Mid-ocean Ridge Basalt Magmas Research Articles

Formation and Evolution of Chromitites in the Hongshishan Complex, Beishan Orogenic Collage, NW China: Constraints from Mineralogical Compositions, Re‐Os Isotopes and Platinum‐Group Element Geochemistry

AbstractThe Hongshishan chromitite deposits are situated to the north of the Beishan orogenic collage, in the southern part of the Central Asian Orogenic Belt. This study describes the mineral chemistry, Re‐Os isotopes and platinum‐group elements geochemistry of the Hongshishan chromitites for the purpose of constraining the origin, evolution and composition of their parental melts. The restricted ranges of Al2O3, Cr2O3 and Cr#‐Mg# variation of chromite‐cores and chromites fall within the field of the mid‐ocean ridge and ophiolitic podiform chromite settings. The (187Os/188Os)i ratios of the chromitites are in the range of 0.12449–0.12745 (average 0.12637) and the γOs are from –1.92 to –0.06 (average –0.83). In the Re‐Os isotope diagrams, all the samples fall in the field of chromitites and show a residual peridotitic trend. The range of Os isotopic compositions and γOs values indicate that they overlap the depleted MORB mantle (DMM) as well as being close to global Os isotopic data and γOs of ophiolite chromitites. The characteristics of the PGE contents can be roughly subdivided into two groups: podiform chromitites and Ural‐Alaskan type complexes. For the ferritchromite cores, the calculated Al2O3 concentrations of the parental melt are higher (average 16.65 wt%) in high‐Cr than high‐Al chromitite (average 16.17 wt%) and for the chromite, the calculated Al2O3 concentrations are even higher (average 16.48 wt%) in the high‐Cr than the high‐Al examples (average 15.38 wt%). In the (TiO2)melt vs. TiO2 diagrams, most high‐Al melts fall in the MORB, while the high‐Cr melts fall in the ARC field. The calculated FeO/MgO ratios for the parental melt show the closest resemblance to a MORB magma composition. The inferred parental melt composition for studied chromitites falls in the field of mid‐ocean ridge basalt (MORB) magmas and far away from boninite. The calculated degrees of partial melting producing the chromitites are 16%–22% (average 19%), which is around the range of those of the MORB magmas. The chromitites are suggested to have been formed in a MORB setting. The chromites and ferritchromite cores are mostly scattered along the MORB and SSZ harzburgite–dunite fields. Ferritchromite rims and ferritchromites with high YFes formed as a result of alteration during serpentinization. .

Iron isotope compositions of subduction-derived rocks: Insights from eclogites and metasediments of the Münchberg Massif (Germany)

The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/ΣFe = 0.06 to 0.30). Their δ56Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ56Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments.It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ56Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ56Fe values throughout the subduction-exhumation cycle, whereas δ56Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Multiphase ophiolite formation in the Northern Altyn Tagh Orogen, southeastern Tarim

Early Paleozoic ophiolitic mélanges in the Altyn Tagh Orogen, southeastern Tarim, yield a large range of formation ages and geochemical affinities. This study focused on the Hongliugou ophiolitic mélange in the North Altyn Tagh subduction-accretion belt and involved mineral chemical, zircon geochronological, and whole-rock elemental and isotopic investigations of the ultramafic and mafic rocks. In the studied lherzolite samples, Cr-spinel, olivine, and pyroxene show mineral chemistry akin to that of abyssal peridotite. Subhedral-anhedral Cr-spinel grains with high Cr# values (100*Cr/[Cr+Al] of 30–40) and negative oxygen fugacity (fO₂) values represent original spinel formed during mid-ocean ridge basalt (MORB) melt extraction. Other subhedral-anhedral and euhedral Cr-spinel with low Cr# values of &lt; 30 to 20 display a wide range of negative to positive fO₂ values, indicating interaction of peridotite with MORB magma. New zircon U-Pb dating results record ages of <i>ca.</i> 490 Ma for gabbro and <i>ca.</i> 486 Ma for diabase in the Hongliugou ophiolitic mélange. Whole-rock geochemical compositions suggest that the gabbro and diabase samples are tholeiitic and show MORB affinities or transitional affinities between MORB and island arc tholeiite. These mafic rocks might have been derived from a depleted MORB-source mantle that was variably metasomatized by subduction-induced fluids based on their variable Th/Yb, high La/Nb, low Th/Nb, and depleted isotopes (initial ⁸⁷Sr/⁸⁶Sr ratios of 0.703073 to 0.703385 and <i>ε</i>_Nd(<i>t</i>) values of +6.2 to +6.3). New results, when integrated with previous work, clarify that <i>ca.</i> 520 to 510 Ma ophiolites formed in the initial subduction setting and <i>ca.</i> 490 to 480 Ma ophiolites in a back-arc setting. Minor <i>ca.</i> 450 Ma ophiolite probably represents a late phase of ophiolite formation due to the presence of a very minor remnant ocean.

Sulfide in dunite channels reflects long-distance reactive migration of mid-ocean-ridge melts from mantle source to crust: A Re-Os isotopic perspective

The Earth's major heat and mass fluxes are output from the mantle to surface via mid-ocean-ridge-basalt (MORB) magmatism. The variations of MORBs are determined both by the heterogeneity of mantle source and the melt migration processes from mantle source to seafloor. However, before the MORB magmas enter crust, the role of the major melt migration passageways within the mantle (dunite channel systems) in regulating the compositions of MORBs has been rarely evaluated, especially from the perspective of Re-Os isotopes. Here, we report new Re-Os isotopic compositions of base-metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/(Cr3++Al3+) ≤0.66] (products of interaction between MORB-like melts and upper-mantle harzburgites) from the Zedang ophiolite (South Tibet). Rhenium-Os isotopic compositions of low-Cr# dunites from the Samail and Wadi Tayin massifs (typical MOR segments) of the Oman ophiolite are also shown for comparison to reveal the melt plumbing processes at mantle depths beneath mid-ocean ridges. Mineralogical evidence suggests that the Zedang sulfides and desulfurized alloy portions were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios (0.1191-0.1702) and low 187Re/188Os (<0.22) of the sulfides suggest that the chromite acted as a sink for Os-bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub-PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os-rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os-poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Samail and Wadi Tayin massifs of the Oman ophiolite and present-day mid-ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os-bearing sulfides with chromite, leaving high Re/Os components into the residual MOR melts. Once such channel systems were built up at the Moho transition zone, the newly incoming MOR magmas would preferentially dissolve the volumetrically abundant radiogenic BMS and retain Os-rich alloys/sulfides in the channels, further amplifying the Os-isotope mismatch observed between global oceanic crust (more radiogenic) and lithospheric mantle (less radiogenic).This study reveals that the long-distance reactive migration of melts from mantle source to at least Moho depths can result in strong Re-Os fractionation and isotopic mismatch between primary MORBs and mantle residues. It also shows that, unlike other lithophile tracers (e.g., Nd-Hf isotopes) whose signatures are almost completely transferred from mantle source to crust, the sub-crustal melt plumbing processes beneath mid-ocean ridges can be finely constrained by the multistage evolution of chalcophile and siderophile elements (e.g., Re-Os) and their isotopes (e.g., 187Re-187Os) with sulfides and chromites. Our study supports that the MORB compositions are actually regulated by their reactive migration processes occurred between melt generation at depth and eruption on seafloor.

Refractory chromitites recovered from the Eretria mine, East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe2+) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.

Felsic Plutonic Rocks from IODP Hole 1256D, Eastern Pacific: Implications for the Nature of the Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges

Although the axial melt lens (AML) beneath fast-spreading mid-ocean ridges has been detected by seismic reflection for decades, its nature and role in the accretion of lower oceanic crust and the evolution and eruption of mid-ocean ridge basalts (MORB) are still poorly constrained. Plutonic rocks consisting of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML, have for the first time been recovered from an intact fast-spreading oceanic crust section by Integrated Ocean Drilling Program (IODP) Hole 1256D. Whole-rock major elements show a wide and continuous compositional range (e.g. Mg# 24–70) and apparent enrichments in Ti and Fe at intermediate MgO contents (4–6 wt %). Trace element characteristics are coherent for the different lithology groups defined by petrography and mineral modes; that is, gabbro, clinopyroxene-rich diorite, amphibole-rich or oxide-rich diorite and tonalite. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements (especially with respect to Eu) support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization. Enrichments in rare earth elements (REE) in clinopyroxenes from the gabbroic and dioritic intrusive rocks suggest strong assimilation of REE-rich tonalitic components by evolved MORB magmas. Hydrothermal alteration was pervasive during cooling of the plutonic system, which can be traced by petrography, mineral compositions and bulk-rock geochemistry. The upper part of AML, largely composed of low-density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML.

Trace element partitioning between plagioclase and silicate melt: The importance of temperature and plagioclase composition, with implications for terrestrial and lunar magmatism

Trace element partition coefficients between anorthitic plagioclase and basaltic melts (D) have been determined experimentally at 0.6GPa and 1350–1400°C in a lunar high-Ti picritic glass and a mid-ocean ridge basalt (MORB). Plagioclases with 98mol% and 86mol% 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 DRa/DBa 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 DREE during MORB differentiation, but (2) the temperature effect is especially significant and can vary anorthite-melt DREE 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.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

Behavior of MORB magmas at uppermost mantle beneath a fast-spreading axis: an example from Wadi Fizh of the northern Oman ophiolite

Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# [Mg/(Mg + Fe2+) atomic ratio] clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as “MORB-saturated harzburgite” for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt/harzburgite ratios. The modified melt left clinopyroxene (not clinopyroxene + plagioclase as in plagioclase-impregnated abyssal harzburgite) to form the MORB-saturated harzburgites in the vicinity (harzburgite) of the fracture, which are left as gabbro-clinopyroxenite bands. This local modification mimics the whole lithological and chemical variation of the MTZ and makes chemical variation of MORB suite at fast-spreading ridge.

The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle

Micro-analytical determination of Fe3+/∑Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure (μ-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe3+/∑Fe ratios of 0.16±0.01 (n=103), which trace back to primary MORB melts equilibrated at the conditions of the quartz–fayalite–magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe3+/∑Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or <10%) of the difference between Fe3+/∑Fe ratios determined by micro-colorimety and XANES can be attributed to the Mössbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe3+/∑Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe3+ behaving incompatibly in shallow MORB magma chambers. MORB Fe3+/∑Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na2O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe3+ may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at ~QFM. Both explanations, in combination with the measured MORB Fe3+/∑Fe ratios, point to a fertile MORB source with greater than 0.3wt.% Fe2O3.

The Case for Reactive Crystallization at Mid-Ocean Ridges

Evidence from abyssal peridotites suggests that significant chemical reaction with peridotite can occur during the early stages of cooling and crystallization of mid-ocean ridge basalt (MORB) magmas. We evaluate the hypothesis that reactive crystallization (crystallization influenced by such melt^rock reaction) could cause magma compositions to evolve along a different chemical trajectory than expected for fractional crystallization, and that reactive crystallization might be common in MORB petrogenesis. If correct, this hypothesis implies that a component of major element variability in fractionation-corrected MORB, commonly interpreted solely in terms of mantle source composition and potential temperature, could reflect reactive crystallization. The compositional evolution of MORB magmas undergoing reactive crystallization is predicted using thermodynamic calculations.We find that the decreasing melt MgO content during reactive crystallization is accompanied by nearly constant Mg-number [defined as molar MgO/ (MgOþFeO)], whereas melt SiO2 and Na2O contents evolve to higher values than in fractional crystallization. However, the extent of crystallization as a function of temperature is essentially identical during the initial 30^40% of both fractional and reactive crystallization. Comparison of melt transport and reaction timescales in a 1-D, steady-state, porous flow column shows that melt migration via grain-scale porous flow at the transition from melting to crystallization beneath ocean ridges will most probably give rise to reactive crystallization, whereas melt transport through the thermal boundary layer in larger conduits (dikes) will lead to fractional crystallization. Scatter in fractionation-corrected major element compositions could therefore reflect sample-to-sample variations in melt transport dynamics. Using a global compilation of MORB glass compositions, we show that 40^70% of the variability in fractionation-corrected MgO contents observed worldwide is also typically present in groups of samples collected from within 30 km of each other. Such short length-scale variability in MgO, and hence in the temperature of primitive magmas, cannot be due to variability in mantle potential temperature. There is a negative correlation in the variability of fractionation-corrected MgO (and most other compositional variables) with spreading rate.We infer that this negative correlation reflects a greater role for reactive crystallization in the thicker thermal boundary layers present beneath slow-spreading ridges. We demonstrate the ability of combined reactive and fractional crystallization to account for major element variability at several case-study locations, and argue that reactive crystallization can explain many observations of 30 km scale variability. Interpreted in terms of reactive crystallization, fractionation-corrected MgO variability could potentially bound geodynamic parameters such as the depth of onset of diking and the fraction of gabbro emplaced into residual mantle peridotite beneath the igneous crust.

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

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

Helium deficit in high‐3He/4He parent magmas: Predegassing fractionation, not a “helium paradox”

We evaluated He‐Ne‐Ar elemental compositions of mid‐ocean ridge basalt (MORB) glasses and basalt glasses from the Hawaiian hot spot (Loihi Seamount, Kilauea) to determine the (3He/22Ne)solar, 4He*/21Ne*, and 4He*/40Ar* MORB‐type and “plume”‐type end‐member compositions within a binary mixing model. Although the 3He/22Ne ratios of “plume” magma and MORB magma marginally agree with current estimates of solar compositions, the calculated 4He*/21Ne* ratios are lower than the theoretical production ratio and indicate that the “plume” magma component experienced a predegassing fractionation event. This fractionation event resulted in a deficit in both radiogenic 4He* and primordial 3He and accounts for the differences in 3He/22Ne, 4He*/21Ne*, and 4He*/40Ar* ratios and 3He concentrations for both mantle domains. This resolves an apparent contradiction previously termed “mantle He paradox,” though the derived 3He concentrations are also at odds with a primitive mantle concept that requires still higher concentrations. We propose that the primary fractionation of He/Ne and He/Ar ratios is related to a more compatible behavior of He relative to Ne, Ar during partial melting. One model is presented: at depths below a shallow MORB source region a rising mantle plume causes formation of low‐degree partial melts, which are deficient in He. During melt ascent, these “plume”‐derived melts mix with melts of the shallow (MORB‐type) mantle source that experienced a higher degree of melting and hence show no He deficit. Depending on the relative proportion of mixing the resulting oceanic basalts will encompass the whole observed range from Loihi‐type and MORB‐type noble gas isotope compositions.

Depths of Partial Crystallization of H2O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7–11°S)

Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7–11°S). The results indicate that mid-ocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100–900 MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100–300 MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200–900 MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0·4–1 wt% H2O in the parental melts. These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0·04–0·09 and 0·30–0·55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200 MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water.

Cryptic crustal contamination of MORB primitive melts recorded in olivine-hosted glass and mineral inclusions

The origin of compositional heterogeneities among the magmas parental to mid-ocean ridge basalts (MORB) was investigated using a single rock piece of the olivine-phyric basalt from 43 degrees N, Mid-Atlantic Ridge (AII D11-177). The exceptional feature of this sample is presence of very primitive olivine crystals (90-91 mol% Fo) that are significantly variable in terms of CaO (0.15-0.35 wt%). A population of low-Ca olivine (0.15-0.25 wt% CaO) is also notably distinct from high-Ca olivine population in AII D11-177, and primitive MORB olivine in general, in having unusual assemblage of trapped mineral and glass inclusions. Mineral inclusions are represented by high-magnesian (Mg# 90.7-91.1 mol%) orthopyroxene and Cr-spinel, distinctly enriched in TiO2 (up to 5 wt%, c.f. < 1 wt% in common MORB spinel). Glass inclusions associated with orthopyroxene and high-Ti Cr-spinel have andesitic compositions (53-58 wt% SiO2). Compared to the pillow-rim glass and "normal" MORB inclusions, the Si-rich glass inclusions in low-Ca olivine have strongly reduced Ca and elevated concentrations of Ti, Na, K, P, Cl, and highly incompatible trace elements. Strong variability is recorded among glass inclusions within a single olivine phenocrysts. We argue that the observed compositional anomalies are mineralogically controlled, and thus may arise from the interaction between hot MORB magmas and crystal cumulates in the oceanic crust or magma chamber.

Magma dynamics at mid-ocean ridges by noble gas kinetic fractionation: Assessment of magmatic ascent rates

Abstract Despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism, magma dynamics at mid-ocean ridges are poorly known. Here, we propose a new method to assess ascent rates of mid-ocean ridge basalt (MORB) magmas, as well as their pre- and sin-eruptive dynamics. It is based on the idea that a rising magma can reach a variable degree of both CO2 supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust. To quantify the relationship, we have used a model of multicomponent bubble growth in MORB melts, developed by extending the single-component model of Proussevitch and Sahagian [A.A. Proussevitch, D.L. Sahagian, Dynamics and energetics of bubble growth in magmas: analytical formulation and numerical modeling, J. Geophys. Res. 103 (1998), 18223–18251.] to CO2–He–Ar gas mixtures. After proper parameterization, we have applied it to published suites of data having the required features (glasses from Pito Seamount and mid-Atlantic ridges). Our results highlight that the investigated MORB magmas display very different ranges of ascent rates: slow rises of popping rock forming-magmas that cross the crust (0.01–0.5 m/s), slightly faster rates of energetic effusions (0.1–1 m/s), up to rates of 1–10 m/s which fall on the edge between lava effusion and Hawaiian activity. Inside a single plumbing system, very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic crust on a small scale. Constraints on how the systems of ridges work, as well as the characteristics of the magmatic source, can also be obtained. Our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics. An effort should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected. Enhanced knowledge of a number of physical properties of gas-bearing MOR magmas is also required, mainly noble gas diffusivities, to describe multicomponent bubble growth at a higher confidence level.

Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

One of the most persistent questions regarding the phase equilibria of mid‐ocean ridge basalts (MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (&gt;An90) that are characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been discovered to be in equilibrium with plagioclase more calcic than An85. We have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions: an N‐MORB, an E‐MORB, and a continental high‐alumina basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthite‐bearing basalt, the experiments were run in An93‐6 capsules with Fo92 olivine added to the starting glass. The compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al2O3 contents of 61–68 wt%) was produced at 1290°C in all compositions and chromites (Al2O3 contents of 33–42 wt%) at lower temperatures in N‐MORB‐derived liquids despite no spinel having been added to the starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both &gt;Fo89 olivine and &gt;An85 feldspar at temperatures of 1230° and 1210°. These liquids have compositions with Mg# (at% Mg/Mg + FeT*100) that range from 63 to &gt;85. The TiO2‐MgO correlation indicates large (∼16–23%) amounts of crystallization for each percent decrease in MgO. These results suggest the possibility that dry, anorthite‐bearing basaltic magmas are the product of the interaction between primary melt and Al‐spinel‐bearing upper mantle. In addition, the results indicate that MORB magmas can undergo a large amount (&gt;50%) of crystallization prior to reaching 8% MgO. Further, although anorthite‐bearing magmas have characteristics consistent with their being a significant volumetric component of MORB “parent” magmas, the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas.

Tectonic implications of boninite, arc tholeiite, and MORB magma types in the Josephine Ophiolite, California-Oregon

Abstract The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.

Production of CO2 and H2 by Diking-Eruptive Events at Mid-Ocean Ridges: Implications for Abiotic Organic Synthesis and Global Geochemical Cycling

We have calculated the amounts of CO2 and H2 produced by complete degassing of mid-ocean ridge basalt (MORB) magma, and by degassing during transient diking-eruptive events. Our CO2 calculations are based on a model estimate of an initial CO2 content of 1800 ppm in MORB magma, which is equivalent to 2.2 × 1012 mol CO2 per year for magma production at worldwide ocean ridges. Observations indicate that many MORB magmas are emplaced in numerous small pulses of dikes and associated lava flows with very short emplacement times, which would result in release of relatively large amounts of CO2 over short intervals. For example, a dike injected into the oceanic crust that extends from the top of its magma chamber at 2 km depth to the seafloor would degas 2.3 × 104 mol CO2 per m2 surface area of dike, and produce another 4.0 × 104 mol CO2 per m2 on complete crystallization. Unlike CO2, which is not strictly governed by crystallization-alteration processes, H2 is produced from MORB by the reduction of H2O by ferrous iron in the magma to form magnetite and H2 as the magma cools and crystallizes. From published paired analyses of MORB glass and crystalline rock, we estimate that the amount of H2 produced from one cubic meter of rock averages 301 mol. We suggest that the oxidizing agent is H2O dissolved in the magma, which results in rapid generation of H2. The amount of pre-alteration oxidation may be limited by the amount of H2O dissolved in the magma; thus relatively water-rich magmas will undergo greater oxidation. For the case of the two-kilometer-high dike reaching the seafloor, the amount of H2 released is 6.2 × 105 moles H2 per m2 surface area of the dike. This is 10 times greater than the total CO2 released by degassing and crystallization of the dike. Assuming that the H2 generation rate for the entire basaltic layer of the oceanic crust is the same as for MORB lavas (312 mol/m3), then the annual global H2 production rate is 6.3 × 1012 mol H2 per year. This amount is about three times greater than our calculated annual CO2 production from MORBs. Given that the annual CO2 production rate from MORBs over 3.3 Ga can account for all CO2 found in the Earth's crust, hydrosphere, and atmosphere, it is likely that the H2 produced at mid-ocean ridges plays a significant role as a reducing agent in the global redox state of the Earth's surface. In contrast to time-averaged global production rates, the rapid release of CO2 and H2 in diking-eruptive events may locally result in formation of a separate gas phase containing H2-CO2-H2O in that order of abundance. The amounts of CO2 and H2 produced could provide a significant energy source for autotrophic microorganisms. It has been demonstrated that such a CO2-H2-H2O gas mixture yields methanol in magnetite-surface catalyzed reactions at seafloor hydrothermal conditions. Such abiotic synthesis reactions could have been important in early Earth processes.