Local Bulk Composition Effects on Metamorphic Mineral Assemblages

1区 地球科学 Q1 Earth and Planetary Sciences
P. Lanari, M. Engi
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引用次数: 125

Abstract

Plate tectonic forcing leads to changes in the physical conditions that affect the lithosphere. In response to such changes, notably the local temperature ( T ) and pressure ( P ), rocks evolve dynamically. Processes mostly involve mineral transformations, i.e., solid-state reactions, but (hydrous) fluids are often involved, and partial melting may occur in the Earth’s middle and lower crust. While these chemical reactions reflect the tendency of natural systems to reduce their Gibbs free energy, metamorphic rocks commonly preserve textural and mineralogical relics, such as compositionally zoned minerals. Where relics are present, thermodynamic equilibrium clearly was not attained during the evolution of the rock. Petrochronology seeks to establish a temporal framework of petrologic evolution, and for this purpose it is essential to determine the P–T conditions prevailing at several stages. When analyzing a rock sample it is thus critical: 1. to recognize whether several stages of its evolution can be discerned, 2. to document the minerals that formed or were coexisting at each stage, and 3. to estimate at what physical conditions this happened. If (and only if) a chronometer then can be associated to one of these stages—or better yet several chronometers to different stages—then the power of petrochronology becomes realizable. This chapter is concerned with a basic dilemma that results directly from steps (b) and (c) above: P–T conditions are determined on the basis of mineral barometers and thermometers, which mostly rest on the assumption of chemical (or isotopic) equilibrium, yet the presence of relics is proof that thermodynamic equilibrium was not attained. One way out of the dilemma is to analyze reaction mechanisms and formulate a model based on non-equilibrium thermodynamics and kinetics (Lasaga 1998). While this can be fruitful for understanding fundamental aspects of metamorphic petrogenesis, there are more direct ways to address the limited scope needed for petrochronology. The alternative pursued …
局部体积成分对变质矿物组合的影响
板块构造强迫导致影响岩石圈的物理条件的变化。为了响应这些变化,特别是局部温度(T)和压力(P),岩石动态演化。过程主要涉及矿物转化,即固态反应,但也经常涉及(含水)流体,部分熔融可能发生在地球的中地壳和下地壳。虽然这些化学反应反映了自然系统减少吉布斯自由能的趋势,但变质岩通常保留了结构和矿物学遗迹,例如成分分带矿物。在有遗迹的地方,很明显岩石在演化过程中没有达到热力学平衡。岩石年代学旨在建立岩石学演化的时间框架,为此目的,确定在几个阶段普遍存在的P-T条件至关重要。在分析岩石样本时,这是至关重要的:要认识到其演变的几个阶段是否可以辨别,2 .记录每个阶段形成或共存的矿物;来估计这发生在什么物理条件下。当(且仅当)一个计时器可以与这些阶段中的一个相关联,或者更好的是几个计时器与不同的阶段相关联,那么岩石年代学的力量就可以实现了。本章关注的是上述步骤(b)和(c)直接导致的一个基本困境:P-T条件是根据矿物气压计和温度计确定的,它们主要依赖于化学(或同位素)平衡的假设,然而遗迹的存在证明热力学平衡没有达到。摆脱困境的一种方法是分析反应机理,并建立一个基于非平衡热力学和动力学的模型(Lasaga 1998)。虽然这对于理解变质岩石成因的基本方面可能是有益的,但有更直接的方法来解决岩石年代学所需的有限范围。另一种选择是……
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来源期刊
Reviews in Mineralogy & Geochemistry
Reviews in Mineralogy & Geochemistry 地学-地球化学与地球物理
CiteScore
8.30
自引率
0.00%
发文量
39
期刊介绍: RiMG is a series of multi-authored, soft-bound volumes containing concise reviews of the literature and advances in theoretical and/or applied mineralogy, crystallography, petrology, and geochemistry. The content of each volume consists of fully developed text which can be used for self-study, research, or as a text-book for graduate-level courses. RiMG volumes are typically produced in conjunction with a short course but can also be published without a short course. The series is jointly published by the Mineralogical Society of America (MSA) and the Geochemical Society.
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