Stretching chemical heterogeneities by melt migration in an upwelling mantle: An analysis based on time-dependent batch and fractional melting models
Stretching chemical heterogeneities by melt migration in an upwelling mantle: An analysis based on time-dependent batch and fractional melting models
复制标题
上涌地幔中熔体迁移导致的化学不均匀性拉伸：基于时间依赖性批量和分数熔融模型的分析
DOI：
10.1016/j.epsl.2018.06.042
复制
发表时间：
2018
影响因子：
5.3
通讯作者：
Liu, Boda
中科院分区：
文献类型：
--
作者：
Liang, Yan;Liu, Boda
上地幔玄武岩的源区是不均匀的，可能由亏损的背景地幔和富集型地幔斑块组成。上地幔中富集斑点的大小、形状和分布尚不清楚，但可能对控制玄武岩和残余橄榄岩中同位素比值和微量元素丰度的变化起到重要作用。在减压熔融过程中，间隙熔体的质量通量增加，而残留固体的质量通量从固相线向上减小，导致残渣中不相容的微量元素的有效迁移速度加快。因此，化学不均匀的斑点在其通过熔融柱的过程中被拉伸。在这里，我们通过允许地幔来源中的微量元素丰度和同位素比率作为时间和空间的函数来量化熔体迁移引起的尺寸变化。我们使用与时间相关的间歇熔融和分批熔融模型的简单解析解来说明在上升流和化学不均匀的熔融柱中，微量元素或同位素比率是如何在空间和时间上变化的。我们发现，以同位素或不相容的微量元素异常为标志的富集团在通过熔融柱的过程中，沿着熔体流动的方向被可变地拉伸。拉伸量取决于熔融程度、熔体提取的类型(间歇与分批)、熔融柱的孔隙率和分配系数，并且可以通过称为拉伸系数的无量纲参数来量化。对于放射性同位素U、Th、Pb、Sr、Nd和Hf，残留物的伸展系数预计为2∼8，通道熔体的伸展系数至少为30。对于大洋中脊之下的近分数次熔融，浓缩的Nd同位素信号通过低孔隙度基质所需的时间大约是通过高孔隙度通道的时间的10倍。因此，在残留橄榄岩和提取的熔体中观察到的化学不均质性在空间和时间上是解耦的，这对解释玄武岩和残留橄榄岩的同位素和微量元素特征具有重要意义。
The source region of basalt in the upper mantle is heterogeneous and may consist of depleted background mantle and blobs of enriched mantle. The size, shape, and distribution of the enriched blobs in the upper mantle are unknown but may play an important role in controlling variations in isotope ratios and trace element abundances in basalts and residual peridotites. During decompression melting, the mass flux of interstitial melt increases while the mass flux of residual solid decreases upward from the solidus, resulting in an acceleration of the effective transport velocity for an incompatible trace element in the residue. Consequently, a blob of chemical heterogeneity is stretched during its transit through the melting column. Here we quantify the melt migration induced size change by allowing trace element abundances and isotope ratios in the mantle source to vary as a function of time and space. We use simple analytical solutions for the time-dependent batch melting and fractional melting models to illustrate how a trace element or an isotope ratio varies spatially and temporally in an upwelling and chemically heterogeneous melting column. We show that an enriched blob as marked by isotope or incompatible trace element anomaly is variably stretched along the direction of melt flow during its transit through the melting column. The amount of stretching depends on the extent of melting, style of melt extraction (batch vs. fractional), porosity of the melting column, and partition coefficient, and can be quantified by a dimensionless parameter called the stretching factor. For radiogenic isotopes U, Th, Pb, Sr, Nd, and Hf, a factor of 2∼ 8 stretching is expected for the residue and a factor of at least 30 is found for the channel melt. For near fractional melting beneath mid-ocean ridge, an enriched Nd isotope signal takes approximately 10 times more time to transit through the low-porosity matrix than through the high-porosity channel. Hence chemical heterogeneities observed in residual peridotites and extracted melts are decoupled both spatially and temporally, which has important implications for the interpretation of isotope and trace element characteristics of the basalts and residual peridotites.
登录
查看更多内容
不平衡部分熔融模型及其对地幔熔融过程中微量元素分馏的影响
DOI：
--
发表时间：
1992
期刊：
影响因子：
--
作者：
Zhenwei Qin
通讯作者：
Zhenwei Qin
DOI：
--
发表时间：
1989
期刊：
影响因子：
--
作者：
F. Richter;S. Daly
通讯作者：
S. Daly
影响因子：
13.6
作者：
Liu B;Liang Y
通讯作者：
Liang Y
DOI：
--
发表时间：
1985
期刊：
影响因子：
--
作者：
N. Ribe
通讯作者：
N. Ribe
影响因子：
3.5
作者：
J. Warren
通讯作者：
J. Warren