Ontong Java Research Articles

Playing jigsaw with Large Igneous Provinces—A plate tectonic reconstruction of Ontong Java Nui, West Pacific

AbstractThe three largest Large Igneous Provinces (LIP) of the western Pacific—Ontong Java, Manihiki, and Hikurangi Plateaus—were emplaced during the Cretaceous Normal Superchron and show strong similarities in their geochemistry and petrology. The plate tectonic relationship between those LIPs, herein referred to as Ontong Java Nui, is uncertain, but a joined emplacement was proposed by Taylor (2006). Since this hypothesis is still highly debated and struggles to explain features such as the strong differences in crustal thickness between the different plateaus, we revisited the joined emplacement of Ontong Java Nui in light of new data from the Manihiki Plateau. By evaluating seismic refraction/wide‐angle reflection data along with seismic reflection records of the margins of the proposed “Super”‐LIP, a detailed scenario for the emplacement and the initial phase of breakup has been developed. The LIP is a result of an interaction of the arriving plume head with the Phoenix‐Pacific spreading ridge in the Early Cretaceous. The breakup of the LIP shows a complicated interplay between multiple microplates and tectonic forces such as rifting, shearing, and rotation. Our plate kinematic model of the western Pacific incorporates new evidence from the breakup margins of the LIPs, the tectonic fabric of the seafloor, as well as previously published tectonic concepts such as the rotation of the LIPs. The updated rotation poles of the western Pacific allow a detailed plate tectonic reconstruction of the region during the Cretaceous Normal Superchron and highlight the important role of LIPs in the plate tectonic framework.

Revisiting diagenesis on the Ontong Java Plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

AbstractThe calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio‐chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their paleoceanographic interpretation. A previous study of Globorotalia tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher‐Mg chamber calcite and the preservation of a low‐Mg crust on the tests significantly reduced whole‐test Mg/Ca and Sr/Ca. Here we revisit specimens with a combination of synchrotron X‐ray computed tomography (sXCT) and electron probe microanalyses to reevaluate the nature of their diagenetic alteration. The dissolution of higher‐Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high‐Mg chamber calcite and precipitation of low‐Mg crust from the resulting modified pore water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and reprecipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting paleoproxy signals.

Aptian giant explosive volcanic eruptions in the Songliao Basin and northeast Asia: A possible cause for global climate change and OAE-1a

Volcanism is a natural climate force that causes variations in temperatures. The Aptian Oceanic Anoxic Event 1a (OAE-1a) was preceded by a prominent negative C-isotope excursion attributed to major volcanism of the Ontong Java plateau (OJP), which presumably resulted in a pCO2 increase and a climatic change. However, the OJP alone may not adequately explain some important isotopic signatures such as the negative strontium-isotope excursion from 125 Ma to 113 Ma that is recorded in the corresponding marine deposits. We present an independent and hitherto undocumented case, the giant Aptian volcanism in the Songliao Basin and northeast Asia (SB-V) on the Cretaceous active continental margin between the Eurasian and the Pacific plates, which covered an area of ca. 2.3 × 106 km2, nearly matching the simultaneous case of the OJP. Intensive strong, explosive volcanic eruptions of the SB-V occurred at 121–109 Ma and introduced a large volume of fine-grained volcanic ash and degassing volatiles into the atmosphere. The Aptian isotopic ratios (87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) of marine carbonates from the Mid-Pacific shift in values between their Barremian pre-excursion high values and the negative magmatic values of the SB-V. The transient global cooling at the onset of the OAE-1a coincided with the beginning of the violent acidic eruption of the SB-V (119.9–120.2 Ma). We therefore infer that the SB-V must have played a role in the Aptian global climatic changes and OAE-1a through the heavy fall of volcanic dust and the outgassing of aerosol and greenhouse gases.

Ultra-depleted melts in olivine-hosted melt inclusions from the Ontong Java Plateau

The Ontong Java Plateau (OJP) represents the largest large igneous province (LIP) preserved in the geologic record. The most voluminous volcanic types of the OJP—the Kroenke and Kwaimbaita groups, which dominate the accessible portions of the plateau—have relatively flat primitive mantle normalized rare earth element (REE) patterns. With the exception of relatively small volumes of late-stage melts—referred to as the Singgalo group—that are characterized by slightly enriched REE patterns relative to a chondritic pattern, the volcanic groups that dominated the eruptive history of the OJP exhibit remarkably homogeneous, flat REE patterns. Here we isolate, for the first time, olivine-hosted melt inclusions from the OJP. We show that the melt inclusions have two clear populations defined by distinct trace element characteristics. The first population has relatively flat trace element patterns that are similar to those observed in whole rock lavas from the most voluminous volcanic groups (Kroenke and Kwaimbaita) recorded in the OJP. In contrast, a second group of melt inclusions, referred to as UDM (ultra-depleted melt) inclusions, exhibit (light-REE) LREE-depleted patterns relative to a chondritic pattern; these trace element patterns are far more depleted than any previously reported lava from OJP. The UDM have unique trace element signatures that preclude an origin by re-melting the depleted mantle source left over after melt extraction during construction of the OJP. We interpret the new UDM compositions to be the result of melting a previously unrecognized (in lavas) ultra-depleted component hosted in the OJP mantle source.

Seismic wave speed structure of the Ontong Java Plateau

The Ontong Java Plateau (OJP) represents the result of a significant event in the Earth's geologic history. Limited geophysical and geochemical data, as well as the plateau's relative isolation in the Pacific ocean, have made interpretation of the modern day geologic structure and its 120 Ma formation history difficult.Here we present the highest resolution image to date of the wave speed structure of the OJP region. We use a data set that combines Rayleigh waves extracted from both ambient noise and earthquake waveforms and an iterative finite-frequency tomography methodology. The combination of datasets allow us to best exploit the limited station distribution in the Pacific and image wave speed structures between 35 km and 300 km into the Earth.We image a region of fast shear wave speeds, greater than 4.75 km/s, that extends to greater than 100 km beneath the plateau. The wave speeds are similar to as observed in cratonic environments and are consistent with a compositional anomaly that resulted from the residuum of eclogite entrainment during the plateau's formation.The combination of our imaged wave speed structure and previous geochemical work suggest that a surfacing plume head entrained eclogite from the deep mantle and accounts for the anomalous buoyancy characteristics of the plateau and observed fast wave speeds.

Black shales – from coolhouse to greenhouse (early Aptian)

Approximately 120 Ma major volcanic outgassing on the Ontong Java Plateau resulted in perturbations of the global carbon cycle, in a change from cool to greenhouse climate conditions and in major changes in oceanography leading to widespread deposition of black shales during the early Aptian Oceanic Anoxic Event 1a (OAE1a). However, onset of black-shale deposition was diachronous and prior to OAE1a occurred under specific paleogeographic and paleoceanographic conditions. The goal of this study is to identify paleoceanographic constraints during coolhouse conditions that resulted in pre-OAE1a black-shale deposition and to investigate if and to what extent the short-term orbitally induced climate changes are also recorded in the sedimentary archive. We compared four lower Aptian pelagic sections from the former southern and northern continental margins of the Alpine Tethys Ocean and traced the evolution of carbon isotopes, carbonate and organic carbon content as well as palynofacies before and at the onset of OAE1a. Throughout the studied interval, the sections record frequent precession-controlled changes in carbonate content, which are reflected by limestone-marlstone alternations in the shallowest and most proximal Cismon core and by limestone-black-shale couplets in the deepest Pusiano section. Depth controlled sub-/anoxia is also suggested by the prominent OAE1a black shales, which occurred first in the deeper Pie del Dosso and Roter Sattel sections and only subsequently in the shallower Cismon core. However, contrary to expectations, the deepest Pusiano section exhibits – instead of an earliest onset of prominent OAE1a black shales – only a minor increase in TOC and a decrease in carbonate content. This suggests that the orbitally driven climate changes most strongly influenced water stratification and hence are most prominently expressed in the deepest sections. Conversely, the volcanically induced long-term climate changes seemed to more strongly affect organic matter production, the extension of the oxygen minimum zone and hence had the strongest impact on sections at intermediate depth.

Climate variability and ocean fertility during the Aptian Stage

Abstract. Several studies have been conducted to reconstruct temperature variations across the Aptian Stage, particularly during early Aptian Oceanic Anoxic Event (OAE) 1a. There is a general consensus that a major warming characterized OAE 1a, although some studies have provided evidence for transient "cold snaps" or cooler intervals during the event. The climatic conditions for the middle–late Aptian are less constrained, and a complete record through the Aptian is not available. Here we present a reconstruction of surface-water palaeotemperature and fertility based on calcareous nannofossil records from the Cismon and Piobbico cores (Tethys) and DSDP Site 463 (Pacific Ocean). The data, integrated with oxygen-isotope and TEX86 records, provide a detailed picture of climatic and ocean fertility changes during the Aptian Stage, which are discussed in relation to the direct/indirect role of volcanism. Warm temperatures characterized the pre-OAE 1a interval, followed by a maximum warming (of ~ 1.5–2 °C) during the early phase of anoxia under intense volcanic activity of the Ontong Java Plateau (OJP). A short-lived cooling episode interrupted the major warming, following a rapid increase in weathering rates. Nannofossils indicate that mesotrophic conditions were reached when temperatures were at their highest and OJP volcanism most intense, thus suggesting that continental runoff, together with increased input of hydrothermal metals, increased nutrient supply to the oceans. The latter part of OAE 1a was characterized by cooling events, probably promoted by CO2 sequestration during burial of organic matter. In this phase, high productivity was probably maintained by N2-fixing cyanobacteria, while nannofossil taxa indicating higher fertility were rare. The end of anoxia coincided with the cessation of volcanism and a pronounced cooling. The mid-Aptian was characterized by highest surface-water fertility and progressively decreasing temperatures, probably resulting from intense continental weathering drawing down pCO2. The lowest temperatures, combined with low fertility, were reached in the middle–late Aptian across the interval characterized by blooming of Nannoconus truittii. The prolonged cooling was followed by significant warming across the Aptian–Albian boundary. The data presented suggest that OJP activity played a direct role in inducing global warming during the early Aptian, whereas other mechanisms (weathering, deposition of organic matter) acted as feedback processes, favouring temporary cooler interludes.

Re‐Os isotope and platinum group elements of a FOcal ZOne mantle source, Louisville Seamounts Chain, Pacific ocean

AbstractThe Louisville Seamount Chain (LSC) is, besides the Hawaiian‐Emperor Chain, one of the longest‐lived hotspot traces. We report here the first Re‐Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245–0.1314 that are remarkably homogeneous and do not vary with age. A Re‐Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar‐39Ar data. Isochron‐derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271–0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124–0.139) and Samoan shield (0.1276–0.1313) basalts and lower than those of Cook‐Austral (0.136–0.155) and Hawaiian shield (0.1283–0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide‐saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle‐like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re‐Os isotope and PGE composition of LSC basalts reflect a relatively pure deep‐sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites.

The origin of Cretaceous black shales: a change in the surface ocean ecosystem and its triggers.

Black shale is dark-colored, organic-rich sediment, and there have been many episodes of black shale deposition over the history of the Earth. Black shales are source rocks for petroleum and natural gas, and thus are both geologically and economically important. Here, we review our recent progress in understanding of the surface ocean ecosystem during periods of carbonaceous sediment deposition, and the factors triggering black shale deposition. The stable nitrogen isotopic composition of geoporphyrins (geological derivatives of chlorophylls) strongly suggests that N2-fixation was a major process for nourishing the photoautotrophs. A symbiotic association between diatoms and cyanobacteria may have been a major primary producer during episodes of black shale deposition. The timing of black shale formation in the Cretaceous is strongly correlated with the emplacement of large igneous provinces such as the Ontong Java Plateau, suggesting that black shale deposition was ultimately induced by massive volcanic events. However, the process that connects these events remains to be solved.

Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example

We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnöitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2–32 ppm, 162–362 ppm and 159–459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet lherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced ‘stealth’ metasomatism, and/or (2) to a late ‘fleeting’ hydrogen metasomatism, which would hydrate the rock after this first ‘stealth’ metasomatism event. In the second case, the composition of the ‘fleeting’ percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume activity and resulting in the downwelling of the lithosphere asthenosphere boundary. The difference in hydrogen concentration between harzburgites and lherzolites could be linked to variation in trace elements in olivine induced by the first ‘stealth’ metasomatism alone or associated to the late ‘fleeting’ hydrogen metasomatism, triggered by the lack of secondary crystallization of clinopyroxenes in the harzburgite. To conclude, H concentrations in upper mantle minerals may thus be correlated to metasomatic events, with the later yielding variation of the mineral assemblage and trace element composition that constitute the peridotites.

Microstructures, composition, and seismic properties of the Ontong Java Plateau mantle root

AbstractTo study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber‐[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P wave velocities of the peridotites decrease weakly with depth. S wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P and S waves anisotropy is variable (2–12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]‐fiber CPO patterns, if foliations are horizontal, vertically propagating S waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy‐induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.

Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc.

Large igneous province subduction is a rare process on Earth. A modern example is the subduction of the oceanic Hikurangi Plateau beneath the southern Kermadec arc, offshore New Zealand. This segment of the arc has the largest total lava volume erupted and the highest volcano density of the entire Kermadec arc. Here we show that Kermadec arc lavas south of ~32°S have elevated Pb and Sr and low Nd isotope ratios, which argues, together with increasing seafloor depth, forearc retreat and crustal thinning, for initial Hikurangi Plateau-Kermadec arc collision ~250 km north of its present position. The combined data set indicates that a much larger portion of the Hikurangi Plateau (the missing Ontong Java Nui piece) than previously believed has already been subducted. Oblique plate convergence caused southward migration of the thickened and buoyant oceanic plateau crust, creating a buoyant 'Hikurangi' mélange beneath the Moho that interacts with ascending arc melts.

Porosity and sonic velocity depth trends of Eocene chalk in Atlantic Ocean: Influence of effective stress and temperature

We aimed to relate changes in porosity and sonic velocity data, measured on water-saturated Eocene chalks from 36 Ocean Drilling Program drill sites in the Atlantic Ocean, to vertical effective stress and thermal maturity. We considered only chalk of Eocene age to avoid possible influence of geological age on chalk compaction trends. For each depth, vertical effective stresses as defined by Terzaghi and by Biot were calculated. We used bottom-hole temperature data to calculate the time–temperature index of thermal maturity (TTI) as defined by Lopatin. Porosity and compressional wave velocity data were correlated to vertical effective stresses and to TTI. Our porosity data showed a broader porosity trend in the mechanical compaction zone, and the onset of the formation of limestone at a shallower burial depth than the porosity data of the Ontong Java Plateau chalk show. Our porosity data do not show or at least it is difficult to define a clear pore-stiffening contact cementation trend as the Ontong Java Plateau chalk. Mechanical compaction is the principal cause of porosity reduction (at shallow depths) in the studied Eocene chalk, at least down to about 5 MPa Terzaghi׳s effective stress corresponding to a porosity of about 35%. This indicates that mechanical compaction is the principal agent of porosity reduction. Conversely, at deeper levels, porosity reduction is accompanied by a large increase in sonic velocity indicating pore-filling cementation. These deep changes are correlated with TTI. This indicates pore-filling cementation via an activation energy mechanism. We proposed a predictive equation for porosity reduction with burial stress. This equation is relevant for basin analysis and hydrocarbon exploration to predict porosity if sonic velocity data for subsurface chalk is available.

Louisville Seamount Chain: Petrogenetic processes and geochemical evolution of the mantle source

AbstractThe Louisville Seamount Chain is a ∼4300 km long chain of submarine volcanoes in the southwestern Pacific that spans an age range comparable to that of the Hawaiian‐Emperor chain and is commonly thought to represent a hot spot track. Dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 Myr of the chain's history. All samples are alkalic, similar to previous dredge and drill samples, providing no evidence for a Hawaiian‐type tholeiitic shield‐volcano stage. Major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting. Isotopic values define only a narrow range, in agreement with a surprising long‐term source homogeneity—relative to the length scale of melting—and overlap with proposed fields for the “C” and “FOZO” mantle end‐members. Trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism, except for a small number of lavas from the westernmost Louisville Seamounts built on young (<20 Ma old) oceanic crust. The Louisville hot spot has been postulated to be the source of the ∼120 Ma Ontong Java Plateau, but the Louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for Ontong Java Plateau basalts. On the other hand, this signature can be correlated with that of samples dredged from the Danger Islands Troughs of the Manihiki Plateau, which has been interpreted as a rifted fragment of the “Greater” Ontong Java Plateau.

THE EVOLUTION OF UPPER WATER IN THE CENTER OF WEST PACIFIC WARM POOL DURING THE LAST 360KYR

The Core KX97322-4 was taken from the Ontong Java Plateau,western equatorial Pacific.Quantitative analysis of planktonic foraminifera,combined with oxygen isotope data and spectral analyses of each index,revealed the glacial-interglacial variations of the oxygen isotope of planktonic foraminifera and the vertical structure of the upper water in the center of the West Pacific Warm Pool(WPWP) during the last 360kyr.Our results indicate that,since MIS10,the oxygen isotopes of both the surface dwelling species G.ruber and subsurface dwelling species N.dutertrei's showed obvious glacial-interglacial cycles.The surface and subsurface waters were influenced by the volume change of high latitude ice sheets in a large scale generally,and the subsurface water is even more sensitive.The variation in upper water structure also imitates the glacial-interglacial cycle.The thermocline(DOT) was getting deeper during MIS10 to MIS8,but abruptly became shallower in MIS8-MIS7/8,then deepened again after MIS7/8 and turned to stable fluctuation when entered the last glacial stage.The spectral analyses indicate that the joint effect of high latitude forcing and tropic-driving caused the variation in the upper water characters.Of course,ocean circulation has also impact on the variation in the upper water structures.

Radiocarbon dating of planktonic foraminifer shells: A cautionary tale

[1] Sedimentation rate, bioturbation, winnowing, and calcite dissolution produce significant radiocarbon age offsets among multiple species of coexisting planktonic foraminifers and pteropod fragments. We compare the radiocarbon age of foraminifer species and pteropod fragments with estimates of percent calcite dissolved made with a sedimentary proxy (Globorotalia menardii fragmentation index—MFI) to delineate the effect of dissolution on radiocarbon age of foraminifers. Data from two core top transects on the Rio Grande Rise (RIO) and Ontong Java Plateau (OJP) and from down core sediments of varying sedimentation rates in the tropical Pacific (ME-27, MD98 2177, and MW91-9 56GGC) reveal that sediments with the greatest accumulation rates produce the least age offsets among coexisting species. Age offsets among coexisting foraminifers are about 3500 years on RIO, and 1000 years on OJP. Two core tops from RIO yield an age of the Last Glacial Maximum possibly due to mass displacement of younger sediments downslope. Foraminifer age increases with increasing dissolution and there is a consistent pattern of older foraminifer fragments coexisting with younger whole shells of the same species. The only exception is sediments which have experienced high dissolution where fragments are younger than whole shells. The age offset between fragments of G. menardii and its coexisting whole shells does not exceed the age offset among other coexisting foraminifer species in the same core tops.

Paleo-elevation and subsidence of [formula omitted] Shatsky Rise inferred from CO2 and H2O in fresh volcanic glass

Shatsky Rise, a large Mesozoic oceanic plateau in the northwest Pacific, consists of three massifs (Tamu, Ori, and Shirshov) that formed near a mid-ocean-ridge triple junction. Published depth estimates imply that Shatsky Rise has not subsided normally, like typical oceanic lithosphere. We estimated paleo-eruption depths of Shatsky Rise massifs on the basis of dissolved CO2 and H2O in volcanic glass and descriptions of cores recovered from five sites of Integrated Ocean Drilling Program Expedition 324. Initial maximum elevations of Shatsky Rise are estimated to be 2500–3500 m above the surrounding seafloor and the ensuing subsidence of Shatsky Rise is estimated to be 2600–3400 m. We did not observe the anomalously low subsidence that has been reported for both Shatsky Rise and the Ontong Java Plateau. Although we could not resolve whether Shatsky Rise originated from a hot mantle plume or non-plume fusible mantle, uplift and subsidence histories of Shatsky Rise for the both cases are constrained based on the subsidence trend from the center of Tamu Massif (∼2600 m) toward the flank of Ori Massif (∼3400 m). In the case of a hot mantle plume origin, Shatsky Rise may have formed on young (∼5 Ma) pre-existing oceanic crust with a total crustal thickness of ∼20 km. For this scenario, the center of Shatsky Rise is subsequently uplifted by later (prolonged) crustal growth, forming the observed ∼30 km thickness crust. For a non-plume origin, Shatsky Rise may have formed at the spreading ridge center as initially thick crust (∼30 km thickness), with later reduced subsidence caused by the emplacement of a buoyant mass—perhaps a refractory mantle residuum—beneath the center of Shatsky Rise.

Geothermobarometry of basaltic glasses from the Tamu Massif, Shatsky Rise oceanic plateau

We present the results of a petrological study of core samples from Tamu Massif (Site U1347), recovered during the Shatsky Rise Integrated Ocean Drilling Program (IODP) Expedition 324. The basaltic glasses from Site U1347 are evolved tholeiitic basalts containing 5.2–6.8 wt% MgO, and are principally located within the compositional field of mid‐ocean ridge basalts (MORBs) but they have systematically higher FeO, lower Al2O3, SiO2, and Na2O concentrations, and the CaO/Al2O3 ratios are among the highest known for MORBs. In this sense, glasses from Site U1347 more closely resemble basaltic magmas from the Ontong Java Plateau (OJP), although they still have lower SiO2 concentrations. In contrast to MORB and similar to OJP, our fractionation corrected values of Na2O and CaO/Al2O3 indicate more than 20% of partial melting of the mantle during the generation of the parental magmas of Tamu Massif. The water contents in the glasses, determined by midinfrared Fourier transform infrared (FTIR) spectroscopy, are MORB‐like, and vary between 0.18 and 0.6 wt% H2O. The calculated pressure (P)‐temperature (T) conditions at which the natural glasses represent cotectic olivine‐plagioclase‐clinopyroxene compositions range from 0.1 to 240 MPa and 1100 to 1150°C reflecting magma storage at shallow depth. The variation of the glass compositions and the modeled P‐T conditions in correlation with the relative ages indicate that there were at least two different magmatic cycles characterized by variations in eruptive styles (massive flows or pillow lavas), chemical compositions, volatile contents, and preeruptive P‐T conditions. Each magmatic cycle represents the progressive differentiation in course of polybaric crystallization after the injection of a more primitive magma batch. Magma crystallization and eruption episodes are followed by magmatic inactivity reflected in the core sequence by a sedimentary layer. Our data for Tamu Massif demonstrate that, similar to Ontong Java ocean Plateau, the crystallization beneath Shatsky Rise occurs at different crustal levels.

Tracking the Australian plate motion through the Cenozoic: Constraints from 40Ar/39Ar geochronology

AbstractHere we use geochronology of Australian intraplate volcanoes to construct a high‐resolution plate‐velocity record and to explore how tectonic events in the southwest Pacific may have influenced plate motion. Nine samples from five volcanoes yield ages from 33.6 ± 0.5 to 27.3 ± 0.4 Ma and, when combined with published ages from 30 to 16 Ma, show that the rate of volcanic migration was not constant. Instead, the results indicate distinct changes in Australian plate motion. Fast northward velocities (61 ± 8 and 57 ± 4 km/Ma) prevailed from 34 to 30 (±0.5) and from 23 to 16 (±0.5) Ma, respectively, with distinct reductions to 20 ± 10 and 22 ± 5 km/Ma from 30 to 29 (±0.5) Ma and from 26 to 23 (±0.5) Ma. These velocity reductions are concurrent with tectonic collisions in New Guinea and Ontong Java, respectively. Interspersed between the periods of sluggish motion is a brief 29–26 (±0.5) Ma burst of atypically fast northward plate movement of 100 ± 20 km/Ma. We evaluate potential mechanisms for this atypically fast velocity, including catastrophic slab penetration into the lower mantle, thermomechanical erosion of the lithosphere, and plume‐push forces; none are appropriate. This period of fast motion was, however, coincident with a major southward propagating slab tear that developed along the northeastern plate margin, following partial jamming of subduction and ophiolite obduction in New Caledonia. Although it is unclear whether such an event can play a role in driving fast plate motion, numerical or analogue models may help address this question.

Cryptic lower crustal signature in the source of the Ontong Java Plateau revealed by Os and Hf isotopes

Previous studies of Ontong Java Plateau (OJP) basalts reveal that they are products of 20–30% degree of melting and possess ocean-island-like Pb, Nd, and Sr isotopic signatures that fall within a limited range yet define two distinct isotopic groups represented by the Kroenke- and Kwaimbaita-type basalts and by the Singgalo-type basalts. The origin of the two groups is not clear, although the Kroenke–Kwaimbaita-type signature has been suggested to represent the plateauʼs main mantle source, probably originating from the lower mantle. In this study, samples from central Malaita, Solomon Islands and Ocean Drilling Program (ODP) Sites 807, 1185B, and 1187 were analyzed for Os and Hf isotopic composition to further investigate the origin of the bimodal isotopic composition of OJP basalts. The Kroenke-type basalts, which are parental to Kwaimbaita-type basalts, show the least effect of post-emplacement alteration on their Os isotopic composition and have εHf(t)=+10.8 to +13.3, Os = 118–174 ppt, Re = 161–1111 ppt, and near-chondritic (Os187/Os188)t=0.1322±0.0029 [γOs(t)=2.7±2.2 and 4.3±2.3 (n=6; 1σ) relative to primitive upper mantle (PUM) and average chondrite values, respectively], suggesting a near-primitive mantle source for the OJP. Up to 25% assimilation of altered Jurassic Pacific MORB crust into fractionating Kroenke-type magma could explain the lower, 11–51 ppt, Os contents and more radiogenic, (Os187/Os188)t=0.1395±0.0020 [γOs(t)=8.4±1.5 and 10.0±1.5 (n=3; 1σ)], composition of some Kwaimbaita-type basalts. These compositions are markedly different from those of the least altered Singgalo-type basalts with εHf(t)=+10.2 to +11.5 and (Os187/Os188)t=0.3301±0.0175 [γOs(t)=156±14 and 160±14 (n=8, 1σ)] and low, 21–37 ppt, Os contents that are lower oceanic crust-like. Despite the scatter in the Re–Os isotope data, meaningful isochron ages and initial values were obtained. These ages are 121.4±4.6 Ma for Malaitan Kwaimbaita Fm. basalts and 123±24 Ma for Singgalo-type basalts, with initial 187Os/188Os of 0.129±0.025 and 0.325±0.098, respectively. The Re–Os ages agree very well with the average ∼122 Ma 40Ar–39Ar age for the OJP. The Re–Os and Lu–Hf results reinforce the interpretation of a two-component mantle source for the OJP consisting of a dominant near-primitive mantle similar to that involved in the sources of several ocean islands, represented by the Kroenke- and Kwaimbaita-type basalts, and old, recycled lower continental crustal material expressed subtly in the isotopic signature of the Singgalo-type basalts. A lower crustal influence may be explained by incorporation of ancient, delaminated mafic continental crust into a thermochemical plume; incorporation could provisionally involve dense ex-crustal material that had accumulated in the lower mantle or less dense material that had stagnated at mid-mantle depths.