The earliest stage of Izu rear‐arc volcanism revealed by drilling at Site U1437, International Ocean Discovery Program Expedition 350

The International Ocean Discovery Program Expedition 350 drilled between two Izu rear‐arc seamount chains at Site U1437 and recovered the first complete succession of rear‐arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear‐arc seamounts came into existence at this site, were recovered in the deepest part of the hole (∼15–16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear‐arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear‐arc seamount chains. It is suggested that following the opening of the Shikoku back‐arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at ~ 9 Ma by the start of rear‐arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back‐arc Basin opening.


Site U1437 of the International Ocean Discovery Program (IODP)
Expedition 350 is located between the Manji and Enpo rear-arc seamount chains. At this site, the recovery of an in situ volcanic succession subsequently covered by > 1400 m of sediments provides the first information on the magmatism before the formation of the rear-arc seamount chains.
In this study, we present whole rock chemical compositions collected from the deepest part of the drilling site, which we interpret as not derived from the Izu rear-arc volcanism in its current form. This study thus reveals important information on the previous stage of Izu rear-arc volcanism and on the complexity of arc and rear-arc magmatism.

| Background
The Izu-Ogasawara (or Izu-Bonin) arc crust, south of Honshu Island, Japan, has been formed by arc magmatism related to the subduction of the Pacific plate beneath the Philippine Sea plate. It is an excellent example of the possible formation of juvenile continental crust in an intra-oceanic convergent margin system (Kodaira et al., 2008;Suyehrio, Takahashi, Ariie, & Yokoi, 1996;Tamura et al., 2016;Tamura, Ishizuka, Sato, & Nichols, 2019). Figure 1b shows the present Izu arc system, which consists of three types of volcanic environments: (i) Quaternary arc-front volcanoes that are parallel to the Izu-Ogasawara trench (volcanic front); (ii) Miocene-Pliocene rear-arc seamount chains consisting of volcanoes lying at a high angle (60-70 ) to the arc; and (iii) < 2.8 Ma (Ishizuka, Uto, & Yuasa, 2003;Ishizuka, Uto, Yuasa, & Hochstaedter, 1998) bimodal rift-type volcanoes in the extensional zone between the volcanic front and rear-arc seamount chains.
The rear-arc seamount chains have been interpreted to have formed after cessation of seafloor spreading in the Shikoku back-arc Basin at~15 Ma (Okino, Ohara, Kasuga, & Kato, 1999). The seamount chains show a trend toward becoming younger from west to east (Ishizuka, Uto, & Yuasa, 2003). The formation of the seamount F I G U R E 1 (a) The structure of the Izu-Ogasawara-Mariana arc system (Taylor, 1992). Double lines indicate spreading centers, active in the Mariana Trough and relic in the Shikoku and Parece Vela Basins. The Izu-Ogasawara, West Mariana, and Mariana arcs are outlined by the 3 km bathymetric contour and other basins and ridges are outlined by the 4 km contour. (b) Seafloor bathymetry of the northern Izu-Ogasawara arc. The Quaternary arc-front volcanoes (Oshima, Toshima, Miyakejima, Mikurajima, Kurose, Hachijojima, Aogashima, Myojin Knoll, Myojinsho, Sumisu, Torishima) and Pliocene-Miocene rear-arc seamount chains (Kan'ei, Manji, Enpo, and Genroku Chains and Horeki Seamount), and Sumisu rift in the extensional zone are shown. The summits of many frontal volcanoes have grown above sea level to form the Izu-Ogasawara (Bonin) islands. White stars, the locations of IODP EXP350 Sites U1436 and U1437; dashed lines, eastern and western margins of extensional zone chains might be related to compression caused by collision between the southwest Japan and Izu arcs associated with the Japan Sea opening (Bandy & Hilde, 1983;Karig & Moore, 1975), or they might overlie Shikoku Basin transform faults (Yamazaki & Yuasa, 1998). Kodaira et al. (2008) showed that the north-south spacing of rear-arc volcanoes is not linked to the underlying undulating thick and thin crustal structure, suggesting that thick parts of rear-arc crust had been produced in Oligocene before the formation of the Shikoku Basin . Another possible hypothesis is that the seamount chains overlie hot regions in the mantle wedge, such as the hot fingers proposed for northeast Japan that intruded into the mantle wedge from west to east (Honda, Yoshida, & Aoike, 2007;Tamura, Tatsumi, Zhao, Kido, & Shukuno, 2002).
IODP Expedition Site U1437 is located in a volcano-bounded basin between the Manji and Enpo rear-arc seamount chains at a depth of 2117 m below sea level, and~90 km west of the arc-front volcano Myojinsho (Figure 1b). Figure 2 shows the age-depth diagram of Site U1437. A seismic reflection profile, showing the nature of the studied units, is presented in Yamashita, Takahashi, Tamura, Miura, and Kodaira (2018). Drilling at Site U1437 reached 1806.5 m below seafloor (mbsf). During the expedition, the recovered rocks were divided into seven lithostratigraphic units from Unit I to Unit VII and one igneous unit (Igneous Unit 1) within lithostratigraphic Unit VI that consists of a peperitic rhyolite intrusion (Tamura et al., 2015b). Overlying volcaniclastic sediments (Units I to Unit VI) were derived from nearby rear-arc and/or distal volcanic front volcanoes (Tamura et al., 2015b； Gill et al., 2018. Magnetic stratigraphy derived from polarity reversals was used down to 1300 mbsf at the base of Unit V, suggesting~9 Ma (Tamura et al., 2015b).
Secondary ion mass spectrometry U-Pb zircon ages obtained from Igneous Unit 1 and Unit VII are (13.9 ±0.2) Ma and (15.4 ±0.8) Ma, respectively (Schmitt et al., 2018). Unit VII records the oldest magmatism discovered during Expedition 350, and it occurred after the end of the adjacent Shikoku back-arc Basin spreading. Parts of Unit VII are thick monomictic breccia interpreted to be hyaloclastite deposits showing macroscopic textural evidence of quench fragmentation (e.g., quenched concave margins, and emplacement at high temperature). In the lower part of Unit VII, Core Section 70R2 (1721 mbsf) is made of metric domains of very angular, moderately vesicular lava surrounded by jigsaw fit breccia of the same composition, in which alteration pipes occur, all attesting to in situ hyaloclastite. Alteration pipes in Core Section 68R3 (1703 mbsf) suggest another in situ facies. Core Section 59R2 (1623 mbsf) contains numerous coarse, poorly vesicular lava clasts with delicate quench margins, and may also represent in situ facies. Cores 42R to 54R consist of a > 100 m-thick, black, framework-supported, hyaloclastite breccia that broadly grades from being dense clast-dominated to pumice clast-dominated. With the exception of some hydrothermally altered red clasts, the breccia is very homogeneous in type of components and grain size, and is considered monomictic overall. Most of the unit is nonstratified, although its top includes normally graded beds. The presence of stratification attests to at least partial resedimentation, but the overall homogeneity indicates that it is a proximal hyaloclastite facies, with minimal resedimentation involved.
These lines of evidence suggest that these are near-vent facies and, therefore, representing magmas emplaced at or at close distance from the drill site.
F I G U R E 2 Lithostratigraphic units and age-depth plot of Site U1437. Lithostratigraphic unit depths (Units I to VII and Igneous Unit 1) are from Tamura et al. (2015b). Plus signs show biostratigraphic and paleomagnetic age data from Tamura et al. (2015b). Filled circles show 40 Ar/ 39 Ar and U-Pb zircon age data from Schmitt et al. (2018).    Trace element (ppm) by ICP-MS  Pr How much transport affected the rest of Unit VII is more ambiguous.
Some intervals are clearly graded, stratified, and/or polymictic, attesting to lateral transport and accidental pick-up from the substrate. The provenance of other intervals cannot be assessed with confidence because of poor core recovery. Nevertheless, apart from a few fine-grained intervals, most of Unit VII is made of thick beds of angular, coarse ash-size to lapillisize hyaloclasts and lava clasts, strongly suggesting that they are derived from sources less than a few hundred meters away, thus not associated with long-distance transport. Most of the analyzed clasts are atypically large (2-42 cm) and dense (non-to moderately vesicular). The provenance of these coarse clasts may be linked to accidental pick-up from the substrate during local avalanching of the hyaloclastites, or they may represent in situ subsurface intrusive hyaloclasts. The large range in textures shown in these clasts therefore makes them representative of a broader region upslope (hundreds of meters to a few kilometers), and not only derived from a single locality or vent. Large chemical variations of clasts from Unit VII are consistent with this provenance model.

| Samples and methods
Samples were selected from Unit IV to Unit VII of Site U1437 (Figure 2;

| Sample preparation
Rocks were sawn and polished to extract individual clasts. These clasts were crushed into pebble sizes (5-10 mm) and soaked in plenty of slowly flooded hot water of~40 C for 2 weeks. Then, these clasts were put into a glass beaker with distilled water, which was boiled in a microwave oven. The addition of fresh distilled water and boiling were repeated until an addition of silver nitrate solution stops to precipitate silver chloride. After the desalinization, all samples were washed with pure water and acetone using an ultrasonic cleaner.
Coarse samples were pounded in an iron mortar, and altered pieces were removed by hand picking as much as possible. All samples were pulverized using a polycarbonate tube and alumina rod.

| Major and trace element analyses
For analysis of major elements, a mixture of 0.4 g of sample powder and 4 g of Li 2 B 4 O 7 was fused and vitrified. This glass bead was analyzed using a Rigaku Simultix 12, X-ray fluorescence (XRF) analyzer, following the method of Tani, Kawabata, Chang, Sato, and Tatsumi (2006). Accuracy and reproducibility for major elements are better than 1 % and better than 2 % relative standard deviation (2 standard deviation), respectively.

| RESULTS
Major and trace element concentrations in the samples from Unit IV to Unit VII (Figure 2) are presented in Table 1 Sun and McDonough (1989). The depletions in Nb and Ta of Unit VII are much larger than those of Unit V of Mariana rear-arc, respectively (Tamura et al., 2014). Thus, in terms of the negative anomalies of Nb and Ta, Unit VII and Unit V are similar to the volcanic front and the rear-arc, respectively. Interestingly, Igneous Unit 1 is rhyolitic in composition, and its incompatible elements are generally lower than in basalts and basaltic andesites of Units V and VII.
Igneous Unit 1 is highly depleted in Sr and Ti in particular ( Figure 5).

| Volcanic front vs rear-arc
The geochemical differences between volcanic front and rear-arc volcanoes could be related to spatial variations in subduction components within the subducting plate, which were added at different depths in the overlying mantle wedge during subduction before or during its melting (Ishizuka et al., 2006;Ishizuka, Taylor, Milton, & Nesbitt, 2003;Tamura et al., 2007;Tamura et al., 2014). The mantle F I G U R E 6 Legend on next column.   intruding into the mantle wedge. In this scenario, the magma is first extracted beneath the rear-arc and then extracted again as the depleted mantle moves toward the volcanic front where it is fluxed by fluids from the slab (Hochstaedter et al., 2000;Hochstaedter et al., 2001). The geochemical signature of the generated magmas reflects the subduction components and the original mantle wedge (Hochstaedter et al., 2001;Straub, Woodhead, & Arculus, 2015;Tollstrup et al., 2010). Tamura et al. (2014) found that these distinct subduction components generate two primary magmas at the single volcanic front volcano of Pagan, Mariana arc, and suggested that these two subduction components can exist separately, because the hydrous fluid is a hydrous carbonatite, which is immiscible with a silicate melt (sediment melt) (Tamura et al., 2014).
In summary, the volcanic front and rear-arc magmas are generated through partial melting of the underlying mantle wedge that con-  (Figures 4a and 6), and the~8 Ma stratigraphic age of these samples (Figure 2) corresponds to the radiometric age of nearby rear-arc seamount chain volcanoes (Ishizuka, Uto, & Yuasa, 2003). This indicates that these Unit V samples have been reworked from neighboring rear-arc seamounts at~8 Ma.
Unit VII samples have flat patterns or concave REE patterns with a maximum at Nd (Figure 4b). Some of the extensional zone basalts (Tollstrup et al., 2010) between the volcanic front and the rear-arc seamount chains have concave REE patterns with a maximum at Nd ( Figure 4c). Their low Ba/La and La/Sm ratios also are similar to some of Unit VII ( Figure 6). Extensional zone basalts are related to rifting within the Izu arc. They have been influenced by subduction components depending on the distance from the volcanic front (Hochstaedter et al., 2001).

| The early Izu rear-arc magmatism
The rear-arc seamount chain-type volcanism occurred at about 17-8 Ma in the western part of this broader zone (i.e. East Shikoku Basin seamounts and the westernmost seamounts of the chains), and migrated to the eastern part of the chains over time during 8-3 Ma (Ishizuka et al., 1998;Ishizuka et al., 2009;Ishizuka, Uto, & Yuasa, 2003). Ishizuka et al. (2009) propose that this migration was caused by steepening of the subducting slab. At Site U1437, volcanic front and/or rift-type magmatism (Unit VII) is overlain by sediments from the rear-arc seamount chain-type magmatism (Unit V).
The difference between Units VII and V indicates a temporal change of the subduction components, and supports the slab steepening hypothesis. Moreover, intrusions of hot fingers into the mantle wedge could play an important role in the genesis of arc volcanoes (Tamura, 2003;.