Satoru Haraguchi

Reply to comment by I. Pineda‐Velasco, T. T. Nguyen, H. Kitagawa, and E. Nakamura on “Diverse magmatic effects of subducting a hot slab in SW Japan: Results from forward modeling”

In response to the subduction of the young Shikoku Basin of the Philippine Sea Plate, arc magmas erupted in SW Japan throughout the late Cenozoic. Many magma types are present including ocean island basalt (OIB), shoshonite (SHO), arc-type alkali basalt (AB), typical subalkalic arc basalt (SAB), high-Mg andesite (HMA), and adakite (ADK). OIB erupted since the Japan Sea back-arc basin opened, whereas subsequent arc magmas accompanied subduction of the Shikoku Basin. However, there the origin of the magmas in relation to hot subduction is debated. Using new major and trace element and Sr-Nd-Pb-Hf isotope analyses of 324 lava samples from seven Quaternary volcanoes, we investigated the genetic conditions of the magma suites using a geochemical mass balance model, Arc Basalt Simulator version 4 (ABS4), that uses these data to solve for the parameters such as pressure/temperature of slab dehydration/melting and slab flux fraction, pressure, and temperature of mantle melting. The calculations suggest that those magmas originated from slab melts that induced flux melting of mantle peridotite. The suites differ mostly in the mass fraction of slab-melt flux, increasing from SHO through AB, SAB, HMA, to ADK. The pressure and temperature of mantle melting decreases in the same order. The suites differ secondarily in the ratio of altered oceanic crust to sediment in the source of the slab melt. The atypical suites associated with hot subduction result from unusually large mass fractions of slab melt and unusually cool mantle temperatures.

REY-Rich Mud: A Deep-Sea Mineral Resource for Rare Earths and Yttrium

Abstract This chapter focuses on newly discovered extensive deposits of deep-sea mud containing high concentrations of rare-earth elements and yttrium (REY). The deep-sea REY-rich muds are found in pelagic region of the Pacific Ocean and very recently also in the Indian Ocean. REY-rich muds are characterized by the following five advantages: (1) tremendous resource potential by virtue of their wide distribution, (2) high REY concentrations with significant heavy REE enrichment, (3) a stratiform distribution that allows relatively simple and cost-effective exploration, (4) very low concentrations of radioactive elements such as Th and U, and (5) ease of extraction of REY by acid leaching. These features demonstrate that the REY-rich mud could constitute a highly promising REY resource for the future. A system to mine REY-rich muds is also presented. This system can be developed based on a system developed and tested for sulfide-rich muds in the Red Sea and manganese nodules in the Pacific Ocean, although pressurized air-lift system will be needed to lift REY-rich muds from very deep water.

Genesis of ultra-high-Ni olivine in high-Mg andesite lava triggered by seamount subduction

The Kamchatka Peninsula is a prominent and wide volcanic arc located near the northern edge of the Pacific Plate. It has highly active volcanic chains and groups, and characteristic lavas that include adakitic rocks. In the north of the peninsula adjacent to the triple junction, some additional processes such as hot asthenospheric injection around the slab edge and seamount subduction operate, which might enhance local magmatism. In the forearc area of the northeastern part of the peninsula, monogenetic volcanic cones dated at <1 Ma were found. Despite their limited spatiotemporal occurrence, remarkable variations were observed, including primitive basalt and high-Mg andesite containing high-Ni (up to 6300 ppm) olivine. The melting and crystallization conditions of these lavas indicate a locally warm slab, facilitating dehydration beneath the forearc region, and a relatively cold overlying mantle wedge fluxed heterogeneously by slab-derived fluids. It is suggested that the collapse of a subducted seamount triggered the ascent of Si-rich fluids to vein the wedge peridotite and formed a peridotite–pyroxenite source, causing the temporal evolution of local magmatism with wide compositional range.

Classification of geochemical data based on multivariate statistical analyses: Complementary roles of cluster, principal component, and independent component analyses

Identifying the data structure including trends and groups/clusters in geochemical problems is essential to discuss the origin of sources and processes from the observed variability of data. An increasing number and high dimensionality of recent geochemical data require efficient and accurate multivariate statistical analysis methods. In this paper, we show the relationship and complementary roles of k-means cluster analysis (KCA), principal component analysis (PCA), and independent component analysis (ICA) to capture the true data structure. When the data are preprocessed by primary standardization (i.e., with the zero mean and normalized by the standard deviation), KCA and PCA provide essentially the same results, although the former returns the solution in a discretized space. When the data are preprocessed by whitening (i.e., normalized by eigenvalues along the principal components), KCA and ICA may identify a set of independent trends and groups, irrespective of the amplitude (power) of variance. As an example, basalt isotopic compositions have been analyzed with KCA on the whitened data, demonstrating clear rock‒tectonic occurrence‒mantle end-member discrimination. Therefore, the combination of these methods, particularly KCA on whitened data, is useful to capture and discuss the data structure of various geochemical systems, for which an Excel program is provided. This article is protected by copyright. All rights reserved.

Origin of felsic volcanism in the Izu arc intra-arc rift

An intra-arc rift (IAR) is developed behind the volcanic front in the Izu arc, Japan. Bimodal volcanism, represented by basalt and rhyolite lavas and hydrothermal activity, is active in the IAR. The constituent minerals in the rhyolite lavas are mainly plagioclase and quartz, whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite. Both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range. The petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts. The hydrous mineral-free narrow mineral compositions and low-Al2O3 affinities of the IAR rhyolites are produced from basaltic middle crust under anhydrous low-temperature melting conditions. The IAR basalt lavas display prominent across-arc variation, with depleted elemental compositions in the volcanic front side and enriched compositions in the rear-arc side. The across-arc variation reflects gradual change in the slab-derived components, as demonstrated by decreasing Ba/Zr and Th/Zr values to the rear-arc side. Rhyolite lavas exhibit different across-arc variations in either the fluid-mobile elements or the immobile elements, such as Nb/Zr, La/Yb, and chondrite-normalized rare earth element patterns, reflecting that the felsic magmas had different source. The preexisting arc crust formed during an earlier stage of arc evolution, most probably during the Oligocene prior to spreading of the Shikoku back-arc basin. The lack of systematic across-arc variation in the IAR rhyolites and their dry/shallow crustal melting origin combines to suggest re-melting of preexisting Oligocene middle crust by heat from the young basaltic magmatism.