A younger and protracted emplacement of the Ontong Java Plateau

The submarine volcanic emplacement of the Ontong Java Plateau (OJP) is the suggested cause of Ocean Anoxic Event 1a (OAE 1a). However, no precise timing and duration exists for the formation of OJP, and its connection to OAE1a relies mainly on proxies in the sedimentary record. We provide high-precision ⁴⁰Ar/³⁹Ar data from OJP drill and dredge sites that considerably improve OJP's eruptive history. The ages determined from this work are as much as 10 million years younger than previous dates and indicate a protracted formation over at least 6 million years. OJP now appears too young to have caused OAE1a, but we suggest that it may have had a role in the later OAE1b. The protracted eruptive sequence has implications for the emplacement dynamics of OJP and other large igneous provinces.

New evidence for the Ontong Java Nui hypothesis

The formation of the Ontong Java Nui super oceanic plateau (OJN), which is based on the model that the submarine Ontong Java Plateau (OJP), Manihiki Plateau (MP), and Hikurangi Plateau (HP) were once its contiguous fragments, could have been the largest globally consequential volcanic event in Earth's history. This OJN hypothesis has been debated given the paucity of evidence, for example, the differences in crustal thickness, the compositional gap between MP and OJP basalts and the apparent older age of both plateaus relative to HP remain unresolved. Here we investigate the geochemical and ⁴⁰Ar-³⁹Ar ages of dredged rocks recovered from the OJP's eastern margin. Volcanic rocks having compositions that match the low-Ti MP basalts are reported for the first time on the OJP and new ~ 96-116 Ma and 67-68 Ma ⁴⁰Ar-³⁹Ar age data bridge the temporal gap between OJP and HP. These results provide new evidence for the Ontong Java Nui hypothesis and a framework for an integrated tectonomagmatic evolution of the OJP, MP, and HP. The isotopic data imply four mantle components in the source of OJN that are also expressed in present-day Pacific hotspots sources, indicating origin from (and longevity of) the Pacific Large Low Shear-wave Velocity Province.

Early Cretaceous Plume–Ridge Interaction Recorded in the Band-e-Zeyarat Ophiolite (North Makran, Iran): New Constraints from Petrological, Mineral Chemistry, and Geochronological Data

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and Neo-Tethyan oceanic tectonic units. Among these, the Band-e-Zeyarat ophiolite consists of (from bottom to top): ultramafic cumulates, layered gabbros, isotropic gabbros, a sheeted dyke complex, and a volcanic sequence. Sheeted dykes and volcanic rocks are mainly represented by basalts and minor andesites and rhyolites showing either normal-type (N) or enriched-type (E) mid-ocean ridge basalt affinities (MORB). These conclusions are also supported by mineral chemistry data. In addition, E-MORBs can be subdivided in distinct subtypes based on slightly different but significant light rare earth elements, Th, Nb, TiO2, and Ta contents. These chemical differences point out for different partial melting conditions of their mantle sources, in terms of source composition, partial melting degrees, and melting depths. U-Pb geochronological data on zircons from intrusive rocks gave ages ranging from 122 to 129 Ma. We suggest that the Band-e-Zeyarat ophiolite represents an Early Cretaceous chemical composite oceanic crust formed in a mid-ocean ridge setting by partial melting of a depleted suboceanic mantle variably metasomatized by plume-type components. This ophiolite records, therefore, an Early Cretaceous plume–ridge interaction in the Makran Neo-Tethys.

Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia

The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO2-undersaturated or SiO2-saturated series rocks, with the former having higher TiO2 and FeOT contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeOT contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al2O3 contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 °C (average 1557 ± 43 °C, 1σ), and pressures of 2.9–5.1 GPa (average 3.8 ± 0.7 GPa), with the SiO2-undersaturated series forming at higher melting temperatures and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere.

Cryptic evolved melts beneath monotonous basaltic shield volcanoes in the Galápagos Archipelago

Many volcanoes erupt compositionally homogeneous magmas over timescales ranging from decades to millennia. This monotonous activity is thought to reflect a high degree of chemical homogeneity in their magmatic systems, leading to predictable eruptive behaviour. We combine petrological analyses of erupted crystals with new thermodynamic models to characterise the diversity of melts in magmatic systems beneath monotonous shield volcanoes in the Galápagos Archipelago (Wolf and Fernandina). In contrast with the uniform basaltic magmas erupted at the surface over long timescales, we find that the sub-volcanic systems contain extreme heterogeneity, with melts extending to rhyolitic compositions. Evolved melts are in low abundance and large volumes of basalt flushing through the crust from depth overprint their chemical signatures. This process will only maintain monotonous activity while the volume of melt entering the crust is high, raising the possibility of transitions to more silicic activity given a decrease in the crustal melt flux.

In this study the authors show that monotonous basaltic volcanoes can host a range of melts in their sub-volcanic systems, extending to rhyolitic compositions. The study implies that volcanoes which have produced monotonous basaltic lavas on long timescales could transition to more explosive, silica-rich eruptions in the future.

High mantle seismic P-wave speeds as a signature for gravitational spreading of superplumes

New passive- and active-source seismic experiments reveal unusually high mantle P-wave speeds that extend beneath the remnants of the world's largest known large igneous province, making up the 120-million-year-old Ontong-Java-Manihiki-Hikurangi Plateau. Sub-Moho Pn phases of ~8.8 ± 0.2 km/s are resolved with negligible azimuthal seismic anisotropy, but with strong radial anisotropy (~10%), characteristic of aggregates of olivine with an AG crystallographic fabric. These seismic results are the first in situ evidence for this fabric in the upper mantle. We show that its presence can be explained by isotropic horizontal dilation and vertical flattening due to late-stage gravitational collapse and spreading in the top 10 to 20 km of a depleted, mushroom-shaped, superplume head on a horizontal length scale of 1000 km or more. This way, it provides a seismic tool to track plumes long after the thermal effects have ceased.

Haida Gwaii (British Columbia, Canada): a Phanerozoic analogue of a subduction-unrelated Archean greenstone belt

Understanding the formation and evolution of Precambrian greenstone belts is hampered by gaps in the rock record and the uncertainty of the tectonic regime that was operating at the time. Thus identifying a modern analogue of a Precambrian greenstone belt can be problematic. In this paper we present geological, geochemical and petrological evidence outlining the case for Haida Gwaii (British Columbia, Canada) as a modern example of a greenstone belt. Haida Gwaii is comprised of two rift-related volcano-sedimentary sequences. The older (Early Triassic) Karmutsen volcanic sequence consists of subaqueous ultramafic-mafic volcanic rocks that are capped by marine carbonate and siliciclastic rocks. The younger (Paleogene) Masset bimodal volcanic sequence consists of tholeiitic and calc-alkaline basalt along with calc-alkaline silicic volcanic and intrusive rocks that are capped by epiclastic sandstones. The Karmutsen and Masset volcanic rocks have indistinguishable Sr-Nd-Pb isotopes demonstrating they were derived from a similar mantle source. Some of the Masset calc-alkaline rocks are compositionally similar to magnesian andesites (SiO2 = 56-64 wt%; Mg# = 0.50-0.64) that are typical of subduction-related Archean greenstone belts. We show that the calc-alkaline signature observed in the bimodal sequence of the Masset Formation is likely due to fractional crystallization of a tholeiitic parental magma under relatively oxidizing (ΔFMQ + 0.7) conditions indicating that a calc-alkaline signature is not prima facie evidence of a subduction setting. Given the geological and geochemical evidence, Haida Gwaii represents one of the best analogues of a modern subduction-unrelated Archean greenstone belt.

Interactions of the Greater Ontong Java mantle plume component with the Osbourn Trough

The Ontong Java-Manihiki-Hikurangi plateau (OJMHP) is considered to have originated from a starting mantle plume, and have been rifted apart by two spreading ridges. However, the ages of these spreading ridges and their possible interactions with the presumed mantle plume are unclear. The Manihiki-Hikurangi plateau has been rifted apart by the Osbourn Trough which formed the southwestern Pacific crust to the east of the Tonga-Kermadec trench. Here we report Pb-Hf-Os isotopes of the basaltic crust (Site U1365 of IODP Expedition 329) formed by the Osbourn Trough. Linear regression of Re-Os isotopes results in an age of 103.7 ± 2.3 Ma for Site U1365 basalts, indicating that the Manihiki-Hikurangi plateau was rifted apart by the Osbourn Trough with a spreading rate of ~190 mm/yr. The superfast spreading rate supports the Osbourn as an abandoned segment of the early Pacific spreading ridge, which initially overlapped with the giant starting plume. Moreover, the Pb-Hf isotopes of some of Site U1365 basalts show distinct differences from those of the Pacific mid-ocean ridge basalts, while they are similar to the basalts of the Ontong Java and Manihiki plateaus. We suggest that the OJMHP mantle plume components has been involved by the Osbourn spreading center.

How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot

Increasingly, spatial geochemical zonation, present as geographically distinct, subparallel trends, is observed along hotspot tracks, such as Hawaii and the Galapagos. The origin of this zonation is currently unclear. Recently zonation was found along the last ∼70 Myr of the Tristan-Gough hotspot track. Here we present new Sr–Nd–Pb–Hf isotope data from the older parts of this hotspot track (Walvis Ridge and Rio Grande Rise) and re-evaluate published data from the Etendeka and Parana flood basalts erupted at the initiation of the hotspot track. We show that only the enriched Gough, but not the less-enriched Tristan, component is present in the earlier (70–132 Ma) history of the hotspot. Here we present a model that can explain the temporal evolution and origin of plume zonation for both the Tristan-Gough and Hawaiian hotspots, two end member types of zoned plumes, through processes taking place in the plume sources at the base of the lower mantle.

Striped geochemical zonation has been observed along parts of hotspot tracks, although its origin is not well-understood. Here, the authors present Sr–Nd–Pb–Hf isotope data and present a model that can explain the evolution of zonation in both Tristan-Gough and Hawaiian hotspots, reflecting two end members.

Motion of the Ontong Java Plateau in the hot-spot frame of reference: 122 Ma-present

A new model of Pacific absolute plate motion between 140 and 0 Ma, generated in the fixed hot-spot frame of reference, has been used to track palaeogeographic positions of the Ontong Java Plateau (OJP) from the time (c. 122 Ma) and location (c. 43°S) of its formation to its present location north of the Solomon Islands. The resulting OJP seafloor flow-line suggests that changes in Pacific plate motion, passage over hot spots and Pacific Rim tectonism all have influenced the continuing structural development and deformation of the plateau. Satellite-derived gravity, bathymetry and Rayleigh-wave tomography potentially reveal the structural fabric of the OJP and adjoining Nauru Basin, including the orientation of probable fracture zones, location of possible relict spreading centres and the presence of a thick lithospheric root, as well as possible later hot-spot-related modification of the fabric. The most recent phase of OJP deformation, which began about 6 Ma, accelerated at 2.6 Ma and continues today, has resulted in the uplift of the islands of Malaita and Santa Isabel, and the formation of the Malaita Anticlinorium, with slip along the old fracture zones possibly triggering submarine canyon formation on the NE side of the OJP. This collision-related deformation also is probably responsible for the ongoing uplift and tilting of the islands of Nauru and Banaba NE of the OJP high plateau.

Large igneous province magma petrogenesis from source to surface: platinum-group element evidence from Ontong Java Plateau basalts recovered during ODP Legs 130 and 192

A total of 16 Ontong Java Plateau (OJP) basalt samples from Ocean Drilling Program Legs 192 and 130 were analysed for major, trace and platinum-group elements (PGEs; Ir, Ru, Rh, Pt and Pd). Major- and trace-element compositions determined by our study confirm Leg 192 shipboard analyses that indicated a new group of more primitive or ‘Kroenke-type’ basalts, with higher MgO, Ni and Cr, and lower incompatible-element, abundances than the more common Kwaimbaita-type basalts. The PGE abundances quantified here extend the range of the continuum of compositions found in previously analysed OJP basalts and are similar to those present in some komatiites. The PGEs, therefore, cannot be used to differentiate definitively between OJP basalts groups. The two samples analysed from Leg 130 (one from Site 803 and one from Site 807) are akin to the Kwaimbaita-type basalts.

Low-temperature alteration has not affected Pd abundances in the Leg 192 basalts as it has in the Solomon Island and the Leg 130 samples. Elemental abundances and ratios along with petrography reveal that the OJP basalts have not experienced sulphide saturation. Positive correlations of Ir and Ru with Cr and Ni attest to the lithophile behaviour of the PGEs and lend more credence to studies suggesting compatibility of these elements in oxide and silicate phases, such as Cr-spinel and olivine. Estimates of sulphur abundance in the mantle, degree of partial melting and pressure of melt initiation were used in conjunction with the model of Mavrogenes & O’Neill to calculate a minimum initial excess temperature of +185–+235°C (1465–1515°C at 3.5–4.0 GPa) above ambient mantle for the OJP source. This is in broad agreement with a fossil geotherm preserved in megacrysts and peridotite xenoliths found in pipe-like intrusives of alnöite that outcrop on the island of Malaita, Solomon Islands. Using the PGEs as a guide, the OJP basalts were modelled using a three-source component melt mix: a 10% garnet peridotite melt of primitive mantle composition, which then passed through the garnet-spinel transition and melted a further 20%, a 30% partial melt of fertile upper mantle and 0–1% of outer core material. The core component was included only in the plume source, and the ratio of plume source to upper mantle source was 19: 1. It is evident from this study that the PGE contents of at least some of the OJP basalts are too high to be generated by primitive mantle sources alone. A PGE-enriched component is required and we suggest that this is outer core material. While a sulphide-rich mantle component could also increase the PGE abundances (assuming that the sulphide is exhausted during partial melting), the sulphur-undersaturated nature of these basalts argues against this. Variations in the amount of outer core in the source (from 0 to 1 wt%) and degree of fractional crystallization can account for the entire range in PGE abundances of OJP basalts.

Experimental petrology of basement lavas from Ocean Drilling Program Leg 192: implications for differentiation processes in Ontong Java Plateau magmas

Melting relations of the basement lavas drilled from the Ontong Java Plateau during ODP Leg 192 were experimentally determined at 1150–1250°C and 0.1–190 MPa under the oxygen fugacity along the fayalite-magnetite-quartz (FMQ) and cobalt-cobalt oxide (CCO) buffers. The basement lavas were classified into two types according to phenocryst assemblage and whole-rock composition: one type is low in MgO (<8 wt%) and olivine + plagioclase + augite-phyric (Kwaimbaita type); and the other is rich in MgO (>8 wt%) and olivine-phyric (Kroenke type). One sample was chosen from each type as a starting material of the melting experiments. The experimental results demonstrate that the variations in phenocryst assemblage and whole-rock composition in the basement lavas can be modelled adequately by fractional crystallization processes in a shallow magma chamber (<6 km in depth). The experimentally determined mineral-melt equilibria, in combination with detailed petrographical investigation, revealed that the vast majority of phenocrysts are in equilibrium with their host magma composition, but some are not. The latter include unusually An-rich parts of plagioclase phenocrysts in the Kwaimbaita-type lavas. These An-rich parts probably crystallized in a mushy boundary layer along the wall of the magma chamber where the melt was relatively rich in H2O. Some olivine phenocrysts in the Kroenke-type lavas show reverse zoning, with core compositions that can be in equilibrium with the Kwaimbaita-type magmas. The cores of these olivine phenocrysts were most probably assimilated from a solidified pile of the Kwaimbaita-type lavas when the Kroenke-type magmas ascended through it.

Accretionary-lapilli-bearing pyroclastic rocks at ODP Leg 192 Site 1184: a record of subaerial phreatomagmatic eruptions on the Ontong Java Plateau

Detailed analysis of lithologies and lithofacies associations within the 337.7 m thick basement volcaniclastic succession, recovered during Ocean Drilling Program (ODP) Leg 192 at Site 1184 on the Ontong Java Plateau, shows that in bulk it is made up of pyroclastic deposits of phreatomagmatic origin. The succession is essentially made up of two lithologies: lapilli tuff (59% of the total recovered core length) and tuff (34%), consisting almost entirely of juvenile clasts (>97%) and containing significant amounts of matrix-supported accretionary and/or armoured lapilli clasts. The evidence indicates that the succession was formed by at least six (and perhaps as many as 10) major phreatomagmatic eruptions that were subaerial and associated with the main phase of volcanism on the Ontong Java Plateau.

Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40Ar/39Ar age for volcaniclastic rocks at ODP Site 1184

The discovery of a thick (337.7 m drilled) succession of volcaniclastic sediments at Ocean Drilling Program (ODP) Site 1184, on the eastern salient of the Ontong Java Plateau (OJP), shows that at least part of the plateau formed at or near sea level. All the other parts of the OJP that have been drilled or are exposed on land are composed of basaltic lava flows erupted in a deep-marine environment. The composition of the volcaniclastic rocks at Site 1184 is essentially indistinguishable from that of basaltic lava flows recovered from the other drill sites on the OJP, suggesting that the eastern salient formed during the same magmatic event (at approximately 120 Ma) that formed the main part of the plateau. A steep (−54°) magnetic inclination preserved in the volcaniclastic succession is consistent with an Early Cretaceous age, but rare nannofossils recovered from the volcaniclastic rocks suggest a much younger Eocene age. In order to resolve this paradox, we attempted to date the succession by the 40Ar/39Ar technique, even though the rocks are highly altered. Two samples of feldspathic material separated from basaltic clasts give minimum age estimates of c. 74 Ma. The basaltic clasts have compositions similar to that of their host rocks and are therefore probably cognate. A weighted average of the results of total fusion experiments on four or five small euhedral plagioclase crystals separated from the matrix of the volcaniclastic rocks at each of four levels in the lower part of the succession gave a value of 123.5 ± 1.8 Ma, and this probably represents a reasonable estimate of the age of eruption. Although not very precise, the 40Ar/39Ar results are the best that presently can be obtained from such altered rocks. They rule out an Eocene age for the volcaniclastic succession at Site 1184, and we suggest that the Eocene nannofossils were introduced through contamination, probably along fractures.

Compositional variability in lavas from the Ontong Java Plateau: results from basalt clasts within the volcaniclastic succession at Ocean Drilling Program Site 1184

Tholeiitic basalts have been recovered from drill sites in different locations on the Ontong Java Plateau (OJP) and are remarkably homogeneous across this large igneous province. The most abundant basalt type is represented by the Kwaimbaita Formation on Malaita in the Solomon Islands, where it is capped by the isotopically distinct and slightly more incompatible-element-enriched basalt of the Singgalo Formation. Ocean Drilling Program (ODP) Leg 192 drilled five sites on the OJP, four of which penetrated basement lava successions. All basalt recovered during Leg 192 is chemically and isotopically indistinguishable from Kwaimbaita-type lavas.

Site 1184 of ODP Leg 192 is situated on the eastern salient of the OJP, and is unique because the recovered volcaniclastic succession contains the first conclusive evidence for emergence of part of the OJP above sea level. Within this succession are clasts of basaltic material. We report the major element-, trace-element and isotopic compositions of 14 moderately to highly altered basalt clasts. On the basis of incompatible-element concentrations, specifically high field strength elements (HFSE) and rare earth elements (REE), four groups of clasts are defined. Group 1 clasts are similar to basalt from the Kwaimbaita Formation. Group 2 clasts show variable composition, but the heavy rare earth element (HREE) concentrations are similar to those of basalts from the Kwaimbaita Formation. Group 3 clasts have compositions similar to the high-MgO Kroenke-type basalt recovered during ODP Leg 192. Group 4 clasts are more evolved than the Kwaimbaita or Singgalo lavas, and contain deep negative Eu and Sr anomalies on primitive-mantle (PM)-normalized diagrams, as well as high concentrations of Nb, Ta and Th. Group 4 clasts also show a large fractionation of Nb from La and have (Nb/La)PM ratios of approximately 2. Sr-, Nd- and Pb-isotope ratios were measured on five clasts covering all four groups. Although the Sr- and Pb-isotope ratios exhibit some variability, which we attribute to alteration, the Nd-isotope ratios are within the field defined for Kwaimbaita-type lavas.

We conclude that most of the compositional variability displayed by these clasts is a result of alteration and that Ta appears to be the most immobile incompatible trace element. All of the clasts were derived from the mantle source that produced the Kwaimbaita-type and Kroenke-type basalts. Our data emphasize the widespread nature of Kwaimbaita-type basalt and show that the source region was active under both the eastern salient and the high plateau of the OJP.

Volatiles in submarine basaltic glasses from the Ontong Java Plateau (ODP Leg 192): implications for magmatic processes and source region compositions

Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H2O, CO2, S, Cl). At four of the sites (1183, 1185, 1186, 1187) the glass is from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. Glassy pillow rims from Site 1187 and the upper group of flows at Site 1185 have 8.3–9.3 wt% MgO compared with values of 7.2–8.0 wt% MgO for glasses from Sites 1183, 1184 1186, and the lower group of flows at Site 1185. Low-MgO glasses have slightly higher H2O contents (average 0.22 wt% H2O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H2O (average 0.16 wt%). Average S concentrations are 910 ± 60 ppm for the high-MgO glasses v. 1030 ± 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeOT. This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K2O and S/TiO2 with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S Kα peak in the glasses indicate low oxygen fugacities comparable to MORB values. Chlorine contents of the glasses are very high compared with MORB, and Cl/K ratios for all glasses are relatively high (>0.7). This ratio is sensitive to assimilation of hydrothermally altered material, so the high values indicate assimilation during shallow-level crystallization of OJP magmas. Ratios of H2O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H2O/Ce values are probably also caused by the same assimilation process that results in high Cl. The water content of the high MgO-magmas before contamination is estimated to be approximately 0.07 wt% H2O, corresponding to H2O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H2O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H2O content of the OJP mantle source region (170 ± 30 ppm H2O) is similar to that of the depleted MORB source (140 ± 40 ppm H2O). Instead, large extents of melting beneath the OJP must have been caused by a relatively high mantle potential temperature, consistent with upwelling of a hot mantle plume.

Pin-pricking the elephant: evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts

Age-corrected Pb, Sr and Nd isotope ratios for early Aptian basalt from four widely separated sites on the Ontong Java Plateau that were sampled during Ocean Drilling Program Leg 192 cluster within the small range reported for three earlier drill sites, for outcrops in the Solomon Islands, and for the Nauru and East Mariana basins. Hf isotope ratios also display only a small spread of values. A vitric tuff with εNd(t) = +4.5 that lies immediately above basement at Site 1183 represents the only probable example from Leg 192 of the Singgalo magma type, flows of which comprise the upper 46–750 m of sections in the Solomon Islands and at Leg 130 Site 807 on the northern flank of the plateau. All of the Leg 192 lavas, including the high-MgO (8–10 wt%) Kroenke-type basalts found at Sites 1185 and 1187, have εNd(t) between +5.8 and +6.5. They are isotopically indistinguishable from the abundant Kwaimbaita basalt type in the Solomon Islands, and at previous plateau, Nauru Basin and East Mariana Basin drill sites. The little-fractionated Kroenke-type flows thus indicate that the uniform isotopic signature of the more evolved Kwaimbaita-type basalt (with 5–8 wt% MgO) is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers, but instead must reflect the signature of an inherently rather homogeneous (relative to the scale of melting) mantle source. In the context of a plume-head model, the Kwaimbaita-type magmas previously have been inferred to represent mantle derived largely from the plume source region. Our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event (e.g. a small amount of partial melting) approximately 3 Ga or earlier, and subsequently evolved in nearly closed-system fashion until being tapped by plateau magmatism in the early Aptian. These results are consistent with current models of a compositionally distinct lower mantle and a plume-head origin for the plateau. However, several other key aspects of the plateau are not easily explained by the plume-head model. The plateau also poses significant challenges for asteroid impact, Icelandic-type and plate separation (perisphere) models. At present, no simple model appears to account satisfactorily for all of the observed first-order features of the Ontong Java Plateau.

Partial melting below the Ontong Java Plateau

Primary magma compositions for Kroenke-type basalts from the Ontong Java Plateau (OJP) have been estimated using a hybrid forward and universe model. For accumulated fractional melting of a fertile peridotite source, the primary magma had 16.8% MgO and lost 18% olivine by fractional crystallization to produce Kroenke-type basalts; the melt fraction was 0.27 and the potential temperature was 1500°C. For equilibrium melting of a fertile peridotite source, the primary magma had 19.3% MgO and lost 25% olivine by fractional crystallization to produce Kroenke-type basalts; the melt fraction was 0.30 and the potential temperature was 1560°C. The model peridotite source composition, melt fraction and potential temperature required to produce the primary OJP magmas are in excellent agreement with those that have been independently estimated from incompatible trace-element concentrations.

Phreatomagmatic eruptions on the Ontong Java Plateau: chemical and isotopic relationship to Ontong Java Plateau basalts

The compositions of glass clasts in volcaniclastic rocks recovered from drilling at Site 1184 on the eastern salient of the Ontong Java Plateau (OJP) are investigated using microbeam analytical methods for major, minor and trace elements. These data are compared with whole-rock elemental and isotopic data for bulk tuff samples, and with data from basalts on the high plateau of the OJP. Three subunits of Hole 1184A contain blocky glass clasts, thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks. The glass clasts have unaltered centres, and are all basaltic low-K tholeiites, with flat chondrite-normalized rare earth element (REE) patterns. Their elemental compositions are very similar to the Kwaimbaita-type and Kroenke-type basalts sampled on the high plateau. Each subunit has a distinct glass composition and there is no intermixing of glass compositions between subunits, indicating that each subunit is the result of one eruptive phase, and that the volcaniclastic sequence has not experienced reworking. The relative heterogeneity preserved at Site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau, and suggests that the eastern salient of the OJP had a different type of magma plumbing system. Our data support the hypothesis that the voluminous subaerially erupted volcaniclastic rocks at Site 1184 belong to the same magmatic event as the construction of the main Ontong Java Plateau. Thus, the OJP would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans, and these factors may have influenced the contemporaneous oceanic anoxic event.