Mineral composition, major element oxides and trace element concentration of sediments from ODP Leg 180 sites

Middle Miocene to Holocene fine-grained argillaceous sediments (clays, claystones/muds, and mudstones), which volumetrically dominated the sediment recovery in the Woodlark Basin during Leg 180, were chemically analyzed for major elements, trace elements, and some rare earth elements by X-ray fluorescence. Selected samples also underwent X-ray diffraction (XRD) analysis for mineral determination. The results shed light on sediment provenance when combined with shipboard sediment descriptions, smear slide study, and XRD. The oldest sediments recovered (Site 1108) of middle-late Miocene age include volcanogenic muds with distinctive high MgO and K2O, indicative of a relatively basic calc-alkaline source related to an inferred Miocene forearc succession. The forearc basement, composed of diabase and basalt, was locally exposed (Site 1109) and eroded in the late Miocene (<5.4-9.93 Ma), giving rise to fluvial conglomerates (Sites 1109, 1115, and 1118). Chemically distinctive fine-grained claystones and siltstones (with relatively high Ti, low K) are compatible with derivation from tropically weathered basic igneous rocks, correlated with the Paleogene Papuan ophiolite. Overlying latest Miocene-Pleistocene fine-grained sediments throughout the Woodlark Basin were partly derived from calc-alkaline volcanic sources. However, relatively high abundances of Al2O3 and related element oxides (K2O and Na2O) and trace elements (e.g., Rb and Y) reflect an additional terrigenous input throughout the basin, correlated with pelitic metamorphic rocks exposed on Papua New Guinea and adjacent areas. In addition, sporadic high abundances of Cr and Ni, some other trace metals, and related minerals (talc, crysotile, and chlorite) reflect input from an ophiolitic terrain dominated by ultramafic rocks, correlated with the Paleogene Papuan ophiolite. The source areas possibly included serpentinized ultramafic ophiolitic rocks exposed in the Papua New Guinea interior highlands. Chemical evidence further indicates that fine-grained terrigenous sediment reached the Woodlark Basin throughout its entire late Miocene-Holocene history. Distinctive high-K volcanogenic muds rich in tephra and volcanic ash layers that appear at <2.3 Ma (Sites 1109 and 1115) are indicative of high-K calc-alkaline volcanic centers, possibly located in the Dawson Strait, Moresby Strait, or Dobu Seamount area. Chemical diagenesis of fine-grained sediments within the Woodlark Basin is reflected in clay neomorphism and localized formation of minerals including dolomite, ankerite, and zeolite but has had little effect on the bulk chemical composition of most samples.

中中新世至全新世细粒泥质沉积物（黏土、黏土岩/泥质、泥岩）在大洋钻探第180航次伍德拉克海盆的沉积物回收总量中占绝对优势。本次研究采用X射线荧光光谱法（X-ray fluorescence）对该类沉积物开展常量元素、微量元素及部分稀土元素的化学分析，并选取部分样品通过X射线衍射（X-ray diffraction, XRD）完成矿物鉴定。结合现场沉积物描述、涂片薄片研究与XRD分析结果，可明确沉积物物源信息。本次回收的最古老沉积物（1108站位）形成于中晚中新世，为具有显著高氧化镁（MgO）和氧化钾（K₂O）特征的火山成因泥质沉积物，指示其物源为与推断的中新世弧前层序相关的相对基性钙碱性源区。由辉绿岩和玄武岩组成的弧前基底在晚中新世（<5.4-9.93 Ma）期间局部出露于1109站位并遭受侵蚀，进而形成分布于1109、1115和1118站位的河流砾岩。具有独特地球化学特征的细粒黏土岩和粉砂岩（钛（Ti）含量较高、钾（K）含量较低），其物源与热带风化的基性火成岩一致，可对应古近纪巴布亚蛇绿岩套。遍布伍德拉克海盆的上覆晚中新世末-更新世细粒沉积物，部分来源于钙碱性火山物源。然而，沉积物中较高的氧化铝（Al₂O₃）及相关氧化物（氧化钾K₂O、氧化钠Na₂O）和微量元素（如铷Rb、钇Y）的丰度，反映整个海盆存在额外的陆源输入，该输入与巴布亚新几内亚及邻区出露的泥质变质岩相关。此外，铬（Cr）、镍（Ni）及部分其他痕量金属的局部高丰度，以及相关矿物（滑石、纤蛇纹石（crysotile）、绿泥石）的出现，指示了以超镁铁质岩为主的蛇绿岩地貌区的物质输入，该物源对应古近纪巴布亚蛇绿岩套。物源区可能包括出露于巴布亚新几内亚内陆高地的蛇纹石化超镁铁质蛇绿岩套岩石。地球化学证据进一步表明，细粒陆源沉积物在晚中新世至全新世的整个演化历史中均有抵达伍德拉克海盆。在<2.3 Ma的地层中（1109和1115站位）出现的富含火山碎屑与火山灰层的高钾火山成因泥质沉积物，指示了高钾钙碱性火山中心的存在，该中心可能位于道森海峡、莫尔斯比海峡或多布海山区域。伍德拉克海盆内细粒沉积物的化学成岩作用表现为黏土重结晶作用以及局部形成白云石、铁白云石和沸石等矿物，但该作用对绝大多数样品的整体化学成分影响微弱。