expand on previous studies and thoroughly characterize the extensive xenolith suites erupted from the Mt. Moffett and Mt. Adagdak volcanic centers (Adak Island, Central Aleutians), which range from primitive ultramafic cumulates to more evolved amphibole gabbros and hornblendites. We present detailed petrography as well as in situ trace and major element mineral chemistry. We use these data to calculate pressure, temperature, and fO2 estimates for the xenoliths, and compare these findings to experimental results to understand the crystallization sequence and P–T–H2O–fO2 under which the cumulates formed. The Moffett crystallization sequence is defined by early amphibole fractionation and an abrupt shift in oxide compositions from chromite to magnetite, while the Adagdak suite is characterized by simultaneous saturation of amphibole+plagioclase and oxide compositions that become increasingly aluminous before magnetite saturation. Olivine–spinel oxybarometry of the Adagdak xenoliths indicates that they are oxidized relative to mid-ocean ridge basalt (MORB:FMQ +0.1 to +2.1). Highly fractionated REE and elevated Sr/Y ratios are observed in clinopyroxene from the most primitive cumulates, consistent with a contribution from a basaltic eclogite melt. This basaltic eclogite melt is hypothesized to come from partial melting of the slab or through melting of basalt introduced into the subarc mantle through forearc subduction erosion. These signatures are greatly diminished in the more evolved lithologies, which can be explained through fractionation of plagioclase and amphibole. Our findings support the presence of a complex magmatic plumbing system beneath Adak, with Mt. Moffett and Mt. Adagdak volcanic centers tapping compositionally distinct sources. More broadly, our results are consistent with studies suggesting that low-degree basaltic eclogite melts through slab melting or forearc subduction erosion contribute to arc magmas in the Aleutians, although the associated geochemical signatures are easily obscured by differentiation in the crust.

The origin of Ni and Mn variations in Hawaiian and MORB olivines and associated basalts

The origins of differences in the Mn and Ni contents of olivine phenocrysts from ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are disputed. One prevalent hypothesis is mixing between peridotite melts and melts of an olivine-free pyroxenite that forms by reaction between large extents of melting of recycled ocean crust and peridotite. A second is peridotite melting at various pressures without contributions from recycled materials, and a third is mixing of recycled crust into peridotite sources. All models purport to account for olivine data, but an additional requirement is that the models account for corresponding liquid compositions. Here we demonstrate that none of the proposed models alone are able to account for both olivine and liquid data. The pyroxenite model predicts variations in liquid Mn to account for high Fe/Mn ratios, in contrast to the data where the high ratios result primarily from Fe variations. Using an updated model of Ni partitioning, combined with quantitative modeling of polybaric melting, confirms the important role of pressure on olivine compositions, and in addition shows that extent of melting and mantle fertility are also important. Most MORB are consistent with ∼10% -15% melting of peridotite at lower pressures, but low Ni olivines found on ridges adjacent to hot spots require a depleted mantle source. High Ni olivines can be produced by low degree melting of peridotite at high pressure, but such models do not produce the high Ni liquids that are also observed, nor the trace element abundances such as high TiO2. Mixing basalt and peridotite sources is not successful because it fails to account for the low Na/Ti ratios of OIB. A successful model for OIB involves adding ∼3–5% of a low degree eclogite melt with high trace element abundances and low Na/Ti to mantle peridotite, and then melting this source to small extents at high pressure. The need for a low degree melt of eclogite also rules out models that call upon high degrees of melting of eclogite that react with peridotite to produce OIB sources, and shows that this melting does not occur during upwelling beneath hot spots, where high degree melts of eclogite would occur. A firm conclusion of this work is that olivine compositions cannot be used to calculate the amount of recycled crust in mantle sources. Further work to explore variations within OIB and MORB will benefit from the realization that olivine compositions alone cannot be interpreted reliably without quantitative polybaric melting models and attention to co-existing liquids and trace element abundances.

Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.

Clinopyroxene Compositions of E-MORB-Type Gabbro from Bingdaban Ophiolites in Central Tianshan, NW China: Insights into the Evolution of the Magmatic System and Geodynamic Setting

Clinopyroxene, one of the primary rock-forming minerals in mafic rocks, is the major host of lithophile elements in the mantle lithosphere and plays a crucial role in understanding mantle evolution and rock petrogenesis. Taking the Bingdaban ophiolite as an example, this study employed electron probe microanalysis and in situ trace element analysis to investigate the geochemistry of clinopyroxene in gabbros to determine the magma series and evolution, constrain the physicochemical conditions of the magmatic processes and explore the petrogenesis and tectonic setting. Representative gabbro samples were subjected to zircon U–Pb isotopic analysis, yielding an age of 424.3 ± 5.9 Ma. Geochemical investigations revealed that the Bingdaban gabbros exhibit tholeiitic composition, suggesting a genesis associated with enriched mid-ocean ridge basalt (E-MORB). Mineralogical analyses indicated that the clinopyroxene in the gabbros was Mg-rich, Fe-poor, and alkali-poor, representing a subalkaline series. The compositional end members of clinopyroxene were calculated as Wo38.9–48.0En30.9–48.1Fs10.4–24.4, indicating a predominance of diopside with a minor amount of augite. Temperature–pressure conditions imply that these rocks formed in a high-temperature, low-pressure, and shallow-source environment. Compositional estimates of the melt in equilibrium with clinopyroxene are consistent with the overall characteristics of the host rock, reflecting an E-MORB setting. The Bingdaban gabbro likely originated from an initially depleted mantle source that later received an input of enriched mantle material, indicating formation in either an initial oceanic or immature back-arc basin tectonic setting.

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.