Mid-ocean Ridge Basalt Samples Research Articles

Contribution of recycled oceanic crust to the extremely light molybdenum isotopic compositions of mid-ocean ridge basalts from the South China Sea

The contribution of recycled oceanic crust to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Nd–Hf isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the South China Sea (SCS). The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). The ESB MORBs display extremely light Mo isotopic compositions (δ98/95Mo = −0.80‰ to −0.24‰) with NdHf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show higher δ98/95Mo values (−0.44‰ to +0.05‰) and depleted NdHf isotopic compositions. The δ98/95Mo values of the SCS MORBs are negatively correlated with Ce/Mo ratios and positively correlated with εHf values, suggesting the incorporation of dehydrated slab components into the source of the SCS MORBs. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone, as suggested by mantle tomography. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. This study, therefore, provides compelling evidence for the involvement of recycled oceanic slabs in generating MORB and the influence of an ascending mantle plume on seafloor spreading in a continental marginal sea. Mantle recycling processes in marginal sea basins exhibit distinct characteristics compared to those occurring in open oceans.

Rubidium isotopic fractionation during magmatic processes and the composition of the bulk silicate Earth

Rubidium is a moderately volatile element with high incompatibility and fluid mobility. Its stable isotopes have great potential in tracing various geological processes. For example, lunar rocks are isotopically heavier than terrestrial ones, suggesting volatile loss by evaporation during or following the formation of the Moon. However, these studies rely on a poorly constrained estimate for the composition of the Earth’s mantle and a poor understanding of high-temperature processes which may act to fractionate stable Rb isotopes. It is therefore important to precisely characterize different rock types that sample the Earth’s mantle as well as to evaluate the importance of key isotopic fractionation processes. In order to address these issues, we established a high precision analytical method for Rb isotopic measurements using the Nu Sapphire CC-MC-ICP-MS (collision-cell multi-collector inductively coupled plasma mass spectrometer). In addition, we present a series of Rb isotopic data of volcanic rocks from Hekla volcano (Iceland) and MORB (mid-ocean ridge basalt) samples. We show that our method returns a high Rb sensitivity (∼500 V/μg·g−1 for 85Rb) and a long-term reproducibility of 0.03‰ on δ87Rb (the permil deviation of the 87Rb/85Rb ratio from the SRM 984 standard). This method uses a 2 ng/g Rb solution for analyses, allowing us to consume about 10 times less Rb to achieve similar or better precision than previous studies. Using this method, seven geostandards and one synthetic standard return Rb isotopic data consistent with previous work. Twenty-one Hekla volcanic rocks, spanning compositions from basalt to rhyolite, show limited Rb isotopic variation, with δ87Rb values varying from −0.17‰ to −0.07‰, demonstrating that magmatic evolution has an insignificant effect on Rb isotope ratios. A set of MORB samples (n = 15) from different mid-ocean ridges also span a limited Rb isotopic variation, displaying a range similar to the Hekla rock suite (−0.19 to −0.02‰). Combining our new data together with previously reported OIB data gives an average δ87Rb value of −0.12 ± 0.08‰ (2SD, n = 25), representing the current best estimate of the mantle’s isotopic composition. Considering the δ87Rb values of the upper continental crust (−0.14 ± 0.01‰, 2SE, n = 73), as inferred from recent measurements of granites, loess and sediments, and assuming this value represents the whole crust, the revised Rb isotopic composition of the bulk silicate Earth (and by extension the bulk Earth, assuming no Rb partitioned into the core) is −0.13 ± 0.06‰ (2SD).

Stable Sn isotope signatures of Mid-ocean ridge basalts

The constraint of Sn isotope composition of the Earth's mantle is the prerequisite for understanding planet formation with Sn isotopes and Sn isotope fractionation in terrestrial reservoirs. However, previous estimates of the Sn isotope composition of the Earth's mantle are hampered by the small number of mantle-related samples. In this study, we analyzed Sn stable isotope compositions of 27 basalts from geochemically and geologically diverse mid-ocean ridge segments, to constrain the Sn isotope composition of the depleted mantle source. The spread of Sn isotope compositions for the 27 different mid-ocean ridge basalts (MORBs) is limited, with δ122/118SnBHVO-2 (delta notation of 122Sn/118Sn ratios relative to USGS BHVO-2 basalt standard) values ranging from −0.051 ± 0.015‰ to 0.209 ± 0.016‰ (δ122/118Sn3161a from 0.280 ± 0.015 to 0.540 ± 0.016‰), despite great geochemical and geographical diversities of the samples. The slightly higher δ122/118SnBHVO-2 value (up to 0.209 ± 0.016‰ (δ122/118Sn3161a up to 0.540 ± 0.016‰)) of one MORB sample appears to be caused by post-eruptive alteration of the basalts. Other than that, the δ122/118SnBHVO-2 values of the MORB samples do not show correlations with sample latitude, spreading rate of mid-ocean ridge, MgO content of the bulk rock samples, and partial melting index Na8.0 (defined as [Na2O] + 0.373 × [MgO] − 2.98), implying that Sn isotopes do not fractionate significantly during MORB melt generation and evolution processes. This study confirms the limited Sn isotopic variability between fresh MORBs globally, pointing to the Sn isotopic homogeneity of the depleted mantle source. Using these new MORB data, we proposed an estimate of the δ122/118SnBHVO-2 value for the Earth's depleted mantle to be 0.036 ± 0.087‰ (δ122/118Sn3161a of 0.367 ± 0.087‰) (2SD, N = 12). This value provides a reference point for understanding the planetary and magmatic processes of Earth from a Sn isotope perspective.

Low-degree melt metasomatic origin of heavy Fe isotope enrichment in the MORB mantle

Studies of mid-ocean ridge basalts (MORB) show a variable Fe isotope composition of the oceanic upper mantle. To test a recent hypothesis that heavy Fe isotope enrichment in the MORB mantle results from the same process of incompatible element enrichment, we conduct an Fe isotope study of well-characterized MORB samples from a magmatically robust segment (OH-1) of the Mid-Atlantic Ridge (MAR) at ∼ 35°N. The data show large Fe isotope variation (δ56Fe = +0.03 to +0.18‰) that correlates well with the abundances and ratios of more-to-less incompatible elements and with Sr-Nd-Hf isotopes. Our findings in support of the hypothesis can be detailed as follows: (1) the oceanic upper mantle has a heterogeneous Fe isotope composition on varying small spatial scales with isotopically heavy Fe (high-δ56Fe) preferentially associated with pyroxenite lithologies; (2) such lithologies, which are also enriched in the progressively more incompatible elements, are of low-degree (low-F) melt metasomatic origin; (3) with all the conceivable processes considered, the low-F melt metasomatism takes place at the lithosphere-asthenosphere boundary (LAB) beneath ocean basins through crystallization of incipient (Low-F) melt in the seismic low velocity zone (LVZ) at the base of the growing oceanic lithosphere (i.e., LAB) over the Earth's history since the onset of plate tectonics, forming composite lithologies with geochemically enriched pyroxenite veins dispersed in the depleted peridotite matrix; (4) such mantle of composite lithology when transported to beneath the present-day ocean ridges will undergo decompression melting and produce MORB melts with geochemical trends of “melting-induced mixing” as observed at the MAR and global MORB; (5) we predict all this to be a globally common process and widespread.

A subduction influence on ocean ridge basalts outside the Pacific subduction shield

The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.

Ga isotopic fractionation in sulfides from the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge

This study examines the Ga isotopic compositions of sulfides in the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge, mid-ocean ridge basalts (MORB), and calcareous sediments around the hydrothermal fields. The δ71/69GaNIST-994 values of the MORB samples vary little (+1.20‰ to +1.23‰, with an average of +1.22‰) and are consistent with the δ71/69GaNIST-994 values of two standard basalt samples (BCR-2 and BHVO-2), indicating that Ga isotopes may either not fractionate or fractionate only slightly under high-temperature geological processes; therefore, the δ71/69GaNIST-994 value of oceanic crust may be +1.22‰. The sediments (+1.28‰ to +1.47‰, with an average of +1.38‰) are rich in heavier Ga isotopes than the basalts, and the Ga present in the sediments may have originated from soluble Ga present in the seawater that was adsorbed by (Mn, Fe) oxides/hydroxides. The Ga contribution of basaltic debris to the sediments was almost negligible. Thus, we speculate that the δ71/69GaNIST-994 value of seawater in the study area fell within a range from +1.92‰ to +2.36‰. The δ71/69GaNIST-994 values of the sulfides in the Yuhuang hydrothermal field range from +0.99‰ to +1.57‰, with an average of +1.25‰, and the δ71/69GaNIST-994 values of the sulfides in the Duanqiao hydrothermal field range from +0.93‰ to +1.55‰, with an average of +1.19‰. The δ71/69GaNIST-994 ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar, with the Ga isotopic fractionation reaching 0.58‰ and 0.62‰, respectively. The average δ71/69GaNIST-994 values in the sulfides are close to those in the MORBs. This suggests that Ga within the sulfides in the Yuhuang and Duanqiao hydrothermal fields mainly originated from MORBs, with seawater and sediments making only small contributions. The Ga isotopic fractionation in the sulfides may be related to processes associated with the formation of sulfides, such as rapid precipitation or the admixture of different stages of sulfide. This study is of great significance for understanding the global distribution of Ga isotopes and the Ga cycle in submarine hydrothermal systems.

Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.

Quantitative vesicle analyses and total CO2 reconstruction in mid-ocean ridge basalts

Vesicle textures in submarine lavas have been used to calculate total (pre-eruption) volatile concentrations in mid-ocean ridge basalts (MORB), which provide constraints on upper mantle volatile concentrations and global mid-ocean ridge CO2 flux. In this study, we evaluate vesicle size distributions (VSDs) and volatile concentrations in a suite of 20 MORB samples that span the range of vesicularities and vesicle number densities observed in MORB globally. We provide recommended best practices for quantifying vesicularity, vesicle number densities, and VSDs based on synthetic vesicle populations and comparisons between traditional 2D estimates and X-ray computed micro-tomography results. For 2D measurements, we recommend analyzing multiple polished fragments with a cumulative area >100 times the area of the largest observed vesicle and including >200 vesicles in stereological VSD reconstructions. For 3D measurements, we recommend analyzing sample volumes >0.01 cm3 at resolutions <2.0 μm/pixel for low vesicularity MORB (i.e., <4 vol.%) and sample volumes >0.1 cm3 with resolutions <5 μm/pixel for higher vesicularity samples. Our validation of vesicularity measurements allows reconstructions of total CO2 concentrations in MORB using dissolved volatile concentrations, vesicularities, and equations of state. We assess approaches for estimating the exsolved CO2 concentration in MORB vesicles and find that CO2(g) density is ~40% lower than previously suggested, likely due to melt contraction during quenching. Based on these results, we recommend using sample eruption pressures, magmatic temperatures, and an equation of state that accounts for non-ideality at high temperatures to calculate exsolved CO2 concentrations when independent constraints from Raman spectroscopy or laser ablation are unavailable. Our results suggest that some previous studies may have overestimated MORB CO2 concentrations by as much as 50%, with the greatest differences in samples with the highest vesicularities. These new results imply lower CO2/Ba of undegassed, enriched-MORB and lower integrated global ridge CO2 flux than previously inferred.

Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific

A number of high-temperature processes (e.g., melt-rock reactions, metasomatism, partial melting) can produce significant Ca isotopic fractionation and heterogeneity in the mantle, but the mechanism for such fractionation remains obscure. To investigate the effect of mantle partial melting on Ca isotopic fractionation, we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts (MORBs) from the East Pacific Rise and Ecuador Rift in the northeastern Pacific. The δ44/40Ca of these MORB samples exhibit a narrow variation from 0.84‰ to 0.88‰ with an average of 0.85‰±0.03‰, which are similar to those of reported MORBs (0.83‰±0.11‰) and back-arc basin basalts (BABBs, 0.80‰±0.08‰) in literature, but are lower than the estimate value for the bulk silicate Earth (BSE, 0.94‰±0.05‰). The low δ44/40Ca signatures of MORB samples in this study cannot be caused by fractional crystallization, since intermediate-mafic differentiation has been demonstrated having only limited effects on Ca isotopic fractionation. Instead, the offset of δ44/40Ca between MORBs and the BSE is most likely produced by mantle partial melting. During this process, the light Ca isotopes are preferentially transferred to the melt, while the heavy ones tend to stay in the residue, which is consistent with the fact that δ44/40Ca of melt-depleted peridotites increases with partial melting in literature. The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral (Cpx and Opx) Ca isotopic fractionation and melting mineral modes. Mantle partial melting is one of the common processes that can induce lower δ44/40Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.

Geochemistry of mafic volcanics in the Bayingou ophiolitic mélange, Western Tianshan, NW China: Implications for magma genesis and tectonic setting

This paper reports on the occurrence of mid-ocean-ridge basalts (MORB), boninites, and mafic volcanics with affinities to ocean island basalts (OIB) in the Bayingou Palaeozoic ophiolites, Western Tianshan, NW China, which is one of the best preserved ophiolitic sequences, and are the youngest ophiolites reported in the Western Tianshan during the Late Palaeozoic. The MORB samples have comparatively low TiO2 concentrations (0.84–1.20 wt%) and total rare earth element (REE) contents (25.30–55.97 ppm). The ISr values and εNd(t) values of MORBs range from 0.70480 to 0.70540 and + 3.58 to +6.90, respectively. The boninite sample has high SiO2 (53.79 wt%), very low TiO2 (0.22 wt%), high MgO (14.71 wt%), and a Mg# (100 × Mg2+/(Mg2++Fe2+)) value of 75. The boninite sample also has higher Cr (983 ppm) and Ni (230 ppm) contents and lower high field strength element content (e.g. Zr, Y, V, and heavy REEs) in comparison to the MORBs. The light REE (LREE) content of the boninite is slightly enriched, whereas most MORB samples display a depletion of LREEs ((La/Yb)N = 0.63–0.86), thus indicating that there are normal-type MORBs in this area. The OIB samples are characterized by a high concentration of TiO2 (1.89–3.15 wt%) and total REE content (100.68–151.07 ppm) with enriched LREEs ((La/Yb)N = 5.62–6.80). The ISr values and εNd(t) values of OIBs range from 0.70520 to 0.70541 and − 0.24 to +2.21, respectively. The whole rock Sr, Nd, and Pb isotopic compositions of OIB samples are plotted in the field between enriched mantle II (EMII) and PREMA. Such geochemical variation implies two mantle sources at the spreading zone: possibly, one source from the MORB and several sources from EMII-type mantle (mantle plum) in combination with characteristics of trace elements (Zr, Hf, Nb, Ta, Pb, and Ce).Petrogenetic modelling suggests that MORB may have derived from partial melting (ca. 10–20%) of a mantle peridotite that had previously experienced MORB extraction. For boninites, petrogenetic modelling suggests that these rocks are a product of about 20% partial melting of a source that previously experienced about 12% melt extraction from the source that formed the normal-type MORB. A hot plume may have been the thermal factor influencing the generation of the boninites. Volcanic rocks with OIB affinities in the Bayingou ophiolites are generated by 4.4–5.4% partial melting of the deeper mantle (EMII) and has a garnet lherzolite composition. The source of OIBs is affected by contamination of ancient oceanic lithosphere and terrigenous sediments, and is the same as other OIBs that occur in Palaeozoic ophiolites in Western Tianshan, indicating that these ophiolites may have formed in the same subduction event during the Palaeozoic. Our findings indicate that basaltic rocks in the Bayingou ophiolites formed in the supra-subduction zone (SSZ) environment in response to the subduction of Southern Tianshan Ocean during the late Early Carboniferous. The northward subduction of Southern Tianshan Ocean occurred until at least the late Early Carboniferous. In addition, the Northern Tianshan Ocean developed into a relatively mature ocean basin at the same time as the occurrence of MORB in the Bayingou ophiolites.

Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

Although olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy for mantle composition, these melt inclusions generally show larger elemental and isotopic compositional variation than their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotope data from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions from different crystals within the same sample show >2.5‰ δ18O variation within each sample, which is nearly eight times the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB. Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in the host olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions from each sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higher olivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here we discuss several potential processes that could contribute to these observations, but none satisfactorily explain the olivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear that the variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpreted with caution.

The stable tungsten isotope composition of modern igneous reservoirs

Abstract Tungsten is a highly incompatible and fluid mobile element in the Earth’s crust and mantle, but its geochemical cycle in magmatic systems still remains poorly understood. Tungsten stable isotopes represent a novel tool with the potential to better constrain this cycle, because of possible W isotope fractionation during changes in redox-state (valence states +4 and +6) and coordination (tetrahedral and octahedral). By employing a 180W-183W double spike our analytical protocol for stable W isotope measurements yields an external reproducibility of ±0.018‰ (2 s.d.) in δ186/184W and enables the resolution of small scale stable W isotope variations between different igneous reservoirs. Here, we present the first stable W isotope data for representative volcanic rocks from different igneous settings including mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs) and basalts to dacites from various subduction-related settings. The δ186/184W values of MORB samples from the Mid-Atlantic Ridge (+0.088 ± 0.017‰, n = 8) and OIB samples from the Canary Islands and Reunion Island (+0.084 ± 0.019; 2 s.d.; n = 17) show a narrow range and are analytically indistinguishable. This calls for a homogeneous stable W isotope composition in the mantle. Subduction-related rocks are more variable in their δ186/184W values (−0.009 to +0.195‰). Samples from the Troodos ophiolite complex in Cyprus show elevated W/Th and mainly high δ186/184W values (+0.101 to +0.195‰, n = 16) due to relative W enrichment from a subducted sediment component with heavy W isotope composition. In support of this model, Eastern Mediterranean sediments are isotopically heavier than mantle-derived rocks (+0.085 to +0.144‰ in δ186/184W, n = 5). Lavas from the Sunda arc show regional variability with a tendency towards lower δ186/184W values in the Central/East Java region (as low as −0.009‰). However, sediments from this region show uniformly high δ186/184W values of up to +0.301‰ and thus fail to explain the light compositions of the lavas. Hence, we rather propose that either the local subduction of an oceanic basement relief (Roo Rise) or source overprint by partial melts from deeply subducted crust accounts for low δ186/184W values in respective lavas. In arc settings, where the trace element budget is controlled by fluid-like components (New Britain) or melt-like components (Papua New Guinea) derived from shallowly subducted oceanic crust, we observe mantle-like W/Th and a narrow range in δ186/184W values (+0.109 ± 0.027‰, n = 19), which is only slightly elevated compared to MORBs and OIBs. This might hint towards the preferential release of isotopically heavy W during shallow fluid-induced melting events. Our results demonstrate that the stable W isotope composition of igneous rocks represents a novel tool to trace processes controlling the geochemical cycle of W in the silicate Earth.

Isotopic fractionation of zirconium during magmatic differentiation and the stable isotope composition of the silicate Earth

Abstract High-precision double-spike Zr stable isotope measurements (expressed as δ94/90ZrIPGP-Zr, the permil deviation of the 94Zr/90Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ94/90ZrIPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ94/90ZrIPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ94/90ZrIPGP-Zr, from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme magmatic differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a αzircon-melt of 0.9995 based on the whole rock δ94/90ZrIPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.

Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts

The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing 3He, 22Ne, and 36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10 ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge 3He flux of 800 ± 170 mol/yr and upper mantle 3He/22Ne and 3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of 3He flux, implies an upper mantle CO2 flux of 5.9 ± 1.0 × 1013 g/yr and a CO2 concentration of approximately 110 ppm.Our estimate of the mantle 3He flux is the first determined independently of oceanographic 3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing 3He/22Ne and 3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2 concentrations and the mantle CO2 flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.

Applicability of large-ion lithophile and high field strength element basalt discrimination diagrams

ABSTRACTBasalt discriminant diagrams have been used to identify the tectonic setting of basaltic magmatism since the 1970s and have played an important role in reconstructing paleotectonic environments. However, the significant increase in the availability of geochemical data has led to a reassessment of these diagrams, suggesting that some of the tectonic settings indicated by these diagrams are not accurate. Here, we use a database of global ocean island basalt (OIB), mid-ocean ridge basalt (MORB), and island arc basalt (IAB) geochemistry to propose a series of new tectonic discriminant diagrams based on the ratios of large-ion lithophile elements (LILEs) to high field strength elements (HFSEs). These new diagrams indicate that the LILE can be used to differentiate OIB, MORB, and IAB samples, meaning that LILE/HFSE ratios can discriminate between these basalts that form in different tectonic settings. Our new diagrams can correctly assign samples to OIB, MORB, and IAB categories more than 85% of the time, with the discrimination between OIB and MORB having an accuracy of slightly less than 85%.

High water content in primitive mid-ocean ridge basalt from Southwest Indian Ridge (51.56ºE): Implications for recycled hydrous component in the mantle

The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed (ITF) and Gallieni (GTF) transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust (up to 9.5 km). Although H2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts (MORB) worldwide show strong variation in H2O contents, but with a nearly uniform H2O/Ce ratio. Regionally distinctive H2O contents and H2O/Ce ratios are inferred to be related to the H2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR (51.56°E). The estimated H2O content (1.3 wt.%±0.3 wt.%) in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts (OIB) and back-arc basalts (BABB). In addition, the calculated H2O/Ce ratio (1 672-4 990) are extremely high, bearing “arc-like” signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.

Coexistence of MORB and OIB-type mafic volcanics in the Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, northeast India: Implication for heterogeneous mantle source at the spreading zone

This paper presents the first report of the coexistence of tholeiitic mid-ocean ridge basalt (MORB) – type and alkaline ocean island basalt (OIB) – type mafic volcanics from the Manipur Ophiolite Complex (MOC), Indo-Myanmar Orogenic Belt, northeast India. The MORB-types have comparatively lower TiO2 concentrations (0.6–1.6wt.%), show almost flat REE patterns with depleted LREEs [(La/Sm)N=0.62–1.03]. However, few samples in the MORB group show enrichment in LREE [(La/Sm)N=2.83–2.95] which is the typical composition of P-MORB. Alkaline OIB-types are characterized by high concentration of TiO2 (1.7–3.5wt.%) with highly enriched LREE pattern as compared to their HREE [(La/Sm)N=2.27–3.44, (Sm/Yb)N=2.56–3.29]. Such geochemical variation implies more than two sources. Possibly, one for OIBs (enriched mantle) and several ones for MORBs; from depleted MORB mantle (DMM) source to significantly enriched DMM source by OIB-type components. Petrogenetic modeling suggests that 20% partial melting of depleted mantle within spinel stability facies zone (shallow depth) is responsible for generation of MORB tholeiites and 5–10% partial melting of enriched mantle or plume material at garnet facies stability zone (deeper depth) is responsible for production of alkaline OIB-type. Geochemical signatures of variably enriched MORB and P-MORB samples further suggest possible scenario of mixing of depleted N-MORB and enriched OIB melt. It is therefore likely that mafic volcanics of the MOC were derived from chemically heterogeneous mantle sources erupted at the sea floor spreading zone as MORB generated by partial melting of depleted upper mantle and as OIB generated by partial melting of enriched mantle or a plume source, which was proximal to the spreading axis. Later, due to prolonged subduction of the Indian plate beneath the Myanmar plate and afterward collisional activity, they might have accreted along the Indo-Myanmar Orogenic Belt as upthrust ocean crust.

Implications of pore space characteristics on diffusive transport in basalts and granites

A detailed characterization of the pore space is crucial for understanding of transport and element transfer in rocks. Here, the effect of differences in texture and content of secondary minerals on transport in pore systems was determined for two rocks of widespread occurrence, mid-ocean ridge basalts (MORB) and granites. Pore space characteristics were analyzed by Hg-porosimetry, intrusion of a molten alloy, and synchrotron-based X-ray tomographic microscopy. For evaluating the role of pore space characteristics for the prediction of diffusive transport, data on porosity, and the effective diffusion coefficient (D eff) were compared. Extended connective pore systems due to cracks and mineral dissolution are present in samples of both rocks, indicating high internal specific surface area. Uneven pore size distributions in altered MORB samples can be assigned to secondary minerals. Pore spaces determined by X-ray tomography, used to determine main direction of pores in the 3-D orthogonal system, suggest a slight anisotropy. In log–log plots, both rocks show roughly a linear dependence of D eff for H2O and compounds with comparable diffusivities (D2O, monovalent cations, and anions) on porosity, but at same porosity D eff is clearly higher in granitic than in basaltic samples. This difference is increasing with decreasing porosity, indicating that at low porosities the efficiency of element transport in basaltic samples is diminished, mainly inherited by the presence of small pores slowing down diffusion. The fact that diffusive transport in basaltic rocks is stronger dependent on porosity than in granitic rocks shows that also other rock characteristics such as pore size distribution and tortuosity of the pore network, highly affected by the alteration degree, can markedly affect transport and reactivity of pore solution.

Phase relations and equation of state of a natural MORB: Implications for the density profile of subducted oceanic crust in the Earth's lower mantle

The phase relations and density of a natural mid‐ocean ridge basalt (MORB) were investigated from 28 to 89 GPa and 1600 to 2700 K by in situ X‐ray diffraction measurements and chemical analysis of the quenched samples using transmission electron microscopy (TEM). We observed an assemblage of five phases up to 50 GPa, namely an aluminum‐bearing magnesium perovskite phase, a calcium perovskite phase, a stishovite phase, the new aluminum‐rich (NAL) phase, and a calcium ferrite‐type phase. The NAL phase was no longer observed above 50 GPa. The phase proportions were obtained by Rietveld refinement of the in situ X‐ray diffraction patterns. After the disappearance of the NAL phase beyond 50 GPa, the proportion of each phase remains constant up to 89 GPa. The density of MORB was calculated using the measured volumes, phase proportions, and chemical compositions of the coexisting phases. The thermoelastic parameters of the MORB sample were estimated from the fit of the measured densities at various pressure and temperature conditions. Resulting MORB density profiles were calculated for different subducting slab temperature profiles. MORB densities are 0.5% to 2% greater than those of the surrounding mantle over the entire lower mantle range, suggesting MORB likely subducts to the core–mantle boundary.

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions

Abstract:Mid‐ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium‐bearing MORB sample E13–3B (MgO = 9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13–3B only by fractional crystallization at pressure &gt;4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ∼1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post‐entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at &gt;4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at &gt;4±1 kbar to mainly olivine+plagioclase crystallization at &lt;1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.