Kimberlite petrogenesis: Insights from clinopyroxene-melt partitioning experiments at 6 GPa in the CaO-MgO-Al2O3-SiO2-CO2 system
Kimberlite petrogenesis: Insights from clinopyroxene-melt partitioning experiments at 6 GPa in the CaO-MgO-Al2O3-SiO2-CO2 system
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DOI：
10.1016/j.gca.2005.01.012
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发表时间：
2005-06
影响因子：
5
通讯作者：
S. Keshav;A. Corgne;G. Gudfinnsson;M. Bizimis;W. McDonough;Y. Fei
中科院分区：
文献类型：
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作者：
S. Keshav;A. Corgne;G. Gudfinnsson;M. Bizimis;W. McDonough;Y. Fei
在这项实验研究中，我们研究了矿物熔体分配的主要和微量元素之间的单斜辉石和富CO2金伯利岩熔体在6 GPa的压力和温度为1410°C和1430°C。所产生的熔体含有约28wt%的溶解CO2，并且被橄榄石和单斜辉石饱和。为了评估温度、晶体和熔体成分对微量元素分配的影响，在CaO-MgO-Al 2 O3-SiO2-CO2模型系统中进行了实验。结果表明，除Al、Mg、Si和Ga外，所有元素在单斜辉石中都不相容。分配系数显示出相当大的星等变化范围，从DU的10− 3和DBat到DSi的10 − 2.5。两个实验运行显示出相似的总体分配模式，其中D值在1430°C下较低。稀土元素显示出宽范围的分配系数，DLa（0.012-0.026）比DLu（0.18-0.23）低大约一个数量级。分配系数为2+和3+阳离子进入M2-网站表现出近抛物线依赖于半径的掺入阳离子的晶格应变模型预测。这强调了晶体结构对控制微量元素分布所做出的贡献。使用在这项研究中获得的数据结合在已发表的文献中，我们还讨论了其他重要参数，即熔体组成，压力和温度，可能对分区的影响。我们的分配系数已被用来模拟一代的组I（GI）金伯利岩从南非。数值模拟结果表明，金伯利岩熔体可以产生的0.5%熔融的MORB型亏损源，已被丰富的小度熔体起源于类似的亏损源。这一结果表明，GI金伯利岩的源区可能位于岩石圈-软流圈过渡带。来自软流层的少量熔融物的渗流将在非对流的岩石圈下地幔底部附近产生一个交代层位。这种小程度的熔体的积累与挥发分和传导加热的存在一起将触发周围地幔的熔融，随后导致金伯利岩熔体的喷发。此外，我们的模型表明，GI源可以产生的2Ga古老的MORB源CA的交代。一千年前。假设MORB型地幔是地球上最枯竭的岩浆源，那么这就是GI源可能存在的最古老的年龄。然而，这个年龄更可能反映一系列交代事件的平均年龄，而不是单个事件的平均年龄。
In this experimental study, we examine the mineral-melt partitioning of major and trace elements between clinopyroxene and CO2-rich kimberlitic melts at a pressure of 6 GPa and temperatures of 1410°C and 1430°C. The melts produced contain ∼ 28 wt% dissolved CO2, and are saturated with olivine and clinopyroxene. To assess the effects of temperature, crystal and melt compositions on trace element partitioning, experiments were performed in the model CaO-MgO-Al2O3-SiO2-CO2system. Our results reveal that all the elements studied, except Al, Mg, Si, and Ga, are incompatible in clinopyroxene. Partition coefficients show a considerable range in magnitude, from ∼ 10−3for DUand DBato ∼ 2.5 for DSi. The two experimental runs show similar overall partitioning patterns with the D values being lower at 1430°C. Rare earth elements display a wide range of partition coefficients, DLa(0.012–0.026) being approximately one order of magnitude lower than DLu(0.18–0.23). Partition coefficients for the 2+ and 3+ cations entering the M2-site exhibit a near-parabolic dependence on radius of the incorporated cations as predicted from the lattice strain model. This underlines the contribution made by the crystal structure toward controlling the distribution of trace elements. Using data obtained in this study combined with that in the published literature, we also discuss the effects that other important parameters, namely, melt composition, pressure, and temperature, could have on partitioning. Our partition coefficients have been used to model the generation of the Group I (GI) kimberlites from South Africa. The numerical modeling shows that kimberlitic melts can be produced by ∼0.5% melting of a MORB-type depleted source that has been enriched by small-degree melts originating from a similar depleted source. This result suggests that the source of GI kimberlites may be located at the lithosphere-asthenosphere transition. Percolation of small degree melts from the asthenosphere would essentially create a metasomatic horizon near the bottom of the non-convecting sublithospheric mantle. Accumulation of such small degree melts together with the presence of volatiles and conductive heating would trigger melting of the ambient mantle and subsequently lead to eruption of kimberlitic melts. Additionally, our model shows that the GI source can be generated by metasomatism of a 2 Ga old MORB source ca. 1 Ga ago. Assuming that MORB-type mantle is the most depleted source of magmas on earth, then this is the oldest age at which the GI source could have existed. However, this age most likely reflects the average age of a series of metasomatic events than that of a single event.