Consequences of diffusive reequilibration for the interpretation of melt inclusions
Consequences of diffusive reequilibration for the interpretation of melt inclusions
复制标题
扩散再平衡对熔体夹杂物解释的影响
DOI：
10.1029/2001gc000205
复制
发表时间：
2002
期刊：
影响因子：
3.7
通讯作者：
C. Langmuir
中科院分区：
文献类型：
--
作者：
Elizabeth Cottrell;M. Spiegelman;C. Langmuir
熔体包裹体中的微量元素丰度常被用来解释熔融和熔体提取过程。然而，这些解释往往假设熔体包裹体的化学成分与宿主晶体生长的液体相同，即使捕获后扩散和变质的驱动力是明显的。本文开始利用数值模型来量化扩散对熔体夹杂物的影响。该模型计算了球形包裹体的成分演化，它最初与晶体主体处于平衡状态，然后跳跃到某一距离，并超过平衡状态。特别是，我们考虑了末端成员的情况，其中捕获的熔体最初与邻近晶体不平衡，因为这设置了重新平衡的最短时间。提供了一套数字代码，允许用户探索其他初始条件。该模型计算了包裹体成分的变化，以及当包裹体与周围晶体重新平衡时围绕包裹体生长的扩散晕的结构。在自然形成的包裹体中检测这些剖面可以估计自捕获以来的时间和初始包裹体浓度。再平衡程度受分配系数、扩散系数和夹杂半径的影响最大。具有高矿物/熔体分配系数的快速扩散元素被迅速改变，特别是在小的包裹体中。由于矿物对不同元素具有非常不同的D矿物/熔体，扩散再平衡的效果因矿物而异。例如，重稀土元素(HREE)在橄榄石中的分配系数较高，使得HREE浓度比轻稀土元素(LREE)浓度更容易改变。相反，斜长石包裹体中的锶、Eu和Ba比其他微量元素平衡得更快。对已公布的橄榄石包裹体的微量元素浓度的检查表明，几乎没有证据表明重新平衡，至少对于轻稀土和其他高度不相容的元素来说是这样。然而，由于橄榄石扩散系数和初始条件的不确定性，很难提供明确的限制条件。相反，斜长石中的微量元素扩散系数已被实验测定[Cherniak，2001]，已发表的斜长石包裹体的微量元素浓度表明，以与模型预测一致的方式与宿主进行了广泛的扩散交换。因此，包裹体后改造可能是解释某些熔体包裹体数据的一个重要因素。
Trace element abundances in melt inclusions are commonly used to interpret melting and melt extraction processes. These interpretations, however, often assume that the chemical compositions of melt inclusions are identical to the liquid from which the host crystal grew, even though driving forces for postentrapment diffusion and modification are demonstrable. This paper begins to quantify the effects of diffusion on melt inclusions using a numerical model. The model calculates the compositional evolution of a spherical inclusion which initially is in equilibrium with a crystal host out to some distance rjump and out of equilibrium beyond. In particular we consider the end‐member scenario, whereby the trapped melt is initially out of equlibrium with the neighboring crystal as this sets the minimum time for reequilibration. A package of numerical codes is provided that allows the user to explore other initial conditions. The model calculates the change in inclusion composition and also the structure of diffusion halos that grow around the inclusion as it reequilibrates with the surrounding crystal. The detection of these profiles in naturally occurring inclusions may allow the time since entrapment and the initial inclusion concentration to be estimated. The extent of reequilibration is most strongly influenced by the partition coefficient, diffusivity, and the inclusion radius. Fast‐diffusing elements with high mineral/melt partition coefficients are modified rapidly, particularly in small inclusions. Because minerals have very different Dmineral/melt for the various elements, the effects of diffusive reequilibration differ substantially from one mineral to another. For example, the higher partition coefficients of the heavy rare earth elements (HREE) in olivine make HREE concentrations easier to modify than light rare earth elements (LREE) concentrations. In contrast, Sr, Eu, and Ba in plagioclase hosted inclusions equilibrate more rapidly than the other trace elements. Examination of published trace element concentrations of olivine hosted inclusions show little evidence for reequilibration, at least for the light REE and other highly incompatible elements. It is difficult, however, to provide firm constraints due to the uncertainties in olivine diffusivities and the initial condition. In contrast, trace element diffusivities in plagioclase have been determined experimentally [ Cherniak, 2001 ], and the trace element concentrations of published plagioclase hosted inclusions show evidence for extensive diffusive exchange with the host in a manner consistent with model predictions. Postentrapment modification therefore is likely an important factor in the interpretation of some melt inclusion data.