Mid-ocean Ridge Basalt Glasses Research Articles

A discovery of nanoscale sulfide droplets in MORB glasses: implications for the immiscibility of sulfide and silicate melts

Sulfur forms an immiscible liquid upon saturation in magma, and sulfide droplets are commonly found in fresh mid-ocean ridge basalt (MORB). Scanning electron microscopy (SEM) analysis revealed that the fine-grained and weakly phyric MORB samples exhibited hypocrystalline to vitreous textures. Transmission electron microscopy (TEM) of MORB glasses exhibits nanoscale sulfide droplets (10–15 nm) with rounded shapes and smooth edges, showing crystalline and homogeneous composition. Elemental distribution included S, Fe, Cu, and Ni, while Si, Al, and O were lacking. Prior research clarified the immiscibility between sulfide and silicate melts, impacting the size distribution of sulfide droplets. This is the first report on nanoscale sulfide droplets within MORB glasses, and these results suggest that nanoscale sulfide droplets represent the initial phase of sulfide saturation. Such an insight may prove useful in understanding how siderophile and chalcophile elements behaved during sulfide crystallization. In addition, this study determines the immiscibility of sulfides and silicate melts that occur in the early nanometer stage. Therefore, it is speculated that immiscibility phenomena may occur in the nanometer stage during magma evolution.

Mg and Fe isotope compositions of mid-ocean ridge basalts modified by Mg-Fe inter-diffusion during melt transport

We analysed forty-nine, hand-picked, mid-ocean ridge basalt (MORB) glasses for their Mg isotopic compositions using a high-precision, critical mixture double-spiking approach. Additionally, the Fe isotopic compositions of a subset (twenty-two) of these samples were measured. Samples from the East Pacific, Mid-Atlantic, South East Indian and East Scotia ridges have average δ26Mg ∼ −0.21 ‰ and δ57Fe ∼0.10 ‰, while those from the ultra-slow spreading Gakkel and South West Indian ridges have lighter Mg isotope compositions, δ26Mg as low as ∼ −0.32 ‰, and heavier Fe isotope compositions, δ57Fe up to 0.25 ‰. Overall, the samples show a striking negative correlation between δ26Mg and δ57Fe. Few MORB have δ26Mg as high as predicted by equilibrium models of melting and differentiation; in complementary fashion, measured δ57Fe are typically higher than modelled, equilibrium values. Furthermore, we show that the slope of 26Mg/24Mg and 25Mg/24Mg covariations in MORB is consistent with kinetic not equilibrium fractionation. These observations identify an important diffusive control on both Mg and Fe isotopic compositions of MORB. Fractional transport of melt beneath ridges juxtaposes low Mg/Fe fractional melts from depth with high Mg/Fe residues at the top of the melting column. We argue that diffusively limited, partial Mg-Fe exchange between melt and peridotite through which it migrates leads to melts becoming isotopically lighter in Mg and heavier in Fe. Low melt rock ratios from sub-adiabatic melting beneath the slowest spreading ridges result in the greatest diffusive exchange between peridotite and melt en route to surface.

The relationship between iron redox states and H2O contents in back-arc basin basaltic glasses from the North Fiji Basin

Island-arc basalts (IAB) from convergent plate margins tend to have both higher iron oxidation states (higher Fe3+/Fe2+) and higher H2O contents than the mid-ocean ridge basalts (MORB) produced by seafloor spreading, which raises the question of whether these two characteristics are causally related. Back-arc basin basalts (BABB) may help with this question, in that they are products of seafloor spreading, like MORB, but sourced from supra-subduction mantle, like IAB. Here we examine the relationship between Fe3+/Fe2+ (determined by XANES spectroscopy) and H2O contents in BABB glasses from the North Fiji Basin (NFB). The glasses cover a range of compositions from 6.1 to 8.5 wt% MgO, and from 0.16 to 1.6 wt% H2O, and have also been analysed for trace elements and Sr-Nd-Pb-isotopes. Measured Fe3+/Fe2+ range from MORB-like to slightly higher values and are positively correlated with H2O and heavy-halogen contents (Cl, Br and I). The correlation is due to lower Fe2+ rather than higher Fe3+ compared to the MORB array. This suite of BABBs is not significantly enriched in trace elements other than H2O and the heavy halogens, with MORB-like Ba/Th and other incompatible-element ratios, suggesting that the H2O and halogens may have come from dehydration of subducted lithospheric serpentinites under subsolidus conditions. Na2O is unusually variable for a suite of samples from such a limited area, and is not correlated with H2O, precluding a relationship between H2O and degree of melting. The link between H2O and low Fe2+ may be explained by H2O-rich fluids entering the transcrustal magma plumbing system, where they increase the proportion of olivine and augite to plagioclase crystallizing during the crustal evolution of the magmas. In this situation, relatively small additions of H2O can have an enhanced effect on major-element chemistry if the crustal evolution proceeds by cycles of replenish-mix-tap-crystallize (RMTX) rather than by simple fractional crystallization without repeated replenishment and tapping. The absence of any increase in Fe3+ with H2O, together with the lack of any correlation between H2O and Fe3+/Fe2+ in MORB glasses generally, suggest that various mechanisms, including more extensive olivine fractionation, were responsible for elevating Fe3+/Fe2+ in H2O-rich basalts from convergent margin settings. Proxy methods based on concentrations of elements with redox-variable partition coefficients (V, Eu) are not sensitive enough to record the subtle variations in Fe3+/Fe2+ observable by XANES spectroscopy.

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 < ±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.

A Machine Learning Based‐Approach to Predict the Water Content of Mid‐Ocean Ridge Basalts

AbstractWater is critical in the evolution of the mantle due to its strong influence on the physicochemical properties of mantle rocks. Mid‐ocean ridge basalts (MORBs) are commonly used to study the compositional characteristics of the convecting upper mantle. However, there remains abundant samples in the global MORB data sets without direct measurements of water contents. The commonly observed good correlation between H2O and other incompatible trace components, such as Ce, has been applied to quantify water contents of MORBs. However, this approach assumes constant H2O/Ce in the target samples, which is not always true as the H2O/Ce ratios of MORBs could be rather heterogeneous even in some short ridge segments. Utilizing the present compositional data of global MORB glasses with measured water contents (n = 1,467), we construct a Random Forest Regression model based on machine learning, which can predict water concentrations of samples based on selected major and trace element data, without assuming a ratio between H2O and other trace elements. This model allows us to precisely recover water contents for MORBs with comparable accuracy with traditional analytical methods. The predicted results of MORB glasses from this model (n = 1,931) expand the water content database of global MORBs and indicate a broad existence of high‐H2O MORBs. This new approach allows us to investigate the water content of MORBs from some ridges lacking previous water content measurements (e.g., the Chile Ridge and the Pacific‐Antarctic Ridge) and infer changes in the water content of MORB sources through applying the model to transform fault samples.

Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity, whose origin remains debated. Here we present a systematic study of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Our analyses show significant Mo isotope variations with δ98/95Mo (relative to NIST SRM3134) ranging from −0.23‰ to −0.06‰. We argue that these Mo isotope variations are not caused by processes of MORB melt generation and evolution but reflect mantle isotopic heterogeneity. Taking together with the literature data, we show that MORB Mo isotope compositions vary systematically with geochemical parameters indicating mantle enrichment. These observations are best explained by two-component mixing between an incompatible element depleted endmember (e.g., low La/Sm, Nb/La, Nb/Zr and Th/Yb, and high Sm/Nd and 143Nd/144Nd) with low δ98/95Mo (∼−0.21‰) and an incompatible element enriched endmember (e.g., high La/Sm, Nb/La, Nb/Zr and Th/Yb, and low Sm/Nd and 143Nd/144Nd) with high δ98/95Mo (∼−0.05‰). The association of heavier Mo isotope compositions with the geochemically more enriched MORB is inconsistent with recycled ocean crust with or without sediment being the enriched endmember. Instead, this is consistent with the enriched endmember being of magmatic origin, most likely lithologies of low-degree melt metasomatic origin dispersed in the more depleted peridotite matrix in the MORB mantle. Thus, with MORB Mo isotope systematics, we confirm that recycled oceanic mantle lithosphere metasomatized by low degree melt plays a key role in the formation of E-MORB source lithologies. Our study also highlights Mo isotopes as an effective tool for studying upper mantle processes.

Improving the reliability of Fe- and S-XANES measurements in silicate glasses: Correcting beam damage and identifying Fe-oxide nanolites in hydrous and anhydrous melt inclusions

The redox state of silicate melts influences crystallization, element partitioning, and degassing behavior. Synchrotron-based micro-X-ray absorption near edge structure (μXANES) spectroscopy has emerged as a powerful tool for determining redox conditions through the direct measurement of speciation of multivalent elements such as iron and sulfur in silicate glasses. In particular, the high spatial resolution afforded by synchrotron μXANES makes it one of the few techniques available for determining redox conditions in melt inclusions, which can provide insights into pre-eruptive melt properties. However, the small size of melt inclusions, the deep penetration of X-rays, and irradiation-induced beam damage make μXANES measurements in melt inclusions challenging. Here we present data showing the rapid occurrence of Fe- and S-μXANES beam damage in experimental glasses, mid-ocean ridge basalt glasses, and olivine-hosted melt inclusions from the southern Cascade arc and Kīlauea Volcano and develop approaches to recognize and correct for beam damage through repeated rapid analyses. By applying a time-dependent correction to a series of rapid measurements (~82 s/scan) of Fe-μXANES pre-edge centroid positions, irradiation-induced photo-oxidation (Fe2+ to Fe3+) can be corrected back to undamaged initial Fe3+/ΣFe even in damage-susceptible hydrous glasses. Using this beam damage correction technique, hydrous basaltic melt inclusions from the southern Cascades have Fe3+/ΣFe values that are ~0.036 lower (corresponding to -0.5 log units lower oxygen fugacity) than would have been indicated by standard Fe-μXANES measurements. Repeated, rapid analyses (150–300 s/scan) were also used to identify S-μXANES beam damage (photo-reduction of S6+ to S4+), which was corrected with a peak fitting method to restore initial S6+/ΣS. We observe that S-μXANES beam damage can occur rapidly even in low-H2O mid-ocean ridge basaltic glasses and melt inclusions from Kīlauea Volcano, which are otherwise stable during even prolonged Fe-μXANES analyses. By mitigating and correcting for sulfur photo-reduction, we conclude that some mid-ocean ridge basaltic glasses contain 0.08–0.09 S6+/ΣS, which is more sulfate than might be expected based on the reduced oxidation state of these glasses (near the fayalite-magnetite-quartz oxygen buffer). Using beam damage identification and correction techniques, the valence states of iron and sulfur can be accurately measured even in beam damage-susceptible glasses and melt inclusions. Finally, using Fe-μXANES, we demonstrate the presence of Fe-oxide nanolites within otherwise glassy, naturally quenched melt inclusions, which can complicate determination of iron valence state in affected glasses.

Variations of Source Composition and Melting Degrees of Olivine-Phyric Rocks from Kamchatsky Mys: Results of Geochemical Modeling of Trace Element Contents in Melts

We conducted the geochemical modeling of trace element contents for primary melts of olivine-phyric rocks from Kamchatsky Mys. This modeling reveals substantial chemical heterogeneity of their source while the average source composition is close to the enriched DMM (E-DMM). The average estimation of the melting degree is in the range from 9.1 ± 3.8% for the model of modal batch melting to 15.4 ± 5.2% for the model of accumulated fractional melting, which is slightly higher than the estimation for primitive mid-ocean ridge basalt (MORB) glasses (7.4 ± 2.2% and 12.5 ± 3.8% respectively). It is in a good agreement with high melting degrees estimated earlier for other rocks of the Kamchatsky Mys ophiolites. Low pressure of mantle melting caused by the elevated speed of decompression relative to the average MORB could explain elevated melting degrees estimated for Kamhcatsky Mys ophiolites as well as their characteristic Sr-anomalies and sulfide saturation on the earliest stages of magmatic evolution.

Iron isotope fractionation during mid-ocean ridge basalt (MORB) evolution: Evidence from lavas on the East Pacific Rise at 10°30′N and its implications

Whether the Earth’s mantle has a chondritic δ56Fe (deviation in 56Fe/54Fe from the IRMM-014 standard in parts per thousand) value or not remains under debate. The current view is that the observed average δ56Fe of mid-ocean ridge basalts (MORB) cannot be explained by partial melting of mantle source with chondritic value alone. Here, we report Fe isotope compositions on 29 MORB glasses sampled along a flowline traverse across the East Pacific Rise (EPR) axis at 10°30′N. These glasses show large MgO variation (1.8–7.4 wt.%) that forms a compositional continuum resulting from varying extent of fractional crystallization, which is accompanied by systematic Fe isotopic variation. Fractional crystallization modeling suggests that early crystallization of olivine, pyroxene and plagioclase gives rise to an iron enrichment trend and an increase in δ56Fe. Once Fe-Ti oxides appear on the liquidus and begin to crystallize, the FeOt and TiO2 contents of the residual melt decrease rapidly, which lead to a slight decrease in δ56Fe. These observations indicate that significant Fe isotope fractionation can indeed take place during MORB melt evolution. Hence, δ56Fe values of variably evolved MORB melts do not represent those of primary MORB melts and thus cannot be used to infer mantle source Fe isotope compositions. Importantly, δ56Fe values of primary MORB melts after correction for the effect of fractional crystallization can be well reproduced by mantle melting. Therefore, our study supports the idea that the Fe isotope composition of the accessible Earth is close to be chondritic. We note that conclusion would assume that the core, which takes up ∼90% of the Earth’s Fe, must have a chondritic Fe isotope composition.

Evolution, hydrothermal assimilation, and ascent of magma inferred from volatile contents in MORB glasses: An example from thick lava pile at IODP Site 1256

A continuous section through an 811 m-thick lava pile of mid-ocean ridge basalts at Integrated Ocean Drilling Program Site 1256 provides important constraints on the variation with depth of volatile element (H2O, CO2, S and Cl) contents in fresh matrix glasses to understand evolution of magma, condition of magma ascent, and degree of hydrothermal assimilation. The lava pile is divided into two groups based on eruption location: off-axis basalts of upper portion (250–534 m beneath seafloor, mbsf) and on-axis basalts of lower portion (534–1061 mbsf). The lava pile is composed of pillow, sheet, and massive lavas that include thin (<2 m) hyaloclastite intervals.Variations of H2O and major element contents in Site 1256 matrix glasses can be explained by fractional crystallization of one primitive basalt (MgO, 7.7 wt%) at the pressure of a melt lens (top of axial magma chamber). CO2/Nb and S/Dy in a large majority of fresh matrix glasses are lower than undegassed ratios, and these facts indicate that CO2 and S losses occurred during differentiation within the melt lens and/or magma transport to the seafloor. Saturation pressures calculated from dissolved CO2 and H2O contents give a wide range of pressures from seafloor (~25 MPa) to axial magma chamber (~60 MPa). Many off-axis glasses have saturation pressures of seafloor, but on-axis glasses have high saturation pressures up to axial magma chamber depths. The saturation pressures of off-axis glasses decrease with decreasing depth (i.e., with increasing distance from the axis). These observations indicate that the many off-axis glasses were highly degassed because their source magmas travelled for long time periods from melt lens to off-axis seafloor locations. On the other hand, on-axis glasses were less degassed because magma transport times to on-axis locations were shorter than those for off-axis locations.The most distinctive characteristic for the Site 1256 glasses is their higher Cl/Nb than any other mid-ocean ridge basalt glasses. The strong Cl enrichment is explained by assimilation of hydrothermal brines. Beneath Site 1256, the melt lens was very shallow (<1.2 km) and hydrothermal circulation of high-salinity brines with variable salinities would reach easily to the melt lens.

Determination of the oxidation state of iron in Mid-Ocean Ridge basalt glasses by Raman spectroscopy

AbstractA series of synthetic Mid-Ocean Ridge Basalt (MORB) glasses with Fe3+/FeTOT from 0 to 1, determined previously by Mössbauer spectroscopy, were used to test methods for quantifying Fe3+/FeTOT by Raman spectroscopy. Six numerical data reduction methods were investigated, based on conventional approaches as well as supervised and unsupervised machine learning algorithms. For the set of MORB glass standards, with fixed composition, the precision of all methods was ≤±0.04 (1 St.dev.). However, Raman spectra recorded for 42 natural MORB glasses from a wide range of locations revealed a strong correlation between the spectra and composition, despite the latter varying only over a relatively limited range, such that the methods calibrated using the glass standards are not directly applicable to the natural samples. This compositional effect can be corrected by using a compositional term that links spectral variations to the Fe3+/FeTOT value of the glass. The resulting average Fe3+/FeTOT determined by Raman spectroscopy was 0.090 ± 0.067 (n = 42). This value agrees with the latest Fe K-edge XANES and wet-chemistry estimates of 0.10 ± 0.02. The larger uncertainty of the Raman determination reflects the sensitivity of Raman spectroscopy to small changes in the glass structure. While this sensitivity is detrimental for high precision Fe3+/FeTOT determinations, it allows the major element composition of natural MORB glasses to be determined within 1 mol% through the use of an artificial neural network. This suggests that Raman spectrometers may be used to determine the composition of samples in situ at difficult to access locations that are incompatible with X-ray spectrometry (e.g., mid-ocean ridges).

Evaluating the roles of melt-rock interaction and partial degassing on the CO2/Ba ratios of MORB: Implications for the CO2 budget in the Earth’s depleted upper mantle

Abstract Carbon content in the Earth's depleted upper mantle has been estimated in previous studies using CO2/Ba ratios of CO2 undersaturated depleted mid-ocean ridge basalt (D-MORB) glasses and melt inclusions. However, CO2/Ba ratios in CO2 undersaturated MORB may not necessarily record those of the mantle source, as they may be affected by (1) assimilation of Ba-rich plagioclase-bearing rocks in the oceanic crust and (2) CO2 degassing through partial degassing and mixing. In this study, we evaluate these effects on the CO2/Ba ratios as well as other volatile to refractory trace element ratios (H2O/Ce, F/Nd, Cl/K, and S/Dy) in D-MORBs using the compositions of olivine-hosted melt inclusions and glasses from the Siqueiros and Garrett transform faults. The Siqueiros and Garrett melt inclusions are CO2 undersaturated and highly depleted in incompatible trace elements, and their average CO2/Ba ratios show relatively large ranges of 90 ± 34 and 144 ± 53 respectively. A subset of melt inclusions in lavas from both transform faults show potential signatures of contamination by plagioclase-rich rocks, such as correlations between major elements contents (e.g., FeO, Al2O3, and MgO), and trace element ratios (e.g., Sr/Nd). We find that (1) assimilation fractional crystallization (AFC) of gabbro into D-MORB and (2) mixing between partial melts of gabbro and D-MORB can reproduce the observed range in Sr/Nd ratios as well as the general trends between major elements. However, we find that these processes had limited effects on the CO2/Ba ratio of the melt inclusions and it is unlikely that they can account for the observed range in the CO2/Ba ratio. On the other hand, while a partial degassing and mixing model can generate melts with large range of CO2/Ba ratios (as proposed by Matthews et al. (2017)), it cannot reproduce the Pearson correlation coefficients between CO2/trace element and 1/trace element ratios observed in the Siqueiros and Garrett melt inclusions. Instead, when analytical uncertainties on the elemental concentrations are considered, a model without partial degassing can adequately reproduce the majority of the observed range in CO2/Ba ratio and Pearson correlation coefficients. Hence, we postulate that the Siqueiros and Garrett melt inclusions are undegassed and use their average CO2/Ba ratios to estimate the Siqueiros and Garrett mantle source CO2 contents (21 ± 2 ppm and 33 ± 6 ppm respectively). We also evaluate the effects of shallow level crustal processes on H2O/Ce, F/Nd, Cl/K, and S/Dy ratios, and after which we filter those effects, we estimate the H2O, F, Cl and S contents in the mantle sources of the Siqueiros (40 ± 8 ppm, 8 ± 1 ppm, 0.22 ± 0.04 ppm, and 113 ± 3 ppm) and Garrett (51 ± 9 ppm, 6 ± 1 ppm, 0.27 ± 0.07 ppm, and 128 ± 7 ppm) melt inclusions.

An automated laser fluorination technique for high-precision analysis of three oxygen isotopes in silicates.

The three oxygen isotopes in terrestrial/extraterrestrial silicates can provide geochemical and cosmochemical information about their origin and secondary processes that result from isotopic exchange. A laser fluorination technique has been widely used to extract oxygen from silicates for δ17 O and δ18 O measurements by isotope ratio mass spectrometry. Continued improvement of the techniques is still important for high-precision measurement of oxygen-isotopic ratios. We adopted an automated lasing technique to obtain reproducible fluorination of silicates using a CO2 laser-BrF5 fluorination system connected online to an isotope ratio mass spectrometer. The automated lasing technique enables us to perform high-precision analysis of the three oxygen isotopes of typical reference materials (e.g., UWG2 garnet, NBS28 quartz and San Carlos olivine) and in-house references (mid-ocean ridge basalt glass and obsidian). The technique uses a built-in application of laser control with which the laser power can be varied in a programmed manner with a defocused beam which is in a fixed position. The oxygen isotope ratios of some international reference materials analyzed by the manual lasing technique were found to be isotopically lighter with wider variations in δ18 O values, whereas those measured by the automated lasing technique gave better reproducibility (less than 0.2‰, 2SD). The Δ17 O values, an excess of the δ17 O value relative to the fractionation line, also showed high reproducibility (±0.02‰, 2SD). The system described herein provides high-precision δ17 O and δ18 O measurements of silicate materials. The use of the automated lasing technique followed by careful and controlled purification procedures is preferred to achieve satisfactory isotopic ratio results.

Carbon Fluxes and Primary Magma CO2Contents Along the Global Mid‐Ocean Ridge System

AbstractThe concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO2outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2concentrations for ridges that have Ba and/or Rb data. When combined with quality‐controlled volatile‐element data from the literature (n = 2,446), these data constrain a range of primary CO2abundances that vary from 104 ppm to 1.90 wt%. Segment‐scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 105to 3.0 × 108m3/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km3/year. When combined with CO2/Ba and CO2/Rb‐derived primary magma CO2abundances, the calculated segment‐scale CO2fluxes vary by more than 3 orders of magnitude (3.3 × 107to 4.0 × 1010mol/year) and sum to an integrated global MORB CO2flux of × 1012mol/year. Variations in ridge CO2fluxes have a muted effect on global climate; however, because the vast majority of CO2degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long‐term variations in climate via degassing directly into the atmosphere in shallow‐water areas or where the ridge system is exposed above sea level.

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).

The behaviour of ferric iron during partial melting of peridotite

In order to better understand the behaviour of Fe3+ during partial melting of a mid-ocean-ridge-basalt (MORB) mantle source, we performed partial melting experiments of a spinel peridotite at 1.5 GPa, between 1320 and 1450 °C, and over a range of oxygen fugacities (fO2) varying from FMQ-3.5 to FMQ+6 (log-units relative to FMQ = Fayalite-Magnetite-Quartz oxygen buffer). Changes in fO2 were achieved using different capsule materials and by varying the oxidation state of the starting material. Fe3+ contents of spinels and glasses were examined using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy in full-field mode. Our results show that this method allows Fe3+/ƩFe measurements to be made of spinels in our experimental charges but is not suitable for quantifying Fe3+/ƩFe of glasses, particularly at relatively low fO2 where melt Fe2O3 contents are small. We therefore employed several methods (Fe3+/ƩFe ratios of spinels measured by XANES and calculated by stoichiometry, and Fe-alloy sliding redox sensors) to assess the fO2 of the samples. These were then used to calculate the Fe3+/ƩFe ratios of glasses using published models, which were further constrained by Mössbauer spectroscopy measurements on an additional set of partial melting experiments.For a particular initial redox state, the Fe3+/ΣFe ratio in melts remains approximately constant with partial melting degree. This is explained by a slight decrease of the bulk partition coefficient of Fe2O3 (DFe2O3peridotite-melt) as the degree of partial melting increases. The Fe3+/ΣFe ratios of resulting melts are thus restricted to a relatively narrow range over large degrees of melting, as observed for global MORB glass analyses. This observation is also confirmed by pMELTS calculations employing different bulk Fe3+/ΣFe ratios. We conclude that the Fe2O3 content in MORB can be accounted for through partial melting of a mantle source containing 0.1–0.5 wt% Fe2O3 and a bulk DFe2O3peridotite-melt ranging from 0.3 to 0.1.

Magma ascent and lava flow emplacement rates during the 2011 Axial Seamount eruption based on CO2 degassing

Quantitative metrics for eruption rates at mid-ocean ridges (MORs) would improve our understanding of the structure and formation of the uppermost oceanic crust and would provide a means to link volcanic processes with the conditions of the underlying magmatic system. However, these metrics remain elusive because no MOR eruptions have been directly observed. The possibility of disequilibrium degassing in mid-ocean ridge basalts (MORB), due to high eruptive depressurization rates, makes the analysis of volatile concentrations in MORB glass a promising method for evaluating eruption rates. In this study, we estimate magma ascent and lava flow emplacement rates during the 2011 eruption of Axial Seamount based on numerical modeling of diffusion-controlled bubble growth and new measurements of dissolved volatiles, vesicularity, and vesicle size distributions in erupted basalts. This dataset provides a unique view of the variability in magma ascent (∼0.02–1.2 m/s) and lava flow rates (∼0.1–0.7 m/s) during a submarine MOR eruption based on 50 samples collected from a >10 km long fissure system and three individual lava flow lobes. Samples from the 2011 eruption display an unprecedented range in dissolved CO2 concentrations, nearly spanning the full range observed on the global MOR system. The variable vesicularity and dissolved CO2 concentrations in these samples can be explained by differences in the extent of degassing, dictated by flow lengths and velocities during both vertical ascent and horizontal flow along the seafloor. Our results document, for the first time, the variability in magma ascent rates during a submarine eruption (∼0.02–1.2 m/s), which spans the global range previously proposed based on CO2 degassing. The slowest ascent rates are associated with hummocky flows while faster ascent rates produce channelized sheet flows. This study corroborates degassing-based models for eruption rates using comparisons with independent methods and documents the relationship between eruption dynamics, magma ascent rates, and the morphology of eruptive products. Globally, this approach allows interrogation of the processes that govern mid-ocean ridge eruptions and influence the formation of the oceanic crust.

Determination of Fe3 +/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB

To improve the accuracy of X-ray absorption near-edge structure (XANES) calibrations for the Fe3+/ΣFe ratio in basaltic glasses, we reevaluated the Fe3+/ΣFe ratios of glasses used as standards by Cottrell et al. (2009), and available to the community (NMNH catalog #117393). Here we take into account the effect of recoilless fraction on the apparent Fe3+/ΣFe ratio measured from room temperature Mössbauer spectra in that original study. Recoilless fractions were determined from Mössbauer spectra collected from 40 to 320K for one basaltic glass, AII_25, and from spectra acquired at 10K for the 13 basaltic glass standards from the study of Cottrell et al. (2009). The recoilless fractions, f, of Fe2+ and Fe3+ in glass AII_25 were calculated from variable-temperature Mössbauer spectra by a relative method (RM), based on the temperature dependence of the absorption area ratios of Fe3+ and Fe2+ paramagnetic doublets. The resulting correction factor applicable to room temperature determinations (C293, the ratio of recoilless fractions for Fe3+ and Fe2+) is 1.125±0.068 (2σ). Comparison of the spectra at 10K for the 13 basaltic glasses with those from 293K suggests C293 equal to 1.105±0.015 (2σ). Although the 10K estimate is more precise, the relative method determination is believed to be more accurate, as it does not depend on the assumption that recoilless fractions are equal at 10K. Applying the effects of recoilless fraction to the relationship between Mössbauer-determined Fe3+/ΣFe ratios and revised average XANES pre-edge centroids for the 13 standard glasses allows regression of a new calibration of the relationship between the Fe XANES pre-edge centroid energy and the Fe3+/ΣFe ratio of silicate glass. We also update the calibration of Zhang et al. (2016) for andesites and present a more general calibration for mafic glasses including both basaltic and andesitic compositions. Recalculation of Fe3+/ΣFe ratios for the mid-ocean ridge basalt (MORB) glasses analyzed previously by XANES by Cottrell and Kelley (2011) results in an average Fe3+/ΣFe ratio for MORB of 0.143±0.008 (1σ), taking into account only analytical precision, and 0.14±0.01(1σ), taking into account uncertainty on the value of C293. This revised average is lower than the average of 0.16±0.01 given by Cottrell and Kelley (2011). The revised average oxygen fugacity for MORB based on the database of Cottrell and Kelley (2011) is −0.18±0.16 log units less than the quartz-fayalite-magnetite buffer of Frost (1991) at 100kPa (∆QFM=−0.18±0.16).

A re-assessment of the oxidation state of iron in MORB glasses

The oxidation state of Fe, Fe/3+ΣFe (where ΣFe=Fe2++Fe3+), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) (n=42). This value is lower than that reported previously by XANES, 0.16(1) (n=103), but consistent with the most recent value determined by redox titrations, 0.11(2) (n=104), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of Mössbauer spectra and the resulting Fe/3+ΣFe values of the standards. Our alternative interpretation removes the problem of resolving Fe/3+ΣFe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/ΣFe (as is the case for Fe2+), and gives Fe/3+ΣFe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO2). Fe/3+Fe2+ is related to fO2 for our synthetic MORB composition by the temperature independent expression ΔQFM=4log⁡(Fe3+/Fe2+)+4.23(5), where ΔQFM is the fO2 in log units relative to the quartz–fayalite–magnetite buffer. The average fO2 of natural MORB was estimated to be QFM+0.1.

Spreading rate‐dependent variations in crystallization along the global mid‐ocean ridge system

AbstractWe investigate crustal accretion at mid‐ocean ridges by combining crystallization pressures calculated from major element contents in mid‐ocean ridge basalt (MORB) glasses and vapor‐saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from &gt;9000 MORB glasses across the global range of mid‐ocean ridge spreading rates. Additionally, we estimate vapor‐saturation pressures from &gt;400 MORB glasses from PETDB and &gt;400 olivine‐hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor‐saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on and off‐axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor‐saturation pressures (∼3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO2 content of 50–85 ppm for the depleted upper mantle feeding the global mid‐ocean ridge system.