Serpentinite Seamounts and Hydrated Mantle Wedge in the Izu-Bonin and Mariana Forearc Regions

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Recent studies of forearcs in the circum-Pacific regions have revealed that the widespread serpentinization of mantle wedge peridotite occurs along the subducting slab at depths of 15-30 km due to water supplied from the slab. A huge zone of diapiric serpentinite seamounts along the trench axis in the Izu-Bonin and Mariana forearcs suggests that voluminous and gravitationally unstable low-density serpentinites generated just above the subducting slab have risen to the seafloor to form the seamounts. During ODP Leg 125, metamorphic rock clasts recovered from Holes 778A and 779B at Conical seamount, one of the serpentinite seamounts, have provided essential information on the interaction between forearc material and water. A geochemical study of the 778 A metabasalts indicates that the rocks have a chemical affinity with mid-ocean ridge basalts, some of which have zigzag REE patterns due to intense interaction with seawater. There are two possible origins that are worth considering. One is the trapped oceanic crust in the area between the trench and the volcanic front when subduction of the Pacific plate started, and the other is the accreted oceanic crust supplied directly from descending oceanic slab during subduction. The Hole 778A metabasalts commonly contain quartz veins, which have been produced prior to or during blueschist facies metamorphism, because high-pressure minerals, lawsonite, pumpellyite, and aragonite, were often crystallized in the vein. When the trapped or accreted oceanic crust had been squeezed deep down by the subducting slab, it encountered the pelagic sediments on top of the subducting slab. The SiO2-rich fluids having permeated the Hole 778A rocks were probably derived from these pelagic sediments. A phengite-rich clast, the only clast recovered from Hole 779B, is ultrabasic in composition, but is rich with incompatible elements, such as Zr, Ti and Th, and is relatively poor in compatible elements, such as Cr , Ni, and Co. Rocks with similar geochemical characteristics are found in the metasomatic reaction zone developed at the boundary between serpentinite and pelitic schist in the high-pressure Sanbagawa metamorphic belt, Japan. The clast may have been formed at the boundary between mantle wedge peridotite and subducting slab, where the hydrothermal metasomatic reactions have pervasively occurred between mantle wedge and pelagic sediments.近年,環太平洋前弧域では,沈み込むスラブから供給される水により,深さ15-30 kmにおいてマントルウェッジ・カンラン岩の大規模な蛇紋岩化か発生していることが明らかにされてきた.伊豆-小笠原,マリアナ前弧域において海溝軸に沿って発達する巨大なダイアピル蛇紋岩海山群は,沈み込むスラブ直上に発達した重力的に不安定な低密度の蛇紋岩が上昇し,海底に蛇紋岩海山を形成したことを示している.国際掘削計画第125節において,蛇紋岩海山の一つであるコニカル海山の掘削孔778Aと779Bから回収された変成岩片は,前弧物質と水との相互作用に関する重大な情報を与えてくれる.778Aの変成玄武岩類は,その地球化学的特徴が中央海嶺玄武岩に類似しており,その中には海水の影響を強く受けた希土類元素存在度パターンを示すものがある.これらは,太平洋プレートの沈み込み開始時に,マリアナ前弧域に封じ込められた海洋地殻あるいは直接付加した太平洋プレートの断片である可能性が強い.封じ込められたあるいは付加した海洋地殻は,スラブの沈み込みにより高圧変成作用が発生している深所まで引きずり込まれたと考えられる.778Aの変成玄武岩には通常石英脈が認められる.この石英脈には,ローソナイト,パンペリ一石,アラレ石などの高圧鉱物が再結晶していることから,脈形成の時期は変成作用時,あるいはそれ以前であることがわかる.スラブの沈み込みにより削剥・破砕され断片化した海洋地殻は,沈み込むスラブ直上の泥質堆積物に接触する.778Aの岩石類に浸透するシリカに富んだ流体は,おそらく,この泥質堆積物に由来するであろう.779Bから回収された唯一の変成岩である雲母に富んだ岩片は,超塩基性の組成をもつが,ZrやTiなどの不適合元素に富みCrやNiなどの適合元素に乏しい.同様の岩石が,三波川変成帯の蛇紋岩と泥質片岩の境界部に発達する反応帯に認められことから,この岩片は,交代作用が普遍的に起こりうるマントルウェッジと沈み込むスラブとの境界部で形成された可能性が高い.
2002-02-12

Serpentinite Seamounts and Hydrated Mantle Wedge in the Izu-Bonin and Mariana Forearc Regions

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