Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications
Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications
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DOI：
10.2138/am-2002-0401
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发表时间：
2002-04
影响因子：
3.1
通讯作者：
D. Henry;C. Guidotti
中科院分区：
文献类型：
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作者：
D. Henry;C. Guidotti
摘要缅因州西部广泛的天然黑云母数据限制了温度和晶体化学对变质岩黑云母中饱和钛水平的控制。与缅因州西部阿卡迪亚造山带的M3变质作用有关的地质和岩石学特征良好的变质地形是这种方法的理想选择，因为变质作用发生在3.3kbar的大致等压条件下，并且接近化学平衡。这些变质岩的数据集显示，钛含量在变质级别(从石榴石到硅线石-钾长石带)、矿物组合和整体成分范围的连续体上发生了系统的变化。样品的选择使得竞争取代被限制在含有石英、铝相(绿泥石、十字石或硅线石)、钛相(钛铁矿或金红石)和石墨的变质泥岩中。由于晶体化学因素的影响，在任何特定的变质带中，钛和镁的含量之间都存在着反线性关系。富镁黑云母中八面体钛的减少和四面体硅的增加有助于缓解八面体和四面体片层的尺寸差异。对于特定镁含量的黑云母，钛在硬沸石带条件下最显著地增加。我们的约束数据集允许我们计算天然黑云母的钛饱和面，它是3.3kbar时温度和镁含量的函数。在类似的变质环境中，钛饱和面可以用来建立几个重要的变质特征。这些方法包括达到平衡的一般方法、局部和/或由于绿泥石轻微蚀变而偏离平衡的情况，以及基于难选矿物和基质黑云母中黑云母包裹体中钛的相对和绝对地质测温。
Abstract An extensive natural biotite data set from western Maine constrains the temperature and crystalchemical controls on the saturation Ti levels in biotites from metapelites. The geologically and petrologically well-characterized metamorphic terrain associated with the M3 metamorphism of the Acadian Orogeny of western Maine is ideal for this approach in that metamorphism occurred at roughly isobaric conditions of 3.3 kbar, and chemical equilibrium was closely approached. The data set from these metapelites exhibits systematic variations in Ti contents over a continuum of metamorphic grades (garnet through sillimanite-K-feldspar zones), mineral assemblages, and bulk compositional ranges. Samples were selected so that competing substitutions are restricted to those in metapelites with quartz, aluminous phases (chlorite, staurolite, or sillimanite), Ti phases (ilmenite or rutile), and graphite. Due to crystal-chemical factors, in any given metamorphic zone, an inverse linear relationship exists between Ti and Mg contents. Decreasing octahedral Ti and increasing tetrahedral Si in Mg-rich biotite helps alleviate size disparity between octahedral and tetrahedral sheets. For a biotite with a given Mg content, Ti most dramatically increases above staurolite zone conditions. Our constrained data set allows us to calculate a Ti saturation surface for natural biotite as a function of temperature and Mg content at 3.3 kbar. The Ti saturation surface can be used to establish several important metamorphic features in similar metamorphic settings. These include a general approach to equilibrium, local and/or subtle departures from equilibrium due to minor alteration to chlorite, and relative and absolute geothermometry based on Ti in biotite inclusions in refractory minerals and in matrix biotite.