Origin Of Ocean Island Basalts Research Articles

Origin of ocean island basalts in the West African passive margin without mantle plume involvement

The geochemical variabilities in intraplate basalts (IB) from the West African passive margin (WAPM) region, have generally been employed to indicate the presence of recycled materials in an associated upwelling mantle plume. However, the absence of time-progressive linear hotspot tracks in WAPM-IB make it difficult to explain their genesis solely by the mantle plume hypothesis. Here, we show that the Sr–Nd–Hf–Pb isotopic variations in basalts from most of the WAPM-IB could have mainly attributed to the derivation from two types of fusible regions of the refertilized subcontinental lithospheric mantle (SCLM) and the sub-lithospheric mantle. The locations and magma genesis of WAPM-IB are strongly related to the distance from the Mesozoic rift axis and the structure of the rifted SCLM. The melting of the source region can possibly be attributed to small-scale mantle convection at the base of the SCLM without the involvement of a mantle plume.

Triple oxygen isotope constraints on the origin of ocean island basalts

Understanding the origin of ocean island basalts (OIB) has important bearings on Earth’s deep mantle. Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB’s formation, its exact fraction in OIB’s mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. Here we show, through theoretical calculation, that unlike many known proxies, triple oxygen isotope compositions (i.e. Δ17O) in olivine samples are not affected by crystallization and partial melting. This unique feature, therefore, allows olivine Δ17O values to identify subducted oceanic crusts in OIB’s mantle source. Furthermore, the fractions of subducted ocean sediments and hydrothermally altered oceanic crust in OIB’s mantle source can be quantified using their characteristic Δ17O values. Based on published Δ17O data, we estimated the fraction of subducted oceanic crust to be as high as 22.3% in certain OIB, but the affected region in the respective mantle plume is likely to be limited.

Length scales of mantle heterogeneities and their relationship to ocean island basalt geochemistry

The upper mantle is widely considered to be heterogeneous, possibly comprising a “marble-cake” mixture of heterogeneous domains in a relatively well-mixed matrix. The extent to which such domains are capable of producing and expelling melts with characteristic geochemical signatures upon partial melting, rather than equilibrating diffusively with surrounding peridotite, is a critical question for the origin of ocean island basalts (OIB) and mantle heterogeneity, but is poorly constrained. Central to this problem is the characteristic length scale of heterogeneous domains. If radiogenic osmium signatures in OIB are derived from discrete domains, then sub-linear correlations between Os isotopes and other geochemical indices, suggesting melt-melt mixing, may be used to constrain the length scales of these domains. These constraints arise because partial melts of geochemically distinct domains must segregate from their sources without significant equilibration with surrounding peridotite. Segregation of partial melts from such domains in upwelling mantle is promoted by compaction of the domain mineral matrix, and must occur faster than diffusive equilibration between the domain and its surroundings. Our calculations show that the diffusive equilibration time depends on the ratios of partition and diffusion coefficients of the partial melt and surrounding peridotite. Comparison of time scales between diffusion and melt segregation shows that segregation is more rapid than diffusive equilibration for Os, Sr, Pb, and Nd isotopes if the body widths are greater than tens of centimeter to several meters, depending on the aspect ratio of the bodies, on the melt fraction at which melt becomes interconnected in the bodies, and on the diffusivity in the solid. However, because Fe-Mg exchange occurs significantly more rapidly than equilibration of these isotopes under solid-state and partially molten conditions, it is possible that some domains can produce melts with Fe/Mg ratios reflecting that of the surrounding mantle but retaining isotopic signatures of heterogeneous domains. Although more refined estimates on the rates of, and controls on, Os mobility are needed, our preliminary analysis shows that heterogeneous domains large enough to remain compositionally distinct in the mantle (as solids) for ∼10 9 yr in a marble-cake mantle, can produce and expel partial melts faster than they equilibrate with surrounding peridotite.

Isotope and trace element variations in lavas from Raivavae and Rapa, Cook–Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia

We have examined the major and trace element and Sr–Nd–Hf–Pb–Os isotope variations in two suites of lavas from the islands of Raivavae and Rapa from the Cook–Austral chain to better constrain the nature and origin of the source(s) of these lavas. Combined with data from other Cook–Austral islands and seamounts, the Raivavae and Rapa samples reveal the presence of three distinct components involved in the genesis of these lavas, a HIMU component, a depleted (or DM) component, and an “enriched” (EM) component. Northern Cook–Australs lavas (those north of the Austral Fracture Zone (AFZ)) falling along the “Macdonald” pseudo-age progression reflect mixing of the HIMU and DM components, whereas lavas south of the AFZ reflect mixing of the DM and EM components.Strong depletions in potassium and other fluid soluble elements in HIMU lavas suggest that the HIMU source contains ancient recycled (dehydrated) oceanic crust. The EM lavas have systematically less radiogenic Os-isotopes than the HIMU lavas. Therefore, the enriched or EM component cannot be generated through the addition of pelagic or terrigenous sediment to the HIMU source. Instead, the EM component in the southern Cook–Australs may represent recycled subduction-modified sub-arc mantle. High 208Pb/204Pb values in the DM component relative to East Pacific Rise mid-ocean ridge basalts (MORB) preclude derivation of this component from assimilated lithospheric mantle. The Sr–Nd–Pb isotope characteristics of the DM component resemble Pacific alkalic seamount lavas, and may derive from easily fusible components (e.g., pyroxenite veins) dispersed in the Pacific upper mantle.The Raivavae and Rapa lavas define a Hf–Nd isotope correlation that is considerably steeper than the mantle Hf–Nd isotope array. The unusually low εHf values (for a given εNd value) characteristic of the HIMU source are consistent with the presence of recycled oceanic crust. However, the presence of low εHf values in the Rapa EM lavas reveals that anomalously low εHf values are not restricted to the HIMU source. Regardless of the ultimate origin of the Raivavae and Rapa lavas, the comparatively low εHf values in these lavas indicate that multiple low-εHf components are present in the mantle. The addition of these components may help resolve the apparent terrestrial Hf–Nd mass-imbalance with respect to estimates of the bulk earth Nd- and Hf-isotope composition.Islands and seamounts from the Cook–Austral chain do not follow a clear age progression as predicted by the standard plume model for the origin of ocean island basalts (OIB). Previous models for the origin of the Cook–Austral chain have appealed to the existence of multiple small plumes. However, these models are implausible because they do not explain how several deep-seated plumes become aligned with each other in the direction of plate motion. We propose that volcanism along the Cook–Austral chain (and perhaps in much of French Polynesia) is generated through self-perpetuating melting anomalies that preferentially sample enriched components in a marble-cake mantle. These melting anomalies preferentially form or strengthen where lithospheric boundary conditions are favorable, such as at pre-existing transform faults or where the lithosphere has been thinned by a previous period of volcanism.

Origin of ocean island basalts: A new perspective from petrology, geochemistry, and mineral physics considerations

Consideration of petrology, geochemistry, and mineral physics suggests that ancient subducted oceanic crusts cannot be the source materials supplying ocean island basalts (OIB). Melting of oceanic crusts cannot produce high‐magnesian OIB lavas. Ancient oceanic crusts (>1 Ga) are isotopically too depleted to meet the required values of most OIB. Subducted oceanic crusts that have passed through subduction zone dehydration are likely to be depleted in water‐soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb) relative to water‐insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). Melting of residual crusts with such trace element composition cannot produce OIB. Oceanic crusts, if subducted into the lower mantle, will be >2% denser than the ambient mantle at shallow lower mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser than the ambient peridotitic mantle, perhaps by as much as ∼15%. Neither in the solid state nor in melt form can bulk oceanic crusts subducted into the lower mantle return to upper mantle source regions of oceanic basalts. Deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting volatiles and incompatible elements as a result of metasomatism taking place at the interface between the low‐velocity zone and the cooling and thickening oceanic lithosphere. These metasomatized and recycled deep portions of oceanic lithosphere are the most likely candidates for OIB sources in terms of petrology, geochemistry and mineral physics.

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Current models of ocean island basalt (OIB) Pb isotope systematics based on longterm isolation of recycled oceanic crust (with or without sediment) are not supported by solutions to both terrestrial Pb paradoxes. It follows that the linear arrays of OIB data in Pb isotope diagrams are mixing lines and have no age significance. A new model is presented that takes into account current solutions to both terrestrial Pb paradoxes and that explains combined Pb and He isotope evidence in terms of binary mixing. The key feature of this model is a two‐stage evolution: first, long‐term separation of depleted mantle from undepleted lowermost lower mantle. Mixing between these two reservoirs results in the wide spread in 207Pb/204Pb ratios and generally high (but variable) 3He/4He ratios that typify enriched mantle 1 (EM1) OIBs. The second stage involves metasomatism of depleted upper mantle by EM1 type, lowermost mantle‐derived melts. Evolution in the metasomatized environment is characterized by variable but generally high (Th+U)/(Pb+He) ratio that leads to a rapid increase in 208Pb/204Pb and 206Pb/204Pb ratios and decrease in 3He/4He. Mixing between depleted mantle and melts from metasomatized mantle portions reproduces the characteristics of high μ (HIMU) OIBs. The Sr versus Nd isotope array is compatible with binary mixing between depleted mantle and near‐chondritic lowermost mantle because of the large variation in Sr/Nd ratios observed in EM1 and HIMU OIBs. OIBs contaminated by subcontinental lithospheric mantle (EM2) exhibit more complex isotope systematics that mask their primary geochemical evolution.

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Origin of ocean island basalts: A new model based on lead and helium isotope systematics

Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts

Dehydration experiments on natural amphibolite have been carried out under upper mantle P/T conditions, in order to examine transportation of trace elements during dehydration processes in the subducted oceanic lithosphere. Pb, Nd, and Rb are more readily transported by aqueous fluids during amphibolite dehydration than U-Th, Sm, and Sr, respectively. The results indicates that the dehydration of subducted oceanic crust may result in large increases in U/Pb, Th/Pb and Sm/Nd ratios, and a decrease inRb/Sr ratios of subducted oceanic crust. This ultimately leads to higher Pb and Nd isotopic ratios, and lower Sr isotopic ratios in the subducted oceanic crust than the present MORB source mantle, given a sufficiently long periods of isolation in the mantle. It follows that the very high Pb isotopic ratios observed in some ocean island basalts, known as HIMU, can be readily achieved by incorporation of ancient subducted crust into their mantle source. However, Sr and Nd isotopic ratios cannot be explained by bulk mixture of the ancient subducted oceanic crust with depleted or primitive mantle, but require significant fractionation ofNd/Sr ratios in the subducted oceanic crust before mixing with mantle material. Possible processes to produce Sr and Nd isotopic compositions similar to that of HIMU may involve partial melting of recycled subducted basaltic crust under lower mantle conditions and refertilization of primitive mantle by the partial melt.

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.