Beschreibung: Many oceanic plateaus have been emplaced at or adjacent to mid-ocean ridges. To explain plateau volume and thickened crust compared to normal oceanic crust, hotspot–ridge interaction is commonly assumed, but the manner of interaction remains unclear. The Shatsky Rise oceanic plateau is a large volcanic mountain that formed at a triple junction during Late Jurassic and Early Cretaceous time. Recent drilling and seismic investigations suggest that the intermediate edifice in the rise, Ori Massif, is a central volcano. Paradoxically, magnetic lineations were traced across parts of Ori Massif, implying formation at a spreading ridge. In this study, we examined magnetic anomalies over and around Ori Massif to obtain insights about the formation of this volcanic edifice. Magnetic data from 21 cruises were corrected, combined, and gridded to construct a magnetic anomaly map. Forward and inverse magnetic modeling was done to investigate the magnetic structure of Ori Massif. The results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks, analogous to magnetic anomalies recorded by spreading-ridges. This magnetic structure is not expected for a central volcano that was built by long runout lava flows, implying that Ori Massif eruptions must have been constrained near the ridge axis. Magnetic bights on the north and south boundaries of Ori Massif imply that it was bracketed by triple junctions, indicating complex ridge tectonics during the formation of Shatsky Rise. The surprising finding that Ori Massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness. Other oceanic plateaus also record linear magnetic anomalies, implying a link between divergent plate boundaries and oceanic plateau volcanism.

Beschreibung: A reconnaissance survey of Fe–Mn crusts from the 5000 km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40 mm), and those analyzed varied from 4 mm to 32 mm. However, crusts as thick as 80 mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe–Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific. The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably derived predominantly from weathering of local volcanic outcrops. Fe–Mn crusts from a dredge haul at 3412 m water depth, 2°S latitude, are pervasively phosphatized along with the substrate rocks (site D7). Phosphatization took place through replacement of carbonate, preferential replacement of Fe oxyhydroxide relative to Mn oxide in the crusts, preferential replacement of silica-rich phases relative to Al-rich phases in the crusts, and precipitation of carbonate fluorapatite in pore space. The preferentially replaced silica may have been Si adsorbed on the Fe oxyhydroxide. The enrichment of Ni, Zn, and Cu in the phosphatized crust reflects preferential adsorption into the tunnel structure of todorokite. The rare earth element plus yttrium (REY) patterns indicate a lower oxidation potential during phosphatization of the NER crusts compared to Pacific phosphatized crusts. NER phosphatization occurred in a deeper-water environment than typical for phosphatization of Pacific crusts, occurred post-middle Miocene, a younger age than phosphatization the Pacific crusts, and had in part a different set of chemical changes produced by the phosphatization than did the Pacific crusts. The southern third of NER has Fe–Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146 ppm), Pt (1.5 ppm), Ru (52 ppb), and Rh (99 ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe–Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

Beschreibung: International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Beschreibung: Shatsky Rise in the Northwest Pacific is the best example so far of an oceanic plateau with two potential hotspot tracks emanating from it: the linear Papanin volcanic ridge and the seamounts comprising Ojin Rise. Arguably, these hotspot tracks also project toward the direction of Hess Rise, located ∼1200 km away, leading to speculations that the two plateaus are connected. Dredging was conducted on the massifs and seamounts around Shatsky Rise in an effort to understand the relationship between these plateaus and associated seamounts. Here, we present new 40Ar/39Ar ages and trace element and Nd, Pb, and Hf isotopic data for the recovered dredged rocks and new trace elements and isotopic data for a few drill core samples from Hess Rise. Chemically, the samples can be subdivided into plateau basalt-like tholeiites and trachytic to alkalic ocean-island basalt compositions, indicating at least two types of volcanic activity. Tholeiites from the northern Hess Rise (DSDP Site 464) and the trachytes from Toronto Ridge on Shatsky’s TAMU massif have isotopic compositions that overlap with those of the drilled Shatsky Rise plateau basalts, suggesting that both Rises formed from the same mantle source. In contrast, trachytes from the southern Hess Rise (DSDP Site 465A) have more radiogenic Pb isotopic ratios that are shifted toward a high time-integrated U/Pb (HIMU-type mantle) composition. The compositions of the dredged seamount samples show two trends relative to Shatsky Rise data: one toward lower 143Nd/144Nd but similar 206Pb/204Pb ratios, the other toward similar 143Nd/144Nd but more radiogenic 206Pb/204Pb ratios. These trends can be attributed to lower degrees of melting either from lower mantle material during hotspot-related transition to plume tail or from less refractory shallow mantle components tapped during intermittent deformation-related volcanism induced by local tectonic extension between and after the main volcanic-edifice building episodes on Shatsky Rise. The ocean-island-basalt-like chemistry and isotopic composition of the Shatsky and Hess rise seamounts contrast with those formed by purely deformation-related shallow mantle-derived volcanism, favoring the role of a long-lived mantle anomaly in their origin. Finally, new 40Ar/39Ar evidence indicates that Shatsky Rise edifices may have been formed in multiple-stages and over a longer duration than previously believed.

Beschreibung: Recent research from the Shatsky Rise in the western Pacific Ocean provides new insights on the formation and evolution of this oceanic plateau as well as tests of mantle models to explain anomalous large igneous province (LIP) volcanism. Recent Shatsky Rise studies cored the igneous pile (Integrated Ocean Drilling Program Expedition 324), imaged the interior with seismic refraction and multichannel seismic reflection data, and mapped magnetic anomalies adjacent to the plateau to provide new constraints on its tectonic history. Coring data show that Tamu Massif, the largest edifice within Shatsky Rise, is characterized by massive sheet flows, similar to flows caused by voluminous eruptions in continental flood basalts. Core data also indicate that the massive eruptions waned as the plateau evolved and smaller edifices were built. Seismic data show intrabasement reflectors within Tamu Massif that indicate volcanism from its center, indicating that this is an enormous shield volcano with abnormally low flank slopes and thick crust (~ 30 km). Paleomagnetic data record minimal geomagnetic field variations, consistent with the inference of massive, rapid volcanism. Altogether, the physical picture indicates that Shatsky Rise was built by massive, rapid eruptions that formed enormous volcanoes. Geochronologic data support the previously inferred age progression, with the volcanic massifs formed along the trace of a triple junction starting from Tamu Massif and becoming progressively younger to the northeast. These data weaken support for rapid emplacement because they show that the last eruptions atop Tamu Massif encompassed several million years between the final massive flows as well as a long hiatus of ~ 15 Myr until late stage eruptions that formed a summit ridge. They may also indicate that the last eruptions on Tamu and Ori massifs occurred while the triple junction was hundreds of kilometers distant. Furthermore, magnetic anomaly data indicate that the plate boundary reorganization associated with Shatsky Rise formation occurred several million years prior to the first Tamu Massif eruptions, suggesting plate boundary control of Shatsky Rise initiation. Geochemical and isotopic data show that Shatsky Rise rocks are variably enriched, with the majority of lavas being similar to mid-ocean ridge basalts (MORB). However, the data indicate deeper (〉 30 km) and higher partial degree of melting (15–23%) as compared with normal MORB. Melting models indicate that the magma experienced a mantle temperature anomaly, albeit only a small one (~ 50 °C). Some lava compositions suggest the involvement of recycled subducted slab material. Recent investigations of Shatsky Rise initially envisaged a competition between two end-member models: the thermal plume head and the fertile mantle melting beneath plate extension (aka, plate model). Both hypotheses find support from new data and interpretations, but both do not fit some data. As a result, neither model can be supported without reservation. Noting that most basaltic oceanic plateaus have formed at triple junctions or divergent plate boundaries, we suggest that this dichotomy is artificial. Oceanic plateau volcanism is anomalous and focused at spreading ridges for reasons that are still poorly understood, mainly owing to uncertainties about mantle convection and geochemical reservoirs. Shatsky Rise investigations have vastly improved our understanding of the formation of this oceanic plateau, but highlight that important work remains to understand the underlying nature of this volcanism.

Beschreibung: Oceanic plateaus are huge basaltic constructions whose eruptions may briefly outstrip even global mid-ocean ridge magma production. Although they form great undersea mountains, their origins are poorly understood. A widely accepted explanation is that oceanic plateaus are built by massive eruptions from the head of nascent thermal mantle plumes that rise from deep in the mantle to the surface [e.g., Duncan and Richards, 1991]. An alternative is that plateaus erupt by decompression melting of fusible patches in the upper mantle at plate edges or zones of extension [Foulger, 2007].

Beschreibung: Integrated Ocean Drilling Program (IODP) Expedition 324 cored Shatsky Rise at five sites (U1346–U1350) to study processes of oceanic plateau formation and evolution. Site penetrations ranged from 191.8 m to 324.1 m with coring of 52.6 m to 172.7 m into igneous basement at four of the sites. Average recovery in basement was 38.7%–67.4%. Cored igneous sections consist mainly of variably evolved tholeiitic basalts emplaced as pillows or massive flows. Massive flows are thickest and make up the largest percentage of section on the largest and oldest volcano, late Jurassic age Tamu Massif; thus, it may have formed at high effusion rates. Such massive flows are characteristic of flood basalts, and similar flows were cored at Ontong Java Plateau. Indeed, the similarity of igneous sections at Site U1347 with that cored on Ontong Java Plateau implies similar volcanic styles for these two plateaus. On younger, smaller Shatsky Rise volcanoes, pillow flows are common and massive flows thinner and fewer, which might mean volcanism waned with time. Cored sediments from summit sites contain fossils and structures implying shallow water depths or emergence at the time of eruption and normal subsidence since. Summit sites also show pervasive alteration that could be due to high fluid fluxes. A thick section of volcaniclastics cored on Tamu Massif suggests that shallow, explosive submarine volcanism played a significant role in the geologic development of the plateau summit. Expedition 324 results imply that Shatsky Rise began with massive eruptions forming a huge volcano and that subsequent eruptions waned in intensity, forming volcanoes that are large, but which did not erupt with unusually high effusion rates. Similarities of cored sections on Tamu Massif with those of Ontong Java Plateau indicate that these oceanic plateaus formed in similar fashion.