18. MINERALOGICAL AND GEOCHEMICAL ANALYSES OF SEDIMENTARY SERPENTINITES FROM CONICAL SEAMOUNT (HOLE 778A): IMPLICATION FOR THE EVOLUTION OF SERPENTINE SEAMOUNTS l
18. MINERALOGICAL AND GEOCHEMICAL ANALYSES OF SEDIMENTARY SERPENTINITES FROM CONICAL SEAMOUNT (HOLE 778A): IMPLICATION FOR THE EVOLUTION OF SERPENTINE SEAMOUNTS l
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18. 圆锥形海山（778A 孔）沉积蛇纹岩的矿物学和地球化学分析：对蛇纹石海山演化的影响
DOI：
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
J. Cotten
中科院分区：
文献类型：
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作者：
Y. Lagabrielle;J. Cotten
对778A孔35个样品进行了x射线衍射（XRD）矿物学分析和主微量元素化学分析。大多数选定的样品是粉砂大小的沉积蛇纹岩或微角砾岩，除了基性岩的软碎屑、块状蛇纹岩化橄榄岩的硬碎屑和含有浮游有孔虫的固结的、未变形的蛇纹微角砾岩。矿物学和地球化学分析都可以在分析的样品中区分出三组。这些群对应于沿钻探剖面存在的三个地层层段。A组包含上部样品（岩性单元I）。它们由固结不良的蛇纹岩泥组成，携带着坚硬的岩石碎屑（蛇纹岩化橄榄岩、变质玄武岩）。它们的矿物学组合特征如下：蛇纹石、铁氧化物和氢氧化物、文石和盐石。它们的SiO2、MgO含量变化较大，但SiO2/MgO比值接近1。CaO含量高与文石发育有关。A12O3含量低。相对较高的K20、Na2O和Sr含量可能与海水相互作用有关。B组（30-77 mbsf）包含具有非常均匀的化学和矿物学成分的样品。它们由蛇纹岩微角砾岩组成，具有频繁的剪切构造。硬岩碎屑也存在（蛇纹石化橄榄岩，变质玄武岩，一个可能的燧石碎片）。B组样品的矿物学特征是存在蛇纹石和自生矿物：羟基碳酸盐和hydrogrossular。方解石和绿泥石也存在，但所有样品都缺乏文石。它们的化学成分与母岩的成分非常相似。C组含粉砂级蛇纹岩和蛇纹岩微角砾岩，局部富含红色碎屑，可能为强蚀变（氧化）基性碎屑。恢复的层段中，碎屑的来源比剖面中较高的层段更加多样化。碎屑岩性包括蛇纹化橄榄岩、变质玄武岩、变质火山碎屑岩、变质橄榄岩辉长岩和角闪岩砂岩。矿物学和地球化学反映了这些成分。样品中蛇纹石的含量较前几组少。相对而言，海泡石、坡缕石和绿泥石-蒙脱石是分析样品中存在的矿物相。还发现了副火成岩矿物（角闪石、辉石、赤铁矿）。大多数C组样品的化学成分与块状蛇纹岩橄榄岩不同。主要区别在于(1)SiO2、CaO、TiO2和A12O3含量较高，(2)SiO2/MgO比值大于1，(3)A12O3与MgO、Cr和Ni呈负相关。这些特征提示了锥形海山侧翼流动结构的新约束条件。
Thirty-five samples from Hole 778A were prepared for X-ray diffraction (XRD) mineralogical analyses and for chemical analyses of major and trace elements. Most of the selected samples were silt- and sand-sized sedimentary serpentinites or microbreccias except for a soft clast of mafic rock, a hard clast of massive serpentinized peridotite, and a pebble of consolidated, undeformed serpentine microbreccia that contained planktonic foraminifers. Both mineralogical and geochemical analyses allow discrimination of three groups among the analyzed samples. These groups correspond to three stratigraphic intervals present along the drilled section. Group A contains the upper samples (lithologic Unit I). These consist of poorly consolidated serpentine muds carrying hard-rock clasts (serpentinized peridotites, metabasalts). They are characterized by the following mineralogical assemblage: serpentine, Fe-oxides and hydroxides, aragonite, and halite. They exhibit variable SiO2, MgO contents, but are characterized by a SiO2/MgO ratio near 1. CaO content is high in relation to development of aragonite. A12O3 content is low. Relatively high K20, Na2O, and Sr contents are present, presumably in relation to interactions with seawater. Group B (30-77 mbsf) contains samples exhibiting very homogeneous chemical and mineralogical compositions. They consist of serpentinite microbreccias exhibiting frequent shear structures. Hard-rock clasts are also present (serpentinized peridotites, metabasalts, one possible chert fragment). The mineralogy of the Group B samples is characterized by the presence of serpentine and authigenic minerals: hydroxycarbonates and hydrogrossular. Calcite and chlorite are also present, but all the samples lack aragonite. Their chemical compositions are remarkably similar to compositions of their parent rocks. Group C contains silt- and sand-sized serpentine and serpentine microbreccias, which are locally rich in red clasts, probably strongly altered (oxidized?) mafic fragments. Intervals having clasts of more diverse origin than those higher in the section were recovered. Clast lithology includes serpentinized peridotites, metabasalts, metavolcaniclastite, meta-olivine gabbro, and amphibolite sandstone. Mineralogy and geochemistry reflect these compositions. Serpentine content of the samples is less than in previous groups. Correlatively, sepiolite, palygorskite, and chlorite-smectite are mineral phases present in the analyzed samples. Accessory igneous minerals (amphiboles, pyroxenes, hematite) also were found. The chemical compositions of most of Group C samples differ from that of massive serpentinized peridotites. The main differences are (1) higher SiO2, CaO, TiO2 and A12O3 contents, (2) a SiO2/MgO ratio greater than 1, and (3) a negative correlation between A12O3, and MgO, Cr, and Ni. These characteristics suggest new constraints relative to the flow structure of the flank of Conical Seamount.