Combined thermodynamic and rare earth element modelling of garnet growth during subduction: Examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway
Combined thermodynamic and rare earth element modelling of garnet growth during subduction: Examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway
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DOI：
10.1016/j.epsl.2008.05.018
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发表时间：
2008-07
影响因子：
5.3
通讯作者：
M. Konrad‐Schmolke;T. Zack;P. O'Brien;Dorrit E. Jacob-
中科院分区：
文献类型：
--
作者：
M. Konrad‐Schmolke;T. Zack;P. O'Brien;Dorrit E. Jacob-
测定了挪威西部片麻岩地区超高压榴辉岩石榴石的主量元素和微量元素分带模式。所有研究的石榴石都显示出多个生长带，并保存了关于主要和稀土元素(REE)的复杂生长分带模式。由于寄主岩石化学成分的差异，出现了两种类型的主元素组成分带模式：(1)与生长带对应的阶梯状成分突变；(2)成分均一的内部，与生长带无关，随后向边缘发生突然的化学变化。尽管主要元素分带不同，但所有石榴石的稀土配分模式几乎相同，可划分为四个不同的分带，具有特征的配分模式。为了根据岩石的俯冲历史解释主量元素和微量元素的分布和分带模式，我们结合了适当的块状岩石成分的热力学正演模型，得到了沿着推断的压力-温度路径的稳定相的摩尔比例和主要元素组成，并在高压变质过程中计算了稳定相中稀土元素的质量平衡分布。我们的热力学正演模型再现了天然石榴石中复杂的主要元素分带模式和生长带，预测了石榴石在四个不同反应阶段的生长：(1)绿泥石分解，(2)绿帘石分解，(3)角闪石分解和(4)摩尔单斜辉石在超高压条件下的还原。稀土元素在模拟的稳定相之间分布的质量平衡产生了石榴石中与天然样品非常相似的特征分带模式。在俯冲过程中，石榴石的生长和微量元素的掺入与基质相接近热力学平衡。石榴石中的稀土元素模式显示出明显的富集带，指纹显示参与石榴石形成反应的矿物，以及局部峰，这可以用分馏效应和矿物组合的变化来解释。
Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Western Gneiss Region (Norway). All investigated garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost identical in all garnets and can be divided into four distinct zones with characteristic patterns. In order to interpret the major and trace element distribution and zoning patterns in terms of the subduction history of the rocks, we combined thermodynamic forward models for appropriate bulk rock compositions to yield molar proportions and major element compositions of stable phases along the inferred pressure-temperature path with a mass balance distribution of REEs among the calculated stable phases during high pressure metamorphism. Our thermodynamic forward models reproduce the complex major element zonation patterns and growth zones in the natural garnets, with garnet growth predicted during four different reaction stages: (1) chlorite breakdown, (2) epidote breakdown, (3) amphibole breakdown and (4) reduction in molar clinopyroxene at ultrahigh-pressure conditions. Mass-balance of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in garnet that closely resemble those in the natural samples. Garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction. The rare earth element patterns in garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the garnet-forming reactions as well as local peaks that can be explained by fractionation effects and changes in the mineral assemblage.