Oceanic Basalts Research Articles

Subaqueous effusive and explosive phases of late Deccan volcanism: evidence from Mumbai Islands, India

About 30 m of massive basaltic lava sheets and pillows belonging to the Deccan Traps are exposed at Borivali in the Sanjay Gandhi National Park, Mumbai (India). These lavas are conformably overlain by ~ 100 m of volcaniclastic deposits exposed in the Kanheri Caves. The pillows are plagioclase-clinopyroxene-phyric, very poorly vesicular, tholeiite basalts and commonly develop one to few central large cavities. Pillow rinds are holohyaline with swallow-tailed lanceolate or slender needle-shaped plagioclase and fan-spherulitic or dendritic textures in clinopyroxene that indicate rapid cooling under water. The volcaniclastic succession lying on top of the pillow lavas is chiefly composed of non-vesicular to scoriaceous/pumiceous, basaltic and andesitic clasts. The numerous sediment dykes within the volcaniclastic succession and the conformably underlying pillow lavas imply that the entire sequence was emplaced in a submarine environment. The fluidal texture and quenched margins in some of the volcanic clasts strongly suggest that at least part of the volcanic clasts were produced by underwater explosive eruptions. This study describes subaqueous erupted pillow lavas in context of continental flood basalts and provides a significant analogy to recent submarine-erupted intraplate oceanic basalts. The occurrence of pillow lavas and volcaniclastic deposits well above the present-day high tide line (160 m) indicates either high eustatic sea level during the Danaian age (61.8–62.9 Ma), or post-rifting regional uplift in the western Deccan Traps.

Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts

Pyroxenites are diffuse in fertile mantle peridotites and considered an important component in the mantle source of oceanic basalts. They are rarely documented in abyssal and ophiolitic peridotites representing residual mantle after melt generation, and few studies defining their origin are to date available. We present a field-based microstructural and geochemical investigation of the pyroxenite layers associated with depleted peridotites from the Mt. Maggiore ophiolitic body (Corsica, France). Field and petrographic evidence indicate that pyroxenite formation preceded the melt–rock interaction history that affected this mantle sector during Jurassic exhumation, namely (1) spinel-facies reactive porous flow leading to partial dissolution of the pyroxenites, and (2) plagioclase-facies melt impregnation leading to [plagioclase + orthopyroxene] interstitial crystallization. Pyroxenes show major element compositions similar to abyssal pyroxenites from slow-spreading ridges, indicative of magmatic segregation at pressures higher than 7 kbar. Both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by Na2O-poor, LREE-depleted compositions, consistent with unaggregated melt increments. This implies that they represent the continuous evolution of similarly depleted melts leading to different processes (pyroxenite segregation and later melt–rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt–rock interaction history, we modeled all the documented processes in sequence, i.e.: (1) formation of single-melt increments after 6% mantle decompressional fractional melting; (2) high-pressure segregation of pyroxenites; (3) spinel-facies reactive porous flow, (4) plagioclase-facies melt impregnation. The early fractionation of pyroxenites leads to a decrease in pyroxene saturation that is necessary for the subsequent reactive porous flow process, without any significant change in the melt REE composition.

Ubiquitous ultra-depleted domains in Earth’s mantle

Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle. Depleted mantle is a volumetrically dominant component of the Azores plume and possibly of oceanic basalt sources more generally, according to neodymium isotope compositions of olivine-hosted melt inclusions from lavas of the Azores mantle plume.

Ligurian pyroxenite-peridotite sequences (Italy) and the role of melt-rock reaction in creating enriched-MORB mantle sources

Deep melt intrusion and melt-peridotite interaction may introduce small-scale heterogeneity in the MORB mantle. These processes generate pyroxenite-bearing veined mantle that represent potential mantle sources of oceanic basalts. Natural proxies of such veined mantle are very rare and our understanding of mechanisms governing the chemical modification of mantle peridotite by MORB-type pyroxenite emplacement is very limited. We report the results of detailed spatially-controlled chemical profiles in pyroxenite-peridotite associations from the Northern Apennine ophiolitic mantle sequences (External Liguride Units, Italy), and investigate the extent and mechanism driving the local modification of peridotite by the interaction with pyroxenite-derived melt. Pyroxenites occur as cm-thick layers parallel to mantle tectonite foliation and show diffuse orthopyroxene-rich reaction rims along the pyroxenite-peridotite contact. Relative to distal unmodified peridotites, wall-rock peridotites show i) modal orthopyroxene enrichment at the expense of olivine, ii) higher Al, Ca, Si contents and slightly lower XMg, iii) Al-richer spinel and lower-XMg pyroxenes. Clinopyroxenes from wall-rock peridotites exhibit variable LREE-MREE fractionation, always resulting in SmN/NdN ratios lower than distal peridotites. From the contact with pyroxenite layers, peridotite clinopyroxenes record a REE compositional gradient up to about 15cm marked by an overall REE increase away from the pyroxenite. Beyond 15cm, and up to 23cm, the MREE and HREE content decreases while the LREEs remain at nearly constant abundances. This REE gradient is well reproduced by a two-step numerical simulation of reactive melt percolation assuming variable amounts of olivine assimilation and pyroxene crystallization. Percolative reactive flow at decreasing melt mass and rather high instantaneous melt/peridotite ratio (initial porosity of 30%), combined with high extents of fractional crystallization (i.e. relatively low Ma/Mc ratio), accounts for the overall REE enrichment in the first 15cm. Change of melt-rock reaction regime, mostly determined by the drastic decrease of porosity (Φi=0.01) due to increasing crystallization rates, results in more efficient chemical buffering of the host peridotite on the HREE composition of the differentiated liquids through ion-exchange chromatographic-type processes, determining the observed increase of the LREE/HREE ratio. Emplacement of thin (cm-sized) pyroxenite veins by deep melt infiltration is able to metasomatize a much larger volume of the host peridotite. Hybrid mantle domains made by pyroxenite, metasomatized peridotite and unmodified peridotite potentially represent mantle sources of E-MORB. Results of this work stress the key role of melt-peridotite reactions in modifying the upwelling mantle prior to oceanic basalts production.

Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle

AbstractA major goal in Earth Science has been to understand how geochemical characteristics of lavas at the Earth's surface relate to the location and formation history of specific regions in the Earth's interior. For example, some of the strongest evidence for the preservation of primitive material comes from low 4He/3He ratios in ocean island basalts, but the location of the primitive helium reservoir(s) remains unknown. Here we combine whole‐mantle seismic tomography, simulations of mantle flow, and a global compilation of new and existing measurements of the 4He/3He ratios in ocean island basalts to constrain the source location of primitive 4He/3He material. Our geodynamic simulations predict the present‐day surface expression of plumes to be laterally offset from their lower mantle source locations. When this lateral offset is accounted for, a strong relationship emerges between minimum 4He/3He ratios in oceanic basalts and seismically slow regions, which are generally located within the two large low shear‐wave velocity provinces (LLSVPs). Conversely, no significant relationship is observed between maximum 208Pb*/206Pb* ratios and seismically slow regions in the lowermost mantle. These results indicate that primitive materials are geographically restricted to LLSVPs, while recycled materials are more broadly distributed across the lower mantle. The primitive nature of the LLSVPs indicates these regions are not composed entirely of recycled slabs, while complementary xenon and tungsten isotopic anomalies require the primitive portion of the LLSVPs to have formed during Earth's accretion, survived the Moon‐forming giant impact, and remained relatively unmixed during the subsequent 4.5 billion years of mantle convection.

Potassium isotope fractionation during magmatic differentiation of basalt to rhyolite

High-temperature equilibrium and kinetic stable isotope fractionation during partial melting, fractional crystallization, and other igneous differentiation processes has been observed in many isotope systems, but due to the relative nascence of high-precision analytical capabilities for K, it is still unclear whether igneous processes induce systematic and resolvable K isotope fractionation. In this study, we look to the natural laboratory of Hekla volcano in Iceland to investigate the behavior of K isotopes during magmatic differentiation of basalt to rhyolite. Using a novel MC-ICP-MS method, we analyzed 24 geochemically diverse samples from Hekla, including 7 basalts, 8 basaltic andesites, 3 andesites, 4 dacites, and 2 rhyolites, along with 2 additional samples from Burfell, Iceland, for comparison (1 basalt and 1 trachyte). We observed extremely limited variation of 41K/39K ratios throughout our suite of samples, which is not resolvable within the best current analytical uncertainty. The average value of all samples is δ41KNIST SRM3141a = −0.46 ± 0.07‰ (2sd). This value agrees with the Bulk Silicate Earth value previously defined by average global oceanic basalts in literature. The lack of variation throughout this suite of samples from a single volcano system indicates that K does not fractionate during magmatic differentiation (of basalt to rhyolite) through processes such as partial melting and fractional crystallization. This conclusion is important to the estimation of the Bulk Silicate Earth K isotope composition, to placing a more robust estimate on the composition bulk continental crust, and to fostering a better understanding of the behavior of K isotopes during differentiation of the terrestrial planets.

Velocity Weakening of Simulated Augite Gouge at Hydrothermal Conditions: Implications for Frictional Slip of Pyroxene‐Bearing Mafic Lower Crust

AbstractTo understand the velocity‐weakening behavior of pyroxene as a major constituent in mafic rocks under hydrothermal conditions, we employed augite (clinopyroxene, (Ca0.577Na0.110Mn0.004Mg0.886Fe2+0.193Fe3+0.037AlVI0.173Ti0.019)[AlIV0.137Si1.863O6]) as simulated gouge sample material to run velocity stepping sliding tests at temperatures of 101–607 °C, confining pressure of 127 MPa with 30 MPa pore pressure and axial loading rates of 0.04–1.0 μm/s. The friction coefficient was found to range from 0.71 to 0.73 with no systematic temperature dependence. Velocity‐weakening behavior occurred at temperatures above 203 °C, with low velocity dependence or (b‐a) value around 0.0012, corresponding to b/a values of 1.15–1.18. Inferred b values in the rate and state friction laws exhibited an increasing trend with temperature increase from ~0.0056 to ~0.0098 up to 403 °C, indicating activation of Arrhenius‐type creep at the frictional contacts. Microstructural observations on deformed samples show ubiquitous precipitated particles suggesting intergranular pressure solution, and no evidence for deformation by crystalline plasticity was observed. This fact, together with the comparison between experimental data and model prediction, suggests that the process of intergranular pressure solution at frictional contacts may have governed the evolution effect for augite. The minor velocity weakening of augite combined with previous data on plagioclase is consistent with the modest velocity weakening seen in mafic rocks, such as quartz‐free oceanic basalt. Such a minor velocity‐weakening property tends to favor slow slip events on the downdip portion of the subduction interface below currently locked seismogenic zones, such as in southwestern Japan, northern Cascadia, and Mexico.

Authigenic Mineral Texture in Submarine 1979 Basalt Drill Core, Surtsey Volcano, Iceland

AbstractMicrometer‐scale maps of authigenic microstructures in submarine basaltic tuff from a 1979 Surtsey volcano, Iceland, drill core acquired 15 years after eruptions terminated describe the initial alteration of oceanic basalt in a low‐temperature hydrothermal system. An integrative investigative approach uses synchrotron source X‐ray microdiffraction, microfluoresence, micro‐computed tomography, and scanning transmission electron microscopy coupled with Raman spectroscopy to create finely resolved spatial frameworks that record a continuum of alteration in glass and olivine. Microanalytical maps of vesicular and fractured lapilli in specimens from 157.1‐, 137.9‐, and 102.6‐m depths and borehole temperatures of 83, 93.9, and 141.3 °C measured in 1980, respectively, describe the production of nanocrystalline clay mineral, zeolites, and Al‐tobermorite in diverse microenvironments. Irregular alteration fronts at 157.1‐m depth resemble microchannels associated with biological activity in older basalts. By contrast, linear microstructures with little resemblance to previously described alteration features have nanocrystalline clay mineral (nontronite) and zeolite (amicite) texture. The crystallographic preferred orientation rotates around an axis parallel to the linear feature. Raman spectra indicating degraded and poorly ordered carbonaceous matter of possible biological origin are associated with nanocrystalline clay mineral in a crystallographically oriented linear microstructure in altered olivine at 102.6 m and with subcircular nanoscale cavities in altered glass at 137.9‐m depth. Although evidence for biotic processes is inconclusive, the integrated analyses describe the complex organization of previously unrecognized mineral texture in very young basalt. They provide a foundational mineralogical reference for longitudinal, time‐lapse characterizations of palagonitized basalt in oceanic environments.

GENERAL PB‐ISOTOPE SYSTEMATICS OF SOURCES FOR VOLCANIC ROCKS OF THE LATEST GEODYNAMIC STAGE IN ASIA

The modern theory of the evolving Earth is based on integrated isotopic data obtained for the accessible part of the planet and cosmic bodies, in which the U–Pb isotope system plays a key role. The theory is tested by the isotope systematics of oceanic basalt sources. The origin of continental volcanic rocks is often interpreted in terms of the isotopic systematics of oceanic basalts. However, such interpretations, as a rule, reveal contradictions arising from differences in the history and current mantle dynamics of oceans and continents. Under the oceans, a mantle material has long lost connection with the accessible Earth tectonic units; under the continents such a connection is often established. The nature of the evolution of deep‐seated processes under the continents remains uncertain and, by analogy with the oceans, requires deciphering in terms of the components of the mantle sources for volcanic rocks. In modern lithospheric plates of the Earth, there are regions ranging in width from hundreds to thousands of kilometers, which are characterized by high strain rates and, consequently, at least one to two orders of magnitude lower viscosity relative to that of the internal stable parts of the plates. This gives them a special structural status of “dispersed plate boundaries”. The isotope‐geochemical studies of volcanic rocks from regions of the unstable Asia revealed the different nature of components in sources, for which particular interpretations have been proposed. In this paper, a general systematics of sources is defined for volcanic rocks of the latest geodynamic stage in Asia through estimating the incubation time on the 207Pb/204Pb versus 206Pb/204Pb diagram. Two domains are designated: (1) low 238U/204Pb (LOMU) derived from the viscous protomantle (VIPMA), and (2) elevated 238U/204Pb (ELMU). The mantle domains evolved from the Earth's primary material between 4.51 and 4.36 Gyr ago, 4.0 and 3.7 Gyr ago, 2.9 and 2.6 Gyr ago, 2.0 and 1.8 Gyr ago, about 0.66 Gyr ago and <0.09 Gyr ago. Melting anomalies of ELMU sources characterize the unstable mantle of Southern Asia, and those of LOMU sources belong to the Japan‐Baikal geodynamic corridor of the transitional region between the unstable mantle of Asia and its stable core. The Late Cenozoic evolution of the Japan‐Baikal geodynamic corridor resulted in cutting the LOMU domain by the Jeju‐Vitim ELMU source line.

Identifying volatile mantle trend with the water–fluorine–cerium systematics of basaltic glass

The lithophile elements and isotopic compositions of oceanic basalts suggest they derive from different mantle components. We present new analytical results of un-degassed deep-marine basaltic glasses from various regions, and we find strong linear correlations of R2 ≥ 0.997 between H2O and F as well as H2O and Ce, for which we propose mantle trends. The mantle trends represent global variations of mantle components, ranging from a depleted dry peridotite of mid-ocean ridge basalt (MORB) (DMM, depleted MORB source mantle: H2O = 100 ppm; H2O/Ce = 200; H2O/F = 10) to a hydrous peridotite of Hawaiian ocean island basalt (OIB) of deep mantle origin (FOZO, focal zone: H2O = 750 ppm; H2O/Ce = 200; H2O/F = 18.5). Accordingly, we defined the correlation as a volatile DMM–FOZO trend. Based on our findings, we report novel H2O–F–Ce systematics to discriminate the degree of water depletion in the source mantle and the rehydration of the mantle with recycled surface water through oceanic plate subduction. Using this method, most OIBs are distinguished clearly from the DMM–FOZO trend, and we find that the water in their sources originates from recycled water derived from the hydrated oceanic crust and sediment after various degrees of dehydration (75–95%) in subduction zones.

Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources

The chemistry of erupted magmas provides a crucial window into the composition and structure of Earth's convecting mantle. However, magmatic evolution in the crust makes it challenging to reconstruct mantle properties from volcanic rocks in important but incompletely understood ways. Here we investigate how mantle-derived compositional variability in primary oceanic basalts determines their phase equilibria relations and the nature of the geochemical signals they record. By performing experiments on synthetic analogues of compositionally extreme primitive lavas from the Reykjanes Peninsula of Iceland at realistic magma storage conditions (300 MPa, 1140–1260°C), we show that melts from enriched mantle domains retain higher melt fractions as they cool than those generated by melting of typical fertile lherzolite (i.e. they crystallise less mass over any interval of decreasing temperature). These melt fraction differences arise because plagioclase crystallisation is suppressed in Na- and H2O-rich but Ca- and Al-poor liquids derived from enriched source lithologies. Thus, compositional characteristics inherited from the mantle have a first-order control on the efficiency with which cooling basalts crystallise. This means that enriched melts will be more likely to survive crustal processing than depleted melts. Basalt chemistry will therefore be disproportionately influenced by melts from volumetrically minor enriched lithologies compared with melts from the upper mantle's most common lithology, lherzolite, systematically biasing basaltic records towards melts from recycled mantle sources.We combine our experimental observations from Iceland with thermodynamic simulations on mid-ocean ridge basalt compositions and show that mantle-derived variability in crystallisation efficiency can explain two enigmatic features of the global oceanic basalt record: firstly, the anomalous over-enrichment of incompatible elements during the differentiation of mid-ocean ridge basalts, which may reflect a progressive bias towards enriched compositions as differentiation proceeds; and secondly, the frequently documented cargoes of highly anorthitic plagioclase crystals carried by evolved and enriched liquids from which they cannot have crystallised. These crystals can now be understood as the solidified remnants of depleted, lherzolite-derived melts that have been entrained into melt mixtures from more enriched sources. Increases in the degree of enrichment of cumulate rocks sampled from progressively shallower horizons of the oceanic crust can also be interpreted in terms of enriched melts surviving crustal processing in preference to depleted melts.

Potassium isotope systematics of oceanic basalts

Abstract High-temperature isotope fractionation during partial melting and other igneous differentiation processes has been observed in many non-traditional isotope systems. The potassium (K) isotope system has not been extensively investigated historically due to the lack of high-precision analysis methods; however, the recent development of the Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) now allows for high-precision potassium isotope analysis. In this study, we utilized this new method to analyze 51 geologically, geographically, and geochemically diverse oceanic basalt samples including 32 mid-ocean ridge basalts (MORB), 3 back-arc basin basalts (BABB), and 16 oceanic island basalts (OIB). We observed a limited variation of 41K/39K ratios across our spread of samples. This variation in mantle-derived rocks is restricted compared to the large K isotopic fractionation observed in low-temperature systems. The averages of MORBs, BABBs, and OIBs are −0.44 ± 0.17‰ (2sd), −0.44 ± 0.08‰, and −0.41 ± 0.16‰, respectively, and there is no geographical variation (e.g., Indian vs. Pacific MORBs) in terms of K isotopes. Among all samples, there are two outliers, in which we have observed evidence of secondary mineral formation (i.e., palagonite) due to interaction with seawater. These two outliers have a K isotopic composition significantly heavier than other unaltered samples, close to the K isotopic composition of seawater. The grand average of all pristine samples is −0.43 ± 0.17‰ (2sd) which agrees well with the Bulk Silicate Earth (BSE) value previously defined. This new study indicates the homogeneity of K isotopes in the mantle and suggests that, since K will not resolvably fractionate during partial melting, any observable fractionation of K isotopes in primitive basalts is likely due to low-temperature, post-eruptive alteration processes. This conclusion is critical for understanding the initial bulk composition of the Earth and it is essential for any interplanetary comparison.

Tracing subducted oceanic slabs in the mantle by using potassium isotopes

Substantial quantities of oceanic slabs are known to be recycled into the mantle through subduction. However, tracing their fate in the mantle through studies of mantle-derived samples is not always straightforward. As K represents an omnipresent element in subducting crustal materials, its two stable isotopes (39K and 41K) display outstanding potential in tracing recycled crustal materials in the mantle. Here we report the first set of K isotopic data for a suite of well-characterized continental basaltic lavas from Northeast China. The δ41K of these lavas exhibit large variation from −0.81‰ to −0.15‰, compared with the narrow range of unaltered oceanic basalts (−0.43 ± 0.03‰). The isotopic variations, together with geochemical data obtained in earlier studies, cannot be ascribed to chemical weathering, crystal fractionation, crustal contamination, or partial melting, but are consistent with the presence of diverse crustal materials in magma sources. Recycled oceanic crust and sediments both have contributed to the contrasting K isotopic signature observed in these lavas. Our study here highlights the potential of using K isotopes to trace different types of recycled crustal materials in the mantle and places new constraints on the generation of enriched mantle reservoirs.

Process-related isotope variability in oceanic basalts revealed by high-precision Sr isotope ratios in olivine-hosted melt inclusions

Melting of the proto-Iceland Plume produced the intra-plate lavas of the Faroe Islands. The lava series of the Malinstíndur Formation contains picrite flows which have accumulated up to 50% olivine pheno- and antecrysts. Whole-rocks, minerals and olivine-hosted melt inclusions (MI) from this formation, in addition to other Faroe Island lavas, were analysed for their major-trace element and Sr isotope composition. In comparison to other lavas from the Faroe Islands, the depleted incompatible trace-element composition of the population of inclusions investigated here is equivalent to that of its host-lava, and thus minorly influenced by post-entrapment crystallisation (PEC). High-precision Sr isotope ratios of silicate melt inclusions in olivine, binned by their Fo contents (Fo93–78, in 1 mol% Fo bins), reveal large variations in 87Sr/86Sr within a single lava flow. Both the 87Sr/86Sr and their variability increase with decreasing Fo content, suggesting progressive incorporation of different components during magma evolution, that is, concurrent mixing and crystallisation. The total variation in Sr isotope ratios in the MI population covers about 60% of the variation recorded in the erupted lavas of the entire Faroe Island archipelago, but is slightly biased towards more radiogenic 87Sr/86Sr. The variation of Sr isotope ratios and incompatible trace element concentrations is best explained by mixing-in of melts with different trace element and isotope compositions. Clinopyroxene barometry indicates that mixing of melts likely occurred at pressures higher than 0.6 GPa, in sub-MOHO magma conduits and sills. Hence the chemical and isotopic variability of the melt inclusions is influenced by magmatic processes during melt evolution, which must therefore also influence how source heterogeneity is reflected in the composition of the erupted basalts.

The Itmurundy Accretionary Complex, Northern Balkhash Area: Geological Structure, Stratigraphy and Tectonic Origin

The Itmurundy zone of the northern Balkhash area is a Pacific-type orogenic belt. It possesses a complex geological structure and hosts rocks of mantle, accretionary and post-orogenic associations. The volcanic and sedimentary rocks of the accretionary association belong to three suites: Itmurundy (O1-2), Kazyk (O2-3) and Tyuretai (O3-S1). The suites are separated by tectonic unconformities or faults of three orders: 1) large regional faults; 2) medium faults separating mantle and oceanic accreted rocks; 3) small faults separating packages consisting of oceanic sediments. The Itmurundy Fm. (O1-2) is the most lithologically variable consisting of oceanic basalt, pelagic chert, hemipelagic siliceous mudstone and siltstone, and trench greywacke sandstone. The packages, each consisting of chert-siliceous mudstone, are separated from each other by 2nd and 3rd order faults of probably thrust nature, i.e. they are parts of duplex structures. The presence of duplex structures and the high degree of deformation of Itmurundy Fm. rocks are typical of accretionary complexes. The associations of volcanic and sedimentary rocks under study represent a full section of oceanic plate stratigraphy (OPS): basalt (MORB, OIB)—chert (pelagic)—siliceous mudstone, siltstone and shale (hemipelagic)—trench sandstones (greywacke). The structural position and the lithology of Itmurundy rocks accord well with the model of formation of accretionary complexes at Pacific-type convergent margins, in particular, those in the western Pacific.

Subduction zone geochemistry

Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB).The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.

The Alamos Metamorphic Complex, evidence of late Paleozoic collision between Laurentia and Gondwanan blocks in northwestern Mexico

The Alamos Complex is a metamorphosed volcano-sedimentary sequence exposed south of the late Paleozoic Ouachita–Marathon–Sonora orogen in northwestern Mexico, which in turn, is regarded as a continuation of the Alleghenian–Variscan mountain chain. Mineral assemblages in metavolcanic rocks indicate greenschist facies metamorphism. Thermobarometric studies yield an average metamorphic temperature of 380 °C, whereas andalusite in metapelites indicates pressure less than 0.4 ± 0.05 GPa. The main deformation phase generated pervasive foliation, which in turn was isoclinal to close folded and a second axial planar cleavage was developed. Metavolcanic rocks of the Alamos Complex can be separated in two suites: (1) an anorogenic suite, similar to enriched middle ocean basalts (E-MORB) in the Tesia region, and (2) an orogenic suite, characteristic of volcanic-arc setting in the Mocuzari region. U–Pb detrital zircon geochronology of one sample from Tesia yielded a maximum depositional age averaging 395 ± 28 Ma. One sample from the Mocuzari region yielded a maximum depositional age averaging 1104 ± 24 Ma, with the largest peak at 1428. A clastic sequence correlated with the Middle Triassic–Lower Jurassic Barranca Group, nonconformably cover the Alamos Complex. The basal conglomerate of that group yields a maximum depositional age averaging 372 ± 23 Ma. Accordingly, the Alamos Complex must be in part deposited after the early Devonian time and before the Middle Triassic. The main sources of detritus for the complex are the Meso- and Paleoproterozoic rocks of southwestern Laurentia, whereas the main source of detritus for the Barranca Group is the Rio Fuerte Formation exposed ~ 100 km south of the study area. The metamorphic event is ascribed to the late Paleozoic collision of Gondwanan blocks against the southern margin of Laurentia.

K-rich hydrous mantle lithosphere beneath the Ontong Java Plateau: Significance for the genesis of oceanic basalts and Archean continents

Olivine-free metasomatic mantle-derived xenoliths, frequently recovered from kimberlite and lamprophyre intrusions on the continents, are seldom described from oceanic settings. We report the mineralogy, geochemistry and Sr-Nd-Hf-Pb-Os isotopic compositions of a unique K-rich, hydrous mantle nodule sourced from a 34 Ma ultramafic lamprophyre (alnöite) pipe on the island of Malaita, Ontong Java Plateau, SW Pacific. The Ontong Java Plateau, the most voluminous oceanic large igeous province, has a >100 km thick lithospheric root reminiscent of thick lithospheres that characterize cratonic settings. The phlogopite, amphibole, clinopyroxene and ilmenite-rich nature of the Malaita metasomatic nodule bears striking similarity to cratonic mica-amphibole-rutile-ilmenite-diopside (MARID) suite xenoliths, and it provides a tangible example of heavily overprinted oceanic mantle lithosphere. The nodule phlogopite 40Ar/39Ar age of 44.7 ± 1.8 Ma (95% confidence level) predates the 34 Ma alnöites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnöite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnöite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnöite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (87Sr/86Sr45Ma = 0.70419–0.70423; εNd45Ma = +3.5; εHf45Ma = +5.3; 206Pb/204Pb45Ma = 18.66–18.71; 207Pb/204Pb45Ma = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high 187Os/188Os45Ma (0.1824–0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. K-rich hydrous metasomatized mantle lithosphere could represent an enriched source component that is required for the formation of many oceanic intraplate basaltic provinces. As such, strongly metasomatized oceanic lithosphere provides an alternative to recycled crust components within the convecting mantle to explain enriched mantle signatures observed in oceanic basalts worldwide. However, constraining the volumetric abundance of strongly metasomatized oceanic lithosphere, possibly revealed by seismic mid-lithospheric discontinuities, is required before its importance can be established. The existence of K-rich metasomatic components within oceanic plateau lithosphere may also have implications for Archean continental crust formation models. The K-rich mantle component required to explain sanukitoid magma formation associated with continental crust building TTG magmatism may not be restricted to a subduction channel environment, but it can equally reside within overthickened oceanic lithosphere. Thus, oceanic plateaus away from subduction zones could have served as nucleation points for growth of continental crust during the Archean.

Earth's chondritic light rare earth element composition: Evidence from the Ce–Nd isotope systematics of chondrites and oceanic basalts

Combined Ce and Nd isotope ratios provide a time-integrated record of the light rare earth element (LREE) abundances of their source materials. Here, we present new high precision Ce isotope data for chondrites and basalts from ocean islands (OIB) and mid ocean ridges (MORB). The new Ce isotope ratios in chondritic meteorites define a precise new CHUR reference value. In Ce–Nd isotopic space, the MORB and OIB form a well-defined array that intersects with the Ce–Nd chondritic reference value. The simplest first-order explanation is that the bulk silicate Earth (BSE) has chondritic LREE and Ce–Nd isotope ratios. We show, however, that the intercept and slope of the Ce–Nd isotope mantle array depend on several factors. Perhaps most important are whether the BSE is chondritic and how closely the mantle average reflected in the MORB and OIB data corresponds to the Ce–Nd isotope ratio of the BSE. A significant difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could result from the permanent storage of a considerable proportion of Earth's total LREE budget in the continental crust or potential isolated reservoirs. We show that the formation of isolated reservoirs either has a minor effect on the average Ce–Nd composition of the average mantle, or is geochemically and geodynamically implausible. If, due to formation of the continental crust, a significant shift in the average mantle's Ce–Nd isotope composition relative to BSE occurs, this shift is parallel to the Ce–Nd mantle array, and does not affect its chondritic intercept. The chondritic intercept of the Ce–Nd isotope mantle array therefore is strong evidence that BSE's relative LREE and Ce–Nd isotope composition is chondritic. However, an apparent difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could also result if MORB and OIB do not sample the accessible mantle in a representative manner. Although we cannot entirely exclude the latter, it would require a fortuitous combination of factors to cause the observed chondritic intercept of the Ce–Nd isotope mantle array. We therefore conclude that the bulk silicate Earth has chondritic LREE and Ce–Nd isotope ratios.

High-precision measurement of B isotopes on low-boron oceanic volcanic rock samples via MC-ICPMS: Evaluating acid leaching effects on boron isotope compositions, and B isotopic variability in depleted oceanic basalts

Recent analytical advances have made the high precision (2SD, ±≤1‰) determination of boron (B) isotopic ratios in silicate rocks a possibility, but few such data are as yet available, especially for low B (<2 ppm) samples like oceanic basalts, where issues related to cryptic seawater alteration are a concern. This study carefully assesses the reproducibility of B isotope measurement via MC-ICPMS on silicate rocks. Based on repeated measurements of the standard reference materials B-5, B-6, JB-2, JR-2, AGV-2 and JB-3 and nature samples, the external reproducibility of MC-ICPMS measurements on the same solution is generally between 0.1‰ and 0.5‰ (2SD), a similar level of accuracy to that obtained based on long-term measurement of solution standard NIST SRM 951 of 0.4‰ (2SD). For different digestions, the average external reproducibility is 0.6‰ (2SD), which represents the whole procedure reproducibility, including chemical treatment. We have tested an acid leaching experimental procedure for its effects on the B isotope compositions of ~50 Ma boninites collected from a deep ocean setting. Fresh and moderately altered boninite samples were subjected to cold HCl leaching treatments for 1, 3, 5, 7 and 9 h, using 2 N and 6 N HCl, respectively. δ11B values for the fresh sample remained unchanged within error after leaching, suggesting that leaching did not lead to any B isotopic fractionation. δ11B values for different aliquots of an altered boninite sample show lower and uniform δ11B values after 1 h of 6 N HCl leaching, while aliquots leached in 2 N HCl show similar δ11B to the unleached altered sample. These results suggest that changes in B isotopes caused by at least moderate degrees of alteration, and/or adsorption effects in volcanic rocks preserved on the ocean floor can be effectively eliminated through short-duration 6 N HCl leaches before digestion and analysis. B isotope measurements on three documentably fresh, low-B N-type mid-ocean ridge basalt (MORB) glass samples from the Southwest Indian Ridge yield δ11B values from −6.62‰ to −7.39‰, within the range of the recommended global MORB value of ≈−7.1 ± 0.9‰, determined via SIMS methodologies (2SD at ±1.5‰; Marschall and Monteleone, 2015), and the range of four fresh glass samples from the Eastern Pacific Ridge of −7.3 ± 0.8‰, measured by in-situ laser-ablation ICP-MS (2SD at ±0.7–1.0‰; Le Roux et al., 2004).