[S-CG55] 海洋底地球科学

[SCG55-22] 南チャモロ海山の蛇紋岩ブロックと変成岩片の岩石学的特徴: KM23-05航海による採集

*沢田 輝^1,2、大柳 良介^2,3、宮崎 淳一²、宮崎 征行²、岩本 久則⁴、高井 研² (1.富山大学、2.海洋研究開発機構、3.国士舘大学、4.日本海洋事業株式会社)

キーワード：沈み込み帯、泥火山、青色片岩、アンチゴライト

The Mariana island arc in the western Pacific is a subduction zone where oceanic plates converge, and numerous serpentinite mud volcanoes are present on its landward slope. These serpentinite mud volcanoes mainly consist of serpentinite breccia and serpentine mud, with clasts of high-pressure, low-temperature metamorphic rocks scattered within them. Since the 1980s, various rock sampling methods, including dredging, seabed drilling, and manned/unmanned submersible surveys, have been employed to study these formations, leading to petrological and geochemical analyses. These studies have elucidated the multi-stage serpentinization processes of the arc mantle and the metamorphic and alteration histories of subducted crustal materials. However, the amount of serpentinite seamount samples obtained by humans remains extremely limited compared to studies of terrestrial geological bodies. In March 2023, during the KM23-05 research expedition of the JAMSTEC research vessel Kaimei, drilling surveys were conducted using a seabed-installed Boring Machine System (BMS) for biogeochemical studies of serpentinization reaction zones at two serpentinite seamounts: South Chamorro and Asut Tesoru Seamounts. This study reports the fundamental descriptions of rock samples obtained from six drilling operations near the summit of the South Chamorro Seamount, yielding approximately 80 m core samples in total. The cores contained nine large (>30 cm) serpentinite blocks, with the other consisting mainly of serpentine mud and smaller (<10 cm) serpentinite clasts. The large serpentinite blocks exhibited mesh texture, bastite texture, and multiple generations of lizardite and chrysotile veins, as previously reported samples. One of these blocks contained an approximately 1 cm-wide antigorite vein in lizardite serpentinite displaying a mesh texture. Previously, antigorite at South Chamorro had only been detected in trace amounts via powder XRD analysis of serpentine mud, making this the first discovery of antigorite within a rock sample. Previous studies of Asut Tesoru and other seamounts where antigorite-bearing serpentinites have been reported suggest a process in which rocks initially serpentinized at shallow subduction depths undergo further serpentinization due to deep-seated fluid circulation. The presence of an antigorite vein at South Chamorro Seamount suggests that a similar serpentinization process was active there as well. Metamorphic rock clasts were also found within the serpentine mud, most of which were smaller than 1 cm and primarily composed of amphibole with accompanying chlorite, white mica, and titanite. Notably, two relatively large metamorphic rock clasts (one over 4 cm and another over 3 cm) composed predominantly of amphibole were recovered. These amphiboles exhibited a core composition rich in Ca and a rim composition enriched in Na and Fe, consistent with previous studies. Additionally, we identified clasts of metamorphic rocks containing chlorite, mica, and epidote, as well as small clasts of a rock composed of white fibrous tremolite crystals and another rock primarily consisting of monoclinic pyroxene, likely rodingite, which had been reported in previous studies. On the other hand, a small clast (~1cm) of chlorite schist containing euhedral hematite crystals was discovered, which had not been reported in previous studies. This rock appears to have formed under somewhat oxidizing conditions, but further investigation is required to determine its formation environment. Despite extensive oceanographic research on serpentinite seamounts, deep sea drilling continues to yield novel rock samples, highlighting its potential for future discoveries.