Thermal diagenesis of clay minerals within volcanogenic material from ODP Site 135-841

Clay minerals are examined in detail in the sediment from the Tonga Trench margin at Site 841 (Leg 135 ODP). The changes in amount and nature of secondary clays with depth provide an alternative explanation for the intensive alteration of volcanogenic material at convergent margins. A characteristic distribution of clay minerals with depth shows four distinct zones unexplainable by simple burial diagenesis processes. These are named the upper, reactive, lower and rhyolitic zones. The reactive zone is intercalated with numerous sills and is characterized by the dominant iron-rich clays such as saponite, corrensite and chlorite associated with analcime. The occurrence of such iron-rich clays, mostly associated with a large amount of analcime, yields chemical and mineralogical evidence for thermal diagenesis. The required heat for the diagenetic process was transferred from recently intruded basaltic andesite sills. In the vicinity of these intrusions, the iron-rich clay minerals may have formed at temperatures up to 200°C. A zoning with respect to clay and zeolite minerals indicates that the influence of the palaeoheat flow decreased with the distance from the intrusion. The formation of interlayered I/S, illite, kaolinite and aluminous chlorite, which are recognized as major secondary minerals within the rhyolitic complex, was mainly controlled by both early diagenesis at moderately elevated temperatures, and since the Eocene by burial diagenesis at low temperatures. The occurrence of a steam zone in an early stage of the intrusion is restricted to Miocene tuffs and has overprinted the early alteration of the volcanogenic material within the tuffs and has changed the originally pristine composition of the pore fluids.

本文针对大洋钻探计划（Ocean Drilling Program, ODP）第135航次841站位获取的汤加海沟陆缘沉积物中的黏土矿物（clay minerals）展开了详细研究。次级黏土矿物的含量与种类随深度的变化，为汇聚边缘（convergent margins）火山成因物质的强烈蚀变提供了全新的解释视角。黏土矿物沿深度的特征性分布可划分为四个独立的带域，单纯的埋藏成岩作用（burial diagenesis）无法解释该分布规律，这四个带域分别被命名为上段、反应带、下段与流纹岩带。反应带中发育大量互层岩床，其矿物组合以富铁黏土矿物为主，包括皂石（saponite）、柯绿泥石（corrensite）与绿泥石（chlorite），并伴生大量方沸石（analcime）。这类多与大量方沸石伴生的富铁黏土矿物的产出，为热成岩作用（thermal diagenesis）提供了化学与矿物学证据。成岩过程所需的热量来自近期侵入的玄武安山岩（basaltic andesite）岩床。在这些侵入体附近，富铁黏土矿物可在最高200℃的温度下形成。黏土矿物与沸石矿物的分带特征表明，古热流（palaeoheat flow）的影响随与侵入体距离的增大而逐渐减弱。流纹岩杂岩体中的主要次生矿物包括层间伊/蒙混层矿物（Interstratified Illite/Smectite, I/S）、伊利石（illite）、高岭石（kaolinite）以及富铝绿泥石（aluminous chlorite），这些矿物的形成主要受两类过程调控：一是中等升温条件下的早期成岩作用，二是始新世（Eocene）以来的低温埋藏成岩作用。侵入早期形成的蒸汽带仅分布于中新世（Miocene）凝灰岩中，该蒸汽带叠加改造了凝灰岩内火山成因物质的早期蚀变特征，并改变了孔隙流体原本的原始组分。