New Mineralogical and Geochemical Features of Ferromanganese Ore Deposits in the Izu-Bonin and Mariana Island Arcs

Implications of island arc rotations to the studies of marginal terranes

Complex studies of the mineral composition and petromagnetic properties of the rocks which compose an edifice of the Minami–Khiosi submarine volcano located in the Mariana island arc are carried out for the first time. The Minami–Khiosi Volcano is a part of the Khiosi volcanic complex within the alkaline province of the Idzu–Bonin and Mariana island arcs. All of the rocks analyzed are enriched in K2O (1.34–3.30%), Ba (370–806 ppm), and Sr (204–748 ppm). The basalt has a porhyric texture and contains mosTy olivine phenocrysts as individual crystals and growths with a size up to 2 cm; the groundmass is finecrystalline. The samples studied contain at least three Fe-bearing oxide minerals. These are predominant magnetite and less abundant ilmenite and Fe hydroxides. It is established that the samples studied are magnetically isotropic and have high values of natural remanent magnetization and Konigsberger ratio. Similarly to the other island-arc Late Cenozoic submarine volcanoes in the western part of the Pacific Ocean, the samples studied are strongly differentiated by the value of natural remanent magnetization and magnetic susceptibility. The low-coercivity magnetic minerals (titanomagnetite and magnetite) of the pseudo-single-domain structure, as well as high-coercivity minerals (hematite) are the main carriers of magnetization. The high values of natural remanent magnetization are explained by the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility result from the high concentration of ferromagnetic grains.

Some features of the ferromanganese deposits of the Izu-Bonin and Mariana Island arcs tested in the 1st (1977) and 5th (1978) flights of the R/V Volcanologist are considered. Two types of genetic deposits have been identified - hydrogenic and low-temperature hydrothermal. The main ore minerals of hydrogenic ferromanganese deposits are poorly crystallized structures with a low degree of ordering Fe-vernadite and Mn-feroxygite, with a smaller amount of goethite and bernessite. Low-temperature hydrothermal deposits consist either primarily of bernessite, as well as vernadite and goethite, or of hematite, goethite and feroxygite. Hydrogenic ferromanganese deposits of the Izu-Bonin and Mariana Island arcs are characterized by a Mn/Fe ratio of 0.84-1.36, for low-temperature hydrothermal deposits of the Izu-Bonin arc it is 6.13-13.9. It was found that the content of Co, Ni and Cu is significantly higher in the crusts of the Mariana arc compared with the crusts of the Izu-Bonin arc. The contents of the remaining cations of heavy and rare metals - Pb, Cd, Ba, Sr and others - in the crusts of both arcs are close to each other. The content of most rare earth metal cations in the hydrogenic deposits of the Izu-Bonin and Mariana arcs is comparable to each other. Low-temperature hydrothermal ferromanganese deposits of the Izu-Bonin arc differ significantly from hydrogenic deposits with a low (by 1-2 orders of magnitude) content of cations of non-ferrous, heavy and rare metals. The contents of rare earth metal cations in low-temperature hydrothermal samples of the Izu-Bonin arc are low and range from 0.24 (Tm, Lu) to 32.35 mcg/g (Y). Among all rare earth metal cations, Y, Ce, and Nd are present in the largest amounts in these samples.

Site 1201D of Ocean Drilling Program Leg 195 recovered basaltic and volcaniclastic units from the West Philippine Basin that document the earliest history of the Izu–Bonin–Mariana convergent margin. The stratigraphic section recovered at Site 1201D includes 90 m of pillow basalts, representing the West Philippine Basin basement, overlain by 459 m of volcaniclastic turbidites that formed from detritus shed from the Eocene–Oligocene proto-Izu–Bonin– Mariana island arc. Basement basalts are normal mid-ocean ridge basalt (N-MORB), based on their abundances of immobile trace elements, although fluid-mobile elements are enriched, similar to back-arc basin basalts (BABB). Sr, Nd, Pb and Hf isotopic compositions of the basement basalts are similar to those of basalts from other West Philippine Basin locations, and show an overall Indian Ocean MORB signature, marked by high Pb/Pb for a given 206Pb/204Pb and high 176Hf/177Hf for a given 143Nd/ Nd. Trace element and isotopic differences between the basement and overlying arc-derived volcaniclastics are best explained by the addition of subducted sediment or sediment melt, together with hydrous fluids from subducted oceanic crust, into the mantle source of the arc lavas. In contrast to tectonic models suggesting that a mantle hotspot was a source of heat for the early Izu–Bonin– Mariana arc magmatism, the geochemical data do not support an enriched, ocean island basalt (OIB)-like source for either the basement basalts or the arc volcanic section.

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back‐arc basin intersects the line of arc volcanoes. They develop well‐defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back‐arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault‐controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.

Cycling of sulfur in subduction zones: The geochemistry of sulfur in the Mariana Island Arc and back-arc trough

Twenty‐five microbial communities were sampled from 18 different hydrothermal systems located at seven different sites along the Mariana Island Arc and at a single site from the southern Mariana Spreading Center over a 3‐year period. Terminal restriction fragment length polymorphism (T‐RFLP) analysis of the small subunit rRNA gene revealed that the microbial community diversity is much greater along the Mariana Arc/back‐arc than at either hot spot volcanoes or mid‐ocean ridges along the same spatial scale. Cluster analysis of T‐RFLP fingerprints reveals the microbial communities formed three distinct clusters designated Mariana clusters I, II, and III. Microbial communities in Mariana Cluster I are all associated with iron‐rich microbial mats and are dominated by members of the ζ‐Proteobacteria and by unique phylotypes clustering deeply in the δ‐Proteobacteria and within the Nitrospira division. Mariana Cluster II communities are all from shallow hydrothermal systems and mostly from colder sediments or microbial mats that are dominated by putative heterotrophic phylotypes usually associated with seawater and sediments not generally associated with hydrothermal fluid inputs. Mariana Cluster III is generally from much hotter vent sites and is dominated by sulfur‐oxidizing ɛ‐Proteobacteria. Quantitative‐polymerase chain reaction (Q‐PCR) of Archaeal abundance reveal that all of the microbial communities are dominated by members of the Bacterial domain. Sampling of microbial mats from Iceberg Vent at NW Rota‐1 in 2004 and again in 2006 reveal the community has shown a transition from Caminibacter group ɛ‐Proteobacteria phylotypes to a mixed population of Caminibacter, Sulfurovum, and Sulfurimonas group ɛ‐Proteobacteria.

Trace-element and isotopic constraints on the source of magmas in the active volcano and Mariana island arcs, Western Pacific

A generalization of the available original data and literature data on the geological and geophysical knowledge of the underwater volcano Esmeralda, located in the Mariana Island Arc, has been carried out. As a result of studying the rocks dredged during the 4th and 5th cruises of the R/V Vulkanolog at the present level, new data were obtained on the silicate and rare-element composition of the rock samples that make up this underwater volcano. It has been established that the studied volcanic edifice is composed of five types of rocks: basalts, basaltic andesites, dacites, gabbro, and basanites. For the first time, samples of dacite and basanite have been discovered, indicating that the petrochemical diversity of the underwater volcano Esmeralda is wider than previously thought. All dredged rocks are characterized by a slightly increased content of incoherent elements LILE and HFSE. The studies carried out made it possible to attribute the main part of the dredged rocks to the association of island-arc ferruginous tholeiites (IAB, IAT) and only the composition of a single sample of alkaline basalt (basanite) falls into the field of alkaline basalts of oceanic islands (OIB, OIA). The increased content of iron in plagioclase phenocrysts confirms that the rocks belong to the high-iron tholeiite association.

Volatiles in submarine volcanic rocks from the Mariana Island arc and trough

Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs

The first sampling of mantle horizons underlying the ophiolite peridotite complex of the Mariana back-arc basin was carried out based on a spinel lherzolite xenolith in basanites from the submarine Esmeralda Volcano. The paper shows significant differences between the bulk, petrological, geochemical, and mineralogical compositions of this xenolith and similar characteristics of ophiolite peridotites dredged from the Central Trough of the north part of the Mariana Trench, on the west wall of one of the fault zones of the Santa Rosa Bank Fault (southern part of the trough), and ultramafic rocks dredged from the Conical and Torishima seamounts in the fore-arc part of the Mariana and Idzu-Bonin Island Arc Systems. Using different geothermometers and geobarometers, we revealed the PT conditions for the formation of Esmeralda xenolith’s mineral paragenesis. The optimal temperature is 950–1050°C and the pressure is 13–15 kbar. These parameters significantly exceed those suggested for the compared ophiolite peridotites.

Mt Pagan is an active volcano in the Mariana Island Arc, having produced a large eruption (>0.1 km3) in 1981, and recent smaller eruptions (>0.001 km3) in2011 and 2012. In this study we examine available Interferometric Synthetic Aperture Radar(InSAR) from Envisat and ALOS between October 2004 and June 2010 for precursory deformationleading up to the most recent eruptions. Although we do not observe evidence for shallowmagma reservoir inflation, we do observe a zone of subsidence of up to -3 cm/yr located onthe northern flank likely caused by continuing scoria compaction of 1981 eruptive deposits.CosmoSkyMed InSAR data spanning the October 2012 eruption also shows deposit subsidence,but no detectable co-eruptive deflation. We also present remotely-sensed thermal time series fromthe MODIS and ASTER instruments spanning over a decade. There are two distinct locationsof anomalous surface temperatures on the island, which correlate with previously documented,fumarolically active vents at South Pagan and Mt. Pagan. While elevated temperatures duringthe eruptions are visible from space in both ASTER and MODIS data, we find no clear thermalprecursors before eruption. The available data may have had insufficient spatial and temporalresolution to detect either deformation or thermal precursors if the magmatic system had rapidtransitions between quiescence and eruption. Looking ahead, satellite sensors with improvedspatial and temporal resolution will help characterize transient dynamics at small and remotevolcanic islands such as Pagan.

Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.

Esmeralda Bank is the southernmost active volcano in the Izu-Volcano-Mariana Arc. This submarine volcano is one of the most active vents in the western Pacific. It has a total volume of about 27 km3, rising to within 30 m of sea level. Two dredge hauls from Esmeralda recovered fresh, nearly aphyric, vesicular basalts and basaltic andesites and minor basaltic vitrophyre. These samples reflect uniform yet unusual major and trace element chemistries. Mean abundances of TiO2 (1.3%) and FeO* (12.6%) are higher and CaO (9.2%) and Al2O3 (15.1%) are lower than rocks of similar silica content from other active Mariana Arc volcanoes. Mean incompatible element ratios K/Rb (488) and K/Ba (29) of Esmeralda rocks are indistinguishable from those of other Mariana Arc volcanoes. On a Ti-Zr plot, Esmeralda samples plot in the field of oceanic basalts while other Mariana Arc volcanic rocks plot in the field for island arcs.