Crystallization of Interstitial Liquid and Latent Heat Buffering in Solidifying Gabbros: Skaergaard Intrusion, Greenland
Crystallization of Interstitial Liquid and Latent Heat Buffering in Solidifying Gabbros: Skaergaard Intrusion, Greenland
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DOI：
10.1093/petrology/egu028
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发表时间：
2014-07
影响因子：
3.9
通讯作者：
O. Namur;M. Humphreys;M. Holness
中科院分区：
文献类型：
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作者：
O. Namur;M. Humphreys;M. Holness
利用斯凯尔加德侵入体层状系列(LS)、边缘边界系列(MBS)和上边界系列(UBS)中斜长石颗粒的新成分剖面，研究了堆积岩的固化机制和间隙流体的去向。资料表明，斜长石晶体在整个侵入体中表现出三种成分剖面，如下所示。(1)分带正常的谷物，以MBS和UBS为主。它们被解释为形成于晶体碎屑的顶部，然后被埋在堆积堆中。间隙熔体的结晶导致了液体的分异，并在斜长石岩芯上产生了正常的带状边缘。(2)主宰LS上部的非带状晶体也在糊状物的顶部结晶，然后埋藏在间隙液体分数较低的糊状物中或经历对流运动使液体保持恒定成分的糊状物中。(3)具有递减An含量的地幔和恒定组成的边缘的晶体。显示这种复杂分带的颗粒大多出现在LS的下部。根据侵入体内的地层位置，边缘的组成可以是An56、An51或An40。在主岩浆体中，这些成分(AN含量)与斜长石原晶(如岩芯)的成分(如岩芯)相对应，出现在堆石单斜辉石(An56)、铁钛氧化物(An51)和磷灰石(An40)的外观中。斜长石边缘的成分缓冲被解释为在晶体泥中出现新的间隙相时结晶潜热的增强释放的结果。当一个新的相饱和时，潜热对系统的全局热收支的贡献变得足够高，足以使间隙熔体保持在其液相线温度下一段时间，可能超过数千年。在这些条件下，平衡、堆积生长以及化学成分的扩散和可能的平流导致形成成分恒定的斜长石边缘(AN含量)。糊状液的有效热缓冲取决于糊状物的孔隙度(即，糊状物中液体的比例)和成分均匀度。在非均相和高孔隙率的混合体中，新相的饱和发生在混合体的最冷部分，在该相饱和时增强的潜热释放迅速散失到整个液体体积，包括在新相中尚未饱和的最热部分。因此，没有发生热缓冲，间隙结晶产生具有正常分带的颗粒。因此，各种类型的斜长石颗粒在斯凯尔加德侵入体中的分布可以用来推断晶体泥浆物理性质的空间变异性，如残余孔隙度，既有侵入体范围的，也有毫米到厘米的。
New compositional profiles across plagioclase grains from the Layered Series (LS), Marginal Border Series (MBS) and Upper Border Series (UBS) of the Skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid. The data show that plagioclase crystals display three types of compositional profile over the whole intrusion, as follows. (1) Grains with normal zoning, which dominate the MBS and UBS. These are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile. Crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores. (2) Unzoned crystals, which dominate the upper part of the LS, also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition. (3) Crystals with a mantle of decreasing An content followed by a rim of constant composition. Grains showing this complex zoning mostly occur in the lower parts of the LS. Depending on the stratigraphic position within the intrusion, the composition of the rim can be An56, An51 or An40. In the main magma body, these compositions (An contents) correspond to those of plagioclase primocrysts (e.g. cores) at the appearance of cumulus clinopyroxene (An56), Fe–Ti oxides (An51) and apatite (An40). Compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush. When a new phase saturates, the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years. Under these conditions, equilibrium, adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition (An content). Efficient thermal buffering of the mush liquid depends on the porosity (i.e. fraction of liquid within the mush) and the degree of compositional homogeneity of the mush. In a heterogeneous and highly porous mush, saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid, including the warmest part that is not yet saturated in a new phase. As a consequence, no thermal buffering occurs and interstitial crystallization produces grains with normal zoning. The distribution of the various types of plagioclase grains throughout the Skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush, such as the residual porosity, both at an intrusion-wide scale and at a millimetre- to centimetre-scale.