Jun-Ichi Kimura

The Chichibu deposit is an Au–Cu‐rich Pb–Zn skarn deposit formed in the collision zone between the Izu–Bonin and Central Japan arcs. The Izu–Bonin arc is known to host M‐type granites, epithermal gold deposits, and Au‐rich seafloor hydrothermal deposits, indicating a metal‐rich magmatic province. This study geochemically investigates the origin of the Chichibu granitoid and evaluates how the granitic magma generated the Au–Cu‐rich Chichibu Pb–Zn skarn deposit. The Chichibu granitoids are characterized by low K ₂ O and high CaO contents, together with relatively low LILE abundances, similar to those of the nearby Kofu granitoids, which have been classified as M‐type granites and are considered to have formed from Izu–Bonin arc magmatism. Primitive mantle–normalized trace‐element patterns show relative enrichment in LILEs and depletion in Nb–Ta, accompanied by positive Zr–Hf anomalies, which are the signatures of island‐arc calc‐alkaline magmas. Chondrite‐normalized REE patterns show LREE enrichment, negative Eu anomalies, and relatively flat MREE–HREE patterns, which suggest magma generation under garnet‐absent conditions at pressures below 0.8 GPa in the lower crust. The Sr and Nd isotopic compositions of the Chichibu granitoids suggest that they formed through mixing between low‐K basalt derived from an immature arc and sediments of the Shimanto Group basement rocks. Model calculations using ⁸⁷ Sr/ ⁸⁶ Sr _i and SiO ₂ contents suggest a mixing ratio of approximately 90% low‐K Izu–Bonin arc magma and 10% Shimanto sedimentary rocks. Trace‐element mass‐balance modeling indicates that ~24% partial melting of lower‐crustal amphibolite derived from the basalt–sediment mixture could generate the Chichibu granitoid magma. Large variations in ⁸⁷ Sr/ ⁸⁶ Sr relative to the narrow range of ¹⁴³ Nd/ ¹⁴⁴ Nd values of the North Body indicate the contribution of slab‐derived fluids released from altered igneous oceanic crust within the Philippine Sea plate slab to the source region of the low‐K basalt magma. We argue that the basaltic magma was intrinsically fertile in Au and Cu and was ultimately responsible for the formation of the Chichibu Au–Cu‐rich Pb–Zn skarn deposit.

The origin of extremely diverse arc magmas erupted along the 600‐km‐long Quaternary frontal arc of NE Japan was examined using Sr–Nd–Hf–Pb isotopic and major/trace elemental compositions. The major/trace element chemistry of the magmas varied with the various combinations of low‐K to medium‐K and calc‐alkaline to tholeiitic rock suites, even within a single volcano. Nevertheless, magmas from individual volcanoes showed quasi‐linear isotopic variations in the Pb–Pb and Nd–Pb isotope spaces irrespective of their complex rock suites. The isotope data indicate a binary mixing origin of the magmas derived from a mostly homogeneous mantle source and the others from an isotopically diverse crust represented by Cretaceous to Paleogene basement granitoids. Assimilation‐fractional‐crystallization (AFC) and melting‐assimilation‐storage‐hybridization (MASH) processes formed the tholeiitic and calc‐alkaline series, respectively. These would have occurred in the amphibolitic lower crust that constitutes the roots of the basement granitoids. This study demonstrates that the mafic lower crust extensively contributes to the chemical diversity of arc magmas, even for relatively undifferentiated basaltic andesites.

In this study, the Sr isotope ratios (IRs; ⁸⁷Sr/⁸⁶Sr) of ferromanganese (Fe–Mn) crusts are analyzed through laser ablation inductively coupled plasma multiple-collector mass spectrometry. A sample collected from off Minamitorishima Island showed uniform Sr IRs (0.70906–0.70927) similar to that of present-day seawater with more than 36 mm thickness. Meanwhile, a detritus-rich sample collected from off northeast (NE) Japan showed a wide variation in Sr IRs (0.707761–0.709963). The Sr IR variation in the Fe–Mn crust from off NE Japan suggests detrital influx contributions from both the NE Japan arc (0.718). Detrital flux from the NE Japan arc increases from the bottom to middle layers, possibly due to the uplift of the Ou backbone range that occurred after ~2 Ma. The increased influx of the aeolian dust in the outer layer is attributable to global cooling in the Quaternary that increased the loess dust transportation from China to the western North Pacific Ocean. Meanwhile, the influence of the detrital influx on the sample from off Minamitorishima Island appeared to be negligible. The Sr IR analysis with high spatial resolution proposed in this study possibly improves the burial history of Fe–Mn nodules.

The Samoan mantle plume is thought to host three isotopically (radiogenic) distinct low‐³He/⁴He components including EM2 (enriched mantle 2), dilute HIMU (high μ = ²³⁸U/²⁰⁴Pb), and a depleted mantle (DM) component, which were sampled by shield‐stage lavas from the Malu, Vai, and Upo trend volcanoes, respectively. However, it is unclear whether the isotopically distinct components are present as different lithologies. Using new Fe–Sr–Nd–Pb isotope data for Tutuila basalts (Samoa), combined with literature data for other Samoan basalts, we attempt to infer the lithological structure of the Samoan plume. The results show that “Malu trend” basalts have heavier Fe isotopic compositions (δ⁵⁷Fe = 0.15–0.24‰) than “Vai trend” and “Upo trend” basalts. The latter two groups have average δ⁵⁷Fe of 0.14 ± 0.07‰ (2SD) and 0.11 ± 0.03‰ (2SD), respectively, similar to normal midocean ridge basalts (N‐MORBs, δ⁵⁷Fe = 0.15 ± 0.05‰, 2SD). The fractional‐crystallization‐corrected δ⁵⁷Fe values of all shield lavas are positively correlated with (Gd/Yb)_N, Pb/Nd and ⁸⁷Sr/⁸⁶Sr ratios whereas negatively correlated with Nb/Th and ε_Nd ratios, which cannot be explained by partial melting of a single garnet peridotite but point to heterogeneous source lithologies. The EM2 lavas are characterized with high δ⁵⁷Fe and (Gd/Yb)_N, low Nb/Th, and enriched Sr–Nd isotopic ratios, requiring a pyroxenitic source component with imprints of both recycled terrigenous sediments and oceanic crust. The Vai‐ and Upo‐trend lavas with MORB‐like δ⁵⁷Fe can be explained by partial melting of peridotitic sources, although different extents of refertilization by recycled crust are essential for generating their distinct radiogenic isotope signatures. These observations highlight the lithological heterogeneity of the Samoan plume and relates the EM2 component with a pyroxenitic lithology.

International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian continental margin, in the deep water Mentelle Basin (MB) and on the neighboring Naturaliste Plateau (NP). The drillsites are located on continental crust that continued rifting after seafloor spreading began further north on the Perth Abyssal Plain (PAP) between magnetochrons M11r and M11n (133–132 Ma), ending when spreading began west of the NP between chrons M5n and M3n (126–124 Ma). Drilling recovered the first in situ samples of basalt flows overlying the breakup unconformity on the NP, establishing a magnetostratigraphically constrained eruption age of >131–133 Ma, and confirming a minimal late Valanginian age for the breakup unconformity (coeval with the onset of PAP seafloor spreading). Petrogenetic modeling indicates the basalts were generated by 25% melting at 1.5 GPa and a potential temperature of 1380°C–1410°C, consistent with proximity of the Kerguelen plume during breakup. Benthic foraminiferal fossils indicate that the NP remained at upper bathyal or shallower depths during the last 6 Myr of rifting and for 3–5 Myr after breakup between India and Australia. The limited subsidence is attributed to heat from the nearby Kerguelen plume and PAP spreading ridge. The margin subsided to middle bathyal depths by Albian time and to lower bathyal (NP) or greater (MB) depths by late Paleogene time. Periods of rapid sedimentation accompanied a westward jump of the PAP spreading ridge (108 Ma), rifting on the southern margin (100–84 Ma), and opening of the southern seaway between Australia and Antarctica (60–47 Ma).

IODP Site U1437 is located in the Izu rear‐arc region, approximately 330 km west of the Izu‐Bonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6–15 Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]‐type, Rift‐type, and volcanic front [VF]‐type). RASC‐type has the most enriched Nd and Hf isotope and fluid‐immobile trace element ratios and dominates from 9 to 6 Ma. Rift‐type, dominant from 15 to 9 Ma, is similar to VF‐type in Nd‐Hf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VF‐type. Less common and randomly distributed VF‐type sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15 Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a melt‐rich slab component generated at ~125 km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial post‐Shikoku basin magmatism in the Izu rear‐arc generated Rift‐type magmas for about 6 million years before the distinctive RASC‐type magmatism began, which then became increasingly enriched.

Volcanoes in the East Asian/Pacific region have been the source of some of the largest magnitude eruptions during the Late Quaternary, and accurately evaluating their eruptive histories is essential for hazard assessments. To overcome difficulties in resolving and precisely dating eruptions in the near‐source realm, the high‐resolution (varved) sediments of Lake Suigetsu (central Honshu, Japan) were examined for the presence of non‐visible (cryptotephra) layers from 50 ka up until the 30 ka Aira‐Tanzawa (AT) caldera‐forming event of Aira volcano. Cryptotephra layers are four times more frequently preserved than visible markers in the Suigetsu sediments, meaning that this archive provides a unique and unprecedented record of eruptions that were dispersed over the densely populated regions of central Honshu. Major and trace element volcanic glass chemistry is used to fingerprint the ash layers and pinpoint their volcanic origin. Tephras are found throughout the investigated sediments, but the highest abundance of ash fall events are recorded between 39 and 30 ka, capturing a period of intense volcanism at calderas on Kyushu Island (Japan). The augmented Suigetsu tephrostratigraphy records at least seven eruptions from Aso caldera (southern Kyushu) that post‐date the widespread ACP‐4 Plinian eruption (ca. 50 ka) and four explosive events from Aira (central Kyushu) that occurred leading up to the catastrophic caldera‐forming AT eruption (ca. 30 ka).