Ratios Of Mid-ocean Ridge Basalts Research Articles

Melt-Rock Reaction in the Lower Oceanic Crust and the Influence on the Evolution of Mid-Ocean Ridge Basalts at the Central Indian Ridge (7°50′–8°30′S)

Abstract 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.

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

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

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts (MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge (MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios. We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as “pipelike channels” and “pancake-like channels”. The “pancake-like channels” plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the “pipe-like channels” plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.

The prevalence of kilometer-scale heterogeneity in the source region of MORB upper mantle.

The source regions of mid-ocean ridge basalts (MORB) are heterogeneous, consisting of chemically and lithologically distinct domains of variable size. Partial melting of such heterogeneous mantle sources gives rise to diverse isotopic compositions of MORB and abyssal peridotites. Variations in radiogenic isotope ratios in MORB are attributed to mixing of melts derived from enriched and depleted mantle components. However, melt mixing alone cannot fully account for the difference between the average 143Nd/144Nd in abyssal peridotites and their spatially associated MORB. We show that the more depleted Nd isotope composition in abyssal peridotites is a natural consequence of melt migration-induced mixing or smearing in the melting column. Sub-kilometer scale enriched mantle components or heterogeneities are significantly damped or homogenized in both the residue and erupted melt during their transit through the melting region. Heterogeneities with larger size and higher incompatible trace element abundance are more resistive to the mixing processes. The size-sensitive mixing depends on a parameter called the enrichment strength, which is the product of the heterogeneity size and the ratio between incompatible trace element abundance in the enriched and depleted mantle sources. Observed Nd-Hf isotope variations in MORB and abyssal peridotites can be reproduced if the enrichment strength is 20 to 60 km. These heterogeneities could be on the kilometer scale and have similar isotope ratios to but less incompatible trace element abundances than recycled oceanic crust.

Tectonic influence on chemical composition of ocean island basalts in the West and South Pacific: Implication for a deep mantle origin

Pb, Nd, and Sr isotopic compositions and the 40Ar-39Ar ages of lavas from seamounts in the Joban seamount chain, which is the northernmost Cretaceous seamount chain in the northwest Pacific, suggest that these seamounts were produced by HIMU-type (high-μ or high-238U/204Pb) magmatism around 120 Ma. In addition, seamount chains in the western Pacific, which are coeval with the Joban seamount chain, also exhibit a HIMU isotopic signature. Since the seamount chains in the western Pacific are considered to have formed in the present-day South Pacific region, these isotopic features suggest that HIMU-type magmatism must have been active in the South Pacific since the Cretaceous. A notable geochemical characteristic of this magmatism is a correlation between the elemental ratios (La/Yb, Sm/Yb, Sr/Y, Nb/Zr, Ta/Zr, and Th/Ta) of the seamounts/islands and a “relative age” that is an index of the thickness of the lithosphere beneath hot spots. These correlations suggest the importance of the tectonic environment in determining the chemical composition of the magmas. They also suggest a genetic relationship between oceanic plateaus and island/seamount chains because the elemental ratios of oceanic plateaus follow the same trends. Another important geochemical feature is that the elemental ratios of mid-ocean ridge basalts (MORBs) from the East Pacific Rise (EPR) do not follow these trends. Since the source material for EPR MORBs is probably representative of the composition of the upper mantle beneath the South Pacific, this difference could imply an upward flow of material from the deep mantle.

Xenon isotope systematics, giant impacts, and mantle degassing on the early Earth

The relationships between the major terrestrial volatile reservoirs are explored by resolving the different components in the Xe isotope signatures displayed by Harding County and Caroline CO 2 well gases and mid-ocean ridge basalts (MORB). For the nonradiogenic isotopes, there is evidence for the presence of components enhanced in the light 124–128Xe/ 130Xe isotope ratios with respect to the terrestrial atmosphere. The observation of small but significant elevations of these ratios in the MORB and well gas reservoirs means that the nonradiogenic Xe in the atmosphere cannot be the primordial base composition in the mantle. The presence of solar-like components, for example U–Xe, solar wind Xe, or both, is required. For radiogenic Xe generated by decay of short-lived 129I and 244Pu, the 129Xe rad/ 136Xe 244 ratios are indistinguishable in MORB and the present atmosphere, but differ by approximately an order of magnitude between the MORB and well gas sources. Correspondence of these ratios in MORB and the atmosphere within the relatively small uncertainties found here significantly constrains possible mantle degassing scenarios. The widely held view that substantial early degassing of 129Xe rad and 136Xe 244 from the MORB reservoir to the atmosphere occurred and then ended while 129I was still alive is incompatible with equal ratios, and so is not a possible explanation for observed elevations of 129Xe/ 130Xe in MORB compared to the atmosphere. Detailed degassing chronologies constructed from the isotopic composition of MORB Xe are therefore questionable. If the present estimate for the uranium/iodine ratio in the bulk silicate Earth (BSE) is taken to apply to all interior volatile reservoirs, the differing 129Xe rad/ 136Xe 244 ratios in MORB and the well gases point to two episodes of major mantle degassing, presumably driven by giant impacts, respectively ∼ 20–50 Ma and ∼ 95–100 Ma after solar system origin assuming current values for initial 129I/ 127I and 244Pu/ 238U. The earlier time range, for degassing of the well gas source, spans Hf–W calculations for the timing of a moon-forming impact. The second, later impact further outgassed the upper mantle and MORB source. A single event that degassed both the MORB and gas well reservoirs at the time of the moon-forming collision would be compatible with their distinct 129Xe rad/ 136Xe 244 ratios only if the post-impact iodine abundance in the MORB reservoir was about an order of magnitude lower than current estimates. In either case, such late dates require large early losses of noble gases, so that initial inventories acquired throughout the Earth must have been substantially higher. The much larger 129Xe rad/ 136Xe 244 ratio in the well gases compared to MORB requires that these two Xe components evolve from separate interior reservoirs that have been effectively isolated from each other for most of the age of the planet, but are now seen within the upper mantle. These reservoirs have maintained distinct Xe isotope signatures despite having similar Ne isotope compositions that reflect similar degassing histories. This suggests that the light noble gas and radiogenic Xe isotopes are decoupled, with separate long-term storage of the latter. However, without data on the extent of heterogeneities within the upper mantle, this conclusion cannot be easily reconciled with geophysical observations without significant re-evaluation of present noble gas models. Nevertheless the analytic evidence that two different values of 129Xe rad/ 136Xe 244 exist in the Earth appears firm. If the uranium/iodine ratio is approximately uniform throughout the BSE, it follows that degassing events from separate reservoirs at different times are recorded in the currently available terrestrial Xe data.

The constancy of upper mantle fO 2 through time inferred from V/Sc ratios in basalts

The geochemical behaviors of V and Sc during partial melting are more similar to each other than to most other elements but, in detail, the partitioning of V is redox-sensitive whereas that of Sc is not. The V/Sc ratio in basalts is shown here to be a sensitive and robust recorder of the oxygen fugacity (fO 2) of their mantle source regions. The use of V/Sc ratios differs from the use of V and/or Cr elemental systematics in basalts because the latter, strictly speaking, provide information on the fO 2 of the erupted magma and not necessarily that of the mantle source. V/Sc ratios on the other hand can “see through” magmatic differentiation processes, such as olivine crystallization, and therefore, provide a better memory of the fO 2 of the mantle. The V/Sc ratios of mid-ocean ridge basalts (MORBs) are shown here to average 6.74±1.11 (1σ), which constrains the fO 2 of the modern mantle to roughly 0.3±0.5 (1σ) log units below the fayalite–magnetite–quartz (FMQ) buffer. Archean basalts (up to 3.5 Ga) are shown to have identical V/Sc ratios (6.34±0.62, 1σ) to within error indicating that the fO 2 of the mantle source to Archean basalts differ from that of modern convecting mantle by no more than 0.3 log unit. The constraints based on V/Sc systematics represent an improvement on studies using V or Cr elemental systematics, which constrain the fO 2 of Archean and modern mantles to be identical, but only to within ±0.5 to 1 log unit. These observations show that there has been little or no secular evolution of the fO 2 of the upper mantle, at least since the early Archean.

Chlorine–potassium variations in melt inclusions from Raivavae and Rapa, Austral Islands: constraints on chlorine recycling in the mantle and evidence for brine-induced melting of oceanic crust

Chlorine abundance variations in oceanic basalts can provide insights into the degassing and volatile recycling history of the mantle as well as shallow melt/hydrosphere interaction. We have examined major, trace and volatile element abundances in olivine-hosted melt inclusions from the islands of Raivavae and Rapa in the Austral Island chain. The island of Raivavae sits atop a pre-existing fracture zone and thus provides the opportunity to examine the relationship between melt/hydrosphere interaction and local lithospheric structure. The majority of inclusions from both Raivavae and Rapa have well correlated chlorine and potassium concentrations consistent with a source Cl/K2O ratio of ∼0.04, similar to that of uncontaminated mid-ocean ridge and ocean island basalts. The similarity of chlorine/potassium ratios in mid-ocean ridge basalts, Austral Islands basalts, and basalts from many other ocean islands suggests that chlorine/potassium does not significantly vary in the mantle. Because the plume sources of many ocean island chains contain varying types and quantities of recycled oceanic crust and sediments, this indicates that most of the chlorine added to oceanic crust during seafloor alteration is removed during subduction and is not recycled into the deep mantle. High chlorine contents (up to 0.14 wt%) and chlorine/potassium ratios in melt inclusions from an early-erupted Raivavae lava derive from assimilation of Cl-rich brines or brine-impregnated oceanic crust. A small subset of inclusions from the same lava show more extreme chlorine enrichment (up to 2.5 wt%), are depleted in incompatible trace elements relative to normal inclusions, and show extreme fractionation of high-field-strength elements (HFSEs) relative to large-ion-lithophile or rare-earth elements. These latter inclusions derive from partial melting of the pre-existing oceanic crust under brine-saturated conditions. HFSE depletions in these inclusions reflect the stabilization of a HFSE-bearing phase in the lower crust, probably due to high chlorine fugacity. HFSE anomalies are also associated with high chlorine content in mid-ocean ridge basalts. We suggest that these anomalies are also generated by the stabilization of HFSE-bearing phases in high-chlorine-activity melts or fluids. This process may also provide a means of stabilizing rutile in the sub-arc mantle wedge.

A hafnium isotope and trace element perspective on melting of the depleted mantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform ( 176Hf/ 177Hf≈0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have 176Hf/ 177Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low 176Hf/ 177Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.

Oxygen-isotope evidence for recycled crust in the sources of mid-ocean-ridge basalts

Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiations. Variations in (18)O/(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that (18)O/(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.

A Tale of Two Planets

Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information.

Uranium–thorium disequilibria and partitioning on melting of garnet peridotite

THE abundances of isotopes in the 238U decay series can be used as both tracers and chronometers of magmatic processes. In the subsolidus asthenosphere, the activity of each daughter isotope (defined as the product of its concentration and decay constant, and denoted by parentheses) is assumed to be equal to that of its parent. By contrast, (230Th/238U) is greater than unity in most recent mid-ocean-ridge and ocean-island basalts1, implying that thorium is more incompatible (that is, it is partitioned into the melt phase more strongly) than uranium. Melting of spinel peridotite cannot produce the (230Th) excesses, because measured partition coefficients for pyroxenes and olivine demonstrate that uranium is more incompatible than thorium for this rock2. Here I report garnet–melt partitioning data which show that for this mineral-melt pair thorium does behave more incompatibly than uranium, thus supporting the suggestion that mid-ocean-ridge basalts (MORB) are produced by melting initiated at depths where garnet is stable3–6. Using these data, I show that the observed (230Th/238U) ratios of MORB and most ocean-island basalts can be explained by slow, near-fractional melting initiated in the garnet stability field.

231Pa and 230Th chronology of mid-ocean ridge basalts

Mass spectrometric measurements of 235U 231Pa and 238U 230Th disequilibria are used to further constrain processes producing U-series disequilibria in young mid-ocean ridge basalts (MORB) and to determine eruption ages for samples from 9–10°N East Pacific Rise and Juan de Fuca-Gorda ridges. 231Pa/ 235U activity ratios for axial basalts are large and relatively uniform within ridge segments. The systematically large 231Pa/ 235U , 230Th/ 238U , and 226Ra/ 230Th activity ratios in MORB are inconsistent with simple equilibrium melting models and crustal contamination or seawater alteration processes. More complicated melting processes involving disequilibrium or heterogeneous melting of mantle sources and disequilibrium transport of melts to the crust could produce the large U-series disequilibria in MORB. 231Pa and 230Th model ages were determined by estimating initial 231Pa/ 235U and 230Th/ 232Th ratios from axial samples and by assuming constancy of these ratios over time. All 231Pa and 230Th ages agree to ±0–7 ka for samples with ages of 0–130 ka. These concordant ages validate the model assumptions of the two dating methods and suggest that mantle sources and melt generation processes have been uniform for these areas during the past 130 ka. Consequently, 231Pa and 230Th age-dates can be used to quantify the temporal and spatial dependence of volcanism at oceanic spreading centers during the past 375 ka.

The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts

MID-OCEAN-RIDGE basalts (MORBs) are thought to result from melting in the mantle at depths of less than 60 km, in the spinel stability field1–3. MORBs have 176Hf/177Hf ratios indicating derivation from a mantle reservoir with a long-term Lu/Hf ratio greater than that of Cl chondrite meteorites, yet the measured Lu/Hf ratios in MORB are lower than in Cl chondrites: this is the 'hafnium paradox'4. Here we show that the Lu–Hf and Sm–Nd systematics of MORBs require garnet to be a residual phase in MORB melt genesis. This places the onset of melting beneath a mid-ocean ridge at depths greater than 80 km. A sequential melting model, in which melting starts in the garnet stability field and then continues at shallower levels, best explains the combined Nd and Hf isotope systematics, and is compatible with our present geophysical and geochemical knowledge of mid-ocean-ridge magmatism.