Major and trace element composition of sepentine muds and metabasic rocks of ODP Leg 195 and Leg 125 sites@en

New geochemical data on serpentinite muds and metamorphic clasts recovered during Ocean Drilling Program Legs 195 (Holes 1200A-1200E) and 125 (Holes 778A and 779A) provide insights into the proportions of rock types of various sources that compose the serpentinite mudflows and the fluid-rock interactions that predominate in these muds. We interpret the metamorphic rock fragments as derivatives of mostly metamorphosed mafic rocks from the descending Pacific oceanic crust. Based on their mid-ocean-ridge basalt (MORB)-like Al2O3, TiO2, CaO, Si/Mg, and rare earth element (REE) systematics, these metamorphic rocks are classified as metabasalts/metagabbros and, therefore, ~30-km depths represent an active subduction zone setting. The serpentinite muds from Holes 1200A and 1200E have slightly lower REE when compared to Hole 1200D, but overall the REE abundance levels range between 0.1-1 x chondrite (CI) levels. The chondrite-normalized patterns have [La/Sm]N ~ 2.3 and [Sm/Yb]N ~ 2. With the exception of one sample, the analyzed metamorphic clasts show flat to slightly depleted light REE patterns with 1.0-15 x CI levels, resembling MORBs. Visually, ~6 vol% of the serpentinized muds are composed of 'exotic' materials (metamorphic clasts [schists]). Our mixing calculations confirm this result and show that the serpentinite muds are produced by additions of ~5% metamafic materials (with flat and up to 10 x CI REE levels) to serpentinized peridotite clast material (with very low REE abundances and U-shaped chondrite-normalized patterns). The preferential incorporation of B, Cs, Rb, Li, As, Sb, and Ba into the structure of H2O-bearing sheet silicates (different than serpentine) in the Leg 125 and Leg 195 metamorphic clasts (chlorite, amphibole, and micas) have little effect on the overall fluid-mobile element (FME) enrichments in the serpentinite muds (average B = ~13 ppm; average Cs = ~0.05 ppm; average As = ~1.25 ppm). The extent of FME enrichment in the serpentinized muds is similar to that described for the serpentinized peridotites, both recording interaction with fluids very rich in B, Cs, and As originating from the subducting Pacific slab.本数据集包含大洋钻探计划（Ocean Drilling Program）第195航次（钻孔1200A至1200E）与第125航次（钻孔778A与779A）所获取的蛇纹岩泥（serpentinite muds）及变质岩碎屑（metamorphic clasts）的全新地球化学数据，可为揭示构成蛇纹岩泥流的不同源区岩石类型占比，以及这类泥岩中占主导的流体-岩石相互作用过程提供研究视角。我们将这些变质岩碎屑解释为主要源自俯冲太平洋洋壳经变质作用形成的镁铁质岩石的衍生物。基于其类洋中脊玄武岩（mid-ocean-ridge basalt, MORB）的Al₂O₃、TiO₂、CaO、Si/Mg比值及稀土元素（rare earth element, REE）系统特征，这些变质岩被归为变玄武岩/变辉长岩，因此~30 km的深度代表一个活跃的俯冲带环境。钻孔1200A与1200E的蛇纹岩泥的稀土元素丰度略低于钻孔1200D，但整体稀土元素丰度介于0.1~1倍CI型球粒陨石（CI）水平之间。球粒陨石标准化配分模式的[La/Sm]ₙ约为2.3，[Sm/Yb]ₙ约为2。除一个样品外，所分析的变质岩碎屑均呈现轻稀土元素平坦至轻微亏损的配分模式，丰度为1.0~15倍CI型球粒陨石水平，与洋中脊玄武岩特征相似。经目视观察，约6%体积占比的蛇纹石化泥岩由“外源”物质（即变质岩碎屑[片岩]）构成。我们的混合模型计算验证了这一结果，并表明蛇纹岩泥是由约5%的变镁铁质物质（配分模式平坦，稀土元素丰度最高可达10倍CI型球粒陨石水平）添加至蛇纹石化橄榄岩碎屑物质（稀土元素丰度极低，且球粒陨石标准化配分模式呈U型）中形成的。在第125航次与第195航次的变质岩碎屑（绿泥石、角闪石与云母）中，B、Cs、Rb、Li、As、Sb及Ba优先赋存于含H₂O的页硅酸盐（不同于蛇纹石）的结构中，但这对蛇纹岩泥中整体的流体活动性元素（fluid-mobile element, FME）富集程度影响极小（平均B≈13 ppm；平均Cs≈0.05 ppm；平均As≈1.25 ppm）。蛇纹石化泥岩中的流体活动性元素富集程度与已报道的蛇纹石化橄榄岩相似，二者均记录了与源自俯冲太平洋板块的富B、Cs及As流体发生的相互作用。