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首页> 外文学位 >The Origin of Mid-Ocean Ridge Basalts: Insights from Trace Element Contents in Anorthite, Anorthite-hosted-melt Inclusions, and Ocean Core Complexes.
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The Origin of Mid-Ocean Ridge Basalts: Insights from Trace Element Contents in Anorthite, Anorthite-hosted-melt Inclusions, and Ocean Core Complexes.

机译：中洋洋脊玄武岩的起源：钙长石，钙长石熔体包裹体和海洋核心复合物中痕量元素含量的见解。

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摘要

The primary goal of this study is to access the processes that alter primary melts after segregation from a mantle source and ultimately form petrologic Layers 2 and 3 of the Ocean Crust. Mineral, melt inclusion, and whole rock chemical compositions are utilized to further the understanding of 1) the behavior of trace element partitioning between high anorthite plagioclase and basalt melt, 2) the nature of the record preserved by melt inclusions in anorthitic plagioclase in MORB, 3) the variability of melt compositions generated at slow spreading ridges, and 4) the composition of melt and crystals present in an axial magma chamber extrapolated from plagioclase-ultraphyric basalts.;The major and trace element compositions of melt inclusions hosted in high anorthite feldspar from Gorda Ridge and Southeast Indian Ridge are used to evaluate the degree to which melt inclusions represent original melts trapped at the time of crystallization versus melts that have been affected by entrapment processes or modified by post-entrapment processes. Melt inclusions that are 'real' melts will display trace element partitioning relationships with the adjacent mineral host that fit known models. Melt inclusions that have been modified by inclusion-specific processes, will not conform to known partitioning models. Using anorthitic plagioclase from pillow basalts dredged from the Gorda Ridge and the Southeast Indian Ridge, our results indicate that there is no apparent correlation between the calculated mineral-inclusion partition coefficients and the size or composition of the melt inclusion. The data is generally consistent with experimentally determined partition coefficients for high anorthite feldspar. However, some samples contain trace element patterns that are consistent with some degree of diffusive re-equilibrium of compatible elements, i.e., Sr, Eu, and Ba. Therefore, although plagioclase-hosted melt inclusions are broadly representative of the magmas from which with anorthite phenocrysts formed, these melts do not represent the unmodified array of primary mantle melts, and are produced by a complex set of processes.;The variability of magmas produced at a slow-spreading plate boundary is evaluated using mineral analyses, combined with whole rock compositions of gabbroic and basaltic lithologies and is conducted within a spatial context. Samples are drill cores halves obtained during IODP Legs 304/5 from Sites 1309, 1310, and 1311 that are located on Atlantis Massif, at 30°N on the Mid-Atlantic Ridge. Gabbroic samples span the full range of compositions collected from ocean ridges: primitive troctolites through evolved leuco- and oxide-gabbros and whole rock Mg ;Plagioclase ultra-phyric basalts from Southwest Indian Ridge contain greater than 20% plagioclase phenocrysts. Plagioclase An content is generally higher than (molar Ca/[Ca+Na+K] ranges from An94 to An84) the calculated equilibrium composition for respective host melts. Additionally, crystals within a single sample exhibit a variety of zoning patterns, morphologies, and textures. Crystal size distributions of 3 samples are linear and suggest that phenocrysts within a single sample grew in a similar thermal environment. Similar CSD slopes are obtained from each sample indicative of a fairly continuous thermal environment along ridge axis. We make an argument that the best fitting model for the petrogenesis of plagioclase ultra-phyric basalts invokes a magma chamber where convection physically separates low density plagioclase phenocrysts and gradients in intrinsic variables (temperature, pressure, and composition) produce the suite of textures and zoning exhibited by individual crystals within a sample. The bimodal trace element composition of crystals from one sample indicates that melts can exist in chemical isolation until just prior to eruption.
机译：这项研究的主要目标是获得从地幔源中分离后改变初生熔体并最终形成大洋地壳岩石层2和3的过程。利用矿物，熔体包裹体和整个岩石化学成分进一步了解以下内容：1）高钙长斜长石斜长石和玄武岩熔体之间微量元素分配的行为，2）MORB中无斜斜长石斜纹岩中熔体包裹体保留的记录的性质， 3）在缓慢扩散的山脊处产生的熔体成分的可变性，以及4）从斜长石-超玄武岩推断出的轴向岩浆腔中存在的熔体和晶体的成分;;高钙长石长石中所含熔体包裹体的主要和微量元素组成来自Gorda Ridge和Southeast Indian Ridge的熔体被用来评估熔体夹杂物代表在结晶时被捕获的原始熔体与已受截留过程影响或被包埋后改性的熔体的程度。 “真实”熔体的熔体包裹体将显示与已知模型相匹配的与相邻矿物主体的痕量元素分配关系。已通过包含物特定过程修改的熔体包含物将不符合已知的分区模型。使用从戈达岭和东南印第安岭挖出的枕形玄武岩中的斜生斜长石，我们的结果表明，计算出的矿物-夹杂物分配系数与熔体夹杂物的大小或组成之间没有明显的相关性。该数据通常与实验确定的高钙长石长石的分配系数一致。但是，一些样品包含的痕量元素图案与兼容元素（即Sr，Eu和Ba）的某种程度的扩散再平衡相一致。因此，尽管斜长石包裹的熔体包裹体大致代表了形成钙长石斑晶的岩浆，但这些熔体并不代表未修饰的原始地幔熔体阵列，而是通过一系列复杂的过程产生的。利用矿物分析，结合长辉岩和玄武质岩性的整个岩石成分，评估在慢速扩展板块边界处的变形，并在空间范围内进行。样本是在IODP腿304/5期间从位于大西洋中脊30°N的Atlantis Massif上的站点1309、1310和1311获得的钻头半部分。废屑样品涵盖了从海脊收集的全部成分：通过演化的无色和氧化物辉长岩和整个岩石形成的原始闪闪石；来自西南印第安岭的斜长石超玄武岩含有斜长石的隐伏岩含量超过20％。斜长石的An含量通常高于（计算的Ca / [Ca + Na + K]摩尔比范围从An94至An84），计算出的各主体熔体的平衡组成。另外，单个样品中的晶体表现出各种分区模式，形态和纹理。 3个样品的晶体尺寸分布是线性的，这表明单个样品中的隐晶在相似的热环境中生长。从每个样本获得相似的CSD斜率，表明沿着脊轴的环境相当连续。我们认为，斜长石超物理玄武岩成岩的最佳拟合模型调用了一个岩浆室，对流将低密度斜长石玄武岩物理对流，并且固有变量（温度，压力和组成）中的梯度产生了一组纹理和分区样品中的单个晶体所表现出来的。来自一个样品的晶体的双峰微量元素组成表明，熔体可以化学隔离的形式存在，直到喷发之前。

著录项

  • 作者

    Weinsteiger, Allison B.;

  • 作者单位

    Oregon State University.;

  • 授予单位 Oregon State University.;
  • 学科 Petrology.;Geochemistry.;Marine Geology.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 466 p.
  • 总页数 466
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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