Geochemistry of phosphatic crusts from the shelf off Peru@en

Authigenic phosphorite crusts from the shelf off Peru (9°40°S to 13°30°S) consist of a facies with phosphatic coated grains covered by younger phosphatic laminite. The crusts are composed of carbonate fluorapatite, which probably formed via an amorphous precursor close to the sediment water interface as indicated by low F/P2O5 ratios, Sr and Ca isotopes, as well as rare earth element patterns agreeing with seawater-dominated fluids. Small negative Ce anomalies and U enrichment in the laminite suggest suboxic conditions close to the sediment-water interface during its formation. Increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and, consequently, point to episodical development of anoxic conditions during phosphogenesis. The Peruvian phosphorites formed episodically over an extended period of time lasting from Middle Miocene to Pleistocene. Individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide-rich phosphatic layers. Coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions. Episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer. Phosphatic coated grains were later transported to the place of crust formation, where subsequent laminite formation was favored under lower energy conditions. A similar succession of phosphatic layers with varying contents of organic matter and sulfide-rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains.采自秘鲁陆架（南纬9°40′至13°30′）的自生磷质结壳（Authigenic phosphorite crusts）由一类岩相构成，该岩相的磷质包壳颗粒（phosphatic coated grains）被更年轻的磷质纹层岩（phosphatic laminite）所覆盖。该结壳的组分以碳酸盐氟磷灰石（carbonate fluorapatite）为主，结合低F/P₂O₅比值、锶（Sr）与钙（Ca）同位素数据，以及与海水主导流体（seawater-dominated fluids）匹配的稀土元素配分模式（rare earth element patterns），推测其形成于沉积物-水界面（sediment water interface）附近，起源于非晶质前驱体阶段。纹层岩中出现的弱负铈异常（negative Ce anomalies）与铀富集（U enrichment）现象，表明其形成时期沉积物-水界面附近处于次氧化环境（suboxic conditions）。磷质包壳颗粒所在岩相及纹层岩相中均富集亲硫元素（chalcophilic elements）与大量硫化物矿物（sulfide minerals），指示曾发生硫酸盐还原作用（sulfate reduction），进而说明磷矿化作用（phosphogenesis）过程中曾间歇性出现缺氧环境。秘鲁磷质结壳的形成具有间歇性，持续时间从中中新世（Middle Miocene）延伸至更新世（Pleistocene）。单个磷质包壳颗粒可见多层磷质层序，各层的有机质（organic matter）含量与富硫化物磷质层（sulfide-rich phosphatic layers）占比均存在差异。推测包壳颗粒形成于强湍流（high turbulence）引发的间歇性悬浮（episodic suspension）事件及氧化还原条件（redox conditions）波动过程中。孔隙水（pore water）中的间歇性缺氧作用导致最外层的碳酸盐氟磷灰石发生黄铁矿化作用（pyritization）。随后，磷质包壳颗粒被搬运至结壳形成区域，该区域的低能环境更利于后续纹层岩的形成。纹层岩中同样存在磷质层序随有机质与富硫化物层含量变化的特征，表明其形成机制与单个包壳颗粒的形成机制相似。