Intraplate volcanism occurring far from active plate boundaries is commonly attributed to mantle plumes or lithospheric stress reorganization. However, several oceanic rises exhibit magmatic histories that challenge these conventional models. The Conrad Rise in the southern Indian Ocean represents a particularly enigmatic case of oceanic plateau formation. The Conrad Rise was previously interpreted as a Late Cretaceous oceanic plateau, but its origin and magmatic evolution remained poorly constrained.Recent geochronological and isotopic analyses of volcanic rocks from the Conrad Rise (Sato et al., 2024) have significantly revised this perspective. 40Ar/39Ar dating demonstrates that the primary volcanic edifices formed during distinct intraplate episodes in the middle–late Eocene (~40 Ma) and late Miocene (~8.5 Ma), significantly younger than the surrounding oceanic lithosphere (ca. 84 Ma). Furthermore, the Sr–Nd–Pb–Hf isotopic signatures cannot be explained by a single depleted mantle or plume-derived source and instead indicate contributions from enriched reservoirs, including components consistent with lower continental crust compositions.In addition to these volcanic constraints, dredging at the Conrad Rise has recovered granitoid and high-grade metamorphic rocks with clear continental affinities. These rocks record Proterozoic to early Paleozoic crustal histories comparable to those of the Gondwana terranes in East Antarctica and eastern India. The occurrence of continental-derived rocks in such a remote offshore setting recalls similar observations from the Rio Grande Rise in the South Atlantic. While alternative explanations, such as iceberg-rafted debris, must be considered, the size, abundance, and lithological diversity of the recovered rocks, together with the geochemical signatures of the associated volcanism, collectively suggest the involvement of continental material within or beneath the rise.We propose that the unusual episodic intraplate magmatism of the Conrad Rise may result from interactions between mantle upwelling and inherited lithospheric heterogeneity associated with continental components. This “hotspot-less” model, distinct from classical plume-head- or ridge-related mechanisms, drives episodic melt generation and compositional diversity, underscoring the critical influence of inherited lithospheric structures on offshore intraplate volcanism.

Numerous studies have shown that the mid-ocean-ridge basalts (MORBs) distributed in the southern Central Indian Ridge (CIR) exhibit the DUPAL anomaly; yet, the corresponding temporal variations of this signal remain underexplored in this region. Here, we report new Sr-Nd-Pb isotope data for the MORB which forms the southernmost segment of the CIR (CIR-1) and interpret these together with previously reported elemental data. Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust (LCC). This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly. In contrast, the MORB from the off-axis area shows a depleted trace element composition, particularly for highly incompatible elements. This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component. Furthermore, high 87Sr/86Sr, Delta 8/4, and Delta 7/4 values indicate that the source mantle of the MORB in the off-axis area of the CIR-1 segment has undergone at least two melting events, including the formation of the current MORB. The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios (Th/Pb and U/Pb) in the residual mantle, yielding an unradiogenic lead isotope composition. Overall, our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment, contributing to the formation of MORBs with an unradiogenic lead isotopic signature.

We recovered sedimentary and plutonic rocks from the off-ridge portion of the Southwest Indian Ridge (37 degrees 00.02' E, 44 degrees 49.73' 5, 2165 m deep). The petrography, geochemistry, and geochronology of these plutonic rocks were analyzed. Ar-Ar dating of biotite in the plutonic rocks yielded early Paleozoic ages of approximately 475 and 490 Ma, indicating that these rocks are likely related to Antarctic orogenies during the late Neo-proterozoic to early Paleozoic. In addition, the samples exhibited trace element compositions and low initial epsilon Nd values (-8 and -12) similar to those of rocks from western Dronning Maud Land and the Transantarctic Mountains, Antarctica. Such similarities suggest that these plutonic rocks from the off-ridge portion of the Southwest Indian ridge were derived from western Dronning Maud Land or the Transantarctic Mountains and deposited by melting icebergs as dropstones.

中央海嶺は,プレート形成場であり,マントル物質であるかんらん岩が部分融解してメルトが発生し,メルトの分離移動-固化によって玄武岩質海洋地殻と溶け残りかんらん岩が形成されていると考えられている。そのため,海洋底で採 取される深海性かんらん岩は,中央海嶺下での融解を記録していることを想定した議論が多くなされてきた。しかし,近 年の研究成果から,中央海嶺かんらん岩試料の中に,現在の中央海嶺活動以前に融解を経験したことを示唆するものが 含まれていることが指摘されている(Brandon et al., 2000; Standish et al., 2002; Harvey et al., 2006; Liu et al., 2008 )。マン トルの不均質性を生み出すモデルとして,沈み込んだ海洋地殻物質のマントルへの再混入モデルは広く受け入れられて いる(例えば,Hofmann, 1997)。沈み込んだ海洋地殻再混入モデルを受け入れるならば,沈み込んだ過去の溶け残りか んらん岩が混在していることも十分に可能である。 インド洋および南大西洋の海洋底玄武岩類は,他の地域と比較して,玄武岩の同位体比組成が異なることが知られ ている (例えば,Iwamori & Albarede, 2008; Iwamori et al., 2010) 。インド洋中央海嶺からは深海性かんらん岩も多く採取 されてい (Hellebrand et al., 2002; Seyler et al., 2003; Morishita et al., 2009, 2014; Zhou and Dick, 2013 ;Yi et al., 2014) 。イ ンド洋中央海嶺は北から南に向かい拡大速度が速くなることが知られており (DeMets et al., 2010) ,拡大速度と深海性か んらん岩の相関を検討する海域として,ひいては,広域的な海洋マントルの多様性とその要因に関する研究に適した海 域であると考えている。また,インド洋の深海性かんらん岩は,他の海域のものと比較してOs同位体比が高い傾向にあ る (仙田ら,2012)。 海洋研究開発機構の調査船により中央インド洋海嶺最南端領域から深海性かんらん岩が多く採取された。本地域のか んらん岩は,中程度にメルト成分に枯渇し,その後,分化したメルトによる組成改変を受けているという特徴を持つ。最 南端部の結果と,インド洋中央海嶺北部から採取されたかんらん岩と比較してみると,拡大速度や,海嶺セグメントの 中心部-末端部という関係に関わらず相関が見られず,比較的メルト成分に枯渇したかんらん岩が採取されている。また, 最南端部の中央海嶺軸に産する地形的な高まりから採取された試料は,記載岩石学的には斜方輝石が多く(つまりケイ素 に富むかんらん岩)であり,Os同位体組成的には高い特徴を持つことが明らかになってきた。これらの試料は,沈み込 む海洋プレートの影響を受けたマントルウエッジかんらん岩に期待される特徴である。これらを総合して考えると,イ ンド洋中央海嶺は,ゴンドワナ大陸が分裂した場所に形成された海洋であり,パンジアやゴンドワナ大陸を形成した時 (およびそれ以前の)履歴を記録したかんらん岩が中央海嶺下に混合している可能性が指摘できる。このようなかんらん 岩の中には,中央海嶺下での断熱上昇では玄武岩質の地殻を十分に形成するほどメルトを発生しない場合があり,かん らん岩が海洋底近傍まで上昇する可能性が考えられる。このような場合,引き続く海洋プレートの拡大によって断層の 形成,海水の浸透により蛇紋岩化するとことで,海洋底に深海性かんらん岩が露出しやすくなることが起き,マントル の不均質性が海洋プレートの構成そのものに影響を与えることが予想される。

Recent petrological and geochemical investigations of MORB at the southern segments of Central Indian Ridge (CIR) reveal the heterogeneous distributions of MORB-source mantle (Sato et al., 2015). Sato et al. (2015) concluded that MORB from CIR-S2 segment and off-ridge area at the CIR-S1 segment are depleted compositions than typical MORB. Furthermore, deple-tions based on trace element geochemistry of off-ridge MORB from CIR-S1 segment decrease toward present spreading ridge. Because off-ridge MORB was recovered from several dredge sites parallel to the flow line, these distributions might indicate spatial distributions of mantle heterogeneity beneath CIR-S1 segment.

Petrographic and geochemical characteristics of drill core and surface rock samples from the four hydrothermal vent sites of the Yamanaka, Snail, Archaean, and Pika sites at the Southern Mariana Trough (SMT) are described in order to clarify the geological background of hydrothermal activities. The core samples were drilled by the Benthic Multi-coring System (BMS), penetrating up to ~8 m below seafloor. The recovered samples included both basement rocks and sulfide ores. The basement rocks are further subdivided into basaltic andesite and andesite, most of which are recovered from on-axis and off-axis regions of the SMT, respectively. All of the rocks are characterized by relative enrichment of large-ion lithophile elements with noticeable depletion of Nb and Ta, suggestive of significant influence of subducted slab-derived components into the basement rocks both at the on- and off-axis hydrothermal vent sites. Major and trace element variations of the samples suggest that the Yamanaka, Snail, and Archaean rocks can be explained by a sequence of fractionation of an on-axis magma, whereas only the Pika rocks may be influenced by a different magma component, i.e., off-axis magmatism.