[S-CG51] ハードロック掘削科学：陸上・深海底掘削、そしてオフィオライト

[SCG51-07] Application of shipboard natural gamma-ray spectra at DV JOIDES resolution for objective description of serpentinite core drilled in IODP LEG 402

*森下 知晃^1,2、安邊 啓明³、マリア フィロメナ⁴、イリーナ フリナ⁵、ブランドン シャック⁶、秋澤 紀克⁷、マロン ビッカート⁸、エミリー カニンガム⁹、アガチャ ディステファーノ¹⁰、チ フ¹¹、スワンヌ ゴッチャレット¹²、ロルナ ケアンズ¹³、ラヴィ コーラパティ¹⁴、チャオ レイ¹⁵、ルカ マグリ¹⁶、ウオルター メナペース¹⁷、アシュトッシュ パンディ¹⁸、ヴィクトリア パルヴィクス⁹、フィリッペ ペダード¹⁹、イリニ ポーラキ⁶、ミレナ ロドリゲスープリコ²⁵、アレッシオ サンフィリッポ²⁰、パオラ バヌッチ²¹、田村 明弘¹、シャンユー チャオ²²、エミリー エステス²³、アルベルト マリンベルノ²⁴、ネヴィオ ジッタリーニ⁴ (1.金沢大学理工研究域地球社会基盤学系、2.海洋研究開発機構、3.国立研究開発法人日本原子力研究開発機構 東濃地科学センター、4.ISMAR、5.University of Nebraska、6.Louisiana State University、7.広島大学、8.Brest University、9.University of Utah、10.University of Catania、11.University of Houston、12.Sorbonne University、13.University of Texas at Austin、14.Binghamton University、15.China University of Geosciences、16.University of Tasmania、17.MARUM、18.Indian Institute of Science Education and Research Thiruvananthapuram、19.Geosciences Montpellier、20.University of Pavia、21.University of Florence、22.Shanghai Jiao Tong University、23.NSF、24.LDEO, Columbia University、25.Texas A&M University at Galveston)

キーワード：IODP EXP402、JOIDES、natural gamma-ray spectra、Serpentinite

A nondestructive description of core samples obtained by drilling is an essential first step in understanding the record of the cored samples. After preparation of the core samples, a variety of whole-round section measurements were carried out as follows: whole-round surface imaging (four surface of quarter image scan orthogonal to angles of 0, 90, 180 and 270 relative to the CRF), loggings using the whole-round multi sensor logger (WRMSL) with the magnetic susceptibility loop sensor (MSL) and the gamma ray attenuation (GRA) detector, and the Natural Gamma Radiation Logger (NGRL). Natural gamma radiation (NGR) emitted when ⁴⁰K, ²³⁸U, and ²³²Th radioisotopes and their daughter products decay has been routinely measured by onboard equipment on the IODP drillship JOIDES Resolution since March 2009 (from Expedition 317) (Vasiliev et al., 2011 Jour. Appl. Geophys.). NGR spectra show sufficient energy resolution to identify the major peaks of the isotope (Vasilev et al., 2011) .
International Ocean Discovery Program Expedition 402 drilled into the Central Vavilov Basin of the Tyrrhenian Sea, where the continental-ocean transition (COT) is exposed Malinverno, Zitellini, Estes and the Expedition 402 Scientists, 2024 Preliminary Reports). Cores over 100 m long, consisting mainly of serpentinite, were recovered from two sites (U1614 and U1616). NGR variations in one of the serpentinite cores (Site 614) were investigated and their implications were discussed. The measurement interval of the NGR detector is about 10 cm. Since recovered cores are usually fragmented, edge effects must be considered. Total NGR shows a maximum of 120 cps in the core, but most are less than 20 cps. High NGRs above 20 cps appear to be associated with granitic rocks. We chose to focus only on variations in NGR below 20 cps. The total NGR aviation appears to be roughly consistent with the variation in the Uranium signal. Several very low NGR (< 5 cps) intervals are observed in the core. These low NGR intervals are characterized by “homogeneous” serpentinite. One of the mineral phases that may contain uranium in serpentinite cores is carbonate minerals. Carbonate veins and replacement by carbonate minerals is observed heterogeneously in the core. In the presentation, we will introduce the relationships between chemical compositions of minerals and NGR spectra of representative core and evaluate NGR analysis in nondestructive description of serpentine cores.