Source Of Ocean Island Basalts Research Articles

The Chondritic Moon: a solution to the 142Nd conundrum and implications for terrestrial mantle evolution

AbstractRecent discoveries that the Earth has a supra-chondritic 142Nd signature have thrown chondritic geochemical Earth models into doubt. Several solutions have been proposed to explain this discrepancy but none has been widely accepted. This paper reviews Nd isotope data for the Moon which bridge the gap between the 142Nd signatures of chondritic meteorites and the accessible Earth. Different chondrite classes define a 142Nd–148Nd correlation line attributed to incomplete mixing of nucleosynthetic components in the solar nebula. Terrestrial standards have 142Nd signatures well above this correlation line, but the 142Nd signature of the Bulk Moon is c. 6 ppm lower than terrestrial (assuming a chondritic Sm/Nd ratio) and falls within error of enstatite chondrites. In view of the demonstrated isotopic similarity between the Earth and Moon, giant impact models require the Moon to be a sample of the early Earth. Therefore, it is inferred that the Earth–Moon system was generated from material similar to enstatite chondrites, but Earth's mantle experienced Sm/Nd fractionation very soon after the Moon-forming collision. Such fractionation processes have been attributed to subduction of early Fe-enriched crust into a deep mantle storage reservoir. Because Sm/Nd fractionation occurred when most 146Sm had already decayed, the hidden incompatible-element-enriched reservoir only became slightly depressed in its 142Nd signature, explaining why this signal has not yet been detected in ocean island basalt sources.

Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earth's mantle conditions

Halogens have, because of their volatile behavior and incompatibility, the potential to act as key tracers of volatile transport processes within the Earth's mantle. A better understanding of halogen behavior during partial melting processes will improve our understanding of volatile input mechanisms into the Earth's mantle and give insight into its evolution over the Earth's history. This study introduces time of flight secondary ion mass spectrometry (TOF-SIMS) as an analytical method for the determination of halogen partition coefficients and significantly extends the available dataset for fluorine and chlorine partitioning between mantle minerals and silicate melts to conditions of partial melting processes in Ocean Island Basalt (OIB) source regions.Halogen partitioning between olivine, orthopyroxene and silicate melt has been determined in experiments at 1.0–2.3GPa and 1350–1600°C. Combining our data with results of recent studies (O'Leary et al., 2010; Beyer et al., 2012; Dalou et al., 2012, 2014; Rosenthal et al., 2015) shows that fluorine and chlorine partitioning between olivine and melt increases by about 1.5–2 orders of magnitude between 1350°C and 1600°C (fluorine: 0.005(3)–0.31(16); chlorine: 0.005(45)–0.17(9)) and does not show any pressure dependence between 1.0 and 2.3GPa. Chlorine partitioning between orthopyroxene and melt increases by about 1 order of magnitude between 1450°C and 1600°C (0.015(8)–0.16(9)) at a constant pressure of 2.3GPa. Fluorine partitioning between orthopyroxene and melt increases by 1.5 orders of magnitude between 1250°C and 1600°C (0.029(6)–0.20(14)) and does not show any pressure dependence.Transmission electron microscopy measurements show that halogens are not incorporated in the form of humite-type defects in olivine. The most reasonable incorporation mechanism for halogens is via point defects in the olivine and orthopyroxene lattice, where they are inferred to be charge-balanced via oxygen defects.By combining our partition coefficients with natural halogen concentrations in oceanic basalts, we are able to give estimates for fluorine and chlorine abundances in Mid Ocean Ridge Basalt (MORB) (F=3–14; Cl=0.6–14ppm) and OIB (F=34–76; Cl=21–71ppm) mantle source regions. Comparing these with estimates of bulk silicate Earth (BSE) concentrations (F=18±8ppm, Lyubetskaya and Korenaga, 2007; F=25±10ppm, Palme and O'Neill, 2003; Cl=30±12ppm Palme and O'Neill, 2003) indicates that the upper mantle is degassed by 22–88% in fluorine and 22–99% in chlorine relative to the primitive mantle. The OIB source mantle region has a chlorine concentration that is similar to primitive mantle estimates, but is enriched in fluorine by a factor of 1.4–4.2 relative to the primitive mantle. An explanation for the relative fluorine enrichment in the OIB source region is that compared to chlorine, fluorine may be incorporated to a greater extent into the crystal structure of minerals that are stable at high P–T conditions and may thus be recycled more efficiently into the deeper mantle through subduction of oceanic crust.

The terrestrial uranium isotope cycle.

Changing conditions on the Earth's surface can have a remarkable influence on the composition of its overwhelmingly more massive interior. The global distribution of uranium is a notable example. In early Earth history, the continental crust was enriched in uranium. Yet after the initial rise in atmospheric oxygen, about 2.4billion years ago, the aqueous mobility of oxidized uranium resulted in its significant transport to the oceans and, ultimately, by means of subduction, back to the mantle. Here we explore the isotopic characteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct, with high (238)U/(235)U ratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts (MORBs) have (238)U/(235)U ratios higher than does the bulk Earth, confirming the widespread pollution of the upper mantle with this recycled uranium. Although many ocean island basalts (OIBs) are argued to contain a recycled component, their uranium isotopic compositions do not differ from those of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600 million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uranium in OIBs are strikingly consistent with previous OIB lead model ages, indicating that these mantle reservoirs formed between 2.4 and 1.8 billion years ago. In contrast, the uranium isotopic composition of MORB requires the convective stirring of recycled uranium throughout the upper mantle within the past 600 million years.

The recycling of marine carbonates and sources of HIMU and FOZO ocean island basalts

© 2014 Elsevier B.V. Many, and perhaps all, oceanic island basalts (OIB) clearly contain a component of crustal materials that have been returned to the mantle through subduction or other processes. One of the first recycled materials to be identified as a potential source of OIB was mid-ocean ridge basalt (MORB), and this was later fine-tuned as having a long time-integrated (b.y.) high U/Pb ratio or high μ (HIMU) and producing OIB with the most radiogenic Pb isotopic ratios (206Pb/204Pb>20). However, it is becoming more evident that the compositional connection between subducted MORB and HIMU basalts is problematic. As an alternative hypothesis, a small amount (a few %) of recycled Archaean marine carbonates (primarily CaCO3) is proposed to be the main source of the distinct 206Pb/204Pb, 207Pb/204Pb and 87Sr/86Sr isotopic and major-trace element compositions of classic HIMU and post-Archaean marine carbonates for younger HIMU or the so-called FOZO mantle source. As an extension of the hypothesis, a conceptual model that combines the separate evolutionary histories of ancient oceanic lithosphere, which is the source of OIB, and upper mantle, which is the source of MORB, is also proposed. The model claims that FOZO mainly consists of the lithospheric mantle portion of the ancient metamorphosed oceanic slabs that have accumulated in the deep mantle. Such an ultramafic source is geochemically depleted due to prior extraction of basaltic melt plus removal of the enriched subduction component from the slab through dehydration and metamorphic processes. Combined with other proposed models in the literature, the conceptual model can provide reasonable solutions for the 208Pb/204Pb, 143Nd/144Nd, 176Hf/177Hf, and 3He/4He isotopic paradoxes or complexities of oceanic lavas. Although these simultaneous solutions for individual paradoxes are qualitative and non-unique, these are unified under a single, marine carbonate recycling hypothesis.

The distribution of H2O between silicate melt and nominally anhydrous peridotite and the onset of hydrous melting in the deep upper mantle

The partitioning of H2O between a mantle peridotite assemblage and low degree hydrous melt has been investigated at 6 GPa (corresponding to ∼180 km depth) at a temperature of 1400 °C. Peridotite mineral phases were analysed from 6 melting experiments performed in a natural chemical system. The experiments contained ∼80 wt% of a low degree hydrous melt that was obtained through a series of experiments where the melt composition was iteratively adjusted until saturation with the appropriate peridotite assemblage was achieved. The melt is fluid-undersaturated at the conditions of the experiment. Ion microprobe measurements of the mineral phases indicate olivine H2O concentrations of 434±61 ppm wt and average clinopyroxene (cpx) concentrations of 1268±173 ppm wt H2O. Orthopyroxene (opx) and garnet contain 700±46 ppm wt and 347±83 ppm wt H2O, respectively. The H2O content of the hydrous melts was determined by mass balance to be 11±0.5 wt% H2O. H2O partition coefficients between minerals and melt (DH2Omin/melt=XH2Omin/XH2Omelt) are 0.0040±0.0006 for olivine, 0.0064±0.0004 for opx, 0.0115±0.0016 for cpx and 0.0032±0.0008 for garnet.Using the determined H2O partition coefficients the onset and extent of melting at conditions equivalent to 180 km below mid-ocean ridges was determined as a function of mantle H2O content. Current estimates for the H2O content of the depleted mantle (50–200 ppm wt H2O) are insufficient to induce mantle melting at this depth, which requires ∼700 ppm wt H2O to produce 0.1% melting and 1600 ppm wt H2O for 1% melting, along an adiabat with a potential temperature of 1327 °C. Melting can occur at these conditions within the mantle source of ocean island basalts, which are estimated to contain up to 900 ppm wt H2O. If adiabatic temperatures are 200 °C higher within such plume related sources, then melt fractions of over 1% can be reached at 180 km depth. In addition, a model for the distribution of H2O between peridotite mineral phases as a function of depth and at H2O-undersaturated conditions is constructed. The model indicates that for a fixed mantle composition containing 150 ppm wt H2O, the olivine H2O content will increase with depth solely due to changes in inter-phase partitioning and modal proportions of minerals. The change in the olivine H2O concentration with depth corresponds to proposed changes in the dominant olivine slip system for deformation by dislocation creep, that might provide an explanation for the reduction in seismic anisotropy observed at depths >200 km.

Simplified mantle architecture and distribution of radiogenic power

The mantle components that represent the source region of ocean island basalts (OIB) and feed hotspot volcanism are predicted to contain 160 ± 20 (2σm) ng/g Th, a heat‐producing element. This critical model composition indicates that the OIB source region (OSR) comprises a significant amount of recycled oceanic crust and constitutes 19+3−2 (2σm)% of the mantle by mass. The mass fraction of this reservoir supports a mantle architecture with a basal thermochemical layering at an average depth of 2000 ± 100 (2σm) km or two thermochemical piles that extend up to midmantle levels. The hotspot source described here generates 10 pW/kg of radiogenic heat and supplies 7.3 TW to the planet's total surface heat flux. Given that the silicate portion of the Earth produces some 20.4 TW of radiogenic power, with 7.2 TW derived from the continental crust, the mantle source responsible for mid‐ocean ridge volcanism provides only 5.9 TW of radiogenic power (or <2 pW/kg). As a result, the source of hotspots generates >5x more radiogenic heat than the source of mid‐ocean ridges, thus contributing to the energetics that drive mantle convection and potentially the formation of long‐lived plumes via bottom heating of the modern mantle. The potential for a sequestered or unsampled mantle reservoir would impact the relative mass fractions of the source regions of OIB and mid‐ocean ridge volcanism but not the compositional model of the OSR presented here.

Precise and accurate determination of boron concentration in silicate rocks by direct isotope dilution ICP-MS: Insights into the B budget of the mantle and B behavior in magmatic systems

We determine the bulk boron concentration of silicate rocks using a new method that does not require sample evaporation. After spiking (10B enriched NIST SRM 952) and HF attack, the rock solution is directly aspirated into the ICP-MS, and B concentration is determined by isotopic dilution. This technique is applied to samples with B contents in the range of 0.1–30ppm. The very low blank (<0.5ng B) makes this procedure particularly suitable for samples with low B content. Repeated analysis of the basaltic reference sample BHVO-2 yields a B concentration of 2.81±0.09ppm (2σ), giving a 3% relative error. This concentration, like those obtained for other basaltic and andesitic reference materials (BIR-1a, BCR-2, JB-1 and JA-1), agrees well with previously published values.The method is applied to 1) peridotite xenoliths from Mongolia and Russia, and 2) fresh lavas from Piton de la Fournaise, the active volcano on Réunion Island. We use the xenolith data to examine the behavior of boron during mantle melting and enrichment processes (i.e. metasomatism), and to estimate the primitive mantle B content (0.26±0.04ppm), while the lava data allows us to quantify B loss due to magmatic degassing (between 10 and 30%), using the composition of a primitive melt inclusion as the reference for an initial magma composition. Our knowledge of the degassing history of the samples allows us to quantify loss during lava emplacement and cooling (4–10%), continuous open-system degassing (10%) and shallow closed-system degassing (5–8%).Our results indicate that 1) B abundances of ocean island basalts are ca. 60–75% less than estimates based on elements with similar incompatibility (e.g., Nb and Ta), 2) this deficiency could reflect source depletion and/or magmatic degassing, and 3) magma degassing accounts for less than half of the inferred boron anomaly seen on primitive mantle-normalized patterns, implying that the mantle sources of ocean island basalts must be depleted in boron. The robust technique presented in this study could provide useful constraints on boron distribution within the Earth's mantle as well as on its behavior in magmatic systems.

Heterogeneities from the first 100 million years recorded in deep mantle noble gases from the Northern Lau Back-arc Basin

Heavy noble gases can record long-lasting heterogeneities in the mantle, because Ne, Ar, and Xe isotopes are produced from extant (U, Th, K) and extinct (129I and 244Pu) radionuclides. However, the presence of ubiquitous atmospheric contamination in basalts, particularly for ocean island basalts (OIBs) that sample the Earth's deep mantle, have largely hampered precise characterization of the mantle source compositions. Here we present new high-precision noble gas data from gas-rich basalts erupted along the Rochambeau Rift (RR) in the northwestern corner of the Lau Basin. The strong influence of a deep mantle plume in the Rochambeau source is apparent from low 4He/3He ratios down to 25,600 (3He/4He of 28.1RA).We find that the Rochambeau source is characterized by low ratios of radiogenic to non-radiogenic nuclides of Ne, Ar, and Xe (i.e., low 21Ne/22Ne, 40Ar/36Ar, and 129Xe/130Xe) compared to the mantle source of mid-ocean ridge basalts (MORBs). High-precision xenon isotopic measurements indicate that the lower 129Xe/130Xe ratios in the Rochambeau source cannot be explained solely by mixing atmospheric xenon with MORB-type xenon; nor can fission-produced Xe be added to MORB Xe to produce the compositions seen in the Rochambeau basalts. Deconvolution of fissiogenic xenon isotopes demonstrate a higher proportion of Pu- fission derived Xe in the Rochambeau source compared to the MORB source. Therefore, both I/Xe and Pu/Xe ratios are different between OIB and MORB sources. Our observations require heterogeneous volatile accretion and a lower degree of processing for the plume source compared to the MORB source. Since differences in 129Xe/130Xe ratios have to be produced while 129I is still alive, OIB and MORB sources must have been processed at different rates for the first 100 million years (Myr) of Solar System history, and subsequent to this period, the two reservoirs have not been homogenized.In combination with recent results from the Iceland plume, our noble gas observations require the formation and preservation of less-degassed, early-formed (pre-4.45Ga) heterogeneities in the Earth's deep mantle. Consequently, the primitive noble gas reservoir sampled by mantle plumes cannot be created solely through sequestration of recycled slabs or undegassed melts at the base of the mantle during the past 4.4Ga. Finally, if the more primitive, less degassed heterogeneities reside in the Large Low Shear Wave Velocity Provinces (LLSVPs), then LLSVPs must be long-lasting features of the deep mantle and are not composed exclusively of recycled material.

Heterogeneous upper mantle Ne, Ar and Xe isotopic compositions and a possible Dupal noble gas signature recorded in basalts from the Southwest Indian Ridge

Variations in heavy noble gas (Ne, Ar, Xe) isotopic compositions provide unique insights into the nature of heterogeneities in the mantle. However, few precise constraints on mantle source heavy noble gas isotopic compositions are available due to ubiquitous shallow-level atmospheric contamination. As a result, the extent of heterogeneity in mid-ocean ridge basalt (MORB) mantle source Ne, Ar and Xe isotopic compositions is unknown. Basalts from the ultra-slow spreading Southwest Indian Ridge (SWIR) between 7°E and 25°E exhibit remarkable variability in He isotopic composition: SWIR 4He/3He spans half the total range observed in all mantle-derived basalts. Therefore, basalts from the SWIR provide a unique window into upper mantle heterogeneity and present an ideal opportunity to characterize variations in upper mantle heavy noble gas isotopic composition. Here we present new high-precision Ne, Ar and Xe isotopic compositions as well as He, CO2, Ne, Ar and Xe abundances measured in basalt glasses from the SWIR. After correcting the measured values for shallow-level atmospheric contamination, significant and systematic variations in mantle source Ne, Ar and Xe compositions are observed. We note that large variations in source 40Ar/36Ar and 129Xe/130Xe are observed in basalts removed from the influence of known hotspots, indicating a heterogeneous mid-ocean ridge basalt source. Thus, SWIR heavy noble gas data reveal a greater degree of source heterogeneity than is evident in the 4He/3He systematics alone. The observed heavy noble gas isotopic heterogeneities imply that the average MORB source 40Ar/36Ar and 129Xe/130Xe ratios are not yet well-determined.Variation in MORB source 40Ar/36Ar and 129Xe/130Xe at a given 4He/3He and 21Ne/22Ne may reflect heterogeneous recycling of atmospheric Ar and Xe. In particular, we find low mantle source 40Ar/36Ar and 129Xe/130Xe ratios in the eastern region of the study area, which may reflect the noble gas signature of the Dupal mantle domain. Our observations require that the sampled mantle domain either is very ancient (>4.45Ga) or has been metasomatized by subduction zone fluids carrying recycled atmospheric Ar and Xe. However, our Xe isotopic measurements indicate that differences between MORB and ocean island basalt (OIB) source noble gas compositions cannot be explained by recycling of atmospheric noble gases alone. Instead, a relatively undegassed mantle reservoir is required to account for OIB noble gases. The SWIR data demonstrate that the reservoir supplying primordial noble gases to mantle plumes differentiated from the MORB source early in Earth history, and the two reservoirs have not been homogenized over 4.45Ga of mantle convection.

A trace element perspective on the source of ocean island basalts (OIB) and fate of subducted ocean crust (SOC) and mantle lithosphere (SML)

We analyze the first-order observations, basic concepts and explicit/implicit assumptions built into the three major hypotheses for the enriched component(s) in the source of ocean island basalts (OIB) in terms of incompatible trace elements: (1) subducted ocean crust (SOC), (2) subducted continental sediments, and (3) mantle metasomatism. SOC is compositionally too depleted (i.e., [La/Sm] N <1) to be the major source material for OIB that are highly incompatible element enriched (e.g., [La/Sm] N »1). We cannot rule out the contribution of continental sediments as an enriched OIB source component; however, except for two known cases that are yet to be further investigated, there is no convincing evidence for any significant sediment contribution to the petrogenesis of global OIB. Continental materials through subduction erosion can certainly contribute to mantle compositional heterogeneity and may contribute towards some enriched component in OIB source regions. Overall, OIB are not only enriched in incompatible elements, but also enriched in the progressively more incompatible elements, with their inferred source material being variably more enriched than the primitive mantle. These observations require that the OIB sources are pre-enriched by lowdegree (Low-F) melt metasomatism. The interface between the growing oceanic lithosphere and the top of the seismic low velocity zone (LVZ) represents a natural peridotite solidus and is the ideal site for major low-F melt induced metasomatism. The ∼ 70 Myr history of oceanic lithosphere growth to its full thickness of ∼ 90 km records the history of mantle metasomatism, resulting in the deep portion of the oceanic lithosphere being an important enriched geochemical reservoir. We argue that ancient subducted metasomatized mantle lithosphere (SML) provides the major source component for OIB. The metasomatic agent is an H 2 O-CO 2 -rich silicate melt derived from within the LVZ. Upward migration and concentration of the melt at the lithosphere-LVZ interface (i.e., the lithosphere-asthenosphere boundary or LAB) results in chemical stratification in the LVZ with the deeper portion being more depleted (i.e., DMM), providing the source for MORB. The widespread metasomatized peridotites, pyroxenites and hornblendites from xenolith suites exhumed from the deep lithosphere (both oceanic and continental) and in orogenic peridotite massifs confirm the role of a low-F silicate melt phase as the metasomatic agent. The SOC, if subducted into the lower mantle, will be too dense to return in bulk to the upper mantle source regions of oceanic basalts, and may have contributed to the two large low shear wave velocity provinces (LLSVPs) at the base of the mantle beneath the Pacific and Africa over Earth's history.

Early differentiation and volatile accretion recorded in deep-mantle neon and xenon

The isotopes (129)Xe, produced from the radioactive decay of extinct (129)I, and (136)Xe, produced from extinct (244)Pu and extant (238)U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon ((129)Xe/(130)Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir. However, the low (129)Xe/(130)Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and (20)Ne/(22)Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower (129)Xe/(130)Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because (129)I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth's mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation.

Crust recycling in the sources of two parallel volcanic chains in Shandong, North China

Recycled crustal components have been documented for many sources of hotspot-related ocean island basalts, but they have been difficult to identify in continental basalts, because in continental settings, hotspots are often obscured and recycled crustal sources are difficult to distinguish from crustal contamination. We show major, trace element and Sr–Nd–Hf isotopic compositions for two parallel chains of Cenozoic volcanoes from Shandong Province, North China, which are free of crustal contamination and show clear evidence for recycling of mafic lower crust. Sr, Nd, and Hf isotopes in the two volcanic chains form separate binary mixing arrays, which converge on the composition of Dashan, an isolated, nephelinitic volcano with the most depleted isotopic signature. The two chains have lower CaO values and significantly diverging isotope enrichment trends from this common endmember. Both trends deviate from the normal Sr–Nd and Hf–Nd mantle array toward lower 87Sr/ 86Sr and higher ε Hf values, all features that point to a (recycled) eclogitic source. We invoke a two-stage evolution model to generate the endmembers of these two mixing trends. In the first stage, recycled mafic crust (eclogite) is depleted by earlier (late Mesozoic) melt extraction, which elevates the Lu/Hf of the residue relative to Sm/Nd due to garnet control during melting. Subsequently, these silica-deficient residues are transported to the deeper mantle. Finally, in Cenozoic time, upwelling mantle (possibly a plume) transports lenses of residual eclogites into the shallow asthenosphere. The recycled crustal components beneath the two chains differ somewhat in isotopic composition due to different degree of the earlier melting. The upwelling mantle spreads beneath the lithosphere and flows toward regions of thinned lithosphere, e.g. the Tan–Lu Fault Zone in North China, where the recycled crust undergo remelting and mix with peridotite-derived melts to produce the two mixing trends observed.

Mafic dykes at the southwestern margin of Eastern Ghats belt: Evidence of rifting and collision

The southwestern margin of the Eastern Ghats Belt characteristically exposes mafic dykes intruding massif-type charnockites. Dykes of olivine basalt of alkaline composition have characteristic trace element signatures comparable with Ocean Island Basalt (OIB). Most importantly strong positive Nb anomaly and low values of Zr/Nb ratio are consistent with OIB source of the mafic dykes. K-Ar isotopic data indicate two cooling ages at 740 and 530 Ma. The Pan-African thermal event could be related to reactivation of major shear zones and represented by leuco-granite vein along minor shear bands. And 740 Ma cooling age may indicate the low grade metamorphic imprints, noted in some of the dykes. Although no intrusion age could be determined from the present dataset, it could be constrained by some age data of the host charnockite gneiss and Alkaline rocks of the adjacent Prakasam Province. Assuming an intrusion age of ∼1.3Ga, Sr-Nd isotopic composition of the dykes indicate that they preserved time-integrated LREE enrichment. In view of the chemical signatures of OIB source, the mafic dykes could as well be related to continental rifting, around 1.3Ga, which may have been initiated by intra-plate volcanism.

Emeishan large igneous province (SW China) and the mantle-plume up-doming hypothesis

Abstract: The Middle Permian ( c . 262 Ma) Emeishan Basalt Formation of SW China is a commonly cited example of a large igneous province (LIP) that formed as a result of a deep-mantle plume impinging on the base of the lithosphere and generating large regional-scale up-doming prior to volcanism. Recently, however, this assertion has been challenged on the grounds that some lava flows close to the centre of the LIP were erupted in a submarine setting. Here we analyse all the available biolithostratigraphical, petrological, geochemical and volcanological information and show that this supports the idea that the terrain was generated by a plume that originated in the mantle. Emeishan basalt magma melted from a hot mantle source with trace element contents that are similar to the source of Icelandic and intraplate ocean island basalts. However, the amount and lateral extent of uplift is significantly less than is predicted by conventional deep-mantle plume models. We conclude that large-scale doming is not a diagnostic feature of mantle plumes; surface topography can be greatly influenced by the type of lower mantle plume (thermal or thermochemical), how it passes through the transition zone, and how it interacts with the lithosphere.

Formation of enriched mantle components by recycling of upper and lower continental crust

The origin of enriched mantle (EM) sources remains an unsolved problem for constraining the composition and chemical evolution of the Earth's mantle, because a wealth of different, often mutually exclusive models has been suggested. To address this predicament and to re-investigate the origin of EM sources on a global scale, this study is based on combined chemical and isotopic literature data for more than 530 samples from 16 key locations from worldwide ocean islands. The combined Sr, Nd, Pb isotope and trace element systematics of global ocean island basalts suggest that each EM source contains a unique enriched additive. Systematic variations between Th/Nb, K/La, Rb/La, and Ce/Pb ratios and 87Sr/ 86Sr ratios in all EM basalts suggest that all EM-type end-members share a common heritage from the continental crust. The observed coupling of relative Eu enrichments or deficits with 87Sr/ 86Sr isotope ratios further indicates that the inferred compositional differences of EM-type sources are caused by the addition of different proportions of lower and upper continental crust. Recycling of marine sediment and oceanic lithosphere in subduction zones accounts for the isotopic and chemical composition of EM sources with high 87Sr/ 86Sr and relatively constant 206Pb/ 204Pb ratios (e.g. Samoa), which have a high affinity for the upper continental crust. Sources with a coupled 87Sr/ 86Sr– 206Pb/ 204Pb isotope evolution that extend to low 206Pb/ 204Pb but less radiogenic 87Sr/ 86Sr ratios (e.g. Pitcairn) are dominated by lower continental crust. Transfer of the lower continental crust into the mantle can occur either by subduction erosion or by crustal delamination. Here we propose that one common process, the recycling of upper and lower continental crust and oceanic lithosphere at destructive plate margins and their subsequent re-melting as part of the mantle sources of ocean island basalts, can account for the entire range of chemical and isotopic signatures in EM-type oceanic basalts. This implies that the compositional heterogeneity in the Earth's mantle is induced by, and intrinsic to the recycling process and not principally dependent on intra-mantle stirring of a limited number of originally distinct and physically separate mantle reservoirs.

Noble gases in the dynamic mantle

It is proposed that noble gases in the mantle reside mainly in a “hybrid pyroxenite” component that is formed as melt from old subducted oceanic crust reacts with surrounding refractory peridotite under mid‐ocean ridges. The gases are presumed to have been acquired early in Earth history, and mixing under mid‐ocean ridges of melts from pyroxenites and old crust would pass the noble gases to successive generations of pyroxenites. Much of the complement of noble gases would not be erupted at mid‐ocean ridges and would recirculate in the mantle, so the hybrid pyroxenite would degas rather slowly over Earth history. The hybrid pyroxenite would also be denser than average mantle and would tend to settle into the D″ zone at the base of the mantle, in the same way as subducted oceanic crust. Because residence times in D″ are longer than in the rest of the mantle, the D″ pyroxenite would be less degassed and its noble gases would be less radiogenic than that in the mid‐ocean ridge basalt (MORB) source. Plumes could therefore tap a mixture of old, degassed oceanic crust and less degassed hybrid pyroxenite. This could resolve the long‐standing question of the source of unradiogenic helium in many ocean island basalts (OIBs). Abundances of noble gases are not very well constrained and are plausibly larger than conventional estimates, which would remove the need for a large, deep, “undegassed” reservoir. The 40Ar mass balance may be satisfied if account is taken of uncertainties in the potassium content of the continental crust and the bulk silicate Earth and in the Earth's total budget of 40Ar. A relatively simple quantitative theory for the evolution of the noble gases is presented that accounts for the present concentrations and isotopic compositions of helium, neon, and argon in the MORB and OIB source regions. In particular, it confirms that longer residence times in D″ can account for the less radiogenic noble gases in OIBs.

Major element chemistry of ocean island basalts — Conditions of mantle melting and heterogeneity of mantle source

We estimate average compositions of near-primary, ‘reference’ ocean island basalts (OIBs) for 120 volcanic centers from 31 major island groups and constrain the depth of lithosphere–asthenosphere boundary (LAB) at the time of volcanism and the possible depth of melt–mantle equilibration based on recently calibrated melt silica activity barometer. The LAB depth versus fractionation corrected OIB compositions (lava compositions, X, corrected to Mg# 73, X OIB #73, i.e., magmas in equilibrium with Fo 90, if olivine is present in the mantle source) show an increased major element compositional variability with increasing LAB depths. OIBs erupted on lithospheres < 40 km thick approach the compositions (e.g. SiO 2 #73, TiO 2 #73, [CaO/Al 2O 3] #73) of primitive ridge basalts and are influenced strongly by depth and extent of shallow melting. However, X OIB #73 on thicker lithospheres cannot be explained by melt–mantle equilibration as shallow as LAB. Melt generation from a somewhat deeper (up to 50 km deeper than the LAB) peridotite source can explain the OIB major element chemistry on lithospheres ≤ 70 km. However, deeper melting of volatile-free, fertile peridotite is not sufficient to explain the end member primary OIBs on ≥ 70 km thick lithospheres. Comparison between X OIB #73 and experimental partial melts of fertile peridotite indicates that at least two additional melt components need to be derived from OIB source regions. The first component, similar to that identified in HIMU lavas, is characterized by low SiO 2 #73, Al 2O 3 #73, [Na 2O/TiO 2] #73, and high FeO⁎ #73, CaO #73, [CaO/Al 2O 3] #73. The second component, similar to that found in Hawaiian Koolau lavas, is characterized by high SiO 2 #73, moderately high FeO⁎ #73, and low CaO #73 and Al 2O 3 #73. These two components are not evenly sampled by all the islands, suggesting a heterogeneous distribution of mantle components that generate them. We suggest that carbonated eclogite and volatile-free, silica-excess eclogite are the two most likely candidates, which in conjunction with fertile mantle peridotite, give rise to the two primitive OIB end members.

Some basic concepts and problems on the petrogenesis of intra-plate ocean island basalts

Basaltic magmatism that builds intra-plate ocean islands is often considered to be genetically associated with “hotspots” or “mantle plumes”. While there have been many discussions on why ocean island basalts (OIB) are geochemically highly enriched as an integral part of the mantle plume hypothesis, our current understanding on the origin of OIB source material remains unsatisfactory, and some prevailing ideas need revision. One of the most popular views states that OIB source material is recycled oceanic crust (ROC). Among many problems with the ROC model, the ocean crust is simply too depleted (e.g., [La/Sm]PM <1) to be source material for highly enriched (e.g., [La/Sm]PM ≫ 1) OIB. Another popular view states that the enriched component of OIB comes from recycled continental crust (RCC, i.e.; terrigenous sediments). While both CC and OIB are enriched in many incompatible elements (e.g., both have [La/Sm]PM ≫1), the CC has characteristic enrichment in Pb and deletion in Nb, Ta, P and Ti. Such signature is too strong to be eliminated such that CC is unsuitable as source material for OIB. Plate tectonics and mantle circulation permit the presence of ROC and RCC materials in mantle source regions of basalts, but they must be volumetrically insignificant in contributing to basalt magmatism. The observation that OIB are not only enriched in incompatible elements, but also enriched in the progressively more incompatible elements indicates that the enriched component of OIB is of magmatic origin and most likely associated with low-degree melt metasomatism. H2O and CO2 rich incipient melt may form in the seismic low velocity zone (LVZ). This melt will rise because of buoyancy and concentrate into a melt rich layer atop the LVZ to metasomatize the growing lithosphere, forming the metasomatic vein lithologies. Erupted OIB melts may have three components: (1) fertile OIB source material from depth that is dominant, (2) the melt layer, and (3) assimilation of the metasomatic vein lithologies formed earlier in the growing/grown lithosphere. It is probable that the fertile source material from depth may be (or contain) recycled ancient metasomatized deep portions of oceanic lithosphere. In any attempt to explain the origin of mantle isotopic end-members as revealed from global OIB data, we must (1) remember our original assumptions that the primitive mantle (PM) soon after the core separation was compositionally uniform/homogeneous with the core playing a limited or no role in causing mantle isotopic heterogeneity; (2) not use OIB isotopes to conclude about the nature and compositions of ultimate source materials without understanding geochemical consequences of subduction zone metamorphism; and (3) ensure that models and hypotheses are consistent with the basic petrology and major/trace element geochemistry.

Tungsten isotopes as tracers of core–mantle interactions: The influence of subducted sediments

The postulated difference in W isotopic composition of the Earth’s core of ∼2 ε W units, compared to the bulk silicate earth (BSE) has previously been used to search for evidence of core–mantle interaction (CMI) in ocean island basalts (OIB). The absence of W isotope anomalies has thus been taken as evidence that CMI does not occur. However, the addition of subducted sediment with high W to the sources of OIB could obscure a core signature. This possibility brings into question the utility of W isotopes as tracers for CMI. To accurately consider the effects of sediment addition to mantle sources of OIB with respect to W requires improved constraints on the abundances of W in subducting sediment. Here, we present high-precision W abundance data (and other HFSE) for a suite of sediments from the Banda subduction regime in East Indonesia. Subducting East Indonesian sediments have trace element concentrations that resemble those of average upper continental crust (UCC), making these sediments valuable to consider as typical of subducted sediments. Average W abundances of 2.1 ppm, corrected for carbon content coupled with current models of 0.5% core addition and 1% sediment addition to EM1 or HIMU plume, suggest that a model hybrid source should exhibit values of ε W = −0.24 with ∼25 ppb W. Prior studies have not reported such low W isotopic compositions or high estimated W concentrations present in the sources of either Hawaiian or French Polynesian lavas, so such large additions of core material to these plume sources seems unlikely. Given these constraints, core contributions to these source, if present, can be no more than ∼0.1%.

Subduction-related Volatile Recycling and Magma Generation beneath Central Mexico: Insights from Melt Inclusions, Oxygen Isotopes and Geodynamic Models

The subduction-related Michoaca¤n^Guanajuato Volcanic Field (MGVF) in central Mexico contains 900 cinder cones and numerous larger shield volcanoes of Late Pliocene to Holocene age.We present data for major, trace and volatile (H2O, CO2, S, Cl) elements in olivine-hosted melt inclusions from eight calc-alkaline cinder cones with primitive magma characteristics and one more evolved alkali basalt tuff ring.The samples span a region extending from the volcanic front to 175 km behind the front. Relationships between H2O and incompatible trace elements are used to estimate magmatic H2O contents for 269 additional volcanic centers across the MGVF and central Mexico. The results show that magmatic H2O remains high (3^575 wt %) for large distances (150 km) behind the front. Chlorine and S concentrations are strongly correlated with melt H2O and are also high across most of the arc (700^1350 ppm Cl, 1500^2000 ppm S).The alkali basalt, located far behind the front (175 km), has much lower volatile contents (515wt%H2O, 200 ppm Cl, 500 ppm S), and is compositionally similar to other melts erupted in this region. Oxygen isotope ratios of olivine phenocrysts (56^6o) from the calc-alkaline samples are higher than for typical mantle-derived magmas but do not vary systematically across the arc. Calc-alkaline samples have high large ion lithophile element concentrations relative to Nb andTa, as is typical of subduction-related magmas, but alkali basalt samples far behind the front have high Nb and Ta and lack enrichments in fluidmobile elements. Modeling based on volatiles and trace elements suggests that the calc-alkaline magmas were generated by 6^15% partial melting of a variably depleted mantle wedge that was fluxed with H2O-rich components from the subducted slab. In contrast, the alkali basalts formed by small degrees of decompression melting of an ocean island basalt source that had not been fluxed by slabderived components. Based on high d18Oolivine values and trace element characteristics, the H2O-rich subduction components added to the mantle wedge beneath the MGVF are likely to be mixtures of oceanic crust derived fluids and sediment melts. Integrating these results with new 2-D thermo-mechanical models of the subduction zone beneath the MGVF, we demonstrate that the present-day plate configuration beneath theMGVFcauses fluids to be released beneath the forearc and volcanic front, and that sediment melts can be produced beneath the volcanic front by the waning stages of fluid released from the oceanic crust percolating through already dehydrated sediments. Down-dragging of serpentine- and chloritebearing peridotite in the lowermost mantle wedge probably plays a role in fluid transport from the forearc to beneath the arc. H2O-rich magmas located more than 50 km behind the volcanic front can be explained by mantle hydration related to a shallower slab geometry that existed at 3 Ma. Rollback of the slab over the last 2 Myr has resulted in strong mantle advection that forms low-H2O, high- Nb alkali basaltic magmas by decompression melting far behind the present-day volcanic front.