Ocean island basalt (OIB) and OIB-like basalt are widespread in oceanic and continental settings and, contrary to popular belief, most occur in situations where mantle plumes cannot provide a plausible explanation. They are readily distinguished from normal mid-ocean ridge basalt (N-MORB) through ΔNb, a parameter that expresses the deviation from a reference line (ΔNb = 0) separating parallel Icelandic and N-MORB arrays on a logarithmic plot of Nb/Y versus Zr/Y. Icelandic basalts provide a useful reference set because (1) they are by definition both enriched mid-ocean ridge basalt (E-MORB) and OIB, and (2) they represent a larger range of mantle melt fractions than do intraplate OIBs. Virtually all N-MORB has ΔNb < 0, whereas all Icelandic basalts have ΔNb > 0. E-MORB with ΔNb > 0 is abundant on other sections of ridge, notably in the south Atlantic and south Indian oceans. E-MORB and N-MORB from this region form strongly bimodal populations in ΔNb, separated at ΔNb = 0, suggesting that mixing between their respective mantle sources is very limited. Most OIBs and basalts from many small seamounts, especially those formed on old lithosphere, also have ΔNb > 0. HIMU OIB (OIB with high 206 Pb/ 204 Pb values and therefore a high-µ [U/Pb] source) has higher ΔNb on average than does EM (enriched mantle) OIB, consistent with the presence of recycled continental crust (which has ΔNb < 0) in the EM source. Although EM OIBs tend to have the lowest values, most still have ΔNb > 0, suggesting that a relatively Nb-rich component (probably subducted ocean crust) is present in all OIB sources. The OIB source components seem to be present on all scales, from small streaks or blobs of enriched material (with positive ΔNb) carried in the upper-mantle convective flow and responsible for small ocean islands, some seamounts, and most E-MORB, to large mantle upwellings (plumes), inferred to be present beneath Hawaii, Iceland, Réunion, and Galápagos. It is not possible to identify a point on this continuum at which mantle plumes (if they exist) become involved, and it follows that OIB cannot be a diagnostic feature of plumes. The geochemical similarity of allegedly plume-related OIB and manifestly nonplume OIB is the first part of the OIB paradox. Continental intraplate transitional and alkali basalt in both rift and nonrift (e.g., Cameroon line) settings usually has positive ΔNb and is geochemically indistinguishable from OIB. Continental volcanic rift systems erupt OIB-like basalt, irrespective of whether they are apparently plume-driven (e.g., East Africa, Basin and Range), passive (e.g., Scottish Midland Valley) or somewhere between (e.g., North Sea basin). Magma erupted in passive rifts must have its source in the upper mantle, and yet it is always OIB-like. N-MORB–like magma is only erupted when rifting progresses to continental break-up and the onset of seafloor spreading. Continental OIB-like magma is frequently erupted almost continuously in the same place on a moving lithospheric plate for tens of millions of years, suggesting that its source is coupled in some way to the plate, and yet the Cameroon line (where continental and oceanic basalts are geochemically indistinguishable) suggests that the source is sub-lithospheric. The causes and sources of continental OIB-like magma remain enigmatic and form the second part of the OIB paradox.

Abstract This volume summarizes the results of recent research on the Ontong Java Plateau (OJP) in the western Pacific Ocean ( Fig. 1 ). The plateau is the most voluminous of the world’s large igneous provinces (LIPs) and represents by far the largest known magmatic event on Earth. LIPs are formed through eruptions of basaltic magma on a scale not seen on Earth at the present time (e.g. Coffin & Eldholm 1994 ; Mahoney & Coffin 1997 ). Continental flood basalt provinces are the most obvious manifestation of LIP magmatism, but they have oceanic counterparts in volcanic rifted margins and giant submarine ocean plateaus. LIPs have also been identified on the Moon, Mars and Venus, and may represent the dominant form of volcanism in the solar system ( Head & Coffin 1997 ). The high magma production rates (i.e. large eruption volume and high eruption frequency) involved in LIP magmatism cannot be accounted for by normal plate tectonic processes. Anomalously hot mantle often appears to be required, and this requirement has been a key consideration in the formulation of the currently favoured plume-head hypothesis in which LIPs are formed through rapid decompression and melting in the head of a newly ascended mantle plume (e.g. Richards et al. 1989 ; Campbell & Griffiths 1990 ). Eruption of enormous volumes of basaltic magma over short time intervals, especially in the subaerial environment, may have had significant effects on climate and the biosphere, and LIP formation has been proposed as one of the causes of

Abstract 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.

Abstract Formation of the Ontong Java Plateau (OJP), a large igneous province in the western Pacific, has been attributed to a rising plume head in the initial stage of the Louisville hot spot, approximately 120–125 Ma ago. However, the Neal et al. plate reconstruction suggests that the plateau formed approximately 9° north of the current location of this hot spot at 51°S. The magnetization of the plateau’s basement records a palaeolatitude of approximately 25°S which further increases the discrepancy with the plume-head model. Modelling the motion of the Louisville hot spot for the last 120 Ma yields a possible southward motion of up to about 6°. True polar wander (TPW) models also shift the predicted palaeolatitudes of the plateau farther north. Taking into account both hot-spot motion and TPW, formation of the OJP by the Louisville not spot remains a possibility.

Abstract 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; α 95 = 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.

Abstract A rock magnetic study has been performed on rock samples recovered at Ocean Drilling Program Leg 192 sites on the Ontong Java Plateau in the western Pacific. Igneous rocks from the five Leg 192 sites displayed variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Titanomagnetite and titanomaghemite are present in the Ontong Java rocks. Samples with titanomagnetite exhibit Verwey transitions in the vicinity of 120K. Low-temperature curves for samples with multiple magnetic phases do not clearly show the Verwey transition. The inversion of titanomaghemite to a strongly magnetized magnetite is shown by the irreversible thermomagnetic-cooling curve. Despite the geographically widespread locations of the drill sites, variations in rock magnetic properties closely resemble each other, consistent with the fundamental results of the leg that the basement rocks were derived from homogeneous Kwaimbaita-type magma with a single age of approximately 120 Ma. The rock magnetic investigation provides constraints to evaluate the fidelity of the natural magnetic memory in the basalt rocks and corroborates the palaeomagnetic palaeolatitudes determinations for the Ontong Java Plateau. The generally good quality of rock magnetic data exhibited by Leg 192 rocks supports the inference that the characteristic directions of magnetization isolated from the Cretaceous Ontong Java Plateau sites were acquired near the onset of the Cretaceous Long Normal Superchron about 120 Ma. The portion of the Pacific plate containing the Leg 192 sites was in the southern hemisphere during the mid-Cretaceous volcanism.

Abstract This paper presents the most complete results yet published of geological surveys in Malaita, north of latitude 9°05′S between 1990 and 1995. The geology of Malaita reflects its position as an obducted part of the Alaska-size Ontong Java Plateau (OJP). The geology comprises a monolithological Cretaceous basalt basement sequence up to 3–4 km thick, termed the Malaita Volcanic Group (MVG), conformably overlain by a 1–2 km-thick Cretaceous–Pliocene pelagic sedimentary cover sequence. Cretaceous–Pliocene pelagic sedimentation was punctuated by alkaline basalt volcanism during the Eocene and by intrusion of alnöites during the Oligocene. Basement and cover sequences were both deformed by an intense, but short, middle Pliocene event. A number of localized, Upper Pliocene–Pleistocene, shallow-marine–subaerial, predominantly clastic formations overlie the middle Pliocene unconformity surface. The MVG comprises a monotonous sequence of pillowed and non-pillowed tholeiitic basalt lavas and sills with a predominant clinopyroxene–plagioclase–glass–opaques ± olivine mineralogy. The basaltic plateau morphology of the MVG is reflected in the presence of trap-like topographic features exposed in numerous river sections. Remarkably little sediment is present between basalt flows (most interlava contacts are basalt–basalt), indicating high to very high effusion rates. When present, inter-lava sediment is laminated pelagic chert or limestone, millimetres to centimetres thick, reflecting emplacement of the basalt in deep water (near or below the calcite compensation depth). Gabbro intrusions, dolerite dykes and an unusual spherulitic dolerite facies are locally present. The deep-water eruptive environment of the MVG probably was defined by the accumulation of voluminous eruptions from a multi-centred, submarine, possibly fissure-fed, volcanic source. The Malaitan cover sequence largely comprises a series of foraminifera-rich, pelagic calcilutites and calcisiltites with chert and, in the younger formations, arc-derived mudstone interbeds at various stratigraphic levels.

Abstract The Lower Cretaceous sediments of the Ontong Java Plateau of the SW Pacific Ocean provide a depositional history for the period immediately following the termination of one of the largest extrusive igneous events of the Phanerozoic eon. A more complete stratigraphic record is formulated of this critical event in Earth’s history than previously available by integration of previous data and new analyses from DSDP Leg 30 sites combined with shipboard and post-cruise analyses from ODP Leg 192. The oldest sediment occurs within the upper part of the Leupoldina cabri planktonic foraminiferal zone, indicating equivalence with the last half of Oceanic Anoxic Event (OAE) 1a of which the Ontong Java eruption is a postulated cause. The remainder of the Aptian section is marked by major disconformities, with little section in common between central and marginal plateau sites. The Aptian–Albian boundary is conformable at both Leg 192 Sites 1183 and 1186 based on integration of biostratigraphy and preliminary δ 13 C data. However, the overall Albian interval is very incomplete, with regional distribution noted for only the lowermost and upper Albian sections.

Abstract Middle Miocene-upper lower Aptian calcareous nannofossils were recovered from Sites 1183–1187 drilled by Ocean Drilling Project Leg 192 on the Ontong Java Plateau. Nannofossil biostratigraphy indicates the presence of six unconformities among the five Leg 192 sites. These are: (1) between the lowermost Albian and upper middle Albian; (2) between the upper Albian and middle Coniacian; (3) within the lower Maastrichtian; (4) between the lower upper Maastrichtian and basal Danian; (5) within the upper Palaeocene; and (6) between the Oligocene and Miocene. Previous drilling before Leg 192 on the Ontong Java Plateau and in the SW Pacific (Legs 30 and 130) indicated two episodes for major emplacement of basement during the earliest Aptian and Turonian. The Leg 192 drilling was not able to confirm either of these episodes, but instead indicated that the emplacement of basement on the Ontong Java Plateau was relatively continuous from the latest early Aptian to latest Aptian. Results from Sites 1184 and 1185 indicate the possibility of another magmatic episode during the middle Eocene (Zone NP16).

Abstract 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 ε Nd ( 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 ε Nd ( 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.

Abstract 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-, Ndand 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 Fo 90 ) and 20.4% (Fo 92 ) 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.

Abstract 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.

Abstract 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 H 2 O. 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.

Abstract 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.

Abstract Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H 2 O, CO 2 , 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 H 2 O contents (average 0.22 wt% H 2 O) than high–MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low–MgO glasses have lower H 2 O (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 FeO T . This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K 2 O and S/TiO 2 with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S K α 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 H 2 O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H 2 O/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% H 2 O, corresponding to H 2 O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H 2 O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H 2 O content of the OJP mantle source region (170 ± 30 ppm H 2 O) is similar to that of the depleted MORB source (140 ± 40 ppm H 2 O). 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.

Abstract We present a detailed mineralogical and petrological description of the low-temperature alteration patterns in basalts from four new sites drilled during ODP Leg 192 on the Early Cretaceous Ontong Java Plateau. Three main alteration types have been identified: pervasively altered dark grey basalt; black or dusky green halos; and brown halos. Dark grey basalts are the most common and represent the least intensive, but most pervasive, alteration phase. Early interaction of the basalts with low-temperature sea-water-derived hydrothermal fluids lead to the development of black and dusky green halos characterized by the replacement of groundmass and olivine phenocrysts by celadonitic phyllosilicates and smectite. Later interaction of basalts with cold oxidizing sea water produced brown halos characterized by replacement of primary phases and mesostasis by smectite and iron oxyhydroxides. Secondary minerals in order of decreasing abundance include phyllosilicates, calcite, iron oxyhydroxides, pyrite, chalcedony, quartz and zeolites. Veins, resulting from symmetrical infilling of open cracks, commonly contain phyllosilicates, iron oxyhydroxide or pyrite, and late calcite. Carbonate veins cross-cut all other alteration features and stable isotope analyses of vein carbonates indicate formation from marine bicarbonate below about 40°C. A positive correlation between vein density and overall degree of alteration is observed resulting in pervasive development of brown alteration halos in highly fractured rocks. Overall, alteration of basalts from the Ontong Java Plateau is similar to that observed from other DSDP/ODP sites throughout the oceans.

Abstract 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.