The relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers
The relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers
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DOI：
10.1016/j.lithos.2008.12.012
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发表时间：
2009-06
期刊：
影响因子：
3.5
通讯作者：
T. Kuritani
中科院分区：
文献类型：
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作者：
T. Kuritani
在冷却的岩浆房中，岩浆分异既可以通过岩浆体主要熔融部分的晶体分馏（均质分馏），也可以通过主岩浆与来自低温糊状物带的分馏熔体混合（边界层分馏）来进行。在这项研究中，边界层分馏和均匀分馏的玄武岩浆体的相对作用进行了研究，使用的质量平衡模型。模型计算表明，当岩浆房位于低压（<~50 MPa）或岩浆含水量较低（<~1 wt.%）时，边界层分馏不能成为主要分馏机制，如洋中脊玄武岩和板内玄武岩，因为熔体从糊状物区向主岩浆的输送效率低。因此，岩浆主要是以均质分馏作用演化的。如果在主岩浆中没有发生有效的晶体-熔体分离，则岩浆在岩浆演化的早期阶段变得富含晶体。另一方面，当岩浆含水（>~2 wt.%）时，边界层分馏可以有效地发生，岩浆房位于深部（>~100 MPa）。由于来自糊状物区的熔体富含碱金属和H2O，因此由于液相线温度降低，与糊状物熔体混合，抑制了主岩浆的结晶。因此，在边界层分馏作用较活跃的岩浆房中，均匀分馏作用受到更有效的抑制。如果岩浆分异主要通过边界层分馏进行，那么在岩浆演化过程中，岩浆可以长时间保持无晶体状态。
In a cooling magma chamber, magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body (homogeneous fractionation) and by mixing of the main magma with fractionated melt derived from low-temperature mush zones (boundary layer fractionation). In this study, the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics-based mass balance model. Model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures (<~50 MPa) or when magmas are less hydrous (<~1 wt.%), such as mid-ocean ridge basalt and intraplate basalt, because of the low efficiency of melt transport from the mush zones to the main magma. Therefore, magmas evolve principally by homogeneous fractionation. If crystal-melt separation does not occur effectively in the main magma, the magma becomes crystal-rich in the early stages of magmatic evolution. On the other hand, boundary layer fractionation can occur effectively when magmas are hydrous (>~2 wt.%), such as arc basalt, and the magma chambers are located at depth (>~100 MPa). Because the melt derived from mush zones is enriched in alkalis and H2O, crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature. Therefore, homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active. If magmatic differentiation proceeds primarily by boundary layer fractionation, magmas can remain free of crystals for long periods during magmatic evolution.