Subducting volcaniclastic-rich upper crust supplies fluids for shallow megathrust and slow slip

Recurring slow slip along near-trench megathrust faults occurs at many subduction zones, but for unknown reasons, this process is not universal. Fluid overpressures are implicated in encouraging slow slip; however, links between slow slip, fluid content, and hydrogeology remain poorly known in natural systems. Three-dimensional seismic imaging and ocean drilling at the Hikurangi margin reveal a widespread and previously unknown fluid reservoir within the extensively hydrated (up to 47 vol % H2O) volcanic upper crust of the subducting Hikurangi Plateau large igneous province. This ~1.5 km thick volcaniclastic upper crust readily dewaters with subduction but retains half of its fluid content upon reaching regions with well-characterized slow slip. We suggest that volcaniclastic-rich upper crust at volcanic plateaus and seamounts is a major source of water that contributes to the fluid budget in subduction zones and may drive fluid overpressures along the megathrust that give rise to frequent shallow slow slip.

The organic stratigraphy of Ontong Java Plateau Tuff correlated with the depth-related presence and absence of putative microbial alteration structures

Structures in geological samples are often interpreted as fossilised life; however, such interpretations are equivocal, as abiotic processes can be invoked to explain their presence. Thus, additional lines of chemical evidence are invaluable in confirming or refuting such morphological evidence. Glass shards in tuff from the Ontong Java Plateau (OJP) contain microtubular structures that are in close proximity to functionalised nitrogen substituted aromatic compounds that may be indicative of the chemical remnants of biological activity. The organic composition of the OJP tuff containing microtubular alteration structures was compared with tuff without such features. In addition, organic matter associated with horizons with compacted remnants of woody material buried in the OJP tuff and overlying pelagic calcareous foraminifer sediment were also characterised, to ascertain the provenance of the organic matter found in the OJP tuff. As a further control, the organic material in submarine and terrestrial basalts from other locations were also characterised providing further evidence to support the view that the organic matter in the OJP tuff is authigenic. Carbon-nitrogen chemistry was detected across all OJP tuff samples irrespective of the presence or absence of microtubular features, but was not detected in either the wood material, the overlying pelagic sediments or in the basalts from other locations. The results indicate no direct link between the OJP nitrogenous organic compounds and the presence or absence of microtubular features.

Past emergent phase of Shatsky Rise deep-marine igneous plateau

The Cretaceous Period stands out in Earth’s geologic history by ubiquitous and sustained massive eruption of lava, forming several enormous igneous plateaus in the ocean basins worldwide. It has been proposed that the subaerial phases of Cretaceous oceanic plateau formation spurred the global environmental deterioration, yet this view is supported by patchy fossil and/or rock evidence for uplifting of the plateau summits above the sea level. Reported here is by far the most comprehensive case of Cretaceous plateau emergence at northern Shatsky Rise, Northwest Pacific, based on the integration of unique micropalaeontological and seismic evidence. From just above the flat-topped igneous edifice, recent Integrated Ocean Drilling Program (at Site U1346) recovered early Cretaceous (Hauterivian) ostracod and foraminiferal assemblages showing marked shallow-marine preferences. Most intriguing discovery is an ostracod taxon with well-developed eye tubercles, which serves as compelling palaeobiological evidence for a very shallow, euphotic setting. By linking the nearshore biofacies (<20 m water depth) to the basement topography undoubtedly shaped by subaerial weathering and/or erosion, it is obvious that northern Shatsky Rise was remarkably emergent during its final emplacement phase. We suggest that early Cretaceous surface environments might have been affected, at least in part, by Shatsky Rise subaerial volcanism.

Origin and evolution of magmas on the Ontong Java Plateau

The Early Cretaceous Ontong Java Plateau (OJP) represents by far the largest igneous event on Earth in the last 200 Ma and yet, despite its size, the OJP’s basaltic crust appears to be remarkably homogeneous in composition. The most abundant rock type is a uniform low-K tholeiite, represented by the Kwaimbaita Formation on Malaita and found at all but one of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites on the plateau and in the adjacent basins. This is capped by a thin and geographically restricted veneer of a slightly more incompatible-element-rich tholeiite (the Singgalo Formation on Malaita and the upper flow unit at ODP Site 807), distinguished from Kwaimbaita-type basalt by small but significant differences in Sr-, Nd- and Pb-isotope ratios. A third magma type is represented by high-Mg (Kroenke-type) basalt found in thick (> 100 m) successions of lava flows at two drill sites (ODP Sites 1185 and 1187) 146 km apart on the eastern flank of the plateau. The high-Mg basalt is isotopically indistinguishable from Kwaimbaita-type basalt and may therefore represent the parental magma for the bulk of the OJP. Low-pressure fractional crystallization of olivine followed by olivine+augite+plagioclase can explain the compositional range from high-Mg Kroenke-type to Kwaimbaita-type basalt. The Singgalo-type basalt probably represents slightly smaller-degree, late-stage melting of an isotopically distinct component in the mantle source. Primary magma compositions, calculated by incremental addition of equilibrium olivine to aphyric Kroenke-type basalt glass, contain between 15.6% (in equilibrium with Fo90) and 20.4% (Fo92) MgO. Incompatible-element abundances in the primary OJP magma can be modelled by around 30% melting of a peridotitic primitive-mantle source from which about 1% by mass of average continental crust had previously been extracted. This large degree of melting implies decompression of very hot (potential temperature >1500°C) mantle beneath very thin lithosphere. The initiation of an exceptionally large and hot plume head close to a mid-ocean ridge provides the best explanation for the size, homogeneity and composition of the OJP, but is difficult to reconcile with the submarine eruption of virtually all of the basalt so far sampled.

Early Cretaceous Pacific palaeomagnetic pole from Ontong Java Plateau basement rocks

We present new palaeomagnetic data from Ocean Drilling Program Site 1184 on the eastern salient of the Ontong Java Plateau (OJP) where 337.7 m of Early Cretaceous (c. 120 Ma) volcaniclastic rocks were drilled. Alternating field and thermal demagnetizations were equally effective in removing secondary components, allowing the characteristic remanent magnetization directions from a total of 173 samples (out of 183) to be defined. All samples have negative inclinations (normal polarity), and by treating each sample as an independent reading of the palaeomagnetic field a site-mean inclination of −53.9° (N = 173; α95 = 1.0°, k = 109) was obtained. The corresponding palaeo-colatitude is in excellent accordance with previously published time-averaged palaeo-colatitudes from contemporaneous basalts drilled at OJP and the Nauru Basin. Based on the intersection of the seven palaeo-colatitudes a new Early Cretaceous (c. 120 Ma) Pacific palaeomagnetic pole was obtained with co-ordinates 63.0°N, 10.1°E (95% confidence ellipse with a minor semi-axis of 2.9° with an azimuth of 32° and a major semi-axis of 47.7° with an azimuth of 122°). This pole is far more easterly than previously published Early Cretaceous Pacific palaeomagnetic poles. Based on published Pacific palaeogeographic reconstructions in the fixed hot-spot reference frame we were able to calculate different Pacific true polar wander (TPW) poles. All Pacific TPW poles are found to be statistically different from contemporaneous TPW poles obtained in the Indo-Atlantic realm, illustrating motion between the two groups of hot spots.

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.

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.

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.