The Origin of Marginal Compositional Reversals in Basic–Ultrabasic Sills and Layered Intrusions by Soret Fractionation
The Origin of Marginal Compositional Reversals in Basic–Ultrabasic Sills and Layered Intrusions by Soret Fractionation
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DOI：
10.1093/petrology/egg050
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发表时间：
2003-09
影响因子：
3.9
通讯作者：
R. Latypov
中科院分区：
文献类型：
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作者：
R. Latypov
边缘反转是许多基本差异火成体的共同特征，无论其大小和整体成分如何，它作为层状系列的镜子而引人注目。它们的特点是：（1）边缘反转的下部（包括冷却边缘）与侵入体的主体成分之间明显缺乏质量平衡； （2）矿物结晶顺序和（3）矿物成分趋势，两者本质上与层状系列相反； (4) 构成边缘反转的岩石的共晶成分； (5) 由富含斑晶和不含斑晶的母岩浆形成的能力； (6)沿岩浆房底板、近垂直墙甚至顶板发育的能力。目前的岩浆房演化模型都无法对边缘反转的特征提供充分的解释。该问题可以在将岩浆房边缘的薄液体边界层中的索雷扩散和主岩浆体中的剧烈对流相结合的模型中得到解决。关键的建议是，边缘反转的形成是由于寒冷的乡村岩石施加的温度梯度导致液体边界层的非平衡演化而发生的。对边际反转镜像的基本解释是，非平衡索雷分馏的工作方式与产生层状级数的平衡晶体-液体分馏的工作方式相反。
Marginal reversals—a common feature of many basic differentiated igneous bodies regardless of their size and bulk composition—are remarkable in being a mirror of the Layered Series. These are distinguished by: (1) an apparent lack of mass balance between the lower part of the marginal reversals, including chilled margins, and the bulk composition of the intrusions; (2) mineral crystallization sequences and (3) mineral compositional trends, which are both essentially the opposite of those in the Layered Series; (4) the cotectic composition of rocks composing the marginal reversals; (5) the capacity to form from both phenocryst-rich and phenocryst-free parental magmas; (6) the capability to develop along the floor, subvertical walls and even the roof of magma chambers. None of the current models of magma chamber evolution can provide an adequate explanation for the characteristic features of the marginal reversals. The problem can be resolved in the context of a model combining Soret diffusion in thin liquid boundary layers at the magma chamber margins and vigorous convection in the main magma body. The key proposal is that the formation of marginal reversals takes place through the non-equilibrium evolution of liquid boundary layers as a result of a temperature gradient imposed by the cold country rock. The fundamental explanation for the mirror image of a marginal reversal is that the non-equilibrium Soret fractionation works in a manner opposite to that of the equilibrium crystal–liquid fractionation that produces the Layered Series.