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Various crustal processes shape both the lower oceanic crust and mid-ocean ridge basalts (MORBs). To better understand how these crustal processes influence MORB compositions, we conducted comprehensive petrographic and geochemical investigations on gabbroic rocks and erupted lavas dredged from a segment of the Central Indian Ridge (CIR) spanning from 7°50′S to 8°30′S. The petrographic and geochemical analyses of the gabbroic rocks revealed evidence of melt-rock reaction through reactive porous flow in olivine gabbro and gabbro. This process resulted in distinctive features in clinopyroxene, including disequilibrium textures with a troctolite/anorthosite matrix, complex variations in Mg#-Cr-Ti [Mg# = molar Mg/(Mg + Fe2+)] relationships, and considerable enrichment and fractionation of incompatible trace elements. A significant finding of our study is the close resemblance of trace element ratios in MORB and olivine-hosted melt inclusions to those of melts in equilibrium with clinopyroxene from olivine gabbro and gabbro with Sr anomaly (Sr/Sr* = SrN/sqrt[PrN*NdN]; N refers to chondrite-normalized values) greater than ~0.7. This observation strongly indicates that the composition of MORB is influenced by the melt-rock reaction taking place in the lower oceanic crust. Furthermore, our findings suggest that evolved melts in equilibrium with clinopyroxene having Sr/Sr* values lower than ~0.7 are less likely to erupt onto the seafloor and are instead trapped within the lower oceanic crust. Oxide gabbronorite is characterized by coarse-granular, pegmatitic textures and exhibits mineralogically and chemically more evolved characteristics compared to olivine gabbro and gabbro. It is inferred that the oxide gabbronorite formed through the in situ freezing of highly evolved melts within a melt-rich layer. Finally, we present a comprehensive model for melt evolution in the lower oceanic crust at the 7°50′S–8°30′S CIR by integrating all petrological and geochemical data obtained from gabbroic rocks, MORB, and olivine-hosted melt inclusions. This holistic model contributes to a better understanding of the intricate processes governing MORB composition in the context of the lower oceanic crust dynamics at slow-spreading ridges. [ABSTRACT FROM AUTHOR]

Abstract Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust.

Abstract Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift.

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt. With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample's 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample's 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt's Cr isotopic budget during partial melting. [ABSTRACT FROM AUTHOR]

Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift. [ABSTRACT FROM AUTHOR]

Introduction: The North Qilian orogenic belt, as the Northern branch of the original Tethys tectonic domain, is important for reconstructing the tectonic evolution of the ancient Tethys. However, the tectonic history of the North Qilian orogenic belt remains controversial. This study addresses this issue from a geochemical perspective.Methods: In this study, a comprehensive analysis of the geochronology, whole-rock geochemistry, clinopyroxene mineral geochemistry, zircon Ti crystallization temperature, and gabbromagma temperature and pressure in the Yushigou ophiolite of the North Qilian orogenic belt was conducted to provide constraints on its tectonic evolution.Results and Discussion: Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating results reveal that the gabbros have ages of 519 ± 3 Ma and 495 ± 4 Ma, belonging to the Cambrian period. Most of the studied gabbros exhibited geochemical characteristics of tholeiitic basaltic rocks with normal mid-ocean ridge basalt and island arc tholeiite dual geochemical affinities. The gabbros are interpreted to have formed by a high degree of partial melting of the depleted mantle spinel lherzolite. These results suggest that the back-arc basin of the North Qilian tectonic belt may have evolved to a relatively mature stage from 519 to 495 Ma. Overall, this study contributes to our understanding of the tectonic evolution of the North Qilian orogenic belt through geochemical analyses.

• Mid-ocean ridge basalt samples exhibit variable and co-related Mg and Fe isotope composition. • The δ'-Δ' diagram proves the Mg isotope variations in MORB samples are dominated by the kinetic rather than equilibrium process. • The Mg and Fe isotope variations in the MORB samples cannot be explained by partial melting or magma differentiation processes. • Mg-Fe inter-diffusion between melt and residual solid is responsible for the Mg and Fe isotope variations in the MORB samples. 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. [ABSTRACT FROM AUTHOR]

Titanomagnetites in mid‐ocean ridge basalt (MORB) experience variable post crystallization alterations associated with seafloor tectonic and environmental processes. Compared to low‐temperature oxidation, seafloor hydrothermal alteration is thought to be more destructive but its magnetic aftermaths are insufficiently documented. Here we present comprehensive rock magnetic and electron microscopic analyses of fresh and hydrothermally‐altered MORBs dredged from the Longqi and Yuhuang hydrothermal fields, Southwest Indian Ridge. We observe large variations in magnetic properties of fresh MORBs, originated from relative proportions of nano‐scale single‐domain to vortex state and micron‐scale vortex to multi‐domain state dendritic titanomagnetites. Progressive hydrothermal alteration produces secondary magnetite through recrystallization of exsolved and dissolved Fe from primary titanomagnetite. Exsolution is evident by a dual Verwey transition signature and coexisting Ti‐poor titanomagnetites and sphenes in partially chloritized basalts. A schematic model is proposed to explain the variations in magnetomineralogy and magnetic properties with progressive hydrothermal alteration. Intermediate hydrothermal alteration products retain a secondary chemical remanent magnetization (CRM) which is related to the long‐term magnetization variations in oceanic basalts. The established framework allows characterizing MORB hydrothermal alteration and ultimately contributes to resolving the complexity of seafloor magnetism. Plain Language Summary: Seafloor magnetism is essential for understanding geodynamics and resource prospecting. Near the seafloor high‐temperature hydrothermal fields that host "black smokers", the primary titanomagnetites in oceanic basalts experience complicated alterations that profoundly change seafloor magnetic anomalies. This study investigates the alteration of titanomagnetite through fluid‐rock interactions in such seafloor settings. We find titanomagnetite is disassembled into Fe‐rich and Ti‐rich phases when interacting with fluids, causing a series of magnetic property variations. A schematic model is proposed to explain how high‐temperature fluid‐rock interactions gradually destroy seafloor magnetism. This model improves our understanding of seafloor magnetism from the perspective of hydrothermal alterations, which is important for applying magnetic surveys to prospecting seafloor resources and probing subseafloor structures. Key Points: Magnetomineralogy and magnetic property of MORB vary with progressive high‐temperature hydrothermal alterationExsolution superimposed on titanomagnetite dissolution, producing a dual Verwey transition feature in partially chloritized basaltsHydrothermal alteration may have contributed to the long‐term magnetization variations of oceanic basalts [ABSTRACT FROM AUTHOR]

• New Sr-Nd-Hf isotopic data on MORB lavas from OH-1 and OH-3 segments at the Mid-Atlantic Ridge 33–35°N. • Three mantle components are identified in the mantle source of these MAR lavas. • A Sr-Nd-Hf isotopic view on the origin of geological differences in OH-1 and OH-3 segments. We present new Sr-Nd-Hf isotopic data on mid-ocean ridge basalts from two ridge segments (OH-1, OH-3) between the Oceanographer and Hayes fracture zones at the Mid-Atlantic Ridge 33–35°N to constrain the nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge. Together with the major and trace elements data (Niu et al., 2001), the new Sr-Nd-Hf isotopic data illustrate that the mantle sources of these lavas comprise three components, i.e., a depleted mantle with high radiogenic Hf isotopic compositions (ADM), an incompatible elemental enriched component with radiogenic Sr and un-radiogenic Nd-Hf isotopic compositions (E-type I), and an incompatible element depleted component with variably enriched Sr-Nd-Hf isotope compositions (i.e., E-type II). The ADM and E-type I components may be understood as representing ancient mantle melting residues and metasomatic veins developed at the base of the thickening oceanic lithosphere, respectively. The unique E-type II component is best explained as recent mantle melting residues of a geochemically enriched mantle, probably associated with the Azores mantle plume to the north. Taking together, we interpret the E-MORB dominated OH-1 lavas as resulting from a relatively higher degree melting of a mantle source composed of ADM matrix and E-type I material, whereas the N-MORB dominated OH-3 lavas resulting from lower degree melting of the mantle source primarily comprising ADM and E-type II components. Such a petrological and geochemical understanding explains the contrast in crustal thickness, ridge morphology and tomography, and mantle Bouguer anomalies between the two ridge segments. [ABSTRACT FROM AUTHOR]

Abstract Lavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on 15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.

Lavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales. Plain Language Summary: Basalts erupted from submarine volcanoes (seamounts) near mid‐ocean ridges can provide key information about the composition and dynamics of Earth's upper mantle. However, no existing studies examine long chains of seamounts built on the flanks of spreading ridges like the East Pacific Rise as presented here. We measured major and trace element abundances in conjunction with radiogenic isotope ratios to determine the source and origins of lavas from 8°20′N seamounts. Our geochemical investigation of the seamount chain provides us with an opportunity to determine the range of compositions of the mantle beneath a mid‐ocean ridge, their distribution in the mantle, and how and to what degree the mantle melts beneath these seamounts. We analyzed the composition of about 300 rocks from the seamounts and found that the lava compositions on this single chain compare well with the compositions of lavas erupted along the northern East Pacific Rise and from many seamounts scattered about on the northeastern Pacific oceanic crust. We find that this chain is extremely geochemically variable even within 22 km of the ridge axis on the scale of a few kilometers due to different amounts of mixing and melting of a heterogeneous mantle. Key Points: Geochemistry of near‐axis seamounts reveals extreme heterogeneity in the sub‐ridge East Pacific Rise mantleHeterogeneous mantle is preserved within 22 km of the ridge, and on individual seamountsVariations in the seamount lava compositions are explained by melting a heterogeneous mantle to variable degrees [ABSTRACT FROM AUTHOR]

We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab. Plain Language Summary: We use a set of seismic observations recorded globally to investigate the lower mantle beneath Central America. The deepest mantle in this region has been associated with the final resting place of subducted slab material from subduction that initiated approximately 200 million years ago. This ancient subducted material is associated with high seismic wave speeds in the lowermost mantle just above the core‐mantle boundary. We find that patches of highly reduced seismic wave speeds, referred to as ultralow‐velocity zones (ULVZs), appear to be associated with the border of the high wave speed region, along the border of the subducted slab material. These ULVZ patches are consistent with being regions of partial melt. A possible scenario for their creation is that mid‐ocean ridge basalt (MORB), comprising the crust of the subducted slab material, has a low melting point at conditions in the deep earth and may be melting as the slabs reach the bottom of the mantle. Previous experimental work has suggested that MORB will likely partially melt in the deep mantle, yet little evidence for the existence of MORB partial melt has previously been found. Key Points: Subducted slab material resting on the core‐mantle boundary is ringed by ultralow‐velocity zonesA partially molten origin to these ultralow‐velocity zones is likelyMelting of mid‐ocean ridge basalt may give rise to ultralow‐velocity zones [ABSTRACT FROM AUTHOR]

A 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). [ABSTRACT FROM AUTHOR]

Magmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust. [ABSTRACT FROM AUTHOR]

The concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO2 outgassed 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 CO2 concentrations 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 CO2 abundances 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 × 105 to 3.0 × 108 m3/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 CO2 abundances, the calculated segment‐scale CO2 fluxes vary by more than 3 orders of magnitude (3.3 × 107 to 4.0 × 1010 mol/year) and sum to an integrated global MORB CO2 flux of 1.320.85+0.77 × 1012 mol/year. Variations in ridge CO2 fluxes have a muted effect on global climate; however, because the vast majority of CO2 degassed 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. Plain Language Summary: Estimated CO2 contents of primary mid‐ocean ridge basalts (MORB), calculated on a segment‐by‐segment basis, vary from 104 ppm to 1.9 wt%. CO2‐enriched MORB are present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO2 fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary CO2 content. This study provides the most detailed and accurate estimate to date of the integrated total flux of CO2 from mid‐ocean ridges of 1.320.85+0.77 × 1012 mol/year. Key Points: New analyses (753) and compiled data (2,446) for volatiles CO2, H2O, F, S, and Cl are reported in a global suite of mid‐ocean ridge basaltsEstimated primary MORB CO2 contents vary from 104 ppm to 1.9 wt% and are the result of variations in mantle compositionHigh CO2 fluxes at ridges correlate with high primary CO2 contents and are due to the presence of subduction components in MORB sources [ABSTRACT FROM AUTHOR]

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mido-cean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. [ABSTRACT FROM AUTHOR]

• WVZ MORB: lower extents of melting of more depleted source towards the SMZ. • SMZ MORB: shallow melting of metasomatized lithosphere with previous melting history. • High-Ti MORB at WVZ-SMZ boundary suggest recycled ocean lithosphere mantle. • This region may be sampling recycled ocean lithosphere mantle. • Rarity of this component as an isolated source implies efficient mantle mixing during convection. Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. The oceanic lithospheric mantle, the largest volume of recycled plates, however, is often inferred but rarely observed. Here we report a collection of evidence suggesting that the Gakkel Ridge sampled recycled refractory ocean lithosphere. (1) A gradient in composition in the western volcanic zone (WVZ) approaching the sparsely magmatic zone (SMZ) suggests lower extents of melting of more depleted sources. (2) The SMZ itself contains substantial portions of sea floor formed by mantle emplaced at the surface. (3) Sparse enriched basalts from the SMZ are consistent with melting of refractory but metasomatized lithosphere. (4) Unique high-Ti basalts at the WVZ-SMZ boundary originated by deep melting of oxide-gabbro-bearing ocean lithosphere mantle. The volume of recycled ocean lithospheric mantle is enormous– it equals roughly 25 % of the entire mantle per gigayear. However, it is almost never observed as an isolated source. If the crustal portion of the lithosphere is isolated to form discrete sources in the mantle, ten to fifteen times as much depleted mantle lithospheric sources would be formed. The rarity of this source material thus suggests that ordinarily recycled ocean crust and mantle lithosphere become well mixed during mantle convection. The enclosed basin and ultra-slow spreading rate of the Gakkel Ridge may have provided inhibited mixing conditions where recycled oceanic mantle lithosphere was able to be preserved. [ABSTRACT FROM AUTHOR]

Trace element partition coefficients between anorthitic plagioclase and basaltic melts ( D ) have been determined experimentally at 0.6 GPa and 1350–1400 °C in a lunar high-Ti picritic glass and a mid-ocean ridge basalt (MORB). Plagioclases with 98 mol% and 86 mol% anorthite were produced in the lunar picritic melt and MORB melt, respectively. Based on the new experimental partitioning data and those selected from the literature, we developed parameterized lattice strain models for the partitioning of monovalent (Na, K, Li), divalent (Ca, Mg, Ba, Sr, Ra) and trivalent (REE and Y) cations between plagioclase and silicate melt. Through the new models we showed that the partitioning of these trace elements in plagioclase depends on temperature, pressure, and the abundances of Ca and Na in plagioclase. Particularly, Na content in plagioclase primarily controls divalent element partitioning, while temperature and Ca content in plagioclase are the dominant factors for REE partitioning in plagioclase. From these models, we also derived a new expression for D Ra / D Ba that can be used for Ra-Th dating on volcanic plagioclase phenocrysts, and a new model for plagioclase-melt noble gas partitioning. Applications of these partitioning models to fractional crystallization of MORB and lunar magma ocean (LMO) indicate that (1) the competing effect of temperature and plagioclase composition leads to small variations of plagioclase-melt D REE during MORB differentiation, but (2) the temperature effect is especially significant and can vary anorthite-melt D REE by over one order of magnitude during LMO solidification. Temperature and plagioclase composition have to be considered when modeling the chemical differentiation of mafic to felsic magmas involving plagioclase. [ABSTRACT FROM AUTHOR]

Uneven distribution of rainfall and freshwater scarcity in populated areas in the Island of Maui, Hawaii, renders water resources management a challenge in this complex and ill-defined hydrological system. A previous study in the Galapagos Islands suggests that noble gas temperatures (NGTs) record seasonality in that fractured, rapid infiltration groundwater system rather than the commonly observed mean annual air temperature (MAAT) in sedimentary systems where infiltration is slower thus, providing information on recharge sources and potential flow paths. Here we report noble gas results from the basal aquifer, springs, and rainwater in Maui to explore the potential for noble gases in characterizing this type of complex fractured hydrologic systems. Most samples display a mass-dependent depletion pattern with respect to surface conditions consistent with previous observations both in the Galapagos Islands and Michigan rainwater. Basal aquifer and rainwater noble gas patterns are similar and suggest direct, fast recharge from precipitation to the basal aquifer. In contrast, multiple springs, representative of perched aquifers, display highly variable noble gas concentrations suggesting recharge from a variety of sources. The distinct noble gas patterns for the basal aquifer and springs suggest that basal and perched aquifers are separate entities. Maui rainwater displays high apparent NGTs, incompatible with surface conditions, pointing either to an origin at high altitudes with the presence of ice or an ice-like source of undetermined origin. Overall, noble gas signatures in Maui reflect the source of recharge rather than the expected altitude/temperature relationship commonly observed in sedimentary systems. [ABSTRACT FROM AUTHOR]

The change in the stability field of clinopyroxene in mid-ocean ridge basalt (MORB) as a function of pressure has been used widely as a geobarometer. Based on results from crystallization experiments using MORB-like compositions it has been suggested that MORB differentiation occurs at relatively high pressures at ultraslow- and slow-spreading ridges. However, differentiation requires the loss of substantial heat and it is unclear how this is possible at elevated pressures. To better understand the controls on the stability field of clinopyroxene in MORB-like compositions we report a series of experiments performed at 0.1 MPa in which the temperature of clinopyroxene saturation was determined in melts with variable Cr, Ca/Al and f O 2 . The results show that increased Cr and Ca/Al lead to an expansion of the clinopyroxene stability field. Incorporating these results into a new model of MORB differentiation shows that realistic parental melt Cr contents can increase the temperature at which clinopyroxene saturation occurs relative to assuming a Cr-free melt (as is commonly the case). Likewise, high Ca/Al melts will saturate clinopyroxene earlier than low Ca/Al melts and their crystallization may provide an explanation for high Mg# clinopyroxene in oceanic gabbros. The newly calibrated geobarometer gives lower crystallization pressures for MORB at the slow-spreading SWIR than previous calibrations, but still suggests relatively higher pressures of crystallization with decreasing spreading rate. [ABSTRACT FROM AUTHOR]

Basalt lavas comprise the greatest volume of volcanic rocks on Earth, and most of them erupt along the world's mid-ocean ridges (MORs). These MOR basalts (MORBs) are generally thought to be relatively homogeneous in composition over large segments of the global ridge system (e.g., Klein, 2005). However, detailed sampling of two different regions on the northern East Pacific Rise (EPR) and extensive analysis of the samples show that fine-scale mapping and sampling of the ridge axis can reveal significant variations in lava chemistry on both small spatial and short temporal scales. The two most intensely sampled sites within the EPR Integrated Study Site (ISS) lie on and off axis between 9°17'N and 10°N, and from a wide region centered around 9°N where two segments of the EPR overlap (see Fornari et al., 2012, Figure 3, in this issue). The chemical composition of erupted lavas, similar to the genotype of an organism, can be used by igneous petrologists to trace the evolution of magmas from the mantle to the seafloor. The extensive and detailed geochemical studies at the EPR highlight how a thorough understanding of the variability in lava compositions on small spatial scales (i.e., between lava flows) and large spatial scales (i.e., from segment center to segment end and including discontinuities in the ridge crest) can be used in combination with seafloor photography, lava morphology, and bathymetry to provide insights into the magmatic system that drives volcanism and influences hydrothermal chemistry and biology at a fast-spreading MOR.

We report total helium concentrations (vesicles + glass) for a suite of thirteen ultradepleted mid-ocean ridge basalts (UD-MORBs) that were previously studied for volatile contents (CO 2 , H 2 O) plus major and trace elements. The selected basalts are undersaturated in CO 2 + H 2 O at their depths of eruption and represent rare cases of undegassed MORBs. Sample localities from the Atlantic (2), Indian (1) and Pacific (7) Oceans collectively show excellent linear correlations ( r 2 = 0.75 – 0.92 ) between the concentrations of helium and the highly incompatible elements C, K, Rb, Ba, Nb, Th and U. Three basalts from Gakkel Ridge in the Arctic were also studied but show anomalous behavior marked by excess lithophile trace element abundances. In the Atlantic–Pacific–Indian suite, incompatible element concentrations vary by factors of 3–4.3, while helium concentration varies by a factor of 13. The strong correlations between the concentrations of helium and incompatible elements are explained by helium behavior as the most incompatible element during mantle melting. Partial melting of an ultradepleted mantle source, formed as a residue of earlier melt extraction, accounts for the observed concentrations. The earlier melting event involved removal of a small degree melt (∼1%) at low but non-zero porosity (0.01–0.5%), leading to a small amount of melt retention that strongly leveraged the incompatible element budget of the ultradepleted mantle source. Equilibrium melting models that produce the range of trace element and helium concentrations from this source require a bulk solid/melt distribution coefficient for helium that is lower than that for other incompatible elements by about a factor of ten. Alternatively, the bulk solid/melt distribution coefficient for helium could be similar to or even larger than that for other incompatible elements, but the much larger diffusivity of helium in peridotite leads to its more effective incompatibility and efficient extraction from a larger volume of mantle during melting. In either case, partial melting leaves a mantle residue with elevated (U + Th)/ 3 He. Consequently, peridotite residues of mantle melting cannot be the source of high 3 He/ 4 He observed at ocean island hotspots such as Hawaii and Iceland. The extreme effective incompatibility of helium entails that high 3 He/ 4 He mantle sources, isolated before 4.45 Ga based on Xe and W isotopes, have not experienced any melt extraction since they were isolated from convecting portions of the mantle. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge basalts (MORB) provide fundamental information about the composition and melting processes in the Earth's upper mantle. To use MORB to further our understanding of the mantle, is imperative that their crustal evolution is well understood and can thus be accounted for when estimating primary melt compositions. Here, we present the evidence for the occurrence of reactive porous flow, whereby migrating melts react with a crystal mush in mid-ocean ridge magma chambers. This evidence comprises both the textures and mineral major and trace element geochemistry of rocks recovered from the lower oceanic crust, and occurs on a range of scales. Reaction textures include dissolution fronts in minerals, ragged grain boundaries between different phases and clinopyroxene-brown amphibole symplectites. However, an important finding is that reaction, even when pervasive, can equally leave no textural evidence. Geochemically, reactive porous flow leads to shifts in mineral modes (e.g. the net replacement of olivine by clinopyroxene) and compositions (e.g. clinopyroxene Mg-Ti-Cr relationships) away from those predicted by fractional crystallization. Furthermore, clinopyroxene trace elements record a progressive core-rim over-enrichment (relative to fractional crystallization) of more-to-less incompatible elements as a result of reactive porous flow. The fact that this over-enrichment occurs over a distance of up to 8mm, and that clinopyroxenes showing this signature preserve zoning in Fe-Mg, rules out a diffusion control on trace element distributions. Instead, it can be explained by crystal-melt reactions in a crystal mush. The data indicate that reactive flow occurs not only on a grain scale, but also on a sample scale, where it can transform one rock type into another [e.g. troctolite to olivine gabbro, olivine gabbro to (oxide) gabbro], and extends to the scale of the entire lower oceanic crust. Melts undergoing these reactive processes change in composition, which can explain both the major element and trace element arrays of MORB compositions. In particular, reactive porous flow can account for the MORB MgO-CaO-Al2O3 relationships that have previously been interpreted as a result of high-pressure (up to ~8 kbar) crystal fractionation, and for over-enrichment in incompatible elements when compared with the effects of fractional crystallization. The finding of a significant role for reactive porous flow in mid-ocean ridge magma chambers fits very well with the geophysical evidence that these magma chambers are dominated by crystal mush even at the fastest spreading rates, and with model predictions of the behaviour of crystal mushes. Together, these observations indicate that reactive porous flow is a common, if not ubiquitous, process inherent to mushy magma chambers, and that it has a significant control on mid-ocean ridge magmatic evolution. [ABSTRACT FROM AUTHOR]

The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling (DSDP/ODP/IODP) sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge (the East Pacific Rise). The Ontong Java plateau (125-90 Ma) basalts have distinctly lower Na and Nd/Nd, and higher La/Sm and Sr/Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate. [ABSTRACT FROM AUTHOR]

This paper works on the trace and major element compositions of spatially associated basalts and peridotites from the Dragon Bone amagmatic ridge segment at the eastern flank of the Marion Platform on the ultraslow spreading Southwest Indian Ridge. The rare earth element compositions of basalts do not match the pre-alteration Dragon Bone peridotite compositions, but can be modeled by about 5 to 10% non-modal batch equilibrium melting from a DMM source. The Dragon Bone peridotites are clinopyroxene-poor harzburgite with average spinel Cr# ~ 27.7. The spinel Cr# indicates a moderate degree of melting. However, CaO and Al 2 O 3 of the peridotites are lower than other abyssal peridotites at the same Mg# and extent of melting. This requires a pyroxene-poor initial mantle source composition compared to either hypothetical primitive upper mantle or depleted MORB mantle sources. We suggest a hydrous melting of the initial Dragon Bone mantle source, as wet melting depletes pyroxene faster than dry. According to the rare earth element patterns, the Dragon Bone peridotites are divided into two groups. Heavy REE in Group 1 are extremely fractionated from middle REE, which can be modeled by ~ 7% fractional melting in the garnet stability field and another ~ 12.5 to 13.5% in the spinel stability field from depleted and primitive upper mantle sources, respectively. Heavy REE in Group 2 are slightly fractionated from middle REE, which can be modeled by ~ 15 to 20% fractional melting in the spinel stability field from a depleted mantle source. Both groups show similar melting degree to other abyssal peridotites. If all the melt extraction occurred at the middle oceanic ridge where the peridotites were dredged, a normal ~ 6 km thick oceanic crust is expected at the Dragon Bone segment. However, the Dragon Bone peridotites are exposed in an amagmatic ridge segment where only scattered pillow basalts lie on a partially serpentinized mantle pavement. Thus their depletion requires an earlier melting occurred at other place. Considering the hydrous melting of the initial Dragon Bone mantle source, we suggest the earlier melting event occurred in an arc terrain, prior to or during the closure of the Mozambique Ocean in the Neproterozoic, and the subsequent assembly of Gondwana. Then, the Al 2 O 3 depleted and thus buoyant peridotites became the MORB source for Southwest Indian Ridge and formed the Marion Rise during the Gondwana breakup. [ABSTRACT FROM AUTHOR]

The early phase of the 2010 eruption at the Eyjafjallajökull volcano (Iceland) produced poorly evolved mildly alkaline basalts that have a signature more enriched with respect to the typically depleted basalts emitted at ocean ridges. The whole rock geochemistry of these basaltic magmas offers a great opportunity to investigate the mantle source characteristics and reasons leading to this enriched fingerprint in proximity of the ocean ridge system. Some basaltic products of Katla volcano, ∼25 km east of Eyjafjallajökull, have been chosen from the literature, as they display a similar mildly alkaline signature and can be therefore useful to explore the same target. Major and trace element variations of the whole rock suggest a very limited evolutionary degree for the 2010 Eyjafjallajökull products and the selected Katla magmas, highlighting the minor role played by differentiation processes such as fractional crystallization. Nevertheless, effects of the limited fractionation have been erased through re-equilibration of the major and trace element abundances at primary conditions. Concentrations of Th after re-equilibration have been assumed as indexes of the partial melting degree, given the high incompatibility of the element, and enrichment ratios calculated for each trace element. Especially for LILE (Rb, Ba, K, Sr), the pattern of resulting enrichment ratios well matches that obtained from fractional melting of peridotite bearing hydrous phases (amphibole/phlogopite). This put forward the idea that magmas have been generated through partial melting of enriched mantle domains where hydrous minerals have been stabilized as a consequence of metasomatic processes. Refertilization of the mantle has been attributed to intrusion of hydrous silicate melts and fractional crystallization of hydrous cumulates. These refertilizing melts, inherited from an ancient subducted oceanic crust, intruded into a depleted oceanic lithosphere that remained stored for a long time (hundreds of Ma or Ga) before being re-entrained in partial melting. This means that magmas could have acquired their main geochemical differences in response of the variable depletion/enrichment degree of the heterogeneous mantle portion tapped at rather shallow depth (≤100 km). Our finding is another tessera in the open debate on the plume-related vs. non plume-related origin of Icelandic magmatism. [ABSTRACT FROM AUTHOR]

Lithological and isotopic heterogeneity in the mantle archive the mantle's melting history and fingerprint crustal recycling processes coupling the deep Earth with the terrestrial biosphere. Oceanic basalts, consisting of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs), are invaluable tools for studying the chemical evolution of the mantle because they sample mantle heterogeneity at a variety of depths and length scales. In this study, we probe mantle heterogeneity by exploring the systematics of stable Ca isotopes in OIBs and MORBs. We find that OIBs from Iceland have δ 44/40Ca values (∼0.85‰) that are indistinguishable from MORBs, while OIBs from Mangaia, the Azores, the Canary Islands and Hawaii have an average δ 44/40Ca value ∼0.08‰ lower than the MORB-Iceland group. Moreover, MORB and Iceland δ 44/40Ca values are relatively invariable, while OIBs range from 0.70‰ to 0.85‰ and correlate negatively with La/Lu and Nb/Y. The Ca isotope signature of the MORB-Iceland group is consistent with equilibrium fractionation during high-degree melting of average spinel/garnet peridotite with bulk silicate Earth (BSE) δ 44/40Ca. Calcium-isotope variability in OIBs requires two-component mixing of melts with distinct δ 44/40Ca values, one of which is a high-degree melt consistent with derivation from average peridotite with BSE δ 44/40Ca. The melt-mixing endmember with isotopically light Ca (≤0.70‰) must be a deep, low-degree melt. This low-degree melt cannot be derived from a peridotite source with BSE δ 44/40Ca, but instead requires either a source with lower-than-BSE δ 44/40Ca or a different source mineralogy. The isotopically light, low-degree melt is consistent with derivation from a pyroxene- and garnet-rich source, but the effects of mineralogical heterogeneity in the source regions of OIBs cannot be easily distinguished from the possible influence of Ca isotope heterogeneity in the mantle. Regardless, mixing of melts from a shallow peridotite source and a deep source appears to be a ubiquitous part of OIB petrogenesis. • OIBs have lighter and more variable Ca isotope signatures than MORBs. • Ca isotope variability in OIBs is best explained by mixing of a MORB-like melt and an isotopically light melt. • The isotopically light mixing endmember may trace pyroxene- and garnet-rich mantle sources. • Ca isotopes are a promising tracer of lithological heterogeneity in the mantle. [ABSTRACT FROM AUTHOR]

Submarine lava flow morphology is commonly used to estimate relative flow velocity, but the effects of crystallinity and viscosity are rarely considered. We use digital petrography and quantitative textural analysis techniques to determine the crystallinity of submarine basaltic lava flows, using a set of samples from previously mapped lava flow fields at the hotspot-affected Galápagos Spreading Center. Crystallinity measurements were incorporated into predictive models of suspension rheology to characterize lava flow consistency and rheology. Petrologic data were integrated to estimate bulk lava viscosity. We compared the crystallinity and viscosity of each sample with its flow morphology to determine their respective roles in submarine lava emplacement dynamics. We find no correlation between crystallinity, bulk viscosity, and lava morphology, implying that flow advance rate is the primary control on submarine lava morphology. However, we show systematic variations in crystal size and shape distribution among pillows, lobates, and sheets, suggesting that these parameters are important indicators of eruption processes. Finally, we compared the characteristics of lavas from two different sampling sites with contrasting long-term magma supply rates. Differences between lavas from each study site illustrate the significant effect of magma supply on the physical properties of the oceanic upper crust. [ABSTRACT FROM AUTHOR]

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) 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 for HO and compounds with comparable diffusivities (DO, monovalent cations, and anions) on porosity, but at same porosity D 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. [ABSTRACT FROM AUTHOR]

Abstract: No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238U–230Th–226Ra–210Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report 210Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222Rn from 226Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies. [Copyright &y& Elsevier]

Abstract: Detailed major element, trace element and isotopic study of the FAMOUS and North Famous segments within the geochemical gradient south of the Azores platform provides new constraints on controls on chemical variations at the segment scale and the origin of plume geochemical gradients. A comprehensive investigation of 110 samples along the entire length of the FAMOUS segment, coupled with a recent extensive melt inclusion study by Laubier et al. (2012), shows large trace element diversity within a single segment and substantial isotopic variability that largely correlates with trace element variations. Substantial variations are also present along the short (18km) North Famous segment despite the presence of an axial volcanic ridge. These results confirm multiple supply of magmas along the length of these segments, the lack of a centrally supplied magma chamber, and the ability of melting processes to deliver highly diverse melts over short distances and times. With the exception of one group of high Al2O3, low SiO2 magmas (HiAl–LoSi) largely recovered in the original small FAMOUS area, the data can be simply explained by a two-component mixing model coupled with melting variations. The HiAl–LoSi magmas reflect assimilation and mixing in the crust, an interpretation supported by the diverse melt inclusions in these lavas. Since the mantle heterogeneity reflects two-component mixing, the end members can be constrained. Surprisingly, source mixing between the Azores plume and depleted mantle cannot produce the observations. This is evident regionally from the fact that nearly all basalts have highly incompatible trace element ratios (e.g., Th/La, Nb/La) as high or higher than the most plume-influenced MORB near the Azores hotspot, despite being over 300km farther south and much less enriched isotopically. To account for the elevated highly incompatible trace element ratios, a metasomatic component formed by adding deep, low-degree melts of Azores plume material to a depleted mantle is required. The regional gradient south of the Azores then requires different processes along its length. Close to the Azores, plume material mixes with depleted mantle. The pure plume influence is spatially restricted, and enrichment farther to the south is caused by shallow mantle metasomatized by low-degree melts from deep plume flow. North Famous lavas are spatially closer to the Azores and yet are more depleted in trace elements and isotopes than FAMOUS lavas, suggesting delivery of the enriched component to individual segments is influenced by additional factors such as segment size and offset. The extent to which these processes operate in other regions of plume–ridge interaction remains to be investigated. [Copyright &y& Elsevier]

Abstract: Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth, and provides a geochemical window into the mantle. Deriving mantle composition and melting processes from the erupted lavas requires correction to be made for their evolution as they pass through and generate the oceanic crust. This is typically done by assuming that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation. However, extensive mineral major- and trace element data from a full section of fast-spread lower crustal rocks exposed in Hess Deep (equatorial Pacific Ocean) demonstrate that their evolution is instead controlled by reactive porous flow. These reactions lead to a strong enrichment in, and fractionation of, incompatible trace elements in the melt (as recorded by clinopyroxene compositions), leading to melt compositions far outside of the compositional realm of MORB both in terms of trace element abundances and ratios. The reactive signature increases in strength up section, peaking in varitextured gabbros interpreted to represent the fossilised axial melt lens, indicating that reactive porous flow occurred on the scale of the entire lower crust. The enrichment of the melt is coupled with a strong trace element depletion of plagioclase, olivine, and, to a lesser extent, clinopyroxene cores, suggesting that these phases represent the residues of the reactions from which trace elements have been removed. The dominant role of reactive porous flow, and the resulting deviations from fractional crystallisation predictions, suggest that the lower oceanic crust plays a much more complex and significant role in modifying the compositions of MORB than previously expected, with consequent implications for models of mantle processes. [Copyright &y& Elsevier]

Abstract: The extent to which the lithological heterogeneity of the mantle source controls mid-oceanic ridge basalt (MORB) compositions remains largely debated. The recycling of oceanic crust into the mantle is widely considered to be an important cause of mantle source heterogeneity. We provide data for major and trace elements and Sr–Nd–Pb isotopes of thirteen mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR) to investigate the mantle source heterogeneity introduced by recycled oceanic crust (ROC). Relationships of Sr–Nd–Pb isotopes and incompatible trace element ratios, such as Sm/Yb and Hf/Zr, and ratios of Zn/Fe, Co/Fe and (Pb/Pb⁎)N indicate a geochemically and lithologically heterogeneous mantle source. Mixing between components from depleted mantle (DMM) and enriched ROC-derived pyroxenite is required to explain these relationships. The relatively high Sm/Yb and the good correlation of 87Sr/86Sr vs. Sr/Nd are in concert with variable contributions from a pyroxenite component derived from recycled plagioclase-fractionated E-MORB. The narrow range of MgO contents (7.49wt.% to 8.58wt.%) and the correlations of major element contents with 87Sr/86Sr and Zn/Fe ratios indicate the control of a mixed pyroxenite–peridotite source on the major element compositions of these lavas. To further investigate the major element contribution from the lithologically heterogeneous mantle source (peridotite/pyroxenite), major element compositions were corrected for fractional crystallization both to a given MgO wt.% and a given liquidus temperature. After correction for fractional crystallization, the FeOT and Na2O increase and CaO decreases with increasing melt proportion from the enriched end-member. Based on the observations of this study and the results from partial melting experiments on pyroxenite and peridotite, a melting model, in which magma forms by mixing a deep and enriched melt (pyroxenite melt) with a shallow and depleted DMM melt (peridotite melt), provides a likely mechanism that can produce the major element systematics. [Copyright &y& Elsevier]

Abstract: The Integrated Ocean Drilling Program site U1365 drilled into the basement of the southwest Pacific crust formed from the mid-Cretaceous Osbourn Trough that rifted apart the Manihiki and Hikurangi Plateaus (the Greater Manihiki). The basalt geochemistry at this site is crucial for understanding the magmatic processes and mantle source of the mid-Cretaceous Osbourn Trough. The recovered fresh basalts were low-K tholeiitic normal (N) and depleted (D) mid-ocean ridge basalt (MORB). Their trace element and Sr–Nd isotope compositions indicate a Pacific-type mantle source rather than any significant influences from the nearby Louisville Seamount Chain or from the Greater Manihiki Plateau. Despite the presence of a plume head underneath the Osbourn Trough at its initial stage, the insignificance of a plume head could be explained by the long-distance (>1000km) southward migration of the Osbourn Trough. Lavas at site U1365 vary from low-MgO (<6.9wt.%) N-MORB at the bottom to high-MgO (8wt.% to 9.5wt.%) D-MORB and, then, to medium-MgO (7.3wt.% to 8.2wt.%) N-MORB according to their eruption sequences, which was accompanied by magma mixing in the magma reservoir. The D-MORB group lavas have higher melting degrees than those of N-MORB group based on their concentrations of TiO2, Na2O and CaO corrected for crystallization relative to MgO=7.8wt.%. The major element compositions of the high-MgO D-MORB lavas were consistent with partial melting in the spinel–peridotite zone over a pressure interval from ~3.1GPa to 2GPa in the mantle. The significant overlap of N-MORB and D-MORB in Sr–Nd isotopes suggests that chemical differences between the two groups were derived from the mantle melting processes. Based on comparison with lavas from the East Pacific Rise where a positive correlation of mantle melting degree vs. spreading rate is shown, we suggest that the Osbourn Trough might have a full spreading rate of ~140mm/yr. Thus, the slow ridge-like axial morphology of the Osbourn Trough should be a character of an extinct fast ridge. [Copyright &y& Elsevier]

Abstract: Segregation of subducted mid-ocean ridge basalt (MORB) at the CMB has been identified as a potentially important mechanism in the long-term evolution of the mantle and core. Here, three-dimensional (3-D) and two-dimensional (2-D) simulations of a compositionally-stratified slab reaching the CMB are presented, with the goals of characterising the resulting thermo-chemical structures for comparison with seismic studies, and of quantifying the fraction of MORB that is able to segregate and remain near the CMB. Compositional stratification of the slab results in a strong torque due to the relatively high density of basalt and low density of harzburgite, which tends to rotate the slab such that the basalt side faces down. Slab-CMB interaction is characterised by heating up of the slab followed by separation of the basalt and harzburgite layers, with harzburgite rising in vigorous plumes. Plumes form at the edges and sides of slabs at the CMB as well as in their interiors (as previously observed for purely thermal slabs) with plume heads dominated by depleted harzburgitic material (sometimes with small amounts of entrained basalt), while plume tails entrain basaltic material. Segregation of basalt depends strongly on the presence or absence of a preexisting dense layer at the CMB and by dimensionality. A preexisting dense layer greatly increases the fraction of basalt that segregates from the slab: in 3-D this fraction is in the range 0.5–0.7 with a layer present compared to 0.25–0.45 with no preexisting layer. Two modes of basalt segregation are observed for slabs that land basalt side-up (i) hot harzburgite extruding from the sides and edges and (ii) hot, harzburgite-rich plumes bursting through the basalt layer (as previously observed in laboratory experiments), whereas for a slab that lands basalt side down (iii) hot basalt can peel off from its underside, displaying fingering instabilities in 3-D. Furthermore, basalt–harzburgite segregation is sometimes observed in slab segments that have already been heated at the CMB and risen a few hundred km, by a folding mechanism. A range of “interesting” structures are observed in the model CMB region, which may be useful in interpreting seismic observations. Strong lateral gradients in composition and temperature are quite common. Structures include plumes next to vertical slab segments, inverted slab sections perched above the CMB and harzburgite curtains. The presence of a stratified layer above the CMB, as observed in global simulations in which a layer builds up by basalt segregation, strongly suppresses plume formation. 2-D simulations give a good first-order guide to the dynamics obtained in fully 3-D geometry, but inherently 3-D structures are missed (plumes versus sheets, fingering) and there is a quantitative difference in the fraction of basalt segregated. [Copyright &y& Elsevier]

Abstract: Mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR 12°50′N) were analyzed for U-series isotopes and compositions of plagioclase-hosted melt inclusions. The 226Ra and 230Th excesses are negatively correlated; the 226Ra excess is positively correlated with Mg# and Sm/Nd, and is negatively correlated with La/Sm and Fe8; the 230Th excess is positively correlated with Fe8 and La/Sm and is negatively correlated with Mg# and Sm/Nd. Interpretation of these correlations is critical for understanding the magmatic process. There are two models (the dynamic model and the “two-porosity” model) for interpreting these correlations, however, some crucial parameters used in these models are not ascertained. We propose instead a model to explain the U-series isotopic compositions based on the control of melt density variation. For melting either peridotite or the “marble-cake” mantle, the FeO t content, 230Th excess and La/Sm ratio increases and Sm/Nd decreases with increasing pressure. A deep melt will evolve to a higher density and lower Mg# than a shallow melt, the former corresponds to a long residence time, which lowers the 226Ra excess significantly. This model is supported by the existence of low 226Ra excesses and high 230Th excesses in MORBs having a high Fe8 content and high density. The positive correlation of 226Ra excess and magma liquidus temperature implies that the shallow melt is cooled less than the deep melt due to its low density and short residence time. The correlations among Fe8, Ti8 and Ca8/Al8 in plagioclase-hosted melt inclusions further prove that MORBs are formed from melts having a negative correlation in melting depths and degrees. The negative correlation of 226Ra excess vs. chemical diversity index (standard deviation of Fe8, Ti8 and Ca8/Al8) of the melt inclusions is in accordance with the influence of a density-controlled magma residence time. We conclude that the magma density variation exerts significant control on residence time and U-series isotopic compositions. [Copyright &y& Elsevier]

Abstract: Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele''s hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent. [Copyright &y& Elsevier]

Abstract: We study the fine structure of the lowermost several hundred km of the large low shear velocity province (LLSVP) beneath the western Pacific by inverting the transverse components of long period (20–200 s) broadband seismic waveforms. We find a low S-velocity zone in the depth range from about 2550–2750 km, with the greatest decrease (relative to PREM) of about 0.2 km/s at depths of about 2650–2700 km. This suggests the presence of significant amounts of mid-ocean ridge basalt (MORB), for which the phase transitions from pv to ppv and from CaCl2-type to α-PbO2-type SiO2 result in a decrease in S-velocities. We also find an increase in the S-velocity, relative to this minimum, of about 0.15 km/s, peaking in the depth range from about 2750–2800 km, which is consistent with the phase transition from pv to ppv expected for average composition models such as pyrolite. [Copyright &y& Elsevier]

Abstract: Intermediate-spreading Southeast Indian Ridge (SEIR) basalts display geographic gradients from 90°E to 118°E in Th/U, (230Th/232Th) and 238U–230Th–226Ra disequilibria (230Th-excesses of 1 to 21% and 226Ra-excesses of 0 to 160%). Highly correlated (238U/232Th) and (230Th/232Th) (r 2 =0.94) span nearly the entire global MORB range; basalts from three of four ridge morphologies form subparallel, vertically stacked arrays on an equiline diagram. (226Ra/230Th) is inversely correlated with (230Th/238U) within sub-regions of the study area. There is a convex (230Th/238U) along-axis pattern (lower 230Th-excesses in the west and east, at shallowest and deepest depths), such that (230Th/238U) does not correlate with axial depth as described elsewhere, despite the ~2300 m west-to-east axial depth increase. κ and κ Pb also vary along axis from κ < κ Pb in the west to κ ≅ κ Pb in the east. Variations in axial depth, crustal thickness, U-series disequilibria, κ and κ Pb are best explained by forward model scenarios in which the physical characteristics of melting (such as melting rate, degree of melting, porosity and melt initiation depth) vary along axis in response to an inferred long wavelength temperature gradient and small variations in the underlying source lithology (pyroxenite veins). This leads to decreased melt supply from west to east, a concomitant change in axial morphology and axial magma chamber depth, and systematic long wavelength geochemical gradients. Melting rate covaries with melt supply in the western and central regions, decreasing eastward along the SEIR, but supply and rate vary inversely from the central to the eastern regions, because cooler eastern mantle contains a few percent of fusible pyroxenite veins that melt productively to generate melts with high Th–U–Ra concentration but low 230Th-excess and 226Ra-excess. The veins contribute only a small proportion of the total melt volume from mantle that is >95% peridotite. Th, Pb and He isotopic gradients are consistent with a changing proportion of enriched pyroxenite veins along axis having a roughly 0.5 to 1 Gyr reservoir age. [Copyright &y& Elsevier]

Abstract: Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host rock, and olivine is notably absent as a chadacryst despite being abundant in the host rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such reactions are enriched in Al2O3 and MgO and depleted in CaO and SiO2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al2O3–MgO–SiO2 relationships of MORB may result from melt–rock reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–rock reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB. [Copyright &y& Elsevier]

Abstract: We succeeded in determining the Ce isotopic composition (138Ce/142Ce) in seawater with an error of 2σ m =0.3–0.7 of ε unit. In this study, 1000–3000 L of seawater samples were passed through MnO2 fibers to concentrate Ce and Nd for precise measurement of their isotope ratios. Four surface seawater samples of the northwestern Pacific and a coastal sample in Tokyo Bay were analyzed. Most Ce isotope ratios in the surface water showed positive ε Ce values (+0.8 to +1.4) in the northwestern Pacific Ocean. These values indicate that Ce in the surface water originates from the continental crust preferentially over mantle-derived materials. We examined binary mixing model between the continental crust and mid-ocean ridge basalt. However the model could not explain both isotopic compositions and concentrations, which implies that the atmospheric input was a possible pathway for Ce into the ocean. A negative ε Ce value was observed in Tokyo Bay, suggesting mantle-derived sources. [Copyright &y& Elsevier]

Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47°E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf ratios clearly distinguish an Atlantic–Pacific-type MORB source, present west of 26°E, characterized by relatively low ɛHf values for a given ɛNd relative to the regression line through all Nd–Hf isotope data for oceanic basalts (termed the ‘Nd–Hf mantle array line’; the deviation from this line is termed ΔɛHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32°E, characterized by relatively high ΔɛHf values and Lu/Hf ratios. Additionally, two localized, isotopically anomalous areas, at 13–15°E and 39–41°E, are characterized by distinctly low negative and high positive ΔɛHf values, respectively. The low ΔɛHf MORB from 13 to 15°E appear to reflect contamination by HIMU-type mantle from the nearby Bouvet mantle plume, whereas the trace element and isotopic compositions of MORB from 39 to 41°E are most consistent with contamination by metasomatized Archean continental lithospheric mantle. Relatively small source-melt fractionation of Lu/Hf relative to Sm/Nd, compared with MORB from faster-spreading ridges, argues against a significant role for garnet pyroxenite in the generation of most central SWIR MORB. Correlations between ΔɛHf and Sr and Pb isotopic and trace element ratios clearly delineate a high-ΔɛHf ‘Indian Ocean mantle component’ that can explain the isotope composition of most Indian Ocean MORB as mixtures between this component and a heterogeneous Atlantic–Pacific-type MORB source. The Hf, Nd and Sr isotope compositions of Indian Ocean MORB appear to be most consistent with the hypothesis that this component represents fragments of subduction-modified lithospheric mantle beneath Proterozoic orogenic belts that foundered into the nascent Indian Ocean upper mantle during the Mesozoic breakup of Gondwana. [ABSTRACT FROM AUTHOR]