Implications of overstepping of garnet nucleation for geothermometry, geobarometry and P–T path calculations
Implications of overstepping of garnet nucleation for geothermometry, geobarometry and P–T path calculations
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DOI：
10.1016/j.chemgeo.2019.119323
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发表时间：
2019-12
期刊：
影响因子：
3.9
通讯作者：
F. Spear;Oliver M. Wolfe
中科院分区：
文献类型：
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作者：
F. Spear;Oliver M. Wolfe
像石榴子石这样的变斑晶，当它在相当大的超越之后成核和生长时，其成分不受平衡关系的支配。这里所遵循的方法是假设成分由最大驱动力（Maximum Driving Force）或平行切线模型控制，并检查假设平衡的潜在陷阱。Fe-Mg分配模型保证了基质相和生长的变斑晶之间的分配平衡，但控制钙铝榴石含量的净转移平衡具有与平衡关系不同的化学计量。本研究的一个重要结果是，使用该模型并假设化学分馏生成的石榴石环带剖面与假设连续平衡生长的环带剖面几乎相同，并且不可能通过单独检查环带剖面来辨别石榴石是在接近平衡的条件下生长还是仅在相当大的超越之后生长。使用来自模拟的相交等值线方法计算的温度路径，其中石榴石在超步进之后生长，并且假设该模型典型地显示出接近等温的加载或具有较小加热路径的加载，即使当石榴石在等温和等压条件下生长时。石榴石的核心和边缘上的石榴石斜长石气压也表明加载在石榴石生长，即使是等温和等压生长的晶体。然而，石榴石-黑云母Fe-Mg交换测温法确实将石榴石核心和边缘的正确温度恢复到10 °C以内。天然样品与理论计算的比较表明，石榴石附近的局部有效体积成分除FeO、MnO和CaO外，还缺乏Na 2 O，因此，依赖于体积成分知识的计算方案难以管理。在一定程度上，石榴石已经成核和生长后，相当大的超越，应用的方法提取的P-T路径从石榴石的化学分区将产生不正确的结果，如果这些方法假设连续平衡与基质。主要的差异来自于石榴石芯的P-T条件的计算。模拟和自然样本表明，在典型情况下，石榴子石边缘与基质组合处于接近平衡的状态，因此平衡方法（热压法或相交等值线）应该能够合理地恢复峰值变质条件。
The composition of a porphyroblast such as garnet is not governed by equilibrium relations when it nucleates and grows after considerable overstepping. The approach followed here is to assume the composition is controlled by the maximum driving force (MDF) or parallel tangent model and to examine the potential pitfalls for assuming equilibrium. The MDF model ensures Fe-Mg partitioning equilibrium between the matrix phases and growing porphyroblast, but the net transfer equilibrium controlling grossular content has a different stoichiometry from the equilibrium relations. An important result of this study is that garnet zoning profiles generated using this model and assuming chemical fractionation are nearly identical to zoning profiles grown assuming continuous equilibrium and it is impossible to discern whether garnet grew under near-equilibrium conditions or only after considerable overstepping by examination of the zoning profile alone.Pressure–temperature paths calculated using the method of intersecting isopleths from simulations in which garnet was grown after overstepping and assuming the MDF model typically display nearly isothermal loading or loading with minor heating paths, even when the garnet was grown under isothermal and isobaric conditions. Garnet-plagioclase barometry on the garnet core and rim also suggests loading during garnet growth, even for crystals grown isothermally and isobarically. Garnet-biotite Fe-Mg exchange thermometry does, however, recover the correct temperature to within 10 °C for both the garnet core and rim.Comparison of natural samples to the theoretical calculations suggests that the local effective bulk composition in the vicinity of garnet is depleted in Na2O, in addition to FeO, MnO and CaO, thus rendering calculation schemes that rely on knowledge of the bulk composition difficult to administer. To the extent that garnet has nucleated and grown after considerable overstepping, application of methods of extracting P–T paths from the chemical zoning in garnet will yield incorrect results if those methods assume continuous equilibration with the matrix. The major discrepancy comes from calculation of the P–T conditions of the garnet core. Simulations and natural samples suggest that under typical scenarios the garnet rim is in near equilibrium with the matrix assemblage so equilibrium approaches (thermobarometry or intersecting isopleths) should recover the peak metamorphic conditions with reasonable assuredness.