Abstract 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.

Abstract Volcanic ash represents a serious hazard to communities living in the vicinity of active volcanoes in developing countries such as Indonesia. Geoscience Australia (GA), the Australia–Indonesia Facility for Disaster Reduction (AIFDR) and Badan Geologi (Indonesia’s Geological Agency) have adapted an existing open-source volcanic ash dispersion model for use in Indonesia. An application example is presented here for Gunung Ciremai in West Java, Indonesia. A stochastic set of eruption events was simulated using eruptive parameters within the acceptable range of possible future events for this volcano: granulometry and a meteorological dataset that represents the complete range of possible wind conditions expected during seasonal wind conditions for the region. Implications for nearby communities of dry v. rainy season conditions on volcanic ash hazard were investigated. Communities on the western side of Gunung Ciremai were highly susceptible to volcanic ash ground loading regardless of the season: communities on the eastern side, however, were more susceptible during the rainy season months. This is attributed to prevailing wind conditions during the rainy season that include a strong easterly component. Disaster risk reduction workers can use hazard maps like those produced here to inform decision-making, and to focus mitigation efforts on communities most at risk.

A substantial portion of the Pacific basin is composed of seafloor formed during the Cretaceous Normal Superchron (CNS). Because this region lacks the magnetic lineations typically required to constrain tectonic reconstructions, we employ additional methods for interpreting CNS Pacific history, involving seafloor fabric, basement paleolatitudes, and age data. We utilize seafloor fabric, including fracture zones and the rift margins of large igneous provinces, to derive quantitative rotations. The timing of such rotations is constrained using rock ages, bounding magnetic isochrons, and estimates of interactions with surrounding terrains. The method relies on high-resolution shipboard bathymetry and rock ages, as much fine-scale seafloor fabric useful for reconstructions is not visible in satellite altimetry data. We show that the Ontong Java, Manihiki, and Hikurangi oceanic plateaus likely originated as one large superplateau, the Ontong Java Nui (OJN). Reconstructions of OJN at 123 Ma reveal large offsets between observed and predicted paleolatitudes. Observed paleolatitudes exhibit a systematic bias, which may be attributed to large-scale rotation of the entire plateau. Such a rotation would imply either that OJN was initially decoupled from the Pacific plate and able to rotate independently or that the orientation of the Pacific plate at 123 Ma differed from conventional model predictions. However, large uncertainties in absolute plate motion models prior to ca. 80 Ma preempt a conclusive interpretation for OJN formation. Given an ~10 km resolution limit for satellite altimetry, continued investments in seagoing research will be needed to investigate tectonic events in magnetic quiet zones.

High-Mg Kroenke-type basalts containing 9–11 wt% MgO from the Ontong Java Plateau (OJP) include spinels. These spinels have nearly identical Cr/(Cr + Al) ratios (0.45–0.54), and are generally hosted by olivine phenocrysts with relatively primitive compositions, with Fo contents [100 × Mg/(Mg + Fe 2+ ) in atomic ratios] as high as 88. This implies that the primary OJP magmas were in equilibrium with refractory peridotites (i.e., harzburgites) and were homogenized by large-scale melting and magmatic evolutionary processes. Oxygen geobarometry indicates that the OJP primary magmas record uniform oxygen fugacity ( f O 2 ) conditions and are slightly more oxidized (0.6 log units) than normal mid-ocean ridge basalts, but record less-oxidized f O 2 conditions than ocean island basalts. These data are consistent with previous studies that suggest the OJP primary magmas were generated by large-scale and extensive melting of a mantle source region under relatively oxidized conditions.

The Lyra Basin is believed to be a contiguous part of the Ontong Java Plateau (OJP), based on geophysical studies. Volcaniclastic rocks dredged at two sites in the Lyra Basin document another post-plateau episode of magmatism on the OJP; they are olivine-titanaugite-phyric alkali basalts with as much as ~30% modal phenocrysts. Lyra Basin basalts have compositions that vary from picritic (MgO ~22 wt%) to more evolved (MgO ~5 wt%) and have low SiO 2 (41–46 wt%), high TiO 2 (2–4 wt%), and high Na 2 O + K 2 O (1–5 wt%) contents that are distinctly different from tholeiites that compose the main OJP. The 40 Ar- 39 Ar weighted mean age of Lyra Basin basalts is 65.3 ± 1.1 Ma, determined using a single-grain laser fusion method of the ground-mass from the least altered alkali basalt and of biotite separates from differentiated samples. This age is interesting because it is much younger than the main stage of OJP formation (122 Ma) and no ca. 65 Ma alkaline basalts have been found previously near or on the OJP. Incompatible trace element modeling suggests that the volcanic rocks of the Lyra Basin may have been formed by a low degree of partial melting (~3%), predominantly at the garnet-lherzolite stability field from the same OJP mantle source preserved in its thick lithospheric root. However, major and trace elements and isotopic compositions can be better explained by magma mixing of Rarotongan alkali magma and magma derived from OJP-source mantle melting (12% partial melting at garnet stability field) in the ratio of 1:2. Although the trace element compositions of Lyra basalts can be reproduced by OJP-source mantle melting with or without contribution from the Rarotongan hotspot, the lower potassium content of the calculated Rarotongan hotspot-influenced melt is more compatible with that of an average composition of Lyra basalt. These results are consistent with previous reconstruction of the OJP path from 120 Ma to its present position, indicating that it may have passed over the Rarotongan hotspot at 65 Ma. In either case, the petrogenesis of Lyra Basin basalts highlights the role of the thick lithospheric root of the OJP in the late-stage development of the plateau. Additional evidence for episodic late-stage magmatic activity on the OJP helps to elucidate the magmatic evolution of the plateau and may provide insights into the origins of other large igneous provinces.

The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with ( 87 Sr/ 86 Sr) t = 0.704513–0.705105, ( 143 Nd/ 144 Nd) t = 0.512709–0.512749, ε Nd (t) = +3.0 to +3.8, ( 206 Pb/ 204 Pb) t = 18.488–18.722, ( 207 Pb/ 204 Pb) t = 15.558–15.577, and ( 208 Pb/ 204 Pb) t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected ( 187 Os/ 188 Os) t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their ( 176 Hf/ 177 Hf) t = 0.28295–0.28299 and ε Hf (t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.