Garnet growth after overstepping
Garnet growth after overstepping
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DOI：
10.1016/j.chemgeo.2017.06.038
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发表时间：
2017-09
期刊：
影响因子：
3.9
通讯作者：
F. Spear
中科院分区：
文献类型：
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作者：
F. Spear
为了给成核和生长提供必要的驱动力，需要超越变质反应。提出了石榴石成核和生长的正演模型，假设成核的亲和力对应于几十度或几千巴的超越。成核和生长的石榴石成分被认为是提供最大自由能减少的成分。根据这些假设，在等温、等压条件下生长的石榴石的分带预测与在连续近平衡条件下生长的石榴石的分带预测足够相似，仅根据分带剖面的形状进行区分似乎是不可能的。此外，在超越后，初始石榴石晶体在成核后的生长耗尽了通过将Mn隔离到现有石榴石晶体中的后续成核的亲和力。因此，渐进式成核需要额外的能量输入，最有可能是通过压力和温度的变化来补充亲和力。在第一个成核石榴石大量生长后，预测的成核石榴石的分带与瑞利分数的典型钟形剖面不匹配，而是根据假设的生长速率限制步骤显示出明显的诊断分带。扩散控制生长导致相对更宽和平坦的轮廓，而界面控制生长导致后期石榴石具有峰值核心Mn轮廓。后一种剖面会在相对较短的时间内（小于1毫安）通过扩散而松弛，因此不太可能被保存下来，但似乎在佛蒙特州中部的石榴石带样品中观察到了。该样品的计算还表明，当绿泥石耗尽时，石榴石停止生长，此时相当大的亲和力仍然存在，表明没有达到平衡。基于P-T路径计算的构造解释假设石榴石的近平衡成核和生长，因此可能需要根据成核所需的超越幅度和变质峰未达到平衡的可能性进行重新评估。
Overstepping of metamorphic reactions is required to provide the driving force necessary for porphyroblast nucleation and growth. Forward models of garnet nucleation and growth are presented assuming an affinity for nucleation that corresponds to several tens of degrees or several kilobars of overstepping. The composition of garnet that nucleates and grows is assumed to be that which provides the largest decrease in free energy. With these assumptions, the zoning predicted for a garnet grown under isothermal, isobaric conditions is revealed to be sufficiently similar to the zoning predicted for garnet grown under continuous near-equilibrium conditions that distinction based on the shape of zoning profiles alone does not appear to be possible. Furthermore, the growth of an initial garnet crystal following nucleation after overstepping depletes the affinity for subsequent nucleation by sequestration of Mn into the existing garnet crystal. Progressive nucleation, therefore, requires additional energy input, most likely through changes in pressure and temperature, to replenish the affinity. The predicted zoning in garnets nucleated following substantial growth of the first nucleated garnet does not match the typical bell-shaped profile of Rayleigh fraction but rather displays distinctly diagnostic zoning depending on the assumed rate limiting step for growth. Diffusion controlled growth results in relatively broader and flattened profiles whereas interface controlled growth results in later garnets having a peaked core Mn profile. This latter profile would relax by diffusion in relatively short times (less than 1 Ma) and would thus not likely to be preserved, but appears to have been observed in garnet zone samples from central Vermont. Calculations for this sample also indicate that garnet ceased growing when chlorite was exhausted, at which point considerable affinity remained indicating that equilibrium was not achieved. Tectonic interpretations based on P–T paths calculated assuming near-equilibrium nucleation and growth of garnet are thus likely to require reevaluation in light of the magnitude of overstepping needed for nucleation and the possibility that equilibrium was not attained at the metamorphic peak.